72 chevy 350 running rough under load!
 

ok, so I don't think my problem is exactly unique, but im having trouble finding posts that share the exact same symptoms.
I have a 72 chevy c10 with a recently rebuilt 350. it has HEI distribution. also it has edelbrock e-street aluminum heads, mild crane came. edelbrock performer intake and a 1406 carb. also has headers and a dual exhaust.

it seemed to run pretty good most of this summer, but slowly got worse and worse over the course of a couple months. I found the spark plugs to show that it was running really rich. so I changed the jets and metering rods a stage leaner. it ran a bit better for a few days, and then seemed to be running rich again. it even started dieseling from too much carbon build up. so I leaned it out again to the number 13 on edelbrocks little chart, basically as lean as it gets. it still ran rich, but it wasn't so bad. then more recently it started running really rough under load. like if i was cruising on the highway in 4th (it has a 700r4) and then gave some gas it would start shuddering and shaking and there just wouldnt be any power. if I gear down and rev it up it still has a lot less power than it should and you can hear it backfiring out the tail pipes. it seems to idle and cruise ok, but sometimes the idle is high and sometimes its really low. also the gas mileage has become absolutely terrible. at the beginning of summer i got about 550km to the tank, a month later i got about 250km to the tank.

i also found that my total timing was at 44 degrees (probably because it used to be on propane) so I bought some new weights and springs and set it on the middle weight spring. idles at about 9 degrees and totals at 36 now.

I figured it must be a vacuum leak because the carb was making a whooshing sound like tons of air rushing in. it almost whistles. so i disassemble the entire intake manifold and carb setup. all the manifold gaskets were really stuck on, they peeled off like a sticker on both sides and didnt show any signs of any leaks. the carb and spacer gaskets were the same. I found that my brake booster port fitting was loose, so I re-sealed that and installed it nice and tight. also the rubber grommet on the brake booster was cracked, so I replaced that as well. I also cleaned out the carb and blew it out with the compressor.

upon putting it all back together it ran great for the first hour or so, maybe even a little lean. but now its back to missing and backfiring under load, sucking back tons of gas, and appears to be running rich again. there are definitely no vacuum leaks that i can find at this point. the carb was quiet for the first while, but now its making that loud whooshing sound, almost sounds like a turbo on a diesel. possibly the carb is getting dirty again, but i replaced the fuel filter at the same time as redoing everything else. I could try replacing the spark plug wires, but they were new with the rebuild and it feels more like a fuel problem to me. i'm at a loss here...

Re: 72 chevy 350 running rough under load!
 

A couple of possibilities>
Your fuel pressure might be too high. Eddy's like 5.5 max. Even though Edelbrock says 6.5 max, real world says 5.5 max.
It's running rich because it's flooding a hair. Especially when the carb heats up. Look in the carb with a light when it's warmed up and idling. You shouldn't see any fuel droplets.

Another reason for a rich smell is your idle is set too high. You're trying to idle on the power circuit. Bump your timing to 14 initial. Hook your vac advance to manifold vacuum. Your idle will speed up. Turn the idle down and reset your mixture screws.
Another indication of too high an idle setting is the deisiling. It'll go away with a closed throttle plate.

Re: 72 chevy 350 running rough under load!
 

like many times it is probably more than one thing going on at the same time...

this might be too simple, but 1st I would check the frame gas filter below the tank and before the fuel pump( these things can be forgotten sometimes) I think in '72 they still used the cannister housing with a cartridge in it...it might have some junk in it, that is breaking free & overwhelming the other filters... and was why it ran OK first then went south again...degraded ethanol jelly can congregate there. The problem you describe sounds just like a similar one my friend had from an old gas tank and fuel that stayed in there over a winter...possibility?

loss of power sounds like a vacumn leak like you already diagnosed ( especially if you can hear it) ...maybe when it gets warmer it opens up cracks that may be tight when cooler?? an older body might have been tweaked rebuilding?

the aftermarket ignition has given friends of mine w/ old chevys problems too??I still run old school points etc.

all you can do is isolate one thing at a time until its figured out...you will be a expert small block guy before its all over...good luck...and please post your results...lots to be learned here. This is old knowledge...

Re: 72 chevy 350 running rough under load!
 

Ill definitely look into the fuel pressure, but it did run fine before with this same setup, why would it be a gradually building problem if the pressure has always been to high?
As for Idle set screws, it doesnt smell rich at the exhaust at idle, or at least it didnt just after i redid everything and it was running great for that few hours. I already have the vacuum advance on the manifold port. and i've tried advancing the timing, but anytime i get over 10 degrees hot starts become a problem.

Re: 72 chevy 350 running rough under load!
 

the vacuum leak sounds like its coming from INSIDE the carburetor, so not really a leak but just a malfunction in the air circuitry inside the carb.

I don't think my truck has that filter under the tank, but i'll look for it. the tank had some fuel in it for 30 years since the po converted it to propane, but i replaced the gas lines and cleaned out the tank, so there shouldn't be any old sludge, plus once again, it ran fine last summer.

Re: 72 chevy 350 running rough under load!
 

THings get worn out. The crap gas we buy eats the needle/seat a bit over time. It might have controlled it till now but now it won't hold it back.

Re: 72 chevy 350 running rough under load!
 

that cannister filter is inside the frame about even with the rear body joint on the pass side...if its a chevy between 1965 -1972 it should be there...its worth a look...the cannister comes apart and there is an old fashioned cartridge inside (at least until '68...might be a replaceable metal filter after that...)if its the original tank its probably there...hey it might just be another little thing to get OK...especially with the moonshine they put in the gas nowadays...ethanol developes into a gel after 30-45 days...and has caused more havoc on old cruisers than can be imagined...

maybe it was removed(that filter) if not it will be obvious...

Re: 72 chevy 350 running rough under load!
 

ok new update on this very frustrating on-going issue. let me review of what goes on with my truck. at any temperature, but worse when it's cold the truck falls on it's face when i get too much into the throttle. from a dead stop, if i try to punch it, instead of roasting the tires like it used to it just fails. it pretty much completely stalls and then backfires out the carb. if im cruising in a low rpm and try to give it some throttle it will miss and backfire out the carb and just stumble in general, until it kicks down a gear and then it rockets to life. also gas mileage has gone to ****. the 7 and 8 cylinders seem to have a much much heavier carbon build up than the rest as well. those spark plugs were pretty much solid black when i pulled them out, all other six were clean, maybe a little rich, but still an extreme difference from 7 and 8. also when i pulled the heads off a while back i noticed the carbon build up was extreme on 7 and 8, and then gradually/progressively got better as you looked at the cylinders closer to the front 1 and 2.

things I've done to diagnose this;

- re-did the grounding on the engine block with a heavy gauge wire, cleaned off all connection points and made sure they were tight
- re-adjusted the valves while running, they were all fine to begin with
- re-torqued the heads, all was fine
- installed a new MSD distributor. (this made a big improvement. better curve, higher output etc. definitely ran better but main issue was still there)
- new spark plug wires (also a big help as one had burnt open, but issue is still there)

most recently this past week i finally gave in and brought the truck to a dyno shop. he had air fuel ratio sensors on both exhausts, vacuum gauge, fuel pressure gauge etc.
the first thing we noticed was that my air fuel mixture was incredibly rich, especially under load and low rpm's. now part of this I know is due to the terrible 700r4 that i have. normally a great transmission, but i let the shop convince me to get a hydraulic lock up. which is awful. it locks up every gear and makes the transmission extremely slow to decide when to switch gears either up or down. but with some testing we figured the spark plug gap was too large and as the edelbrock carb soaked the spark plugs they would begin to miss. sounded logical to me. we decided the edelbrock carb was junk and I would spring for a holly. so he has my truck for a few days and i get back there, 1900 bucks later. it definitely ran better. the holly has much better response and the air fuel ratio was where it needed to be. he said he couldn't get it to miss on the dyno, but its also hard to hear in there because the dyno is so loud. oh, he also found that the spark plugs i had didn't have a long enough reach. so he replaced them with the proper NGK plugs and gapped them properly.

so i drive it home and it seemed to run good, i thought i still heard an issue but i wasn't sure. by the end of day two of driving it it seems im back to square one. i can hardly give more than a quarter throttle when cruising without it stumbling and backfiring. this is so god damn frustrating.

any ideas what i should check? i'm at my wits end here, ready to drive it off a cliff.
my best guess is the intake valves on 7 and 8 aren't sealing properly because of carbon build up. anyone have any ideas why the rear cylinders would get more fuel to begin with? why would 7 and 8 be rich but 1 and 2 be lean?
anyone know where i can get an attachment that will allow me to hook up my compressor to the cylinder head to test the valve seats? any know of a way to clean them without taking the engine apart again?

thanks for any help you send my way. much appreciated.

Re: 72 chevy 350 running rough under load!
 

Almost sound like my old truck when my fuel pump was going bad - except that yours comes to life when it downshifts. Mine would fall flat.

Regardless, a compression check would definitely tell you something. I picked up an adapter (I believe) at NAPA. Or, you can get a leak down kit but it will be more expensive.

Aside from that several things could be happening -cam lobes going flat, valves not seated as you mentioned, valve guides (smoking at startup) or valve stem seals. That all needs to be checked but I would concentrate on the carb and ignition first. You might need to check timing, vacuum advance.

Is the HEI a GM or after-market?

Re: 72 chevy 350 running rough under load!
 

I think i might have figured it out today, or at least a big part of it.

basically it comes down to timing and vacuum advance. I'm running mechanical timing of 10 degrees initial with a 36 over all, so 26 mechanical degrees. and maybe as much as 20 degrees of vacuum (which i know is a little high). so at idle the advance is sitting close to a comfortable 30, and then when i rev it up it nicely climbs to 36 or so as the vacuum advance drops away and the mechanical takes over. at cruise its slightly over 40 i think. I know I'm off on some of my numbers here because i didn't memorize them, so dont be shocked if they sound off.

so basically what is happening here, and i confirmed this with the timing light, is when i tromp on the throttle from a dead stop, the vacuum advance falls completely away. a heavy amount of fuel is now in the cylinders and engine speed hasn't picked up enough yet to bring the advance up. so im basically asking my engine to burn tons of fuel on 10 degrees of advance. so once im up in rpm being heavy on the throttle is fine, as the advance is there to burn it. but from a stop it's just impossible

also, if i'm cruising at a low rpm like 1600-2000rp,, the mechanical advance is low, and if i get too heavy in the throttle, so is the vacuum advance. once again asking the engine to perform on heavy fuel and low advance.

so according to the tuning guy i brought the truck to, I need to get my distributor adjusted. i basically need more initial advance, coupled with less vacuum advance and less mechanical. that way when i'm cruising the total wont get too high, yet when i tromp on it and the vacuum goes away there wont be such a low advance.

this sounds like the most promising solve yet. but it still doesn't explain why the two rear cylinders are getting more fuel than the rest. and it doesn't absolve the valves from a possible sealing problem. that's something i have to look into still. I'll keep you updated.

Re: 72 chevy 350 running rough under load!
 

My truck has a mild 350 with Trick Flow heads, rv cam and Edelbrock 1409. It screams; and it's the first time I have been impressed with an Edelbrock carb. The 1409 comes jetted richer than the 1406. The 1409 has worked great on a couple other vehicles too. I have a new 1406 and it is a dog on every car I try it on; falls on it's face when I tromp on it. I plan to buy a calibration kit for it to richen it like my 1409 but in my experience these 1406's seem too lean. Yours was too rich on the dyno so maybe it is only the needle and seat leaking. I have never had luck with a Holley but I'm sure a dyno guy can set one up properly.

Mine also has a couple bent valves from a past over-rev I suspect. I can not get a smooth idle out of it and a vacuum gauge bounces like crazy. However it does run nice once it's going down the road and I can't feel that roughness at all at highway speeds or tromping on it. I don't think some leaking valves would cause your problem.

For peace of mind you probably should do a compression check and a vacuum check. Your timing specs sound close enough to me. In my experience too much timing advance will cause it to cough. You verified that when you tromp it the vacuum advance goes away and the engine should be happy enough with that 10 degrees of advance. It would be nice if you could try a carb off a good running engine to rule that out. I think I suspect the carb. Good luck

Re: 72 chevy 350 running rough under load!
 

definitely not the carb. the fuel/air ratios are right where they need to be. and the 1406 was always showing a very rich condition on the spark plugs, so a richer 1409 is not where i needed to go. this holly is definitely an improvement. and getting it professionally ported and built was a good idea. im pretty sure timing or valves are my issue. a leak down test is in order, then a recurved distributor. i'll let you know how it goes.

Re: 72 chevy 350 running rough under load!
 

Timing!!
LIke I mentioned right off the bat!
Go figure!!
And your Eddy was rich smelling because of one thing. You where idling on the power circuit.
Which I also mentioned.
But you chose to ignore and keep spending money. And a lot more than I would to cure it. And you're going to waste a bunch more money. THat's good for the mechanic.
Now you need to figure out your timing curve!
I'd tell you what I ran but you'd ignore that too!
At least your mechanic will have a rich xmas!

Re: 72 chevy 350 running rough under load!
 

ok, i can admit when i'm wrong. thank you for so humbly pointing out that you were correct. however, you didn't really explain why my timing was off and what adjusting it would do to fix it. also putting it to 16 degrees would still most definitely give me hot start troubles, not to mention a pre-ignition problem with the amount of timing my distributor already has. but, bumping the initial up to 10-11 has made a big difference as well.

i still disagree with you about putting the vacuum advance to manifold. there is a huge post by a long time GM engineer who worked with distributors on the hotrodders forum who explains why it should always be on ported vacuum unless you are running newer emissions stuff.

also my idle screws on the edelbrock were most definitely not running on the power circuit. the air/fuel ration when i put the truck on the dyno proved that. and whatever anyone says, a professionally tuned holly is a million times better than an edelbrock tuned by me.

please go ahead and tell me what kind of timing you're running.
what's your initial (16 you said?), what's your total mechanical? and what is your vacuum advance? I will gladly take your numbers into consideration. but every application is different so please don't assume you're always right.
thanks for your help though.

Re: 72 chevy 350 running rough under load!
 

A waste to try to explain your timing if you're off trying to find a reason to kill your eddy carb.
Gm engineers don't live at 3500 feet and drive in real world conditions so don't beleive the crap they put out. THey drive and tune absolutely stock motors and tune accordingly. Usually no initial timing plus lots of added vac advance to meet emissions. Wonder what his story would be if he didn't have ported vac available. Cause if he was from the sixties he didn't. Every motor/tranny combo is different. Manifold vacuum for the dizzy is a tuning tool.
Best way to find out whether your carb is running on the power circuit is screw in a mixture screw all the way and see when the motor starts to quit. Try that on your new holley.
Simple cure for hard starts is a power interupter switch to your dizzy (if needed).
My timing curve when I lived in west central Alberta was 22 degrees initial, 12 degrees in the dizzy and 12 in the vac pot. 46 total. 34 all in by 3200 rpm.
No power switch. Fired immediately hot or cold.
350, 11 to 1, holley 3310 750 cfm, edelbrock performer rpm, trick flow heads, 300* duration, .520 lift cam and headers. Dizzy was stock points with pertronix conversion.
I'm not always right, never said I was. Been tuning on motors of all kinds for 50 years so think I've learned a few things.

Re: 72 chevy 350 running rough under load!
 

Ported vacuum is above the butterflies. Manifold is below and always has vacuum. There's a lot of controversy on the subject, mostly because it's misunderstood, but the majority of articles I've read about the subject say to use manifold vacuum.

The reason ported vacuum was introduced was for emission purposes. Engine exhaust runs hotter on ported therefore less hydrocarbon emissions. Good where states have strict exhaust emission standards but that is not necessarily good for the engine performance and even gas mileage.

One thing I found out about these older engines is that no one number works for all. Most of the time they're a starting point and we have to determine which works better by trial and error.

Re: 72 chevy 350 running rough under load!
 

everyone who thinks they know everything about ignition timing should read this article. sorry to insult anyone but everyone i talk to seems to think they always know what they're talking about, yet no one gives the same answers.



http://www.hotrodders.com/forum/dist...1-a-59033.html

sorry i have to note my error here. I am in fact running manifold vacuum, I simply called it by the wrong term assuming it was ported.

Re: 72 chevy 350 running rough under load!
 

your timing is basically what i think i need. although 22 initial sounds super high, i wouldnt put it out of the question.

your mixture screw process is how i always tuned the idle. just gave it a try on the holly as well.

Re: 72 chevy 350 running rough under load!
 

Yeah, I figured that's what you meant. I read the John Hinkley article a few years ago and he really explains the differences well.

Re: 72 chevy 350 running rough under load!
 

You spent $1,900 at the dyno shop and still have the same/similar problem?? The dyno guy should have been able to tell you if you have the wrong timing curve in the distributor. You said it was a new MSD distributor? The paperwork that came with the distributor should tell you what the current timing curve is, giving you a baseline so you know what you need to do to correct the timing issue, if in fact timing is your issue.

As has been said, there are a lot of different points of view when it comes to vacuum advance. In my case, my vacuum advance is not used. The GM engineers said not to hook it up and I get all the total advance I need. My motor runs great. I get way too much total advance when I use the vacuum advance.

Re: 72 chevy 350 running rough under load!
 

Read that before. The subject comes up once in awhile over there. Some say manifold vac to the dizzy, some say ported, some say none but the majority say manifold vac to the dizzy.
You say the engineer said not to use manifold vac but I think you should read it again. He strongly recommends it. Calls 'ported vac' an atrocity.
If you're going to quote something and print it you better be accurate.
So enjoy your ill running motor hooked to ported vac with no initial timing and your new holley that's likely idling on the power circuit already. Everytime you fill up, just pour a gallon or 5 litres on the ground. It's the same thing.

Re: 72 chevy 350 running rough under load!
 

The engineers said not to use manifold vac for only one reason. Warranty!
So put a limiter on your vac pot to cut it in half down to 10 degrees.

Re: 72 chevy 350 running rough under load!
 

My timing curve isn't what you need. I ran a 3.11 first gear 4 speed and 3.73 rear gear. Spent very little time in the lower rpm's.
What tranny you got?
What rear gears?
What's your idle rpm?
How many turns out on your mixture screws?
How far will either one turn in before it starts to quit?

Re: 72 chevy 350 running rough under load!
 

i have a 700r4 with a shift kit and unfortunately a hydraulic lock up.
I'm not sure what the rear end gears are. somewhere around a 2.9 something or a low 3 something. its a 12 bolt posi rear end.
idle is at 750 in neutral, but the tranny pulls it down to 500. (thanks again to the hydraulic lock up).
mixture screws can go about a half turn in before they effect the idle. im pretty sure the dyno guy set the air fuel ratio around 15 at idle. number doesnt mean much to me but to you maybe.

i tried turning up the initial timing to 12 degrees, as it was sitting at 8 (which is lower than i thought i had it at before) anyways, big improvement on the off the line power. but i didn't have much room to test it before i started hitting ice. the only worry is that with the high vacuum setting i have, at a high rpm cruise or even a quick throttle release from a high rpm can bring my total timing over 50. so I don't know at what point pre-ignition would start, but i'm keeping my ears open.

as i mention earlier, i was wrong about what word I used to describe what vacuum i am using. I have been using manifold vacuum this whole time

also how do i reduce the vacuum advance? can i braze a new stop point on it some how?

Re: 72 chevy 350 running rough under load!
 

Can I throw something out? I mean, I'm far from an expert...

Forget the timing for now, 250rpm drop when you put it in gear. IMO, normal (ok, little high, but that can be adjusted. quit farting around adjusting until something works. Currently your going in circles.) My non lock-up trannys do the same, always have. Personally, I shoot for about 750-800 in drive. Forget what it idle's at in neutral for now. (unless it's like 1200 rpm, that would be another issue.)

Have you driven the truck without the OD? How does it act in just drive no OD?
Reading the symptoms you initially posted, you could be just lugging the engine down. With what your describing at cruise, if it was a standard, downshift and go.

The 700r4 may not be doing you any favors in this case. geezer#99 may be on to something when he asked for your rear gear ratio. 2.xx-3.xx with OD may NOT be the combo the engine wants.

What's your highway cruise rpm?

Back in the day my father had an old 6 cyl pickup. About once a month, we'd swap trucks. He was soo gentle driving it, it would load up and start fouling plugs. Let the teen drive it and "blow it out". he used to say it was night and day difference.

This may not be the issue, but currently it seems your chasing your tail, time to consider other possibilities.

Re: 72 chevy 350 running rough under load!
 

You're not an expert Donut but you're thinking like me. I suspected the normal lock up o/d no rear gear scenario too. Good at cruise but till you get there a real dog. My 84 with turbo 350C and 3.42 shifts into lockup real quick and runs the same till I get up past 30mph. I shift mine manually. Put it in second at the lights and shift to 3rd when I get up to 25-30mph.
As for shifting manually it won't hurt it. Some people hook a switch into the lockup circuit to delay it but usually forget to flip the switch.
FWIW your hydraulic lock up if it's working full time will drop your idle rpm from 750 rpm right to zero when you put it in gear.
Don't worry about seeing over 50 degrees at times. Some factory set ups had close to 55 degrees total.
Your dyno guy or you or anybody can't set your a/f ratio with the mixture screws. You do that with jetting and/or metering changes. Your mixture screws are for idle mixture.
The other major problem you got right now is your local weather. Waste of time tuning until it gets back above 15*C.
If your motor tolerates 12*, starts and runs and doesn't ping then try shifting manually until spring. Right now you're banging your head on the wall.
And my apoligies for coming across as such a hard....s!

Re: 72 chevy 350 running rough under load!
 

Couldn't remember exactly how the lock-up worked (only turbo350c I've had was in the mid '90's) But everybody was looking at the engine and missing the elephant in the room.

You mentioned the different curves and got me thinking. If mine was acting like the OP's, drop a gear and go. I had similar issues with an engine when I lost 2nd gear in a 3 speed.

Didn't even consider the air temp. Usually when it got cold enough to need gloves, the tools got put away. Course I have been down south for a couple years now...

Re: 72 chevy 350 running rough under load!
 

ok so i did some more testing and diagnosing and I think I've gotten closer to discovering the main problem...

I took the vacuum off the carb and plugged the port. turned the initial timing up to twenty and then took it for a drive. I was also careful not to rev it too high as i didn't want the total timing to come up too high. the purpose of this was to see if the motor would miss under load and low rpm even with the timing higher. I originally thought it was missing because under load the vacuum advance would disappear and without the engine reving high the timing would be around 8 degrees. well even with it set up at 20 degrees the problem persists, if not even worse. so I checked what my gut has been telling me the whole time.

I rigged up my air compressor line to connect to the spark plug port. so far i've just tested two cylinders, number 1 and number 7. I've often thought 7 and 8 were problem cylinders and the front cylinders were fine, based on the coloring and carbon built up (i think i wrote on that earlier). however, both 1 and 7 had bad leak down problems. they would leak at any pressure I threw at it 30 lbs, 100 lbs. I can hear the air coming up into the valve cover through the lifter galley, but i of course dont know if its leaking past the head gasket or the piston rings. I took the rocker arms off and found top dead center when i tested both cylinders, that way i know the valves were closed. I'm going to find a proper leak down tester and get the numbers and then talk to the engine rebuilder guys here in town. question for you though!...

How can I have a bad leak past the pistons or head gasket and still come up with good numbers on a compression test?
I was getting about 70-90-120-150 on every cylinder when doing the compression check.

Re: 72 chevy 350 running rough under load!
 

A compression test is more or less a quick way to determine ring/valve condition but is not as accurate at pinpointing where the problem is as a leak down test.
At this time, if the engine is still running rough and you have eliminated ignition, timing and/or vacuum leaks, then it's time to think about rebuilding.

If your engine is high mileage it may be rings, valves or both may be worn. Usually a head gasket leak can be detected by two adjacent cylinders having low compression compared to the rest. With a leak down test you can listen for air escaping between two cylinders. However, a head gasket leak may be harder to detect if it only acts up with engine running.

Re: 72 chevy 350 running rough under load!
 

-I'm assuming that the carb float level is correct or a tad low?
-That ignition timing is correct?
-The advance springs and weights you used were plotted and not just thrown in because "the internet said so".
-All 8 cylinders are firing? (for that matter, why keep the dizzy that was set up for propane instead of replacing it with a new one?)
-break in was properly done.
Set the carb back to stock, iirc your about 2 stages leaner now.

Take truck to engine shop, ask them why would those two cylinders are acting like that. One of the techs should be able to diagnose the problem (compression, leak down ...) and decide which direction to go.

This way, you both can decide whose problem it is. (if it's not an engine problem, they can probably test it cheaper than you can.)

Re: 72 chevy 350 running rough under load!
 

ok I'll give as simply as a breakdown as i can to tell you everything i've tested/redone

- engine ground
- full voltage at distributor
- new high output msd distributor
- new spark plugs
- new spark plug cables
- heat protection boots on cables
- new holly carb, professionally dyno tuned by a guy who im told is the best in western canada
- redid the intake manifold gaskets, the originals did not have any leaks
- re-torqued the heads
- tried removing the vacuum advance and bumping initial timing to 20 degrees to see if that would help with the off the line failures. it was worse if anything
- tried running sea foam through the carb and in the engine oil. seemed to change the running a little bit, but not necessarily for the better, and it was a short lived change
- did a vacuum test (at the dyno shop) checked out fine
- fuel pressure test (also at the dyno) also fine
- air/fuel ratio checked out fine after the new carb
- re-did all the valve lash stuff. 3/4 turn with hydraulic lifters
- checked to make sure the timing mark on the harmonic balancer did in fact match up with TDC on the marking plate - it was bang on
- did a leak down test. all cylinders were around the 15-20% mark, which i was told is fine for a cold motor.
- compression test, all cylinders fired approximately 70-90-120-150... so im all good there.

other things of note, i've never had any oil show up in the radiator, no rad fluid in the oil except the one time my heads came a little loose, but i fixed that. i've never had any metal shavings come out with the oil. the break in process was done exactly as they told me to. I had the proper oil with the zinc and everything, ran it for 20-30 minutes varying the rpm from 1500-2500 every few minutes.

any thing else I should check on?

Re: 72 chevy 350 running rough under load!
 

I think you may have a strong point here. i was reading some other forums about hydraulic lock up torque converters and one thing that came up was that with a lower gear ratio set up like mine, engine lugging can be a big problem. also, I've come to realize by reading those posts that my hydraulic lock up converter is set up terribly thanks the the transmission shop that put it in. they are supposed to have a spring pressure that keeps it from locking up until a high speed, but mine locks up the moment it shifts, and it locks up every gear. i've always hated the way my transmission worked, but the shop wanted to charge me 1000 bucks to remove the lock up. I didn't know that it was actually operating incorrectly, and i had no idea it could seriously effect the way my engine runs.
I'm going to put in a new post in the transmission section about removing or adjusting the hydraulic lock up. and once i figure that out we'll see how my truck runs. if it doesnt help the running, at least it will help my transmission issues. it would be nice if it killed both problems at the same time though :)

Re: 72 chevy 350 running rough under load!
 

Don't thank me, geezer#99 hit on it before I did. Glad your exploring other options though. At least rule them out before you go back to the engine.

Re: 72 chevy 350 running rough under load!
 

last update here. on another thread a guy told me to check for a ball blocking a solenoid in the transmission. it was there and I removed it which also removed the locking up function of the transmission. for a couple weeks it still would fall on it's face a bit, but it's been getting better. it's basically healed itself somehow. I guess the lugging on motor had something to do with it, because it seems to run much better now.

Re: 72 chevy 350 running rough under load!
 

Ok, after messing with a simular issue for about 6 months and never finding this solution on line figured I'd share it as it pretty much matches his issues.

Check your Choke!

Someone on my car connected the choke line to the Dizy +12vdc so neither were getting the current they needed, meaning the choke did not fully open and what happens when your choke does not open? You run rich and have a lack of O2 when hot and load up the engine when trying to take off, car sounds like a lawn mower ect....

Hope this helps someone.

Ken:5150::5150:http://67-72chevytrucks.com/vboard/i.../bryan.gif.gif

Re: 72 chevy 350 running rough under load!
 

What became of this?
Having a similar issue. 350 vortec but with carb, ac delco fuel pump, one used with boats.
Seems like it started happening when I changed the condenser in the HEI because the connector clip broke. I grabbed one from Napa thinking it was better quality than other store brands. I use a quadrajet too it’s at about 7psi. I have driven this like this for several years and this issue just popped up.

It’s weird it doesn’t always do it. I went back changed cap, checked to make sure capacitor, coil were okay. Doesn’t smell rich either just hesitation at 2200 rpm, can often accelerate past the spot but not always.
Good information on this thread mine maybe as simple as bad gas.