Need some help (motor guys). Long post

[Psycho71]

Let me start by saying I've always felt like a pretty capable guy when it comes to SBC's. But I'm having some trouble isolating a problem, and want to see if anyone here can point me in the right direction.

I have a 350/350th in my '71 truck. I did a fairly extensive partial rebuild on the truck last winter, but kept the same motor in it for now because it seemed very sound, and I think it still is. It never burned a qt of oil, or fouled a plug in the couple thousand miles I put on it before the teardown.

So here's my problem...It's stumbling when I apply up to 1/4 throttle to it. Then it pics up and runs like a top. When I let off the throttle from just about any speed, it pops thru the muffler. It will do all of this when in park/neutral (no load), and when driving (much worse under load). It even idles like crap when I drop it in gear.

Here's what I've looked at so far:
Plugs....Look great (barely tan on the porcelain)

Inspected wires (new accel wires)

Took cap off of dist. (skip white HEI dist) and inspected everything (all looks good, and nothing is bound up on the dist.. It has a clear cap, so I can see it firing, and see the vacuum advance working).

Sprayed carb cleaner all over the vacuum lines, and around the carb itself (Holley 670 avenger, rebuilt 700 miles ago). Also went inside and sprayed carb cleaner on the vacuum actuators for the a/c controls. No change in idle speed.

Took carb off, replaced the base gasket just for grins (all carb functions seem to be right).

Disconnected the vacuum modulator on the trans, and plugged the line to the carb.

Same with vac. advance.

None of this has made any difference with the problem. I was driving the truck last Friday. I went to work, about 12 miles from the house. Then to the bank. Then to the local walmart to get a hunting license. Everything was great. So I get in it at wally word to come home and kiss the wife by (going hunting), and the problem is there. Drove it home, unloaded all my hunting stuff out of it, and had to actually drive a ford to go hunting (which really pissed me off). So it all started at once! Hasn't gotten any worse. But also, nothing I've done so far has helped it one bit. It does this while cold and at operating temperature. But if I stab the throttle, it wakes right up (after a hard stumble) and runs fine.

The way it's popping thru the mufflers on deceleration makes me think it's in the timing/ignition (not burning all of the fuel, or firing late). I know anything can go bad, sometimes no matter how new it is. But everything except the longblock on this truck has less than 1,000 miles on it right now. All new stuff. And the motor is on fine shape internally.


Any advice????? I'm stumped, without just starting to throw money at things. Which is nothing but a shotgun approach. I'd rather be a little more precise than that.


Thanks for any advice y'all can offer. I do not drive the truck daily, so time is not too much of a factor. But I would like to get this thing going so I can drive my baby when I go back hunting this weekend, if possible.

Again, thanks in advance.

Jay

[HomerJ]

Like you (if you start replacing parts), I'm shooting in the dark here....

It sounds to me like it's running lean at idle and low RPM's. When you did the partial rebuild, did you make any changes to the carb, like maybe closing the butterflies a little bit?

HomerJ

[drbyrdman]

is your distributor of by a tooth or more??

[ratrod67]

Did it backfire through the carb? What carb are you using? If it's a Holley, a backfire will blow the power valve and cause most of your symptoms.

[skchevota]

Have you checked your base timing to see if it jumped. also did you check the mechanical advance in the distibutor( take the rotor off and check the springs and weights). If it is missing a spring or stuck advanced it will cause trouble. Now for the carb, check the powervalve as mentioned, that will usaly cuase it to run rich but is worth checking. You should also check your fuel filter, and fuel pressure. Next take out the site plugs in the side of the carb and check your float bowl levels. If all that checks out fine, and it ran fine before, I would start looking for a vacume leak( possible around the manifold or the brake booster gon bad)

[Longhorn Man]

check to see if the vac advance diaphram is busted.
you said disconnecting it made no chainge.
Plus, my skip white one popped the vac advance diaphram.
plus it goes along with what you were already thinking;

The way it's popping thru the mufflers on deceleration makes me think it's in the timing/ignition

[Psycho71]

It is a Holley carb. But it does have power valve blow-out protection (like all Holleys less than about 10yrs old now have). It never backfired thru the carb. But I am gonna tear into it tonight to verify that the P.V. is still good. No changes to the carb that I made, that would have caused this. It ran just fine for over 700miles, then this problem began suddenly. I rebuilt the carb when I did the build on the truck simply out of precaution. The carb had sat up all winter dry. So I put a kit in it just so I wouldn't have any troubles on start-up. It really didn't even need it. The fuel pressure seems fine. I did check the float levels, and they were perfect. Just a slight dribble when I rock the truck side to side. And, I my alleged brain, if the bowls are staying full, the pump, filter, and general flow to the carb is OK. Fuel is not pouring out of the sight plugs, so I know the needle and seat is not stuck open.

Vacuum advance...I'm gonna tear into the distributer this evening as well. I do have a fairly new, extra, HEI in the garage. So I'll probably throw it in there just to see if it makes a difference. But there again. I can pull the line to the vac. advance, and it still does it. I can suck on the line, and the diaphram will hold vacuum. And I can watch it function thru the clear cap.

I will also inspect the springs on the mechanical advance to verify that operation is correct.

No brake booster, no power brakes. So that isn't it. I went over every vacuum line, and every vacuum operated control with a can of carb cleaner, never changed the idle speed one bit.


I do have an air fuel ratio gauge on the truck, and an O2 sensor in each header. It's always shown a rich condition when either on, or off the throttle. When driving at part throttle, it shows lean. Rarely does it ever settle in the optimal range. But I knew it would never act like it should on a carb'ed vehicle anyway. Just thought it might look cool in the dash. It does, I guess, but it is otherwise pretty much useless.

But, the plugs read out as it is running right on the mixture side of things. The porcelain is a light tan color. No white (indicating lean), and not brown (indicating rich). But i can also see where this problem might not readily present itself on the plugs, due to it's intermittent nature.


I'll mess around with it somemore this evening, and see what I can come up with. If I find anything, I'll post what it is. If not, I'll likely be back here looking for more advice.

Thanks fellas. If you have any more ideas, I'm all ears.

Jay

[haysonj]

Get a vac gage and check it should be 17 to 22 hg and steady at idle. Also do a compression check. these two things will give you a lot more info. something could have changed in the valve train

[soba_03]

I just rebuilt a 355 from ground up and had 2 cylinders that didn't seem to want to fire... I found that my valves were set to tight. just off the seat.

Check compression, if you had the heads off. Throw a gauge on it and check for a valve off it's seat. Should be about 180 if it's fresh, but a SBC will run as low as 50-60.

I just got mine up and running, thanks to those on this board for the help. It only takes about a 1/4 turn or less to push a valve off it's seat.. Good luck

[70rs/ss]

I had a header gasket leak cause a really crappy idle and it popped through the exhaust on throttle let off (cold oxygenated air entering into the exhaust gets burned quickly), I chanaged the header gaskets at the head and no more issue. The module may be weak as well. I am not like everyone else in that I have no fewer than 3-5 of every part that could be wrong including carbs, dizzy, wires, etc, etc so I can pop a carb off and replace to see if that makes a differenc, the same with an hei module, have spares. It helps so I don't spend $$$ replacing stuff that's not broken. Good luck.

[Painter D]

My guess is something to do with the distributor. I'd start there first. Hope this helps.

[drbyrdman]

check the dizzy. I put mine in 180 degrees out one time and off a tooth or so and it will do that. and if you have the spark plug wires on the wrong plug it will do it also. all learning experiences.

[Psycho71]

OK, I finally found the problem.

Here's a pic of it.




I'd heard about a bad batch of these carbs getting out. I guess I got one. Holley's gonna make it right though. Just in case, I ordered an Edelbrock too. The replacement Holley might just sit on the bench a while.


Thanks again for all of the advice fellas.


Jay

[truckdude239]

i was goig to same wires i installed a new set of accel super stocks on ym truck and they urned out bad glad you found the problem

[69ChevyRazorback]

man you guys know some good info on engines. that is one area that im not great on. I wish i had half the motor knowledge you guys have. good stuff. guess I will just have to keep reading and studying.

[71cadc10]

I wonder if the fairly recent addition of up to 10% ethanol has anything to do with this? Did Holley mention it? Was this the chrome plated carb or just a regular finish...looks plated to me.

[ChevLoRay]

Okay, I'm on a learning curve. Tell me (and others) what it is that you found on your Holley. I run Edelbrock Q-Jet on mine, never having owned a Holley. Pointing out the particular problem may help someone else who is unfamiliar with Holleys, like me.

Glad you found your problem.

[Psycho71]

They did not mention anything about Ethanol.

These carbs are not really chrome, but do have a silver finish (might be chrome I guess), as opposed to Holley's old gold colored zinc finish.

I had heard a while back that a in the coating process of a certain run of these carbs had problems, resulting in the plating/coating coming off. I guess I got one.

But I've been in contact with Holley, who says I should have my replacement by the end of the week. And, they're sending me a return shipping label to send my old one back to them (which he said they'd destroy).

I couldn't be happier. This carb is almost three years old. They could have told me to "eat it". But their doing the right thing, and I applaud them for it.


Jay

[Psycho71]

Originally Posted by ChevLoRay

Okay, I'm on a learning curve. Tell me (and others) what it is that you found on your Holley. I run Edelbrock Q-Jet on mine, never having owned a Holley. Pointing out the particular problem may help someone else who is unfamiliar with Holleys, like me.

Glad you found your problem.

Look at the pic I posted above. It's the bowl side of the primary metering block. See all of the finish flaking off? That is the problem. Either some loose material was stopping up a jet, or the same thing is happening inside a small passage of the plate. Either way, the problem's presenting itself as a lean condition on one side of my motor.



Jay

[ChevLoRay]

Okay, I was seeing the inside of the carb and noticed the flakiness, but couldn't really tell for sure what I was seeing. I couldn't discern whether it was a finish issue or flashing on the casting that I was seeing. But, I see now. Ethanol is the culprit? You ain't running Exxon, either? They say they don't use ethanol in there fuels. Yeah, I don't support them either, but I'm happy knowing that it takes 1 gallon of fossil fuel to produce 1.2 gallons of ethanol. Gotta be good for big business. Oh, and raise the price of groceries, too, I betcha! Kinda like buying watermelons for a dollar and selling them for the same price. Need to deal in volume if you ever want to make any money!

[Psycho71]

Update!!!

Got the new carb on Friday. That's outstanding customer service IMO. I called them with the problem Monday morning, Emailed them pics of the problem and shipping info Monday evening, and the replacement showed up Friday evening. The replacement is brand new, in the box, with all of the hardware I'd have gotten if I bought it new.


While I was kinda cheesed off about having the problem to begin with, I couldn't be any happier with the way Holley addressed this issue.

I've bolted on a Edelbrock 1400, and it runs right. Not nearly as torquey as the Holley was, but definitely a bolt-on and go installation. But I think I'm gonna put the Holley back on, as it ran much better, didn't get any worse mileage, and I know how to work on it. Either way, I'll be seeling one of these carbs fairly soon. I just have to decide which one.


Thanks again for y'alls help.

Jay

[kcjones]

I had a very similar problem with my street avenger 670! It was the opposite symptom - to rich, but same core cause...flaky finish inside the bowl lodged in the power valve (and somewhere else), and caused fuel to just spill down the intake. Couldn't idle, ran like crap. It took like 3 or 4 trobleshooting steps to figure it out. Wish I had known this was an issue, would have saved myself hours of time and the neighbor complaining about the gas fumes! Eventually, I just bout a new 4160 and everything has been beautiful since.

One thing you may benefit from (I sure did)....don't run your dist vacuum advance off that special vacuum port, the timed one, run it off normal vacuum. It helps with idle quality, engine temp at idle, and fuel ecomomy. The timed vacuum thing was primarily aimed at emissions, but doesn't help much anyway.