78 Suburban surging then dies on long grade

[RyanAK]

Well... I’ll definitely get a spare module. It did seem to be temp related. Warmer than normal days and after running for a bit. Hasn’t been nearly as warm lately. Aussie mentions the module above as well. Shoulda just bought a new one to pop in when the distributor was apart. But it sounds like I’ll be opening it up again....

...because I used the light springs. I assumed RPM is RPM regardless of vehicle weight. Advance seemed steady at 32* all in at 3000 rpm. That was Saturday. I’ll check again to see if the timing moved. I wouldn’t have thought to mix springs... was just going by the timing curve on the paper and it seemed to give me full advance before 3000.

I also used the Mr. Gasket weights. The do seem like a lesser part than the stock weights... but wasn’t sure if the springs and weights in the kit were somehow calibrated to each other. I’ll likely swap them back to stock weights on your advice when I get a module.

[burnin oil]

When I played with the Mr gasket wieghts they would sling out quick and hang about where you would want it to then at higher rpm, say redline, would give a few more degrees. More importantly they would stick on decel.

Think of the advance springs this way. The lighter the spring the faster they give advance. There should be roughly 25 degrees of advance in the weights. If it comes in super quick its like setting the base timing at that. Can your motor tolerate and use 20 or 30 degrees of timing at less than 1500 rpm? From memory I was advancing at 1200 rpm. This is more in step with a race motor with locked out timing. Now add in around 15 degrees of timing from manifold vacuum hooked to the vacuum advance and you can see where part throttle can get really jacked up quickly. It will still run when you hammer it since vac advance will drop out but part throttle can be a pinging mess that wants more octane. The goal is to bring it in as fast as you can without drivability or pinging issues. This will gain the power you are looking for. Also if the springs are to light the weights are not controlled well and you will get timing that floats around at and above idle speed. So long story short is mix and match the springs all you like but stick to the middle strength springs on atleast one wieght. Also dont be afraid to use a factory spring. Ots all about the curve and takes a little trial and error to get right.

[RyanAK]

I'll go back to the stock weights and try the medium springs from the Mr. Gasket advance kit and see where that takes me. Having a tough time sourcing an ACDelco module locally here. The only thing in stock for local pickup is a PerTronis Flame-Thrower, a Duralast (Wells), or a NAPA Echlin. A Delco is days out.

The vac advance gives 15* at 15" and above. I'm not sure when the mechanical starts to come in, but it's all in before 3000rpm. I guess the springs work together on both weights... I had initially thought each weight worked with an individual spring, so assumed I couldn't mix/match.

If the TVS wasn't working and was cutting off the vac advance, could that have been causing my stumble/surge at cruise?

Appreciate the guidance.

[RyanAK]

New module in. The old one is a GM part and the silicone was almost non-existent. Plus there’s a rather large recess on the back... not completely flat like the replacement. Can a module test good and operate correctly except when it gets hot?

Put the stock weights back in and went to the medium springs. The Mr. Gasket weights weren’t resting at ‘home’ fully retracted when I pulled the rotor. And you were right about the springs... The timing bounced around at off-idle and didn’t always return when the RPMs returned to curb idle.

Timing. I’m at 34* all-in by somewhere just under 3000rpm. (I’m doing this alone and had a tough time working the throttle, watching the timing, and not getting the gun rattled by the A/C compressor pulley...) That put initial timing at 16* @ 850rpm. Vac advance is on manifold vacuum and gives 15* at 15” and over.

So that gives me:

16* base @ 850rpm
18* mechanical advance staring around 2000rpm
34* total all in by ~3000rpm
15* vac advance
31* idle / just-off-idle
49* cruise

No pinging in any situation. Throttle response is good considering this is a Suburban. Still a very slight hesitation going from cruise to WOT.

with everything I’ve done, the truck is running pretty good and I haven’t been able to replicate my stumble-surge-dead scenario. So something must have fixed it.

I did consider going to 36* total, but got nervous. 18* initial seems like a lot.

Now to adjust idle fuel/air and hopefully move on to the next truck project. I REALLY need exhaust!

Appreciate all the help along the way. I’m learnin’!!

R

[burnin oil]

I think 18 base would be fine as long as you are positive that you are only getting another 17 degrees out of the wieghts. You may want to rev up to redline and double check those timing numbers. Theoretically you should have around 25-28 degrees of wieght advance. Sounds like you are on track there. Be aware that the more timing at idle you add the harder on the starter so you may get some kick back warm if the starter is getting weak.

The module, according to GM, either works or it doesn't. In my experience they can go wonky and do wierd things as they get warm before they totally fail.

[RyanAK]

If I get a chance over the next few days I'll try to determine if the advance will increase more if I really ramp it up. I had it to ~3600rpm and didn't see any additional advance. Were some distributors limited in the mechanical advance? This is an HD emissions truck and I'm finding that there are a few differences compared to a 'typical' 350 truck motor from the late 70's. Most are good... like almost no emissions stuff other than PCV, Thermac air cleaner, and the TVS that switched the vac advance between ported and manifold vacuum. Not sure if all HEI distributors are the same of if they can have the advance limited...

Starter is strong, so haven't had any kickback. I'll watch for it though.

I'll try to optimize with:
Checking if mechanical advance will come in any more
If not, give it 2* more timing... hopefully without inducing knock or other issues
See if I get a noticeable difference in idle quality with vac advance on ported
Dial in the idle air/fuel mix
Maybe move accelerator pump linkage to the outside hole of the lever to see if it eliminates the hesitation from cruise to WOT

I threw the old module in the truck. Next time I'm at the parts store I'll have them test it just to see. I'm pretty confident this was part if not all of the issue. There was almost zero silicone compound left on the heat sink and what was there was hard.

This stuff is pretty fun. Thanks for the support.

R

[burnin oil]

solid plan and you are on the right track. As far as HEIs are concerned the emissions ones had the most wieght movement. If its still in a factory vehicle look at the base timing setting. If its 8 degrees initial then it would have to advance 28 degrees to hit the 36 degrees of total mechanical timing that chevys ran with. Emissions distributors had huge slow curves. Don't be surprised if they come back and say the module is good.

[RyanAK]

I haven't had time to look more into the timing and the local parts stores both didn't have the right vac hoses in stock. So... grumble. The truck hasn't shown any indication of the fuel starvation that was the initial symptom that I was chasing. Right now the truck is running pretty strong and consistent. I think it could still be a little better, so I will go through the timing again, fiddle with the idle fuel/air, swap the vac advance around to ported when I can get vacuum hose...

Here's something... After a little cruise around the county the truck was running a little rough. Popped the hood when I got home... and the PCV valve was pushed almost completely out of the grommet. The fit was a little loose, so I got the right grommet and installed it. Haven't had that happen again. But it got me thinking about the PCV in general. When I owned the '71 Suburban I had considered one of the adjustable PCVs from M/E Wagner. With the PCV pushing out on this rig, has me thinking again... There was some oil staining on the PCV inlet filter in the air cleaner. Maybe I could gain something with one of the Wagner units. Anyone have any experience with these?

Gas mileage with mixed driving around the county has been pretty steady at 10mpg. That seems low, even for a Suburban...

Timing, vac advance, any carb adjustments, possibly PCV. Then I can replace my rotten dual exhaust. Then shocks and spring/shackle bushings. Then...

Good truck.


Fun!

[burnin oil]

Full time 4x4 that is pretty good. I had a blazer that was stock 400sb, t350, np203 and 3.07s. 8mpg in town and 6mpg interstate. I swapped ina 4spd and np205 along with rebuilding the 400 and got up to 14mpg interstate.

It sounds like your PVC is clogged up or sticking. I would just throw another one on and go from there. Do some research and you can get different PVC valves that open earlier etc. No sense spending $100 on the adjustable deal in my opinion when $20 will buy 3 or 4 PVC valves.

[RyanAK]

Well, I thought I had this licked, but it’s back. I’m on the home leg of a 1,200 mile round trip to Maine to fish and it’s been slow going. Several pull overs and I’ve had to play the momentum game to get up over the hills. I really need to lick this. My truck is completely drivable around home, but long trips like this aren’t happening again until I get it fixed.

Highway speeds of 60-75mph, at load (uphill or long periods of acceleration), the truck gets starved for fuel. It bogs and goes in quick time, and feels like it’s sucking more gas than what’s available.

I added a “diagnostic tool” in the way of a vacuum gauge to help figure this out. I’m good as long as I keep the load to where there’s 10” or more of vac. Unfortunately this can drop my speed to 50 mph or less on a grade... with traffic zinging by at 75. If I spend any time with my foot in it and vac is below 10”, eventually it will start stumbling. When it bogs, the vacuum drops. So if I’m climbing a hill and engine vacuum is at something like 7”, when it starts lugging the gauge will bounce in time with the lugging between 7” (power) and 4” (stumble).

Engine temps remain steady. Ditto oil pressure. Idles fine with 20” at 750 rpm. It’s been hot and muggy.

Stuff I’ve done on this trip to try to improve things:

Air cleaner lid flip (to bypass the valve in the air cleaner)
Wired the choke open (thought the choke could be closing and choking the engine)
Insulated the hard fuel line between the fuel pump and carb where it sneaks between the block and alternator with rubber fuel line (hot fuel?)

Stuff I’ve done in the last few months chasing this:

Complete ignition tuneup (plugs, wires, cap, coil, rotor, module)
Timing dialed in
Idle air/fuel mixture
New fuel pump
New fuel filter
New soft fuel lines at pump
Confirmed clear lines from tank to pump
New air cleaner
Moved vac advance to manifold source
Double shot of Sea Foam
New PCV valve
New divorced choke

My suspects at this point:
Hot fuel?
Hot carb?
Coolant thermostat too high?
Exhaust heat riser valve?

My MAIN suspicion is that the truck came from Montana and is now on the East Coast. I’m not a carb guru, but I’d guess a rig set up for elevations between 3,000 and 6,000 ft in cool, dry air would need adjustment for a humid summer at sea level to 2,000 ft. Jetting, float level, etc.

I have a fairly decent tools with me and could use some guidance for moving forward. I’m at my non-mechanical in-laws in Connecticut today and tomorrow and would like to get this licked before heading on the last leg home to Pennsylvania. I’ll do just about anything I can under the circumstances, up to and including slapping a different carb on it to get me home. I really, really appreciate the help!!

[burnin oil]

I haven't played with a qjet for along time but as I recall there are a couple of holes on the lever at the front top of the carb. Maybe move it over one hole and enrichen the mixture under acceleration/load. Another thing you can do if you think its lean is there is an adjustment for the secondary air doors ( look like a choke plate for the rear barrels). Tighten the set screw up a little do delay there opening. Its located under the the top plate of the carb and is a small Allen head screw. Its purpose is to control the speed of the rear barrels coming in. Qjets are a mechanical secondary carb but the air doors make it act like a vacuum secondary. Hope that makes sense. If you get that classic 4 barrel sound then the air doors are opening to early. It should be a smooth transition that the engine noise gets louder without the hit.

Others may have a different suggestion but that's all I have at the moment. Qjets usually don't suffer from heat issues to bad so as long as you use the thick funky carb base gasket you shouldn't need a heat shield.

[RyanAK]

Thanks, man. Great suggestions. But it’ll stumble on just the primaries too. Going into the secondaries makes it happen sooner and more ‘violently’. Right now I’m limiting myself to 1/4 throttle at most to stay above 10” vacuum.

[geezer#99]

Unless you’re an expert leave the secondary adjustment alone. Real easy to make that spring become useless. Then you’ll have fun fixing it.
Instead just wire the air door shut so it won’t work at all.
That will eliminate secondary problems.
Had to do that on the father-in-law’s truck. Gave similar symptoms to yours. Eventually found out that some mechanic had hogged out all the secondary stuff (jets etc) to run natural gas.
Only difference on a carb from higher elevation might be it’s jetted leaner.

What’s your carb number?

[RyanAK]

Carb 1705821?. 1978 GM Truck. It’s on a GM crate 350 that the PO installed. 17,000 miles on it.

[geezer#99]

That momentary bog at first throttle movement can be caused by lean metering rods a bit too deep in the jets. That depth setting Can be adjusted by the adjustable part throttle screw that is located in the base of your carb between the mix screws. Covered with a cap. Usually well corroded and stuck into place. Easier to richen your jets a bit.
In the process of trouble shooting the father’s carb we swapped to my original carb from my chevelle. Cured all the problems so we knew it was carb related.
We wired the door shut and he ran it like that for 3 years no problem. That was 30 years ago. Carb parts weren’t readily available then. This was on an 84 3/4 ton hauling a 12 foot overhead camper at normally 3500 foot elevation.

There’s some info here about the ATP. Adjustable part throttle.

http://www.crankshaftcoalition.com/w...4MV_carburetor

[burnin oil]

You should be able to move the linkage on the green arm to give it quicker accelerator pump shot. Adjusting the secondary isn't to hard if you methodical. Find the right Allen key then turn a given amount, say 1/8 turn and evaluate. Keep track of what you did any it can be reversed. Not really different than the adjustable vac canister. A little extra tension is all thats needed. Don't go crazy there. Its not an idle air screw.

[RyanAK]

I thought the inside hole on the pump lever was the bigger shot? And I’ll eventually try to fine tune the secondaries coming in. But right now my drivability issue is high RPMs under load on the primaries. I really feel like I’m sucking the carb dry of fuel. Could the float be hanging up or need adjusted? The inside of carbs are still a bit of a mystery. Appreciate you guys taking the time to make suggestions!

R

[geezer#99]

The Allen key is the lock screw.
The flat headed screw is the adjustment.
When adjusting don’t remove your flathead screwdriver unless the lock screw is tight. If you don’t the spring can rapidly unwind and pop off the retaining pin.

Maybe the float drop is set too low. There’s two settings. Level and drop.

[burnin oil]

Geezer you are the man. You know these suckers better than me so I will let you take the lead. Yesterday I was working on another car and found the qjet off my 72 in a parts vehicle tool box. Good safe keeping as I havent seen it in atleast 5 years.

[geezer#99]

Another possibility is sticky metering rods.
The rod holder can hang up on the gasket.
You should be able to poke a tiny pencil in the vent and push the rods down and feel them push back up. Motor off test.

[RyanAK]

I have video of this happening if anyone is willing to have a look. PM me and I’ll email them along.

Appreciate the help. I’ll see what I can get into in the morning.

[RyanAK]

Could these symptoms be from a bad choke pull-off? I just pushed in the plunger and put my finger over the vacuum nipple and it bled back out. Running at idle with 20” it won’t pull in unless I nudge it. Then it sucks in. Slides out when bringing RPMs up. Back to idle... need to nudge to get it to pull in.

No parts place has one on the shelf and I’m still 300 miles from home. Ugh!

[RyanAK]

New diagnostic. Put the back glass down. When it stumbles the exhaust smells rich. Hard to tell if the rich condition CAUSES the stumble, or it smells rich from when it catches itself. Hmmm.

Slowly rolling home to PA. Lots of hills. Ugh.

[RyanAK]

Still fighting this... I’ve done a LOT of chasing things. Have had more issues. Rigged up some tests. Rebuild kit on the way.

Should a mechanical fuel pump with return line allow fuel to drain back when the engine is shut down? I rigged up a clear fuel filter so I can see what’s happening. On a hot engine the fuel immediately drains back when I kill the motor.

[Ironangel]

Originally Posted by RyanAK

Hi, guys.



Fuel filter finally came in. But without a check valve. Whatever... I put it in. And haven't been able to recreate the bogging. But I'm still nervous that I didn't get it. Could this really have been just a dirty filter and soft fuel hose? Were the two enough to starve the Quadrajet when it was really sucking gas?

The short answer is "YES!"...If we're talking about the tiny filter in the carb inlet then absolutely yes! Once more, a new replacement can clog up in short order if crap in the tank has busted loose for whatever reason. I had to change three one after another getting my second 72 home. They were getting clogged with fine sand that had settled in the tank over the years after sitting parked on a ranch in western Nebraska, probably without a gas cap on I suspect. It wasn't until I added an inline canister filter before the carb that the bogging and dying issue went away to this day. Those tiny filters are great on new clean systems but are overwhelmed when you throw 50 years of dirt at them. Secondly, after reading about the ignition modules...They are indeed very heat sensitive! They use an epoxy matrix to seal the tiny electronics within and a hairline crack undetectable to the eye can break the circuit killing the motor until it cools down enough to close the circuit again allowing the restart. Those modules don't seal down with silicone, they seal down with a heat sinking paste that helps them sink the heat into the dizzy base. It is critical that that paste be used! Surfaces must be cleaned so a good heat transfer can take place. That's all I have to throw in...Good luck!