Hard stumbles in specific situation

[rgunlock]

I've got one of GM Performance's 350/290 crate engines, a built Quadrajet carb on an Edlebrock Performer intake and an NV4500. Its the truck in my signature.

The truck generally is running fine except for one nearly guaranteed situation where it stumbles hard for a while (several seconds). Then it smooths back out and is generally, but not always, just fine for the rest of my trip. There is a convenience store about 8-10 miles from my home that I stop at. The truck runs just fine getting there. I go in for a few minutes, then start it back up and take off. At the high end of 3rd, low end of 4th (maybe 1800-2000 RPM) it starts the stumbling. Occasionally it does it again but less violently in about the same situation further up the road. It hasn't died, and will smooth out if I hold in the clutch.

I've been using this truck as my daily since finishing its build about 1.5 years ago. About 12,000 miles. At that time, pretty much everything mechanical was replaced or rebuilt. Fuel system was all new except the stretch of hard lines. Everything electrical on the truck was replaced except the distributor. While trying to figure this issue out, I have replaced the dist cap and coil (HEI), all of the fuel filters, and added a 1" spacer under the carb. Apparently didn't need any of that.

I've about run out of ideas and could use some suggestions. Thanks guys!

[AussieinNC]

Sounds like you dont have full 12 volts to the power side of the HEI....

Was the original distributor a standard coil system and not HEI?

Also, battery grounds are essential.....check ground wires engine to chassis, engine to cab and cab to chassis.

Also, what fuel tank cap do you have on the truck?

[garyd1961]

Maybe vapor lock?

[rgunlock]

The HEI distributor is OEM as far as I can tell. It may not have been original to the truck but was on the truck when I got it. I rewired the whole truck with an AAW kit during the build. I have all the grounds you mentioned, and oversized battery cables because this truck has been hard to get cranking from day 1. Once it starts cranking it starts right up. I've about decided my battery doesn't have enough cranking amps. I will double-check all the ground connections though.

The gas caps (dual fuel tanks) are original as far as I can tell. They are just the insert and twist type. Tanks were new, as well as the selector switch, rubber portion of the lines and fuel pump. I don't have a return line, but I do have a short hose with a breather valve on each tank. I've never had pressure when I stop and open the cap for a fill-up.

Thanks!

[AussieinNC]

Check that you have 12 volts at the HEI power connector....

Do you have a thick gasket (1/8 " ) between the carb and manifold?

[rgunlock]

I'll check the voltage. I've just installed a 1" phenolic spacer that has a couple of gaskets ~1/8" above and below.

Thanks, Rick

[rgunlock]

Looks like its 11.8 with the ignition on but the truck not running. Jumps to 13.9 with the engine running. This is measured at the connector for the large pink wire going into the "bat" connection on the distributor.

[AussieinNC]

Originally Posted by rgunlock

Looks like its 11.8 with the ignition on but the truck not running. Jumps to 13.9 with the engine running. This is measured at the connector for the large pink wire going into the "bat" connection on the distributor.

Ok..that voltage looks good....only thing left inside HEI now is the module...maybe swap one in and test....

Try driving it without the fuel cap on...say half tank of gas....and see if it still does it...

Thanks for the feedback

[Dead Parrot]

I have a similar gas tank setup and those little vents are pretty fool proof. Same type of dual tab twist caps.

Have you performed the convenience store trip test on both tanks?

Have you tried waiting a similar time at home after getting back from the store and seeing if the stumble happens once on the road again? Is it a one time while warming up issue or a every time when warm problem?

Did this stumble happen from day one or develop some time later?

[rgunlock]

[quote=AussieinNC;8753594]Ok..that voltage looks good....only thing left inside HEI now is the module...maybe swap one in and test..../QUOTE]

Thanks AussieinNC. I had to google what the HEI module was. Looks like a somewhat flat sort of "u-shaped" thing with a couple electrical tabs on each end. I don't even recall seeing one when I've been into my distributor but I'll be checking for one now. I did purchase a (probably crap) HEI distributor when I got this engine but ended up swapping the original back in while chasing original "get it running" issues as I completed the build. I'll open up that new one to scavenge the module.

Originally Posted by Dead Parrot

Have you performed the convenience store trip test on both tanks?

Have you tried waiting a similar time at home after getting back from the store and seeing if the stumble happens once on the road again? Is it a one time while warming up issue or a every time when warm problem?

Did this stumble happen from day one or develop some time later?

I don't recall that it was a "day 1" issue, but thinking back I had it happen pretty early after I started driving the truck. I don't think it happened as frequently or as badly when it does. It seems to happen regardless of which tank I'm on. I payed attention to that to see if I could figure out which fuel filter was clogging when I thought that was the problem. I've got inline filters between the tank and fuel selector valve on each side.

I haven't tried driving again after getting home. Occasionally, I've had the problem happen when accelerating from the 4-way stop about 1/2 mile from my neighborhood. This is still on the way home, but usually only on a really warm day. Once I get past the stumbles, I usually have no problem as long as I'm cruising.

It doesn't seem to happen, or at least very often, in cooler weather. Guestimating that I've only had the issue on 80 degree days or warmer. That's why I thought the carb spacer would be worth trying. But, in the described situation the problem pretty much happens every time.

Thanks so much for helping with this guys. I hope I'm providing enough detail in my answers.

[AussieinNC]

These HEI modules are sometimes very finicky....I would not trust the module from the new one as it may have been the original issue with the new distributor....

I use these from Advance Auto

https://shop.advanceautoparts.com/p/...E&gclsrc=aw.ds

It is always recommended that you carry a spare module in the truck in case of failure...

The new module should include some heat sink compound...it goes on the back of the module where it sits against the dist body...aids heat transfer away from module...if its not included with the module ask the salesperson if they sell it separately.

[rgunlock]

I don't think that my new distributor actually had a problem. Swapping in the original was just part of me trying to get the truck running well originally. One of those "everything is new so you can't trust anything is right" situations.

So, since I already had it, I swapped my original HEI module for the new one from the new JEGs branded distributor. I also removed the carb spacer because it was causing me problems getting idle down where I wanted it and I already knew it didn't solve my problem. The new HEI module didn't help. Same issue in the same situations.

I mentioned earlier that the hard stumbles happen at the high end of 3rd gear or bottom of 4th gear as I accelerate from a stop. It happened in both situations tonight. I guess RPM range isn't directly a factor since those two are totally different. But I was wondering if mechanical or vacuum advance issues could cause this symptom.

I don't know if this helps, but I was also reminded on this test drive that sometimes during the stumbles, I hear short tire screeches. This had caused me to think the engine was dying and essentially clutch-starting itself. However, when this is happening if I press in the clutch the engine smooths out and keeps running fine. When I let the clutch back out the stumbling usually resumes for a bit.

I'll keep your HEI module recommendation in mind, but any other suggestions of things to check? Thanks for the ones you've already provided.

[cadillac_al]

Does the truck still stumble when it's fully warmed up? Does it ever go away?

[rgunlock]

Usually, the truck has already been fully warmed up driving it 8-10 miles. It has only been shut off a few minutes while I was in the store or filling a tank. Then, accelerating away from the store it is almost guaranteed to do it. But yes, occasionally it will do it again miles further down the road when I am again accelerating from a stop. Usually though, it does fine after the first time.

It does seem to be getting worse though lately, stumbling for a longer period or stumbling again at later stops. It might be due to the weather getting hotter. Or possibly because with this COVID-19 thing it is sitting for days between drives. I'm not sure.

[Kudzupatch]

Sure sounds like a fuel issue. I wonder what a cheap fuel pressure gage would cost? Put one in temporarily and see if you have full pressure when it stumbles.

[rgunlock]

I think you are right kudzupatch. Just took that barely used Jegs HEI distributor and swapped it in. Replaced the module I'd borrowed from it. Got the timing set and took the same drive. Behaved herself except for that same situation.

I don't have a fuel pressure gauge in the truck, but I do have an AFR gauge. Today was almost more of a stall than multiple stumbles, but I noticed that the AFR was showing about 22 during the stall. It's usually 12-14ish. So, starved for fuel is what I think.

I'm imagining that the fuel is draining back down the line. While I'm accelerating from that store, I'm using up the gas in the carb's fuel bowl during a relatively hard acceleration. Maybe faster than my manual fuel pump is getting gas back up the line to the carb? Maybe this would also happen after a cold start, but I always have to cruise slow for 1/2 mile or so to get out of the neighborhood or parking garage from work so no hard acceleration right away after starting her up.

I thought that the fuel pump was supposed to have a backflow preventer. This pump was new about the time I started driving her daily, but that doesn't seem to mean a lot these days.

Does this sound likely? Other suggestions? Maybe someone knows a solid brand for manual fuel pumps?

[RyanAK]

Hey, Rick. I had something similar going on with my Suburban. Lots of good suggestions from the forum in my thread here:

http://67-72chevytrucks.com/vboard/s...d.php?t=807508

I seem to have solved it with a new filter in the carb, a new fuel pump, a double shot of Sea Foam, and swapping the vac advance to manifold vacuum.

The biggest improvement was when I moved the vac advance signal. My truck is a HD emissions motor so doesn’t have hardly any smog stuff on it. But, it DOES have a temperature vacuum switch (TVS) at the thermostat housing. This doodad routes vacuum to the vac advance can based on engine coolant temps. At ‘normal’ temps, it puts the advance on ported vacuum. When coolant temps are elevated, it switches to manifold to improve idle and just-off-idle cooling by allowing the spark to advance via full manifold vacuum. I’m suspicious that my TVS isn’t working and basically robbed vacuum to the vac advance can when the motor was hot. Loss of 15* of timing at cruise and light acceleration can’t be good.

Anyway, I’d look and see how your vac advance is plumbed. Try going directly to manifold or ported vacuum and bypassing the TVS if you have one.

Try pulling your in-line filters.

Fuel draining from the carb can be solved with a carb filter with a check valve. But this also restricts flow somewhat at higher RPMs.

Keep the info coming. You’ll get it. Have courage!

[RyanAK]

What fer choke ya got? Is it coming all the way off when warm? If divorced, is the spring and linkage good?

[cadillac_al]

If you have a bad cam lobe new fuel pumps won't help. I only had it happen once in my life but I found it by checking fuel pressure. My truck ran pretty good with 1.5 lbs of fuel pressure for a couple years. You just may have less than that.

[RUN GMC]

You guys are on the right track ......but lets look at one more thing.....is the vapor canister still hooked up......I had this problem when I filled up my tank I couldn't stick the nozzle all the in it would trip the pump on filling the gas tank an indication meaning the vapor can and vent aren't working properly. One day I decided to check the vapor can upon unstrapping it I noticed it was very heavy and smelt like gas (duh I know) but was heavier due to raw gas was getting in it (Im a over filler when getting gas). So what I had was a vapor can that had deteriorated it would allow droplets of raw gas and carbon particles to be suck up to the TBI below the injectors straight into the motor.....my truck would stumble hard in the 40 mph and above range. After changing out the vapor can with a new one(Astro Van sized one off Amazon $60) my truck could pull all the way to 70 mph without the stumble it had before BUT I still have slight miss im on...I plan to calibrate TPS, set idle air, and base rpm to make sure I didnt mess them up when I did it with the bad vapor can.
Long story short is if yours is still hooked up from gas tank to can to carb disconnect it at the carb and cap it off and take it for a test drive to see it your stumble goes away.......if not hooked up then continue with what you are doing but I just wanted tell you what I had went through on my truck......
One more thing I had over looked was the black soot on the concrete in my drive way which I now believe to be carbon partcles out of the vapor can being burned with the raw fuel droplets...Im told they are supposed to be changed out about every 70,000 miles I have never changed one out on a vehicle ever... it is one of the most over looked pieces on a vehicle. The plan is too redo the vents on gas tanks, check and replace gas vent lines, and see if I can put a filter between the vapor can and the TBI suction to use/see when it needs to be changed or see if it has failed at a glance.....

[rgunlock]

Originally Posted by RyanAK

Hey, Rick. I had something similar going on with my Suburban. Lots of good suggestions from the forum in my thread here:

http://67-72chevytrucks.com/vboard/s...d.php?t=807508

I seem to have solved it with a new filter in the carb, a new fuel pump, a double shot of Sea Foam, and swapping the vac advance to manifold vacuum.

Anyway, I’d look and see how your vac advance is plumbed. Try going directly to manifold or ported vacuum and bypassing the TVS if you have one.

Try pulling your in-line filters.

Fuel draining from the carb can be solved with a carb filter with a check valve. But this also restricts flow somewhat at higher RPMs.

Keep the info coming. You’ll get it. Have courage!

My vacuum advance has been hooked up to manifold vacuum. Suppose I could try going the other way and use ported. Any case, I'll check your thread for ideas. I just replaced all of the fuel filters as the first step of trying to troubleshoot this problem.

Originally Posted by RyanAK

What fer choke ya got? Is it coming all the way off when warm? If divorced, is the spring and linkage good?

I've got an electric choke. It appears to be wide open and working as expected in our 90+ heat.

Originally Posted by cadillac_al

If you have a bad cam lobe new fuel pumps won't help. I only had it happen once in my life but I found it by checking fuel pressure. My truck ran pretty good with 1.5 lbs of fuel pressure for a couple years. You just may have less than that.

This was a brand new crate engine 12,000 miles ago. I suppose I could have had a problem with engine break-in but I did follow the procedure and have been using supposedly high zinc motor oil.

Originally Posted by RUN GMC

You guys are on the right track ......but lets look at one more thing.....is the vapor canister still hooked up......

I did not install a vapor canister when I rebuilt this truck. I've just got short pieces of hose with new breather caps on them down at the tanks.

Thanks for all of the suggestions guys!

[RyanAK]

Sounds like you're ok on the vac advace. Have you tested it that it's pulling in? Attach a piece of hose to the can and suck. See if it moves.

I'd still pull the inline filters out and try. Just run straight section of hose between the hard lines where you remove the filter. Just try it on one tank. The sock in the tank, plus the inline, plus the carb filter might be enough restriction to cause issue. What filter is in the carb? Check valve, no check valve, bronze?

If you pull the fuel pump, pull the rod and make sure it isn't damaged.

Sounds like the choke should be ok. Make sure the linkage is holding the plates open. The coil in my divorced choke went bad and in addition to hard cold starts, the choke valve would close under certain conditions and choke out the motor.

What fuel like do you have from the pump to the carb? Is it touching anything hot? Any restrictions in the soft fuel lines going into the fuel pump? Mine had a pretty good kink that restricted flow.

It's a great looking truck. You did a fantastic job. I haven't gotten through all 41(!!) pages of your build thread yet, but I'm taking a look to see if I can see something that might be a clue. You'll find this gremlin eventually. I daily drive the Suburban (16,000 miles since last September) and it's a pleasure. Totally worth getting these rigs into reliable drivers. Keep the faith. You'll get it.

[rgunlock]

Originally Posted by RyanAK

Sounds like you're ok on the vac advace. Have you tested it that it's pulling in? Attach a piece of hose to the can and suck. See if it moves.

I'd still pull the inline filters out and try. Just run straight section of hose between the hard lines where you remove the filter. Just try it on one tank. The sock in the tank, plus the inline, plus the carb filter might be enough restriction to cause issue. What filter is in the carb? Check valve, no check valve, bronze?

If you pull the fuel pump, pull the rod and make sure it isn't damaged.

Sounds like the choke should be ok. Make sure the linkage is holding the plates open. The coil in my divorced choke went bad and in addition to hard cold starts, the choke valve would close under certain conditions and choke out the motor.

What fuel like do you have from the pump to the carb? Is it touching anything hot? Any restrictions in the soft fuel lines going into the fuel pump? Mine had a pretty good kink that restricted flow.

It's a great looking truck. You did a fantastic job. I haven't gotten through all 41(!!) pages of your build thread yet, but I'm taking a look to see if I can see something that might be a clue. You'll find this gremlin eventually. I daily drive the Suburban (16,000 miles since last September) and it's a pleasure. Totally worth getting these rigs into reliable drivers. Keep the faith. You'll get it.

There is a noticable change to my idle when I connect the vacuum advance, so I think its been working on both distributors. I'd put a new vacuum advance on the original distributor early on.

I'm pretty sure I tried bypassing the inline filter on one tank before replacing the filters but its been a while and I might be misremembering. Wouldn't hurt for me to try again. Tanks and senders were new so hopefully the socks are still clear.

The carb filter is one of those paper ones (vs the bronze) that actually install into the inlet on the carb. I replaced it along with the inline filters on each tank. Not sure whether it has a check valve or not. How do I tell?

I bent my own steel line from the fuel pump to the carb and it stands out from the block by at least an inch. I'll double-check for any hard bends in soft lines though.

Thanks a lot for your suggestions, as well as for your comments on my build. I'm very pleased with how the truck turned out being my first build, and love driving it but these last bugs are taking a long time to work out now that the build itself isn't my priority.

[RyanAK]

The paper filters come either with or without a check valve. If you look at the inlet side of the filter, there will either be an open hole, or a plunger in the hole. Plunger = check valve. It’ll keep fuel from draining back out of the carb, but does restrict flow a bit. Pluses and minuses.

Check your soft lines at the pump. Mine were fairly new but already soft and the hard bend on the supply side put a kink in it that would collapse when the pump was sucking hard.

I also sprayed the crap out of my carb with carb cleaner and gave my huge 40 gal tank double dose of Sea Foam. I saw instant improvement.

If you get a gauge, check flow and pressure on the pump.

[rgunlock]

With this virus, I haven't been driving much since my company is enforcing work from home. But, I've made this drive twice now without any problem. Since nothing else made a difference and the only new change is a filter for the carb inlet with a check valve, I'm saying it was the cure. Extra bonus, the truck is also starting much easier for the drive home.

Should have known it would only take a $6.50 part to fix this issue! Thanks to all who made suggestions.

Rick