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GMC V6 and V12 Engines Engine repair and rebuilding

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  #1  
Old April 6th, 2023, 05:07 PM
krazzz krazzz is offline
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Default Stromberg WW- Won't start after sitting/Stumble on acceleration

I have a truck that has been sitting for many years so I have been going through it getting maintenance done on it. So far I have, changed all fluids, plugs, wires, cap, rotor, new fuel pump, rebuilt carb, electronic ignition conversion, checked timing, etc.

The truck now runs and drives quite well but I still have two lingering issues. I am a bit of a novice when it comes to automotive carbs (have rebuilt dozens of small engine carbs.)

1. When the truck sits for a few days it will not start without starting fluid no matter how long you crank. It turns over quickly but will not fire. The choke looks like it is functioning and it is getting fuel. With a half a second shot of starting fluid, it fires up immediately and runs well. It will restart with ease the rest of the day and even the next day or maybe two but then longer than that it will not start. Any ideas? It had an electric fuel pump and I just went back to a mechanical but that didn't impact this situation one way or another.

2. It has a bit of a stumble when pushing the accelerator but after a second or so, it revs up fine and runs well throughout the rest of the RPM range. It isn't horrible but I think it should be running better than that. Idle is set at 550 RPM and the timing and advance are working properly. I suspected an issue with the accelerator pump but looking in the carb I see two healthy squirts of fuel when the throttle linkage is moved. It appears to be adjusted within spec according to the shop manual. What should I check next? I am planning on changing the vacuum advance from the metered port on the carb to the manifold. Not sure if it will help but I have heard that it makes the engine run smoother in general.
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Old April 9th, 2023, 05:22 AM
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AZKen AZKen is offline
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Default Re: Stromberg WW- Won't start after sitting/Stumble on acceleration

Does it also squirt after sitting but before trying to start?. If it squirts, pump accelerator more and faster than you have been while cranking starter.
If in both cases you mention, carb squirts a good stream, then I lean towards timing. These motors do not always run good at the "book spec" timing. Advance/turn distributor, in small increments each starting period to find good results. Obviously if it gets worse, then retard. This may take weeks. Don't rush, let it sit same period of time or the experiments will be misleading. The fact you say an electric fuel pump made no difference, points to timing/weak spark also. Notice if distributor turning helps the after-sit starting or stumble...or both.

More info and your test results may change my mind.
The only carb issue could be a a sticky needle valve, but again you say you have squirt in both cases. So that means there is gas in the bowl and motor should at least fire pretty quickly if choked.

The starter fluid is a little stumping. It is highly combustable. It could be too cold to start otherwise and then when warm it is OK.
Can you advise:
1. After sitting does starter crank slowish?
2. Does the motor not even fire or pop or almost start ....without fluid?
3. Have you checked all vacuum lines?
4. Have you tried squirting gas into carb instead of fluid? Like a good shot/mist of gas from a squirt bottle.
5. Gotta block warmer?

Another cause can be timing chain stretch???? Not sure if that fits this.

Last edited by AZKen; April 10th, 2023 at 02:24 AM.
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Old April 9th, 2023, 07:57 AM
krazzz krazzz is offline
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Default Re: Stromberg WW- Won't start after sitting/Stumble on acceleration

Quote:
Originally Posted by AZKen View Post
Does it also squirt after sitting but before trying to start?. If it squirts, pump accelerator more and faster than you have been while cranking starter.

I will check this, I am not sure if it squirts or not after sitting.


The starter fluid is a little stumping. It is highly combustable. It could be too cold to start otherwise and then when warm it is OK.

I keep it in a heated garage so it is never below 50 degrees.

Can you advise:
1. After sitting does starter crank slowish? No, cranks normal speed.
2. Does the motor not even fire or pop or almost start ....without fluid? No attempt to start at all. That is what is so strange.
3. Have you checked all vacuum lines? I only have one vacuum line and just replaced it.
4. Have you tried squirting gas into carb instead of fluid? Like a good shot/mist of gas from a squirt bottle. I have not tried this but I will do that next time. When the guy took it off the trailer when he delivered it he used gas and it fired right up. He didn't even attempt to start it without adding the gas though.

Another cause can be timing chain stretch???? Not sure if that fits this. I sure hope that is not the case. The truck only has 79k miles on it.
See my comments in blue.
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Old April 10th, 2023, 07:39 PM
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Default Re: Stromberg WW- Won't start after sitting/Stumble on acceleration

OK, will wait for results of those tests before suggesting cause and next steps.
As we know, a motor needs fuel/air, compression and spark at the right time. So with a "no fire" situation we troubleshoot those. Your motor starts with gas or fluid sprayed into carb throat, so we can concentrate on fuel delivery. When helping from afar, we only have the words of the post. We"trust" the description as much as possible. You have been very specific about the history and present issue. That's good. So using your words, logic and experience, we give ideas.

It seems the carb bowl is empty after sitting due to evaporation.. There should be enough for the accelerator pump to give a squirt or two.
Somehow or another there is not enough. THIS CAN BE NORMAL and just takes a special method for starting. We have all had motors that sit. You say a few days. That can mean 2 days or 5 days. You will just need to set to full choke, pump pedal like crazy til it starts. Then gradually open choke.

You tried an electric fuel pump, which if was installed correctly and was the correct PSI...should have started the truck. Absolutely. So this throws me off. Something fishy there. This causes me to think of spark and time, but it starts and runs good....so I bounce back. When you have an electric fuel pump, which I really don't recommend, you have instant pressure. You don't need motor to crank to fill the bowl. Bowl fills and accelerator pump (if working) gives a gas shot. Like the squirt bottle or ether.

Fuel delivery problems have many causes. Empty gas tank, tank not venting, fuel lines or tank clogged, fuel filter clogged, fuel pump, sticky needle valve, float setting.....all that stuff. But again, and I'm sure you are thinking the same thing...it starts with a squirt and starts fast and has new pump and rebuilt carb.

So conclusion:
Most likely,
1. It's normal and is sitting longer than 2 days. Bowl empty. Needs violent start procedure.
2. It's the accelerator pump.


Other, including items in green above,
3. Bad new pump
4. Carb rebuild problem or old rebuild....or Ethanol has attacked needle rubber and accelertor pump rubber. Your careful test, after truck sitting, to observe squirt before trying to start will help to determine carb issues. This will determine if bowl has gas after sitting a few days. Report how many days truck sat when test was done. Also you say the seller used gas to squirt so that is good to know. Kind of weird he needed to do that to start motor, then you put on a new pump and rebuilt carb, among other tune-up items, and it does exactly the same thing. What am I missing there? I think there is a hint there somewhere. Did you ever empty and clean the tank?
5. Choke not fully closed
6. We may want to do a pump flow test at some point.


I should have said check for vacuum leaks. Hoses, gaskets, open ports not noticed on carb or on intake manifold, cracks. You say one hose. That would be vacuum advance?

Theory:
Clogging somewhere causes slow fuel pump flow to fill an empty bowl at starter cranking speed. When artificial starting occurs with squirt in carb, motor runs much faster, pump overcomes clog to fill bowl and stay running except during heavy acceleration. Something to think about. Except electric pump didn't work either.

Last edited by AZKen; April 11th, 2023 at 01:02 AM.
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Old April 25th, 2023, 02:15 AM
Ledslinger Ledslinger is offline
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Default Re: Stromberg WW- Won't start after sitting/Stumble on acceleration

The accelerator pump check ball probably was left out
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