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Ricko1966

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@legozombie Have you been able to test out Ricko1966's tests? I'm currently in the process of installing the fuel system on my 1976 K20 and am curious what you find. I spent most of last week trying to understand how the evap system works. I fully agree it sounds like a vent problem.
What I'm not 100% clear on is the evap system is for capturing fuel vapor from the tank. When the fuel is being consumed while driving, a vacuum is generated and air needs to be able to flow into the tank to equalize the pressure. I'm not clear how the evap system would allow this. If it doesn't then a vented cap is necessary. Or, perhaps the vent line that goes to the air cleaner allows a backflow of air into the tank. (?)

The vent line allows the tank to breathe, in and out. Without releasing evaporated fuel,or drawing in moisture like you would with a vented cap. The evap system let's you burn more of the gas you bought,helps prevent contamination of your fuel,rusting out of your tank, and my own personal theory is the lighter molecules of the fuel evaporate more than the heavier leaving you with a potluck soup of fuel. Not after 1 tank,nut after 1000?
 

legend57

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The vent line allows the tank to breathe, in and out. Without releasing evaporated fuel,or drawing in moisture like you would with a vented cap. The evap system let's you burn more of the gas you bought,helps prevent contamination of your fuel,rusting out of your tank, and my own personal theory is the lighter molecules of the fuel evaporate more than the heavier leaving you with a potluck soup of fuel. Not after 1 tank,nut after 1000?
Thanks, Ricko1966. I wasn't sure if the evap canister allowed flow back into the tank. I agree it is a good thing.
 

legozombie

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If you think the tanks not venting due to cannister problem,run the truck with no gas cap. Problem gone? You have a vent problem. Follow the vent line to the front pull the line at the front,leave it open,did the problem come back. The problem is from that point to the tank. Problem still gone? Verify all routing to the cannister. All good? Replace the cannister. If you need to run temporary with no cannister put a fuel filter on the line at the front to keep dirt and bugs out of the vent line. I've been trying not to post much to avoid personal attacks. I wanted to answer this on day 1.
Got the new canister and test drove it yesterday with everything hooked up. All the EFE/AIR Pump stuff has been deleted. I drove it and it died again, thankfully before I left my neighborhood. I thought it was the venting thing again so I disconnected all the vent lines to the fuel tank and took the gas cap off to test it out. The truck started but whenever I gave it power it would pretty much start dying. I reversed halfway home and managed to turn around and drive in granny gear and 2nd gear down hill to my house. The whole time I had to keep RPMs high or it'd stall out. Once I hit uphill to my house, it died. It's getting spark, air, and fuel. I'm thinking it's a vacuum leak, carb adjustment/rebuild, or new engine (jk but maybe)
@Ricko1966 @legend57
 

Ricko1966

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Got the new canister and test drove it yesterday with everything hooked up. All the EFE/AIR Pump stuff has been deleted. I drove it and it died again, thankfully before I left my neighborhood. I thought it was the venting thing again so I disconnected all the vent lines to the fuel tank and took the gas cap off to test it out. The truck started but whenever I gave it power it would pretty much start dying. I reversed halfway home and managed to turn around and drive in granny gear and 2nd gear down hill to my house. The whole time I had to keep RPMs high or it'd stall out. Once I hit uphill to my house, it died. It's getting spark, air, and fuel. I'm thinking it's a vacuum leak, carb adjustment/rebuild, or new engine (jk but maybe)
@Ricko1966 @legend57
Put a vacuum gauge on it,post your reading. But sounds more like fuel delivery than vacuum. Most vacuum gauges will test fuel pressure,check it,and check fuel volume stick the carbureator end of the fuel line in a bottle and crank. Post results.
 

Snoots

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Just a shot in the dark but, does your truck have a catalytic converter?
 

legozombie

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@Snoots No catalytic converter
@Ricko1966 I haven't run the vacuum test on the carb yet. I was able to turn on my truck and turn it around so I don't get a ticket until it started giving me problems again. This is the fuel pump I have and it's delivering fuel but I did not formally check volume measurements. With the rest of my time, I rebuilt a 4MV carb to throw on for easy testing.
 
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legozombie

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So the 4MV had a different fuel inlet and I didn't want to jank together a makeshift fuel connection. I ended up cleaning up the OEM (I think) M4ME. One thing I noticed during disassembbly, aside from all the bits of charcoal in the bowl, was that the power piston was slammed down and wouldn't open up by itself. The plastic snap ring was fine and all, I just had hard time popping it out. I think the stuck power piston was not allowing enough fuel into the jets. My mechanic buddy swung by and drove it after I installed it back on the truck, finessed the idle mix screws, and had it driving for about 40 minutes with no stalling. When I got back from work I drove it around for about 5-10 minutes and it was giving me no immediate issues at all. I am going to a little further and see if I stall out.

One thing that still alludes me is my fuel vent line or EVAP not allowing the tank to breath and therefore stalling the truck out.
 

Grit dog

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Well if the accelerator pump wasn’t working and now it is and it cured the repeatable issue, that was likely the issue.
Not sure how to address your other concern about the venting and how your fuel system is setup as per your descriptions there now aren’t issues there. I don’t think you confirmed any vacuum in the tank? And hard to tell what’s right wrong or cobbled together.
Fortunately the fuel delivery systems on these are very basic in principle and design.
If it works it works but what fuel pump do you have? Presume it’s appropriate low pressure or you have a regulator?
 

legozombie

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@Grit dog Here's the pump in post #21: Edelbrock Part #17303 (I'm 90% sure). Just for clarity, the accelerator pump was working and moving; it was the power piston, which metered the primaries, that was stuck shut somehow. Maybe there's a connection between the two but I'm just stating it to be clear for future readers/contributors. You're completely right that it's hard to untangle the mess of issues and possible related causes.

I test drove it to and past where I first broke down from the mystery fuel venting issue without any problems at all. Let me say, those secondaries KICKED and I don't think I heard any issues with the primaries. However, I am just weekend warrior who can just now afford a project so I don't have a good ear for engine issues. The engine went vroom vroom and VROOOOOOM and that's about all I understand :laughing1:. My mechanic friend said he was very impressed with how it's running and it's sounding better than before he swapped the blown 350. The engine seems to be running healthy now after cleaning/inspecting the carb and replacing the EVAP can. I'll definitely update this thread if anything EVAP can becomes suspect again. May have been a one off thing since I reseated the hoses and have everything hooked up.

I'm still planning on doing the vac tests just for science and sanity's sake
 

legozombie

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Also don't blow compressed air in the EVAP can, at least while everything is hooked up to the carb...
 

legend57

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Good catch on the power piston. Glad to hear it is running better. I'd highly recommend gathering as many parameters on the truck as possible (vacuum, timing, fuel pressure, AFR, etc..), especially when it is running good. That way, if it starts to run poorly, you have a baseline and can tell when something has changed. It can significantly reduce the frustration, and expense, while debugging. Now, enjoy that truck!
 

legozombie

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Put a vacuum gauge on it,post your reading. But sounds more like fuel delivery than vacuum. Most vacuum gauges will test fuel pressure,check it,and check fuel volume stick the carbureator end of the fuel line in a bottle and crank. Post results.
I used the following set up for fuel pressure and was getting around just over 3 psi while the engine was running and a little more than 4 when the engine was off/shut off valve was closed. I still need to check fuel volume. The engine sounded best at around 15.5-16.5 inHg. At 17 inHg the engine ended up stalling for some reason but that's for another topic. I thought peak vacuum was ideal but I must be missing something or something is off with the engine.
 

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