Need advice on how to tackle an issue on my '86 305

[AMRAAM_Enjoyer]

Basically I have a carbed 305 mated to a TH350 (if that matters at all) with a simple little 2bbl (I assume it is stock based on reference images). The motor and basically everything else around it save for an aftermarket exhaust (Header back) is bone stock. However, I am encountering a very odd sporadic issue when it comes to starting the truck. It seems that when the engine is cold it will start up fine for the most part without stalling out, same for when I have just cut the truck off and try and turn it back on immediately after. But when I try and start the truck after say 30 minutes after cutting it off it will crank, seem like it catches, and then just die out, almost like a lack of fuel or potentially air.

My only experience with carbs is on motorcycles so trying to translate what I know from my Harley and Honda Magna has been a little more difficult than I imagined given all of the extra vacuum components and such (might just be getting into my head a bit about it, but whatever). I have tried to do some digging online for potential similar issues and it seems that it could be quite a few things and I am not really quite sure where to begin to be honest, so I will just list what I have done thus far, and if anyone can point me in the right direction, I would be very grateful.

1. Inspected and adjusted the electric choke to ensure it was adjusted properly and the spindle was seated on the pushrod. (This seemed to fix the issue at first but was a false victory)
2. Adjusted the high speed and low speed screws
3. Inspected the in-carb paper fuel filter (clean and seems to be in near new condition)
4. Inspected fuel lines (no leaks, weeps, or drips)

Other than these issues I am not really quite sure how to proceed. I will state that a few things I noticed that may have something to do with this but am not sure are the following.

1. After driving for a while, when taking the gas cap off, its seems that there is A LOT of pressure behind it, almost like an air compressor. An extremely audible hiss can be heard for a solid 3-4 seconds while unscrewing the gas cap
2. The carb flooded out on me at a stop light the other day while driving. Upon inspection, it was as if the carb was boiling fuel over (visible smoke coming from carb inlet and gas pooled in the chamber recesses on top of the carb.
3. The idle seems to be sporadically rough. Sometimes it idles like a champ, and others it seems almost as if it is fighting to stay running (sometimes even outright stalling on me).

I have seen things saying that it could be vapor lock in the fuel lines, or the fuel pump, or the fuel filter etc. Just wanted to pick the brains of the more initiated on this topic for some assistance. Thanks in advance!

 

[Chevy 88]

I don't buy it.

 

[Turbo4whl]

After driving for a while, when taking the gas cap off, its seems that there is A LOT of pressure behind it, almost like an air compressor. An extremely audible hiss can be heard for a solid 3-4 seconds while unscrewing the gas cap

Is the air blowing out, or is it a hiss because air is being drawn into the tank?

 

[Ricko1966]

Run it with the gas cap loose or off with a rag as a cap for a couple of days. Report back. If the fuel system is building pressure,and it can sitting with heat. If it can't vent it can actually make enough pressure to hold the needle off the seat. If it does this parked the right amount of time your coming back to flooded engine.

 

[Grit dog]

Try driving it with no fuel cap or loose cap.

 

[Grit dog]

Run it with the gas cap loose or off with a rag as a cap for a couple of days. Report back.

Haha you said the same thing at the same time! Hope we’re both not wrong lol.

 

[Ricko1966]

Haha you said the same thing at the same time! Hope we’re both not wrong lol.

It's just a test,and a good one, if the tank is building pressure it will flood. If it has a vacuum it will starve. Let's hope that pin points it so we can help find a vent problem.

 

[AMRAAM_Enjoyer]

Run it with the gas cap loose or off with a rag as a cap for a couple of days. Report back. If the fuel system is building pressure,and it can sitting with heat. If it can't vent it can actually make enough pressure to hold the needle off the seat. If it does this parked the right amount of time your coming back to flooded engine.

Okay, I will give that a go and see what happens. Will report back in a day or so with results.

 

[AMRAAM_Enjoyer]

Is the air blowing out, or is it a hiss because air is being drawn into the tank?

Its air blowing out for sure.

 

[SquareRoot]

I don't buy it.

 

[SquareRoot]

Basically I have a carbed 305 mated to a TH350 (if that matters at all) with a simple little 2bbl (I assume it is stock based on reference images). The motor and basically everything else around it save for an aftermarket exhaust (Header back) is bone stock. However, I am encountering a very odd sporadic issue when it comes to starting the truck. It seems that when the engine is cold it will start up fine for the most part without stalling out, same for when I have just cut the truck off and try and turn it back on immediately after. But when I try and start the truck after say 30 minutes after cutting it off it will crank, seem like it catches, and then just die out, almost like a lack of fuel or potentially air.

My only experience with carbs is on motorcycles so trying to translate what I know from my Harley and Honda Magna has been a little more difficult than I imagined given all of the extra vacuum components and such (might just be getting into my head a bit about it, but whatever). I have tried to do some digging online for potential similar issues and it seems that it could be quite a few things and I am not really quite sure where to begin to be honest, so I will just list what I have done thus far, and if anyone can point me in the right direction, I would be very grateful.

1. Inspected and adjusted the electric choke to ensure it was adjusted properly and the spindle was seated on the pushrod. (This seemed to fix the issue at first but was a false victory)
2. Adjusted the high speed and low speed screws
3. Inspected the in-carb paper fuel filter (clean and seems to be in near new condition)
4. Inspected fuel lines (no leaks, weeps, or drips)

Other than these issues I am not really quite sure how to proceed. I will state that a few things I noticed that may have something to do with this but am not sure are the following.

1. After driving for a while, when taking the gas cap off, its seems that there is A LOT of pressure behind it, almost like an air compressor. An extremely audible hiss can be heard for a solid 3-4 seconds while unscrewing the gas cap
2. The carb flooded out on me at a stop light the other day while driving. Upon inspection, it was as if the carb was boiling fuel over (visible smoke coming from carb inlet and gas pooled in the chamber recesses on top of the carb.
3. The idle seems to be sporadically rough. Sometimes it idles like a champ, and others it seems almost as if it is fighting to stay running (sometimes even outright stalling on me).

I have seen things saying that it could be vapor lock in the fuel lines, or the fuel pump, or the fuel filter etc. Just wanted to pick the brains of the more initiated on this topic for some assistance. Thanks in advance!

Regarding the pressure in the tank, I had until recently a similar issue. Although I have aftermarket EFI, I think the cause and effect are the same. Took me a long time to figure out my issue. The attached is a screenshot of my recent post on another forum.

 

[TotalyHucked]

I agree with what's posted above, sounds like you may have a venting issue. Check the lines headed to/from the charcoal canister under the hood, they may be in such bad condition they're swollen shut or plugged with debris. You may end up having to drop the tank and run new lines. Or if you don't have a charcoal canister/want to do away with it, you can simply run a length of hose to the back bumper along the frame rail and let it vent to atmosphere. That's how my truck is and it doesn't stink up the garage or anything

 

[AMRAAM_Enjoyer]

Regarding the pressure in the tank, I had until recently a similar issue. Although I have aftermarket EFI, I think the cause and effect are the same. Took me a long time to figure out my issue. The attached is a screenshot of my recent post on another forum.

Interesting, so based on this experience, it may very well be possible that the charcoal canister is clogged up, or the vapor line is, maybe even both. It would make a lot of sense. Thank you for the input!

 

[AMRAAM_Enjoyer]

I agree with what's posted above, sounds like you may have a venting issue. Check the lines headed to/from the charcoal canister under the hood, they may be in such bad condition they're swollen shut or plugged with debris. You may end up having to drop the tank and run new lines. Or if you don't have a charcoal canister/want to do away with it, you can simply run a length of hose to the back bumper along the frame rail and let it vent to atmosphere. That's how my truck is and it doesn't stink up the garage or anything

Thats my conclusion as well! Thank you both very much. I have new lines and a new gas cap on order so I am going to give that a shot. In the meantime I will give it a go with drilling a hole in the gas cap and see what that yields.

 

[TotalyHucked]

Interesting, so based on this experience, it may very well be possible that the charcoal canister is clogged up, or the vapor line is, maybe even both. It would make a lot of sense. Thank you for the input!

One thing you can do is unbolt the charcoal canister, pull it out and carefully shake it. If you can hear fuel sloshing around, it's junk. I've always done away with the charcoal canisters and just vented to atmosphere but I remember reading on here that simply letting them dry out doesn't work either, apparently once the charcoal is soaked with fuel, it's essentially ruined. There's some universal charcoal canister setups but I don't think anyone repops an identical one at this point