Vapor lock

[Bonnewagon]

Why are you running a regulator when you have a recirculating style fuel pump? That kind of pump has a built in regulator and sends excess fuel back to the gas tank. That was the factory fix for vapor lock with AC equipped cars. Maybe remove the regulator and see if the problem goes away. FWIW when you have the three port fuel pump and just plug the return port you send all the fuel to the carb. That usually results in a minor flooding condition at idle but otherwise does not affect off-idle or WOT running. That may be a good test to see if you really have vapor lock or something else . When I have a three port pump and suspect insufficient flow to the carb I just take a "T" and plumb the return fuel line right into the supply line. That way all fuel goes to the carb.

 

[Clone TIE Pilot]

Why are you running a regulator when you have a recirculating style fuel pump? That kind of pump has a built in regulator and sends excess fuel back to the gas tank. That was the factory fix for vapor lock with AC equipped cars. Maybe remove the regulator and see if the problem goes away. FWIW when you have the three port fuel pump and just plug the return port you send all the fuel to the carb. That usually results in a minor flooding condition at idle but otherwise does not affect off-idle or WOT running. That may be a good test to see if you really have vapor lock or something else . When I have a three port pump and suspect insufficient flow to the carb I just take a "T" and plumb the return fuel line right into the supply line. That way all fuel goes to the carb.

Many replacement pumps including the return types put out much higher than stock PSI anymore. The current pump pegged the needle at the max reading of my gauge at 10 psi just at idle. Factory manual states fuel psi should not be more than 7-8 psi. Also I have a .135 needle valve seat in my carb and larger valve seats while alliowing more fuel flow are more prone to being forced open by excessive fuel pressure than smaller needle seats.

 

[Bonnewagon]

The current pump pegged the needle at the max reading of my gauge at 10 psi

So what is your pressure now with the regulator? I would think higher fuel pressure would defeat vapor lock.

 

[Clone TIE Pilot]

So what is your pressure now with the regulator? I would think higher fuel pressure would defeat vapor lock.

I have it set at 7 psi. The regulator is between the carb and pump. Generally vapor lock occurs on the suction side of the pump where pressure is low and fuel can boil at lower temps.

 

[Bonnewagon]

So all of us wrapping the pump to carb metal fuel line with aluminum foil are doing it wrong? I had an old VW beetle that it was mandatory to wrap that fuel line or it suffered massive vapor lock. I can't think of any spot on the body mounted fuel line where heat would be an issue. If the suction side is seing that low of pressure then would that not indicate a restriction, probably at the tank? In my experience the spot where the steel body fuel line jumps to the fuel pump with a rubber hose is so low that fuel will siphon from the tank quite vigorously if disconnected. I am on several forums and the vapor lock issue has been popping up quite often even on modern fuel injected vehicles.

 

[Ugly1]

I will try insulating the fuel line and phenolic (heat insulated) fuel pump spacer first. The L69 fuel pump has a built in large fuel reservoir but with it being bolted to the engine block it can conduct heat from it. If that doesn't work I may elect to go to an in tank electric pump setup like the old ZZ3 350 swap kits for 3rd gen LG4/L69 Camaros. Rockauto has Monte Carlo 4.3 TBI fuel senders with electric pump for $47, but still requires a whole mess of additional parts to make it work in a carbed car.

Did find a gas station that sells ethanol free gas buts its far from my house, its more expensive, and its reported they don't always have it in stock.

Yeah, have to add a adjustable pressure regulator for the 6 to 8 psi. You could try an external electric pump at the rubber line near the battery and just bypass the mechanical pump up to a regulator then back to the stock fuel line or bypass that to the carb. “Cheap and dirty “ test. See if better before dropping the tank. If it works and you want to go to the internal one the cheap electric pump can suck the tank dry into gas cans!

 

[Clone TIE Pilot]

So all of us wrapping the pump to carb metal fuel line with aluminum foil are doing it wrong? I had an old VW beetle that it was mandatory to wrap that fuel line or it suffered massive vapor lock. I can't think of any spot on the body mounted fuel line where heat would be an issue. If the suction side is seing that low of pressure then would that not indicate a restriction, probably at the tank? In my experience the spot where the steel body fuel line jumps to the fuel pump with a rubber hose is so low that fuel will siphon from the tank quite vigorously if disconnected. I am on several forums and the vapor lock issue has been popping up quite often even on modern fuel injected vehicles.

I wouldn't say that. Its just a high pressure area will require more BTUs to boil than a low pressure area but engines and especially aircooled ones can generate enough BTUs to do so. Don't forget conduction, heat can conduct from a hot part of a metal fuel line to another part. Its why I am thinking of trying a insulated fuel pump mounting plate. A mechanical fuel pump is in direct contact with the hot engine block. Don't want the pump to be a hotspot.

Also cooler fuel is deniser fuel and increases the mass of fuel that can be pumped into the engine. Mass = volume x density. So its always good to keep heat off the fuel line barring extreme cold environments.

For G bodies there is a hot spot on the suction side near the tank. The passenger side exhaust cat back over the rear axle passes close to the 3 fuel lines. I have mine heat shielded there.

 

[Ugly1]

I am still having an issue with a rough and uneven idle despite everything checking out. No vacuum leaks, fuel mixture dwell at the correct 30 degree range, correct ignition timing, etc. I have come to suspect it may be a vapor lock issue as the symptoms become worse as the engine heat soaks and the idle smooths out if I apply some propane to the air intake. It seems to be starving for fuel. I also have a off idle hesitation that becomes worse the harder I accelerate from a dead stop. With the high fuel prices I suspect my local gasoline may have higher ethanol content as both my Ranger and CVPI are not running quite as well recently either.

From I have read LG4 and L69s had vapor lock issues even back in the 80's when they were new and modern ethanol blended fuels boils even easier than the gasoline of lore. Currently running the stock return style L69 mechanical fuel pump running up to a Holley dead head regulator and to the E4ME Qjet which uses the thick style mounting gasket. Back in the day the dealership fix was to install a in tank electric booster pump to force feed fuel to the original mechanical pump and an electric cooler fan for the Qjet. The old emission legal ZZ4 350 swap kit for F bodies included a new in tank electric fuel pump and return regulator to completely replace the 305 mechanical fuel pump.

Probably the best fix is to convert to a in tank electric fuel pump but I rather not go that extreme if I can help it. Currently thinking of installing a phenolic mechanical fuel pump spacer, fuel line insulation, and maybe one of those external fuel filters with a vapor port. Since I still have the stock EVAP system and charcoal can, I assume I just run the vapor line from the fuel filter and tee it into the tank line into the charcoal can?

Seeing later replies mine went through late. Unless you have an air/vacuum type leak into the line or it is close to a heat source the fuel has no real way to be turned to vapor. Usually it happened as it “wrapped “ the engine or the carb got hot via the intake. The corn syrup could change the temperature of the change but again it needs a type of radiated heat source.

 

[Ugly1]

I wouldn't say that. Its just a high pressure area will require more BTUs to boil than a low pressure area but engines and especially aircooled ones can generate enough BTUs to do so. Don't forget conduction, heat can conduct from a hot part of a metal fuel line to another part. Its why I am thinking of trying a insulated fuel pump mounting plate. A mechanical fuel pump is in direct contact with the hot engine block. Don't want the pump to be a hotspot.

Also cooler fuel is deniser fuel and increases the mass of fuel that can be pumped into the engine. Mass = volume x density. So its always good to keep heat off the fuel line barring extreme cold environments.

For G bodies there is a hot spot on the suction side near the tank. The passenger side exhaust cat back over the rear axle passes close to the 3 fuel lines. I have mine heat shielded there.

Then doing the electric pump and use it as a quick bypass it would give you a fast and “cheap “ test. even better if you could borrow one to do it.

 

[Ugly1]

I can post part numbers later tomorrow, but having built several G and A Body street/show in and around Tucson, 100*F + on some days, delt with this before. The stock FP has the small vent return, if you're does , get rid of it. Get a basic mechanical FP, oreilly/auto zone etc... The basic FP just has the in and out ports. Cap the small line going back to the tank. Depending on how your evap system is setup you might need vented gas cap..., maybe not... I went though the very issue you describe and all new FPs, and found that instead of sending vapor back to the tank it was pumping and sending raw fuel back, and yes pumping air bubbles up to the carb. Quality control is just not what it used to be. You describe symptoms of insufficient fuel reaching the carb. So figuring your carb is good, etc..., etc..., just do the simple first. Holley ... Edelbrock... Excellent carbs but they don't run on bubbles. It sound like you have the fuel regulator with the return. If you do you can tap the return into the line you capped from changing the FP. Also if/when you drop the tank, check all your rubber hoses. The 86 era fuel hose rubber will not handle today's gas and its additives. Seen to many restored 80's vintage burnt to the ground. Use the fuel injection line designed for today's gas. This is also true for the old vacuum lines.

Dam. Didn’t even think in that direction until you mentioned it. I put new lines on my build and it never crossed my mind. Thank you.