The Stovebolt Site Forums The Shop Area Engine Shop FUEL VAPOR LOCK ?

hey guys, 3 weeks ago i bought a 1957 3100 with a 1980 350 4 bolt motor that was rebuilt about 5 years ago.
starts right up, runs smooth.
has a very mild cam, .460 lift, aluminum intake and edelbrock carb, hedders and dual exhaust with flow masters.
temp gauge dont work, but it does not get too hot.
today, after driving for 10 miles, i parked it for about 10 minutes, then it was a little hard to start.
then on my way home at some lights it would seem to run rough and almost stalled out, but didnt. like running on only 6 or 7 cylinders.
then when taking off, it has a little stumble, hesitation, off the line, then smooths out while going down the road over 20mph.
seems like a fuel vapor lock to me.
what do you think ?
standard fuel pump on the passenger side side of front motor.
fuel lines running in the factory positions.
i didnt look under the truck to see if maybe the fule lines are too close to the exhaust, but i dont think they are.
i put a new locking gas cap on it, but thought about it and opened it and it didnt blow anything out, like if it were not vented. that seemed fine.
dont want to get caught out somewhere and it shut off on me.
do you guys got any suggestions ?
thanks much...leonard.

I'd say flooding rather than starving, check the plugs and see what they say

Bill

fuel filter?

i think i did see an external fuel filter under the truck when under it the other day.
so, would a dirty fuel filter act like that onyl when hot ?
i have to change the hedder gaskets soon, so when i do that i will pull all the plugs, but will pull 1 or 2 this coming weekend.
thanks guys... like i said, it felt like vapor lock, but this thing is no race car. dont see how it could be vapor locking... will try filter and plugs...thanks much guys....leonard.

I'm with Red58, the carb's loading up. You don't see much for vapor lock any more. Today's fuels are good enough to vapor lock,
Scott

I had the same problem with an Edelbrock carb on an aluminum manifold. Didn't like heat! After getting it warmed up it wouldn't idle and stumbled off the line. I put an insulator block between the manifold and carb. That helped but I never did get the problem to completely disappear.

A good way to check for vapor lock is to remove the gas cap and blow real hard into the gas tank. If it's vapor locking that most times will clear it up. A good preventive for vapor lock is to clip several wooden clothes pins on the fuel line near the carb.
Always worked for me...45 years ago.

It certainly appears to be a too much fuel problem. Heat percolation is a definite suspect - the suggestion of a heat spacer is sound advice - but perhaps a quick pressure reading of the fuel pump might be in order first. After sitting for 5 years the diaphram could well have cracked under initial start-up and with the resultant build-up of fuel could well lead to carb flooding. Does the oil smell of gas?

Oops! Am I incorrect in assuming the engine sat for 5 years? Still, pump pressure is an easy check in the process of elimination list.

thanks guys.
i was told the engine was rebuilt 5 years ago, driven for 3 years, then sat for 2 years in the restoration shop. so it was setting for 2 years.
when you say heat spacer, are you talking about a 1 inch spacer between the carb and manifold, or the large flat aluminum plate that goes between the carb and intake manifold ?
i am kind of a picky type person, so i might just go ahead and change the fuel pump so i know i will be good for a long time.
also will do a complete tune up and install a heat shield and go from there. but this will take me about 2 weeks to do.
thanks guys....

Vapor lock typically occurs under high fuel demand when a failing fuel pump can't keep up. If yours runs better at higher speed (higher demand) it's likely not vapor lock.

Swapping parts on a WAG is just foolish! Test the pump first, and only change it if it needs to be changed. Vapor lock on anything newer than a 1953 Ford flathead V-8 with the pump mounted high over the intake manifold is highly unlikely, due to the way newer fuel pumps are positioned in relation to the tank. Lifting gas a foot or more above tank level with the suction side of the pump was what caused vapor lock, as the fuel vaporized under high vacuum conditions, and it's hard to pump enough vapor to make an engine run. The early Cadillac OHV V-8's also put the pump above the intake manifold, just like the Fords, and they vapor locked, too.

Run a pressure and volume test, and if you find a bad pump, replace it. Otherwise, find the fuel system restriction if it's starving, or fix the flooding condition if it's loading up. Like Sun Electric Co. used to say, "We test- - - - -not guess!"
Jerry

thanks guys.
i ordered a heat shield today from summit and will do a complete tune up by this weekend, hopefully.
i had a race car in the early 80's, that when it got hot it would vapor lock. so i put one of those moroso cool cans in it. you know the type, its a can with insulation and the fuel line is coiled all around the inside, and you fill it with ice. it worked. but know i dont have to do anything liek that for this truck. its not hot rod enough.
i will let you all know what happens after i tune it up and install the heat shield...thanks to all of you again..leonard

It might work for a while but the base issue is most likely a failing pump.

went today and bought a new fuel pump. also will do a complete tune up, plugs, wires, rotor and cap by this weekend.

with the fuel pump instructions it said,

"small block chevrolet engines with normal crank case pressure, DO NOT FURNISH ADEQUATE LUBRICANT TO THE FUEL PUMP, moisture can form in the pump and cause premature pump failure".
'PACK THE FUEL PUMP CAVITY AROUND THE ACTUATING ARM WITH HEAVY GREASE to prolong the fuel pump life and prevent harmful moisture buildup inside the pump".

anyone ever hear of this ? i can never remember doing this back 25 years ago when i worked on small block chevys. i did what the instructions said, hope that was a good call.

found the part number on my edelbrock carb, and it is #1405, 600cfm.
also, when looking in the distributor cap to inspect, the darn distributor was loose.

it has a small cam shaft in it,a nd i have the peperwork for it, but nowhere does it say what to time it at.
any suggestions there, or jsut do it by ear ?

thanks guys, and i will post after i get it all tuned up..thanks..leonard.

Hmmm. Loose distributor? Perhaps, if that noticably loose, there's the source of your original 'stumbling' problem. How did the old fuel pump check out?
As for timing, well, I rely more upon my vacuum gauge than my aged ears. Heck, my 8-track tapes still sound good to me!

laurance, i didnt have a vacuum gauge, so for 3 bucjs i went and bought a new pump for peace of mind. might have spent that money for nothing, but at least i know it is now new.
where would i hook a vacuum gauge up to to check vacuum, if i were to buy one ?
i just want a hand held one, not installed in the dash.
thanks much..leonard.

sorry, 30 bucks for the pump..

A vacuum gauge is plugged into any clean (unrestricted) source of intake manifold vacuum. Do not use any carburetor fitting, rather, use the intake port for, say, the PVC or brake booster. Then, with the engine warmed to operating temperature,disconnect the distributor's vacuum advance hose and plug it, and at correct engine idle speed slowly rotate the distributor (CCW retard; CW advance) until the vacuum gauge needle reaches highest point. Then drop back one point; tighten distributor. That will give optimum timing at idle speed. Re-connect the distributor vacuum before road testing. This method pre-supposes the carb mixture has been attended along with the customary tune-up observances. The vacuum gauge, and the compression gauge, are invaluable when trouble-shooting engine performance problems. A vacuum gauge mounted in the cab aids in extracting the last ounce of gas mileage driving performance as well. As for that old fuel pump, well, if attached to a siphon hose it beats the heck out of an unwanted drink.

thanks much laurence. great explanation. will add a vacuum gauge to my collection of tools very soon. LOL.
leonard.

well, was finally able to pull the top off my carb today and found one float to be 1/16 off !! suppose to be 7/16 of an inch, one was, and the other was ½ inch gap. That adjustment is holding it upside down, so that means it was too low, not enough fuel in one side. So I adjusted that, and also found one of the two step up piston metering rods to be stuck. Took it apart and sprayed it all down with carb cleaner.
Anyway, just got back from a decent test drive, and so far I think that cured my stumbling. Only cost 20 bucks for gaskets and new fuel inlet washers, and have 4 more gaskets for back up in the future. Not that bad a carb to mess with. I am always afraid of dropping something down the carb and wedging it in the valves and pistons. LOL..
thanks for the help guys. still have nto hooked up my vacuum gauge yet, but running so good i dont want to change anything anywway. LOL...
thanks again.....leonard.

Leonard,
Congrads on a successful tinkering job. That's one more item on your list of parts you're familiar with now, 10,000 to go.:D

thanks BIG.. yea, its kind of cool learning something new.
i have messed with the holleys back in the day, but never these edelbrocks. not too bad. just glad that was the problem. sure beats buying a new one for 250 bucks.
thanks again...leonard

and much thanks to my buddy Fla54Chevy3100.. cant tell you guys how much he has helped me out with a lot of questions i had for him. he is the best.....
leonard

had the same problem with mine. changed the coil problem seems to be cured