[81TATurbo]

1981 Firebird Formula. Running a Butler stroker. 9.1-9.2:1 compression. Running an Edelbrock carb and Performer RPM intake. If I let the car shift itself, it shifts fine. Early. Around 4200 RPM. If I manually shift it, usually around 5200-5400 it bogs into second gear. I have to let off the throttle and then get back on it again. Fuel pump is a Pontiac Carter Hi-Volume Mechanical Pump. Only does it when the car is hot (operating temperature). Doesn’t do it after it’s warmed up a bit not quite at operating temperature which is around 160-170. Just changed out the rubber fuel line to a braided line hoping that would help. Could it be something else?

[78w72]

Way too many variables that could be causing the issue described... and if its only when fully warmed up it could be temp related but running at 160-170 is definitely not anywhere near overheating or too hot to cause true vapor lock. what were outside temps? being in canada i will guess it wasnt very hot out, mid 70's to maybe 80, which should not cause vapor lock... does your fuel line run close to any hot parts of the engine like headers?

with the rpms it happens at, the first thing i would look at or consider changing is the fuel pump... even though it says "high volume" those pumps wont support big cube engines at higher rpms or at the drag strip in most situations. i had the edelbrock equivalent pump that claimed it was good to 600hp, it did just ok on the street, would have cut out & hesitation problems similar to what you described, my car runs at 180 no matter the outside temps ( up to mid 90s & higher humidity at least) or driving conditions, full throttle street runs were mostly ok but had occasional symptoms like yours, at the track right around low 12's it would cut out terribly. i upgraded to a robbmc pump & that fixed all street issues & allows the car to run into the low 11s but on good traction runs it will still have a random fuel starvation issue.

consider upgrading to a robbmc pump & pickup along with bigger fuel line from tank to pump, but the robb pump may cure your issues. or go to a tanks inc type intank pump set up.

[81TATurbo]

Forgot to mention that my gauge reads 6PSI for fuel pressure. Not sure about on the shift. I’m going to see about getting my GoPro in the engine compartment and see if the pressure drops. Atleast if nothing changes, I can rule out a fuel issue

[81TATurbo]

So after doing some digging, as in my vacuum advance had failed and I had to replace it, I noticed some scarring on my rotor. Upon further inspection I noticed that the carbon button had warn unusually. When I replaced it and my rotor, there were two insulator rubber pads under the coil. Late night tinkering one night after a long day with the kids no doubt. Not sure if that would cause it but I had it out today and it runs smoother and no more hesitation or rough running. I'll get out for a good rip tomorrow to test it out more thoroughly. Hopefully that was the cause.

[JBanna]

I have had the vapor lock issue on heavy acceleration in outdoor temps of 80+ and can't really figure it out - I have a Carter street pump set at 6 psi, but have kept the original fuel lines. My car is a 78 T/A with a 455 Pontiac, with 4x heads and a roller cam with moderate increases in duration and lift, so I still have good vacuum; I also switched to headers and full dual exhaust, an Edelbrock Performer manifold, and run a Rochester Quadrajet. I also had my distributor curved. Many people have doubted vapor lock, but the hesitation and faltering does not happen in cooler weather; in fact, I made a 100+ dash one spring at temps around 50 and had no issues, no matter how hard I mashed the accelerator. Finally, the closest my fuel lines get to the headers is about 3 inches, and the thermostat is a 195 degree high flow model.

Any other ideas other than replacing fuel lines and/or fuel pump? How does one even confirm vapor lock?

Thanks, Joe

[78w72]

Originally Posted by JBanna

I have had the vapor lock issue on heavy acceleration in outdoor temps of 80+ and can't really figure it out - I have a Carter street pump set at 6 psi, but have kept the original fuel lines. My car is a 78 T/A with a 455 Pontiac, with 4x heads and a roller cam with moderate increases in duration and lift, so I still have good vacuum; I also switched to headers and full dual exhaust, an Edelbrock Performer manifold, and run a Rochester Quadrajet. I also had my distributor curved. Many people have doubted vapor lock, but the hesitation and faltering does not happen in cooler weather; in fact, I made a 100+ dash one spring at temps around 50 and had no issues, no matter how hard I mashed the accelerator. Finally, the closest my fuel lines get to the headers is about 3 inches, and the thermostat is a 195 degree high flow model.

Any other ideas other than replacing fuel lines and/or fuel pump? How does one even confirm vapor lock?

Thanks, Joe

Vapor lock is hard to confirm since you cant really see the fuel in the lines or carb. True vapor lock is when the fuel is so hot it turns to vapor in the lines/carb, one way to check the fuel at the carb, is when it acts up pull over instantly & shut off the engine, pop the hood & pull air cleaner then look down the carb while operating the throttle, if you see raw fuel squirt out its probably not vapor lock, if its an intermittent broken spray of fuel or looks like an aerated mist then its likely related to overheated fuel. the other option is to use a fuel gauge plumbed to the windshield or like 81turbo said to get a gopro camera to view the gauge, if fuel psi drops from normal then its a fuel supply or vapor related issue.

Vapor lock is not as common as some think, until 81turbo investigated more he would have sworn it was vapor lock but turns out it was ignition related.... at this at this point. I have random fuel cut out issues at the drag strip on hard launches at low 11sec runs, based on the symptoms & that some times its hot summer days many people would say its vapor lock, but in my case its fuel supply related, have confirmed fuel is not evaporating in the carb on the hottest of days.

Big cube or high hp/rpm engines using stock lines or wimpy pumps can have fuel related symptoms in any temps, but heat can exaggerate that just like it can with electrical issues. Do some more testing to narrow it down & consider some heat wrap on the lines near super hot sources like headers.

[JBanna]

Thanks for the new ideas -I like the one recommending a camera (my regulator is in the engine compartment), as well as the quick stop and check the accelerator pump - I could try that with the shaker and air cleaner removed already....I am now wondering though - could an issue in the carb (like the accelerator pump) cause symptoms like vapor lock on a hot day?

[JBanna]

Thanks for the new ideas -I like the one recommending a camera (my regulator is in the engine compartment), as well as the quick stop and check the accelerator pump - I could try that with the shaker and air cleaner removed already....I am now wondering though - could an issue in the carb (like the accelerator pump) cause symptoms like vapor lock on a hot day?

P.S. It probably doesn't make a difference, but my heads are 6x, not 4x...