This is an odd missfire that I have been experiancing when I'm racing my car. Most of the time the car runs like a champ, however it seems like when the motor heats up (lots of racing) the engine begins to stumble badly. It feels like a lean miss almost. Power drops off dramatically, the engine surges and missfires until I let off. The car will run fine in any other condition. This also seems to happen when I'm on the gas for a bit of time. Usually in the middle of 2nd gear or 3rd on a standing start race.. My friend has suggested that my fuel pump is dying which I'm going to try. Does anyone have any other ideas of what I could check out? Saturday is the races, I don't want it to act up on me at the track. Peace!

If the bowls run out of fuel you usually don't get stumbling, but it just flat falls on it's face like you've killed the ignition.... til fuel gets back into the carb.

First thing is to thoroughly clean all the tiny airbleeds on the carb (all four boosters) because they can cause surging/stumbling when they get dirt or moisture in them.

Next would be to check out the fuel/float level to make sure it's not too high & you're not getting a lot of fuel slosh when you accelerate.

Have someone drive behind you when it's doing it and see if you're making black sooty smoke, just to see if it's too much fuel going in.

Of course you want to check your ignition out very well too because most carb problems are really ignition problems. You might have some plugs dying, too wide of gaps, or failing wires.

Hey man I hate to say this but I have the same problem. But of course you may want to get a compression test but my problem resulted from either a blown head gasket or a cracked head. Im sorry to say it but it sounds alot like what Ive been trying to deal with. Im just gonna swap out the 305 TBI and put in a carbed 355 so I dont know if you want to replace the engine to make those races a little more in your favor but I was just adding my pennies cause thats what happened to me.

Tyler

Mine stumbled/stuttered under WOT when I had a bad fuel pump. It would keep enough fuel going in to keep it from acting like it shut off. Try the fuel pump.

Since it seems to be heat related, it could also be ignition related. The stock coil and module are known to be weak/bad actors. Replacing them with aftermarket pieces, as well as a "complete" tune-up (cap, rotor, wires, plugs) wouldn't hurt if it hasn't been done for awhile.

I'm betting fuel pump, though.

There's an old saying something like
90% of carburetor problems are in the ignition.

it's so true... check your ignition out first.

check the wires for restance, . plug gap, and make 100% sure your coils good. even after that.. you may have the ignition module dying on you.

what these fella's said bout checking the fuel pump.. it's true. also make sure the filter's not plugged solid.

Does it crank up normally or does it have a hard start problem as well?

wow ive been tryin to fix this problem too. It only does it after about 20 min of driving and when i try to go over 1500 rpm ive replaced cap,rotor,coil,plug wires plugs,fuel filter fuel pump,and run fuel system cleaner threw it and cleaned the carb with carb cleaner still does it.

Have you put a fuel pressure gage on it? If not, everything is a guess.

A clogged fuel pickup strainer will cause fuel starvation under high demand.

what spark plugs are you using?

I am actually having the exact same problem. It's gotten worse since I installed the Performer intake.

Some people have suggested fuel pump, others have said I may need a phonelic (sp?) spacer.

Any help would be great!


ps. I've got an 86 Z28 - LG4, Performer Intake, Drop base w/ open element, Flowtech headers, hi-flow cat, Flowmasters.

Common problem. If anyone did a search they wouldn't need to be told it's a bad fuel pump. I had the same problem. Car would almost die just at the top of second gear - sometimes but not always. Hooked up rubber line with a pressure gauge and sure enough, just past 3500rpm in second gear the pressure would drop to 0-1 psi. I replaced my pump with a Carter electric AND Carter mechanical redundant setup as well as rerouting my lines and an inline filter. No problems since. Maintains a constant 5-6 at all times. And no more vapor lock when stuck in traffic on hot days

Well I did the fuel pump replacement and it went away for awhile. Now it's back with a vengeance.

I'm certain it is an ignition problem though, at around 3-3500rpm it misses so bad you would swear I was hitting my revlimiter. Pretty much all cylinders are miss firing.
This only happens when your really on the gas though, light load and it will rev up without a problem.

I'm going to do a complete ignition freshen up. wires, cap, rotor and plugs. I'm running an MSD 6al box through a blaster coil. How long are those blaster coils supposed to last?
Any other ideas?

Marc where did you run your fuel lines! I fight this same problem all the time. It is'nt ignition, and thats a fact. New ZZ4 Distributor with Accell Module, Summit coil, Summit wires, and Rapid Fire Plugs. What is happening is that my underhood gets soooooooooo hot from my SLP Headers that my carb vapor locks all the time on hot days at the top of second gear. The muffler also heats the fuel tank sooo much that after I shut the engine off I build 3-4 PSI of pressure before the tank vent valve blows off. My fuel system is a Holley Blue pump with 3/8 aluminum line along the drivers side to the Holley Regulator, then through a fuel pressure guage and through 3/8" hose to the carb routed so that it is nowhere near the headers. I can not think of any way to keep my fuel cooler. If you have any suggestions please let me know but I think our vehicles just have an inherent design flaw. That being not enough room for stuff underhood.

You can easily test the vapor lock/tank vent theories by taking it out cold in the morning with the gas cap off, and trying a run. If the problem is still there with everything cold & no pressure/vacuum in the tank then you can eliminate those possibilities.

You can keep heat off the fuel tank by doubling up on the muffler heat shields or wrapping the muffler & pipe where it goes over the axle with thermotech.

CamaroMike:

I followed the stock routing to the front subframe. I tucked the lines as close to the frame as possible and created a small heat shield for them when they near the headers. The line then goes to the fuel cooler I installed up front next to my auxiliary trans cooler. The line runs from the cooler to the mech pump and from the mech pump I routed the line up by the alternator - going through the inline filter. I held the braided hose from mech pump to inlet with a custom bracket I made the bolts to the head.

Sounds complex buts it's pretty straightforward.

Well I said the hell with it and figured that if I run the fuel line anywhere into the engine compartment along the frame, I will get too much heat from the headers. I placed my K&N fuel filter and regulator under the car along the driver’s sub-frame rail and ran 3/8" hose to the carb over the transmission. The tranny will heat the fuel I'm sure but no worse that any header or line lying on the engine. The fuel cooler sounds like a good idea! Wonder how much that helps? I'll tell ya if anything changes performance wise in the future.