I've been working a lot on my 1986 Firebird. The engine is a 350, vortec heads, edelbrock intake, lunati voodoo camshaft (smaller, little bit of chop at idle), summit racing 4-barrel and some HEI I bought from Amazon. I pitched the original computer controlled QJet, and then also pitched the computer controlled HEI, guessing that the computer would probably be confused. The new HEI is all vacuum advance control only. Transmission is 5-speed and rear end is posi (eaton tru track) with 3.42 gearset.

Here's the issue....the motor idles great and revs just fine in the driveway. The plugs show that it's running a little rich, but I certainly don't smell any fuel in the exhaust. When I stomp on the gas with the car under some load, the motor just rolls over and does just about nothing....seriously, it just falls flat, tach starts dropping...no pinging...just out of power? It runs fine on the highway as long as I lay off the gas pedal too much.

I have a couple of hunches:
1. That cheapo HEI is just not up to the task and won't fire under load
2. Not enough gas flowing under load. I forgot to mention that I *did* have a nice high-volume fuel pump (Holley, I think) but it was leaky (at least I thought so) so I picked up a stock fuel pump replacement. I still have the Holley laying around somewhere, and maybe I'll test it out.
3. Four barrel secondaries opening too quickly? I'm inclined to say no to this one because I can get the motor to misbehave even at lower rpm, just has to be under a good load.
4. Similarly, maybe 4 barrel not flowing enough fuel in the secondaries. I took it apart, cleaned it all up, checked the jet size, and all looks just about normal.

I had a hell of a time with the carburetor this spring. Turned out that it was defective and burned unbelievably rich. Summit was great helping to solve the issue and ultimately sent me a new carb. This one seems okay, but because of the earlier problem, I'm not going to rule it out.

Any ideas?

thanks

gene

 

Sounds like fuel delivery.

Not a problem within the carb (although that's not saying the carb is "perfect", "you don't have a carb problem", etc.); fuel delivery.

"New fuel pump" won't fix the problem if what's REALLY wrong is crud built up in the screen on the pickup in the tank, a broken hose where the tank hooks to the line, a bolt that's too long in the hole that intersects the drive rod, etc.

 

I tend to agree with the lack of fuel comment. I wonder about the flow rate of the replacement fuel pump....

There is no screen in the tank. The float got replaced maybe 2 summers ago. The replacement was electric pump style.... can't find the carburetor floats.... and i just had to abandon the in tank filter. I did plumb in a inline filter just after the tank.

I also have an inline filter between the pump and the carb.

Its the line plugged somewhere? Could be.

If i rev the motor up to several thousand RPM, would you expect the filter to remain full of fuel...or maybe part full... filler is see through? Is that a good 1st cut attempt at isolating a fuel problem.

 

Problem IS NOT the "rating" of the fuel pump.

The stock pump (block mounted, replacement, from AZ or somewhere) was COMPLETELY enough for the 400 I had in my car with a big solid cam, 800 CFM Holley, etc.

An inline filter on the suction side of the pump is a definite potential efff-up. FPs in general SUCK at sucking. They are built to pressurize, not suck. ANY restriction on the suction side is DEATH.

I wouldn't count on the filter's state as an accurate indication. What I'd suggest doing is, operate the car in such a way as to create the behavior; then WHILE IT'S IN THE ACT OF SCREWING UP, cut the ignition off (NOT "Lock") and take it out of gear and coast to a stop; and inspect the carb to see how much fuel is in it. DO NOT let the engine operate in ANY manner WHATSOVER in between observing the symptom and shutting it off!!! The object of the exercise is to CAPTURE the EXACT condition the carb was in while it was failing. Allowing the engine to idle, for example, will remove the load (the attempt by the engine to drain the carb) and allow the pump to catch back up, thereby destroying the evidence you're looking for. You might as well hire a cleaning service to come scrub the door handle before you check for fingerprints.

I know nothing whatsoever about the Summit carb. Summit doesn't "make" anything though, they merely have people who DO "make" things package them for them. So it's probably someone else's product. In the 21st century I'd almost bet money that's Holley or Edelbrock, since AFAIK nobody else still "makes" 4-bbl carbs. I could be wrong though.

 

My money is on the carburetor, but I would start with the recommendation to evaluate your rate of fuel flow because that is a little more simple to start with. Unhook fuel line at carburetor and have friend crank engine. Should see plenty of fuel dumping from the line into your collection can (or whatever you use).

This definitely sounds like a fuel flow or delivery problem, and since the carburetor flows and delivers fuel it is definitely suspect. I didnt see anything mentioned about carb adjustment. These things don't run ideal out of the box, they need tuning to run good.

I actually just fixed a problem like this with my Edelbrock. Been running funky and doing exactly what your car is doing. I knew it was acting like a fuel problem and had no reason to suspect my fuel pump (I even have a filter on the SUCTION side of pump, oh no!). Took carburetor apart and found the problem. Broken gasket caused fuel to flow in wrong places and caused whole problem. My carb is well adjusted so that was never a concern.

 

So far, I tried the test where the motor is shut down immediately after the problem. I noticed 2 things: the fuel level in both the primary and secondary is very high on the sight window. Also noticed the choke is not fully open.
When I adjusted the fuel level, the car was on a very slight incline, so maybe that's what happened...not sure...will readjust the level lower. I don't know why the choke isn't fully open. Either I will repair it, or just wire it fully open until I can figure out the rest.

In regards to the carb tuning...I did adjust the idle-mixture screws for maximum vacuum. I get around 20 inches. The plugs look carbon fouled, so maybe the jetting isn't okay and needs to be leaner. I've got an assortment of jets to test out. What would be a great diagnostic aid is wide-band O2 sensor...been thinking to get one anyway, so maybe now is a good time. I *think* the vacuum advance is connected to the metered port....not 100% sure, and need to verify. I've seen numerous articles and opinions on this one (one supposed GM engineer said the full vacuum is the intended method for numerous reasons.....others say the car works better with ported? I dunno).

Timing is set to about 10 degrees initial advance. I have no idea of the rest of the timing curve. Maybe I'll get some timing tape and measure it all.

Another thought....is it possible that the secondaries open too soon? So that the motor is just way too rich?

 

If the choke is not fully open the engine will virtually always be too rich.

 

Mystery solved....the air filter lid sits too close to the choke, and so the choke cannot full open. Geez!

I've got too little clearance under the hood. With a dropped air filter, 2" element, and one of those fancy flow-through lids, there's just too little clearance for the choke to fully open. I'm not quite sure how to deal with this, but at least I know what's causing the trouble.

 

I ordered a taller filter in the stock diameter.There are tons of options on summit's site.