I have a 91 Formula with the LB9 305 motor. THe car is an automatic with a 2100 stall convertor, 4.11 gears and SLP 1 3/4" headers. Has a Flowmaster 3" system on it. Fuel pressure is set at 50 psi. Ignition consists of Accel 300+, MSD wires cap & rotor and Bosch Platinum plugs. Otherwise, the car is stock.

When the car launches, it bogs for a minute and then goes like hell and chirps the gears. What would cause a momentary bog and the such rapid acceleration?

 

i'd guess the engien vacumn goes to atmosphere when you step on the loud pedal

 

what would cause something like that?

 

suddenly opening up the throttle body ont he tpi will cause the pressure to go to atmospheric (WOT). Sounds like the throttle response needs a little work.

 

Would timing do this? If the car is in neutral and the throttle is revved, it stumbles and then sounds good. At the initial application of the thottle it stumbles though.

Since it has been a while since I have had a 305 in this car, could someone let me know where exactly all the vacuum lines go off of the plenum? I know the back one on the driver's side goes to the brake booster, but the rest are just guesses....please help!

 

Might sound silly, but, does it do this when the fuel tank is full?

 

It did it when it was full and it does it now at around 1/2 tank. Why do you ask?

 

My '89 GMC TPI retro-fit will do that if the tank is below 1/4 of a tank and I really take off hard. It's running out of gas.

 

That is a good point, but his happens while the car is idle in park and gets revved also.

Does anyone know if having the timing off a little will do this?

Thanks again guys

Steve

 

If the engine makes good power and gets decent mileage then the timing is close enough that your should look for a solution to this stumble elsewhere.

 

Another WAG; Maybe a flaky MAP sensor or vacuum line to the MAP? Flat spot in the TSP sensor?

There are of course many other reasons, including needing a PROM tuned for you engine, but I'm sure you've checked the obvious.

 

Yeah, if the PROM is not calibrated for that increase in fuel pressure it could cause problems.

 

When I first got the car, the fuel pressure was set at 50 psi with the stock computer and the car ran really strong at 14.6.

I am thinking either a vacuum problem or a timing problem but am hoping someone can confirm this and/or give me an idea for the vacuum routing.

 

Did you ever solve this? I have recently developed a simmilar problem. I have played with the timing and it made no difference. I'm actually starting to think my problem is the wires. They are so close to the headers that the number 8 wire has been known to smoke.

 

I have an entirely different motor, but mine stumbles when I spike the throttle too. It almost misfires and then revs up. But mine also hangs up twice after a rev before it falls back to idle. I suspect a bad TPS sensor, even though it's new...it might not be calibrated right.

If you figure this out, I'd love to know. I imagine the motors are similar enough that we could be having the same problem.

 

im used to carbs but i think tuned port works the same way. to me it sounds like an accelator pump or on tuned port i think its called pump shot inrichment. cause when you mash the gas the atomospheric pressure changes and the engine need more fuel. any one know what im talking about?

 

Could this possibly be a TPS problem?? Tyr taking a ohm meter and check for any dead spots on the TPS. Ya never know???

 

Maybe the tps is simply out of adjustment. There is an idle voltage that is supposed to indicate the engine is at idle. If it is above this and the engine is idling, the engine will bog just as you said. It senses the rise in voltage and gives an extra shot of fuel, just like the accelerator pump on a carb. When the accelerator pump on a carb goes out it is flat then goes. I think the spec should be around .45 volts or something like that ( If someone could confirm this it would be great as I am looking at a book that only has CCC Quadrajet and TBI stuff, the voltage is the same for both of them so it should be the same for TPI). All you have to do is slightly loosen the allen head bolts then move the tps until it indicates the correct voltage, holding the tps in place, tighten the bolts back up. Hopefully this adjustment works. I had a friend with a 1997 LT1 camaro that did the same thing. Turned out it was the TPS adjustment, just can't remember the spec for it right now probably .45 volts just like the other GM ecms.

 

As dumb as this sounds, one of the problems may be the platinum plugs crapping out under load with the accel 300.