Hey I'm wondering if anyone can offer some advice. My 86 Trans Am 305 LG4 starts bucking under heavy acceleration. I have replaced the following: spark plugs, wires, cap, rotor, fuel filter, fuel pump, egr, pcv, exhaust/cat, smog equipment, o2 sensor, air cleaner, and some minor adjustments/parts on the stock carb. And yes I have read the other threads explaining the timing and fuel pump issues. The timing is correct and there is no problem with the fuel lines/pumps and vacuum lines. The car runs excellent when I drive it normally, its only when I lay the pedal down. Is it the carb not getting enough fuel or possibly my secondaries aren't opening up? Any help/suggestions would be greatly appreciated

 

Possibly the secondary jets are a bit dirty from lack of use? Can you see any fuel coming into the secondaries as you look down the carb opening and move the throttle wide open ? ?

On my 85 Caprice with the Rochester 4 barrel carb, some mechanic must have knocked the throttle cable out of it's holder, and even if I floored it, the secondaries never opened up. That went on for about 8 or 9 years before I discovered it. I reconnected the cable the right way, and the secondaries opened up again, if I floored it, but the car bogged down. I think the air flaps were stuck closed on the secondaries. Anyway, I got it kind of working again with lots of carb cleaner sprayed into the secondaries, and some Techron in the gas tank.

Was your 86 ever working right, at wide open throttle ? Or did the problem start recently ?

 

When I open the throttle I do see fuel jetting into my primaries but not my secondaries. What could that mean? Am I missing something? I have cleaned the carb thoroughly with carb cleaner and running 91 techron. I bought the car about 2 months ago and been rebuilding her everyday. I never noticed it doing it before but then again I wasn't mashing the throttle earlier on either. Now that I've done boatloads of work to her I have noticed it. The car runs extremely well otherwise. I unfortunately don't know enough about these carbs and have purchased the manual to hopefully help me understand them more. They are so damn finicky sometimes and without the proper tools and test equipment then its almost impossible to fix. I'm hoping someone else might have had a similar problem with their Rochester's. Thanks for the reply

 

It might be the same carb that I had on my 85 Caprice with 305 engine. I'm trying to remember what I did to fix the secondaries. The gas only squirts into the secondaries when the throttle gets back far enough to trip a cam which opens the flaps for the secondaries and also ( I think) pumps an extra jolt of gas in to start off the show. ( I'm not a 4 barrel carb expert, I admit it ! ) Just trying to help since no one else has responded yet.

I traded in my Caprice last year so I can't go out and look at the carb.


I remember I was out in the driveway quickly jerking the throttle all the way back with the engine running, and not running, to see what was happening or not happening.


When I mentioned Techron I meant the Techron Concentrate Fuel System CLeaner in a bottle, which I added to the gas tank. $12 I think.

By the way I never had any real carb problems in 424,000 miles, except when the diaphragm inside the choke pulloff valve rotted away. Just had to replace the choke pulloff valve. CHeap fix.


Good luck with it.

 

Hahaha yeah it just might be the same one! My first car was a 87 Firebird that had the same carb on it too. I never had any problems with that one so I never worked on it (minus the cleaning and springs here and there). I was wondering about getting some fuel treatment I think I'll give it a shot. I have done many rebuilds on Holleys and Carters but nothing as complicated as this thing :-/

I did notice that on the passenger side of the carb there isn't a bar or anything pulling back the secondaries. The only bar attached is to the choke so I'm really starting to think some pieces are missing. I'm searching to find pics of the same carb to reference.

I'm starting to wish I would have bought that 89 GTA TPI haha! (though it definitely wasn't as clean) I love 350s! Just such a bullet proof engine. So Im pretty sure once I get the money I'm swapping in a LT1, 6 speed and rearend. **** whom I kidding I just want a 4th gen stuffed under my 3rd gen body :-)

Thanks again i'll get this thing figured out soon enough

 

So I took it to a carb tech and he said it was due to the throttle positioning sensor and the mixture control solenoid being bad and quoted me $300 max to rebuild. Gonna get a few more diagnostics and quotes before I settle.

 

Is your tac by any chance acting fumny when the engine is bucking? Ifso, check the spark module. I just had similar issues.

 

check the pickup coil inside your distributor. mine went bad and had the same symptoms.

 

I was just thinking about replacing the pickup coil too. I'll give it a shot its a cheap fix as opposed to $300 at grampaben- where's the spark module located? Are you referring to a part on the carb?

 

Same part nmbr27 is talking about. It's in the distributor. Mine was $72 out the door. The coil was $69 from parts train. Check the timing when you have them installed.
I put a higher voltage coil in it. Here's the skinny:

On Your Order:
Part #: H584055
1987 Chevrolet Camaro Z28+Iroc-Z Hypertech Ignition Coil Ignition Coil Camaro|Z28 Iroc-Z
Hypertech
Qty: 1.00 Price: $ 59.11

 

Just to clarify the pick up coil is inside the distributor and it what plugs into the esc module. They usually cost $10-15. I'll look it up for your car today when I go into work.

Does your car have an HEI distributor?

 

Yea it's a HEI, I've replaced pretty much everything on the distributor besides the P/U coil. So I went ahead and ordered a pickup coil today it was $21 bucks and some change. I'll install it and check the timing tomorrow. Hopefully this will do the trick. Eventually I will rebuild the carb to just make everything fresh. I'm really tired of the bucking! I can feel her ready to break free but can't. Aggravating.

Thanks for the input/help guys! I'll post once I find the cure (crossed fingers) or if it's still ongoing

 

The ESC was what I had fail. Pickup coil was otay.

 

Joe, a q-jet only has an accelerator pump in the primaries. Bog is prevented when the secondaries open by the secondary air valve.

A bad TPS can cause all sorts of problems. The MCS would cause problems during idle and cruise, rather than WOT (unless it was stuck retracted). What evidence did he provide to show that the TPS and MCS were bad?

Is your SES light coming on?

Does this bucking start immediately upon going WOT, or does it start to pull and then start bucking? Does it make any difference if you just nail it open, or go WOT gradually?

By any chance does your car have an in-tank electric fuel pump? They were sometimes added by dealers as part of a "vapor lock" kit if customers complained about the car dying on them in hot weather. If yours has such a pump, and it's gone bad, the mechanical pump on the engine won't be able to keep up under WOT, but would most likely be fine under normal driving conditions.

 

Five7kid, the evidence or test I should say that the tech did was; take the air cleaner off and hold a rag and his hand over the primaries and secondaries. The car started dying and he suggested that it be rebuilt. He suggested during rebuild to replace the TPS and MCS and that the accelerator pump might be bad.
The car has a semi-rough idle at 900-1000rpm. The only time a SES light comes on is briefly when I first start the car and it's engaging the electric choke. Once I start driving it turns off.
With your knowledge, would this be an indicator of those two not working? Or possibly other carb related problems?
As for the bucking, I can be driving 2nd-4th gears at around 3500rpm-4000rpm if I either punch on it or gradually apply more gas it starts putting slightly then bogging down and bucking, then regain. As soon as I resume wide open throttle it'll start again until I ease the throttle up slowly.
I've read up on the vapor lock issue. I'm going to check and see if I have an in tank one installed. I'm really hoping that I don't have to replace the in tank fuel pump because of time restraints and limited work area. That and dropping the fuel tank is a pain in the butt.
Is it true that I have to separate the rear diff from the chassis to have clearance to remove the tank?
Makes me really miss my old 76 SS Blazer Haha couple bolts and your done!
Thanks!