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q-jet tuning

az-k5

az-k5

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Okay, hi my name is matt and I don't know jack about engines (seriously)


Got a new 1984 3/4 truck with a good rebuilt motor (less than 100 miles on it) and can't get it to run like a champ.


Here are the symptoms.

Starts fine, idles decent.

When taking a test drive back fires like a mother (out the tail and the carb) . This happens as soon as it goes off idle while under load (anywhere above 1000 RPM). You can rev it but can't drive it. Will not drive out of idle. Studders, pops, and trys to stall. It will grunt around in idle just fine.

Doesn't have a traditional APT plug up front (this is where I think the problem is).

Vac = 22hg steady. (set timing by vac gauge)

Idle screws 3.5 turns out

Q-jet, RV cam, no vacume leaks.

Just point me in a decent direction

Thanks
Matt

PS off to check timing with timing light
 
Do you have the vacume advance hooked to the proper port? Ported or manifold? I can't remember which it's supposed to be....somethin to check though.
 
I found the mid throttle screw was still hidden under the carb lid. Got the plug out and it was in full rich. And the timing was way out. Now I have to undo half the things I did before I found the mid throttle set up.

Thanks again for the call paul.
 
There is a rochester carbs book by Doug Roe that is probably available at your library.....really helpful. I have never studied the electronic quadrajet so if you have one of those, then what I am about to say may not apply.

The APT really only limits how lean your cruise mixture can go. It doesn't sound like you are getting anywhere near cruise. Check primary needles and make sure there is a spring under them, needles are in jets, and they appear to travel from normally up to down if you press on them. If the spring is missing or something like that you could be really lean. Next check the enrichment circuit. Look down the primaries (engine off or things get real interesting), open the throttle quickly and you should see a vigorous squirt of fuel -- in fact if it doesn't kind of fog up in the primaries, I would clean things out -- check the accellerator pump and make sure there is a check ball under the screw towards the bottom of the bowl by the accellerator pump. Your Vac advance line should hook up the the port about halfway up on the drivers front side. It almost sounds as if a vacuum leak is developing after you open the throttle blades -- make sure there are no open ports anywhere, some of them don't have vacuum at idle, but at different throttle positions. Also check the base gasket -- if the wrong one is on, bad things can happen. If you think the carb was redone with the engine and may be wrong, get a $12 kit from the parts store and do it again -- just really look at things and use the right gaskets.

I would fix timing/distributor first, then the carb. Also check the vac. modulator on the trans if automatic, and I would adjust the valves just in case.

Good Luck!
 
it is a manual.

I got it better but not perfect. It back fires as you let off the gas when you rev it. If I set the timing below 9° then it runs and stumbles on itself.
 
Got the timing to idle at 6-7°. It runs better but still back fires hop and pops when you try to get on it. Cruise pretty well till you want to play, hit the gas and snap crackle pop.

What should I look for now?

Thanks
 
az-k5, does your q-jet have the tiny check-valve (major restriction) in the fuel filter for the quadrajet? if so, you might try removing that then see if the same backfire will still occur. but it sounds much more like timing...

and so that there's no misunderstanding. APT adjustment - when all the way in(CW) is as lean as the primary rods will allow and when it's out(CCw) it forces to rods to be out of the jets further into the taper for the 'power mode' which richens the part-throttle mix.

also, what's the timing at 3500rpm?
 
Start over. What's the age/ condition of the carb? Has anyone "worked" on it besides youself? What have you done to it? Depending on conditions of the hard parts, it's sometimes more economical to get a new one. People like to torque the crap out of the air horn and warp it, for example. Sometimes it's simple adjustments, but not usually. Typically hard parts are worn out- specifically the throttle bushings.

As a side note, just cause the motor was "rebuilt" doesn't mean it's right- the carb may not be the problem at all. What made you decide to head for the fueling as a problem?
 
Well, I have a lot of unknowns to deal with on this one. The truck ran way to rich to begin with. The idle screws were set full rich. The P/O said it was "some kind of 4 barrel holley he hadn't seen before" . Well it isn't a holley, it is a q-jet. I don't know if it was rebuilt or just put back on. P/O said the engine has an RV cam but I don't have the sticker/papers on the actual model.

He mentioned it had a vacume leak that he couldn't find. It was the main port in the back of the intake (usually used for an auto). He covered it duct tape and never looked at it again (it sucked a hole right through it). Fixed that and got it to idle normal.

Went and adjusted the idle screws since it would backfire will sitting at idle. Set a 3.5 turns and no more idle issues.

Timing: Guy had the dizzy loose (at the crows foot) but would have had no luck setting it with the vacume leak. Now set at 7° (holds 20 hg steady). Still misses slightly though.

Distributor: new cap, wires, plugs. Inners look worn/old. Gunk build up on the dizzy weights with rust color (no pitting) and slight movement from the weigh bushings.

That is where I am at. It will idle and rev pretty well. Drives up to 30 mph if you touch it just right, but will backfire heavily on acceleration under load.
 
I would still set the timing a little lower , like 4 degress . RV cam is also salesman speak for its still stock . Try the coil and ignition module too , since the truck is secondhand , maybe just start with new ones .

BTW , does the dizzy have the weatherpack connector indicating ESC ? Mine was removed for a regular HEI within 3 months of buying my 83 .
 
It has the crane cams sticker on the fender (new sticker) I don't expect it to be all he said ofcourse.
 
Distributor: new cap, wires, plugs. Inners look worn/old. Gunk build up on the dizzy weights with rust color (no pitting) and slight movement from the weigh bushings.
Click to expand...

Clean up the mech advance weights, and everything associated with that. I removed it all for cleaning and re-assembled and it made a hell of an improvement. I did not lube any of that stuff as it just leads to gunking up quicker...

Rene
 
FWP said:
Do a quicky parts store rebuild (factory setup) on it and see what happens next.
Click to expand...

You talking about the distributor or the carb?

I also just went out and counted the turns on the APT, It was nearly 4 turns out. I have it set at 2 now and will do the distributor tommorrow and see how it does.
 
(High Jack on)

I too have the back fireing problem and I dont know what screws to adjust, does some one have a pic?

(High Jack off)

Thanks Shawn
 
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