Hey all,
There's a gimmick rally next weekend that I want to do, so I put the fresh rebuilt quadrajet on my '83 Z28 (5 speed!) today and have this issue:
Anytime I decelerate, the engine stumbles and dies.
This is NOT a vaccum brake issue. I can sit still and stomp the brakes all day with no effect. However, when I'm driving along and hit the brakes, the thing falls on its face and dies. I can keep it running with some fancy footwork (remember, 5 speed) and working the throttle. However, I can make the engine stumble by simply letting off the throttle in gear (making me think it's a deceleration/inertia thing).
The carburetor is a summit rebuilt unit. When I first fired it up it did backfire a few times thanks to the MASSIVE vacuum leak that I forgot to plug (missed the 1/4 ntp fitting on the back of the carb...oops!). I just mention that in case it makes a difference.
It runs like stink under acceleration, though. No problems there.
It seems to me like one of two things are happening when I decelerate:
a) The carburetor is flooding
b) The carburetor is starving
I'm guessing A, but I really don't know.
I appreciate any help you can give...and let me know what other info you need about the car/carb/situation.
Thanks!
Clem
The words "vacuum pull-off" come to mind since it's a high vac situation but I have no real input after that or even how to check the thing. Google I guess, or let other people blow holes in the theory that the choke vacuum pull-off is malfunctioning.
This is why I wait patiently for the day the last carburetor is melted down to be formed into beer cans.
First figure out whether its flooding its dying- Black smoke out the tailpipe and extended crank are your first clues there.
Qjets have a tiny float bowl, and are very sensitive to float level. Too high, and they can slosh out the bowl vents and flood, too low and they uncover the jets and go lean.
If it works well under power, at idle, cold start, etc, and only gives you this stalling problem under braking, I'd try leaning out the idle jets a bit and opening ip the throttle to compensate- might give you a bit more airflow to handle the overfueling issue.
I fought fuel vapor lock problems on mine from 1982 till 1999, when I put a Holley on it.
Never had any trouble from my quadrajets other than mis-adjustments from previous owners.
A lot of people who just don't understand these carbs try to rebuild them and f them up, summit included.
Buy the book on Quadrajets by Cliff Ruggles. Or have him rebuild the carb for you, he's a genuis.
From dealing with my own carbs, it sounds like your float is set too high. Get the spec for your application and set it that way. The Summit carbs are ususally set to a generic spec which is no good for 90% of the applications out there.
Buy a Holley if all you need is WOT performance. Buy a Q-jet or AFB for a street driven car.
Shawn
pres589 wrote:
The words "vacuum pull-off" come to mind since it's a high vac situation but I have no real input after that or even how to check the thing. Google I guess, or let other people blow holes in the theory that the choke vacuum pull-off is malfunctioning.
Except it's NOT the vacuum causing this. The cause is decelleration, but it's not the brakes.
Clem
So there's still some throttle opening and you're slowing down, or foot completely off the throttle and you're coasting down in gear / clutch pedal out?
Yeah...it's hard to describe.
If clutch pedal in or out (in gear or in neutral), it will stumble (and die) with deceleration. If I'm sitting still I can stomp the brakes with zero effect. That's why I ruled out it being a vacuum issue (though I could still be mistaken...but I'm relatively certain it's not a vacuum BRAKE issue).
I can keep it from dying by feathering the throttle but even then it takes a while to recover...feels like flooding to me, intuitively.
Tomorrow I'm going to take the top off the carburetor and see what the float level looks like and if everything else looks kosher in there. I'm guessing/hoping that it's as simple as a float level adjustment. It's such a drastic phenomenon...I hope it's a simple fix.
Thanks,
Clem
Using vac power brakes is not a good test of if something is "a vacuum problem" or not. That system is designed to be used multiple times with the engine not running in case of problems, figure on 3 good stabs, if you don't believe me try it just after turning the engine off in the safety of your garage.
I still think you have a problem with the choke subsystem or possibly an issue with the idle circuit. That said I haven't touched a Q-Jet since about 1995 so what do I know. What you might try is blocking the choke somehow so it cannot go completely shut, like with wire or something.
Good thoughts!
I'll start with the float level and if it persists, I'll disable the choke and see how that does.
But it DOES do this without using the brakes at all...just decelerating by engine braking. So if I'm cruising along say, in second gear (at a relatively high rpm) and let off the throttle to induce enine braking, I see the tach get erratic. If at about that time I push in the clutch, the engine dies (unless I keep it from dying with the throttle).
I need to adjust the idle mixture too, but I have to find a tool for it. I guess I'll try NAPA...or does anyone else know where I can get the tool? It looks something like an inverted clutch head type thing...
I appreciate the help,
Clem
Decel with engine braking is a high vac situation. Make sure there's not a timing retard connected to the wrong sort of vacuum source, ported or the other sort. I just got out of the shower and I'm not thinking really clearly but you want to make sure that high vac isn't pulling out so much timing that the car can't run right. Really should just figure out which vacuum pots on the distributor go to which vacuum source on the carb or intake.
Thanks again for helping think through this.
There is one vacuum line for my distributor and I have it going to the vaccum port on the right front of the carburetor (which I think is the ported vacuum).
Before I take the carb top off tonight, I might try disabling the vaccum advance (and capping the port) to see if it makes a difference. I can try disabling the choke as well.
Thanks!
Clem
The good news is that the float level was set too high.
While fixing it I figured going too far the other way (and definitely proving that that was the problem) was better than not going far enough.
So...now it's starving under acceleration.
I must confess, I've never had to set the float level in a q-jet. The one I rebuilt before...the float level was fine.
Can anyone point me to a proper proceedure for setting the level properly? I can do it iteratively, but it takes a while everytime I have to adjust it (take the top off the carburetor each time).
Thanks,
Clem
Without a spec for your application, it's hard to give you an exact number.
This might help though: http://www.underhoodservice.com/Article/39766/diagnostic_dilemmas_servicing_quadrajet_carburetors.aspx
Shawn
Thanks Shawn, that was a good read.
So, I'll probably just take the iterative approach. I'll eyeball it tonight and hope I get it on the first (second, really) try.
I'm excited to have this thing up and going. Of course, now I'll just need to update the suspension to something respectable 
Clem
Cool...so I got it.
It's running pretty good! I need to adjust the idle mixture and check my timing...but it sure is nice to have a good running F-Body again after all these years (this thing last ran in early '99...well then I got it going last spring but it never ran right).
Clem
