Quadra Jet Problems

chemong55

chemong55

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Supporting Member
I cannot get my 350 to idle,it has a single plane intake with a adapter for spread bore carb.The carb has been rebuilt also.The car idles until the choke comes off then it stalls,had a number of guys try to adjust carb.Yesterday we came to the conclusion that the idle circuit is the problem and even if you increase idle it will still stall,but if you manually close the choke it helps to stop it from stalling.By the way if you rev it up runs perfect.First thought was a air leak but came up without finding one.Any ideas where to go next other than redoing carb.Timeing right on.Could cam,after market Heads,cause problem.Any suggestions welcome this has been my holdup for 2 years.Intake been removed twice looking for leaks nothing.Hope this is enough info as I am trying to relate what my buddy who is a journeyman came up with,Thinks carb or air leak.Thanks.Update running a electric fuel pump and he just called me wants to have a close look at the carb.
 
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If you are going to run a Q-jet get a copy of Doug Roe's book. Rochester Carburetors by Doug Roe, it is invaluable.

My copy is loaned out at the moment so I cant refer directly, but if you are running a hot lumpy cam then the vacuum at Idle is low and you have to jack the butterflies open more. Often so much that you actually start exposing the transition holes and no longer are able to control mixture with the idle circuit. I believe the cure is to drill small air bleed holes in the butterfly plates so they don't have to be opened so far. Start with a small hole and work up if needed. don't get the hole so big that you can't adjust the idle down.

In my opinion the Q-jet is a great carb and easily adjusted for almost any engine size from 250 to 500 cu in. I wouldn't have any other carb. Another area you might look at with the hot cam is the economizer piston spring. With lower vacuum at cruise you may want to reduce the spring tension so that you get the benefit of the leaner mixture during low load cruising.

Good luck.
 
You’ve got a vacuum leak. Likely the carb adaptor.
Borrow a different carb that bolts on to the intake directly.
Holley would be best.
Don’t try an Edelbrock. They don’t like fuel pressure over 5 psi and can overheat easily.
 
All the points above are great things to look into...even changing out the adapter plate (or at least checking for twist/leaks-corrected a few of these with a sheet of ultra fine emery cloth and a sheet of glass).
Another fight that I had with my favorite carb, was the throttle plate rod bores/shafts leaking. I have never taken the time to “repair” this, bought or obtain a different carb so I could go play.
One other point to mention, if you haven’t already put a fuel pressure gauge in line, I suggest this to ensure you are not fighting an out of ‘range’ fuel pressure condition.
I will follow along to see what you find as your fix, as my current build has the Q-Jet as the ‘original’ carb....which may be option 1 as we move forward.
Cheers.
 
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Thanks for the info just ordered all new gaskets so when they arrive we are going to start from the intake upward and hopefully find the problem I will update results.
 
GFaRT's post got me thinking, when i purchased the top end for my sbc i told them i wanted something a little hotter but not over the hill.Started looking at the specs aluminium heads 58cc/210cc,Howards cam Lift.310/.325 duration 225/235.I am no expert on this stuff but this seems to be a hot cam and could be my problem.Howards site says poor idle,hot street,mild bracket racing.Rochester will have idle problems with low vaccum.Opinions?
 
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Here's another article referring to drilling air bleeds in the throttle plates. Cure big cam sag But don't drill the secondaries on a Q-jet.
Your cam should be okay, but it will produce a lumpy idle. Also you don't say, but are you running an automatic and if so what stall speed converter? These cams like to have a higher stall converter than stock. This might help with idle when in gear.
I'll see if I can get my Q-jet book back tomorrow and copy the pages on this for you. I have a Q-jet on my 455 jet boat with a similar cam and single plane intake and it idles fine while spinning the pump.
 
Youtube. There are several videos on youtube, specifically on quadrajets. A couple of these guys explain in great detail exactly what each passage in the carb does, and what it does when its plugged, etc.
Also says that one of the most prevalent idle problems are loose throttle shafts sucking in air. I bought the reamer and bushings to do mine, and I've never done one before.
Easy peasy, and not really expensive. Youtube. You can learn how to do anything with youtube.
I even told a guy a couple years back, if you wanted to learn about having sex with a goat, they probably have a video on that too.
So, one night I got to wondering.......and guess what? NO, they didn't show it, but there was a guy explaining just how it should be done. It was funny.
 
Thanks I have been on youtube for the past few days and you are correct tons of information just hard to sort out what is relevant to my situation.I have a turbo 350 trans with a 2000 stall converter.
 
Here's some excerpts from Doug Roe's book. Note that if the throttle shaft is sloppy in the base you may have an air leak there which would require re-bushing the base. If you need some choke on to idle, it sounds like it's lean and if backing the mixture screws out does not compensate, then it could be the idle holes are too small. Doug addresses this. I assume you've got the mixture screw limiter caps removed so you have full adjustability. As I said before, you need this book if you're running a Q-jet on a modified engine.

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Thanks I will hopefully get back to it after the long weekend decided to finally get out and spend some time at the cottage dam coronavirus.
 
I will give you the same advice I always give...drive it until you break it ..then worry about fixing it....if this doesn’t apply here ..I apologize....
 
Well after a long afternoon we changed all the gaskets from the intake up got the engine to run at 1100 rpm.Did a vacuum leak test nothing,didn't do a vacuum pressure test couldn't find gauge (lol) but once we tried to lower idle below 1100 it stalled.We decided that it is a cam carb problem,just got a call from my buddy and he found his Doug Roe's book and will look at are options.We will do a vacuum pressure test before we drill the butterflies.By the way when idling at 1100 and you hit the throttle it absolutely rocks.Thanks for the help will keep you updated.
 
Before drilling the butterflies or anything else with the carb, I would start by checking the curb-idle port hole size. Remove the idle mixture adjustment needles and see if a 3/32 drill will pass through the hole into the throat. If not, then drill the hole out to 3/32. The needles should still give full adjustment range but now can go richer. You say adding some choke helps, so this makes me think the idle circuit is too lean. Then set the idle as best you can. If still too fast, remove the carb and check the butterfly to transfer slot relationship and if more than .040" is exposed below the butterfly then drill the butterfly and try again. Good luck.
 
GFarT thanks for the help I drilled out the Idle mixture screw ports to 3/32 and now got it idling at between 800 and 900 RPM.It still needs some fine tuning but my buddy is out of town so hopefully he can stop by in a day or 2.
 
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