voscreature

My 427/390 had a strange truck Qjet when I got it. It stumbled quite bad and I simply ordered a "new" carb from Chicago Corvette (Qjet) that was tuned for the 390hp engine. The car runs very good, very strong, very easy hot/cold start with that carb. If I could find an original carb I would be happy since the car is very original in all other aspects.

As a coincidence I noticed that the power piston was moving semi-binded, so when I pulled carb top off the nylon retainer was cocked in the bore causing the power piston not to seat fully. That probably meant the car was running on the primaries at idle. Fixed that so the power piston moves completely bind-free.

After that fix the car behaves bad at warm up, after 3-5 minutes after a cold start it starts to hunt badly for another 5 minutes, then it straightens out and runs fairly OK. After fully warmed up it runs very strong.

After that I decide to fine tune the idle mixture screws. Unfortunately the screws has absolutely no effect at all. The car runs with both screws all the way in. Absolutely no difference with screws all way in or out 6 turns.

I have replaced intake manifold gaskets, carb-to-manifold gaskets, entire ignition is more or less new etc.

One more observation is that when I remove air cleaner the idle goes down approx. 100rpm and becomes more nervous. If I choke it slightly with my hand it runs much better. So I guess it runs too lean at idle.

I now its difficult to fix carbs on forums - but if anyone has any immediate comments or tips I would be glad. Something must be very wrong with my idle circuit?

Thanks.

Martin

MotorHead

If the Q-jet and all around carb guru Lars finds this post he should be able to help.

voscreature

I checked more into this today. Fuel is dribbling out of primaries down onto the primary throttle blades. Tried pushing down the power piston with a small screw driver but no difference, the power piston is definitely bottoming out. After I pulled the top off the carb I checked float level by having an assistant crank the engine (ignition disabled) while holding down the float hanger, float level was approx. 0.31".

Can an slightly incorrect float level cause my dribbling problems? Or could something else be wrong in my setup since it runs from the primaries at idle? I cant close the throttle plates any more, it will not idle at all then.

The main jets are #71 and on the primary rod hanger it is stamped "098" or perhaps "860" (if reading upside down), but I measured the "fat" diameter and it turned out to be 0.051". I think.

Must chase down correct gaskets, I have no carb number to go on (they have grinded away the original number), only the casting numbers 7030782on the throttle plate and 7047687 on the housing.

MotorHead

I posted this thread in one of Lars threads I am sure he will get wind of it and help you out, Qjets I don't know, square bore Holleys and Demons I have a bit better handle on.

Anyway there are other forum members that know Qjets so keep this post at the top, hopefully others will chime in and help

TimAT

If it's running over either the float is too high or it's waterlogged. Not much else can cause that, other that a chunk of garbage in the needle and seat, and the usual result of that is a fountain.

MotorHead

Like I said I don't know too much about Q-jets but if it's got a needle and seat to control fuel level, pull it and blow it out then reinstall and see if it the dribbling / waterfall stops

lars

If the engine will run with the idle mixture screws all the way in, the carb is idling on the main metering circuit. This can be caused by one or more of the following issues:

Float level too high
Leaking needle/seat
Bad float
Binding float
Incorrect float
Carb idle air bypass holes plugged by the commercial carb builder
Lean carb settings (float, APT, or IFRs)
Low speed air bleeds removed by the commercial builder
Other engine tuning issues requiring the carb's idle speed (throttle blade opening) to be "forced" too high. This can include:
Retarded timing
Inoperable vacuum advance
Vacuum advance connected to incorrect port

The carbs from Chicago Corvette are commercially rebuilt carbs. I've repaired several of them, and they all have a variety of problems. You can e-mail me for my "Commercially Rebuilt Q-Jet Problems" paper for a listing of the common problems and issues that I see with those carbs. You can also send me the carb for test and repair.

Lars
V8FastCars@msn.com

voscreature

Thanks. e-mail sent.

I must check the idle air bypass later today. Whats the reason they tend to plug idle air bypass?

Meanwhile, I must find new gaskets etc. Only problem is that the carb number is grinded down, so the only identification I have is the casting numbers on the throttle plate #7030782 and float housing #7047687. Is it possible to correlate this to a carb ID so I can order the correct gasket set? Its difficult to find anything over here without a carb number. :/

Thanks.

Ironcross

you could have used compressed air blown through the idle screws and see if that would help rather then just removing the carb

Indiancreek

Flip the carb over and see if the two metal plug/caps are leaking? They are near the center. Look like to small freeze plugs. JB weld them if they are leaking. :Like you said, fixing one without having hands on it is difficult.
I rebuilt one yesterday that had them both leaking and one of the brass tubes had come loose. So it could be a number of things as Lars has already said.

lars

The idle air bypass holes are located in the float bowl in the "pockets" in the right and left forward corners of the carb - commercial builders will plug these with lead plugs to richen the idle mixture. This however, reduces airflow through the carb, and forces the throttle opening to be greater at idle. This, in turn, can cause the carb to idle on the main metering system and discharge fuel through the main discharge nozzles at idle, as you describe.

There is no way to correlate casting numbers to the carb ID number - the same castings were used for a variety of carb numbers, all with different baseline jetting.

Leaking well plugs, as suggested above, will not cause your main discharge fuel problem.

Lars

rponfick

Lars, I thought you were on vacation. Did tropical storm or Usain Bolt celebration get you?

Ralph

voscreature

Hi,

Just got back from garage. Exactly like you say Lars, the air bypasses are plugged with a lead plug:





Also, even if they were not plugged, the gasket seems to be blocking one of the air holes:



General gasket view:



Some more pics:







Also, the main jets are #71 and the rods are marked "49B". OK for a stock L36?

Should I remove the plugs and try to find a correct gasket and give it a shot? Im concerned about finding a correct gasket though.

Also, the vacuum advance is connected to ported vacuum source, and the can is a B1. I check with my vacuum pump and it starts to pull in at approx. 8", and all in at 23-24". Im considering changing to manifold vacuum instead, and perhaps another can that goes all in a bit earlier. Engine pulled 15-20 at idle, but needle fluctuates like crazy between 15-20", dont know if its because of my dribbling carb nose, or valve guides or....argh.

Martin

scottyp99

Hey, just for grins, bolt the baseplate back onto the carb body, and take a look at how far open the throttle blades are when the throttle linkage is up against the idle speed screw. If the throttle blades are open too much, for whatever reason, it can cause enough velocity in the venturi to activate the primary system, and de-activate the idle system. If we can rule out or confirm this, it will help narrow down the possible causes of the problem, and it's pretty quick and easy to do. According to Lars' post, there are many causes for your problem, and several of them involve the primary throttle blades being too far open, for whatever reason, so I think this would be a good first step.


Keep the shiny side up!
Scott

lars

Yup, you got the commercial hack-job carb - I can't believe they can sell those things and not have somebody go over there and punch them right in the face...

Yes, you should remove the plugs. Take a look at your power piston: Has the bottom stop pin been cut off? Post a pic of the bottom of yor power piston.

Your jet/rod combo is WAY too lean, and is probably one of the reasons the carb won't idle on the idle circuit. Try a 74/43 combo as a good generic starting point.

The B1 vacuum can is no good - it's far too stiff. You need to get a B26 for your application. Only then can you use manifold vacuum and get stable timing and idle quality.

E-mail me if you want me to send you a package with the vacuum advance, jets, rods, and gasket.

Lars
V8FastCars@msn.com

voscreature

Thanks Lars. I will send you and e-mail.

Here is a closeup of the powerpiston:

lars

Well, at least they didn't destroy the power piston, which is usually part of the lead plug mod. Did you check the power piston depressed height as outlined in my paper? I haven't had access to my e-mail yet today, so I'll check it when I get home to get you taken care of.
Lars

scottyp99

Very clean and very organized! I just wanted to show my approval.


Keep the shiny side up!
Scott

voscreature

Scott,

This picture is not the best but the "gap" between blade and throttle plate measures to approx. 0.6mm, i.e. 0.020". With that setting the car idles fairly reliable, but obviously from the primaries.

voscreature

Lars,

No more stupid plugs.



Adjusted the PP height a little, its about 0.020", compared with a feeler gauge. Perhaps a tad more.



Just curious: Did a 4spd manual L36 back in 68 (with a 7028209?) use the air bleeds? I think I read somewhere in your papers that the air bleeds were added some years after the first introduction of the QJet? Also, I guess they used ported vacuum for the advance since I have the AIR-pump?

/Martin