Cliff's Quadrajet Parts and Rebuild Kits

Quadrajet Problem Solving => Diagnose a Quadrajet carburetor problem => Topic started by: mrk3cobra on February 01, 2010, 07:19:01 AM

Title: No ported vacuum???
Post by: mrk3cobra on February 01, 2010, 07:19:01 AM
I have a rochester quadrajet 17080229 on a 1986 chevy truck. The EGR is connected to port J on the quadrajet, according to the emissions lable on the motor this is were it should be connected. The truck will not idle because this port is manifold vacuum. I have tried all the ports on the quadrajet and they all have manifold vacuum.

Why would there be no ported vacuum on this carb. thanks,pete ???
Title: Re: No ported vacuum???
Post by: Schurkey on February 01, 2010, 10:26:34 AM
Throttle open too far?

Incorrectly rebuilt using wrong gaskets or mis-matched components?
Title: Re: No ported vacuum???
Post by: mrk3cobra on February 01, 2010, 10:41:58 AM
The Quadrajet is a new carb. The idle is at 750 RPM'S.

Is there a way to trouble shoot the problem?
Title: Re: No ported vacuum???
Post by: 69ttop on February 01, 2010, 12:27:42 PM
I had a similar vac problem. My q-jet had no vacuum at the side tube because my professional
rebuilder installed the wrong throttle plate gasket which did not have the proper hole punched in
it. I had to rebuild the carb myself for it to function right. On my carb removeing the throttle plate made it much easier to see how the port channels were routed and made it much easier to use carb cleaner and blow air to clean the ports.
Title: Re: No ported vacuum???
Post by: 69ttop on February 01, 2010, 12:31:47 PM
Sorry I forgot to add on the above reply that you might want to remove the throttle plate as well to troubleshoot your vacuum problem.
Title: Re: No ported vacuum???
Post by: mrk3cobra on February 01, 2010, 02:23:44 PM
I have manifold vacuum at all the ports at idle, you said that you had no vacuum.
How do you tell if the plates are to far open at idle.
Title: Re: No ported vacuum???
Post by: Schurkey on February 02, 2010, 10:37:05 AM
How do you tell if the plates are to far open at idle.
Lower the idle speed and see if the vacuum at the ported nipples goes away.
Title: Re: No ported vacuum???
Post by: mrk3cobra on February 02, 2010, 01:16:46 PM
I lowered the idle to 550 RPMS and I still have vacuum at the ported ports.I could not lower the RPM any more then 550 and keep the truck running.
Title: Re: No ported vacuum???
Post by: Cliff Ruggles on February 03, 2010, 04:38:07 AM
"The Quadrajet is a new carb."

"New" from what source?....Cliff
Title: Re: No ported vacuum???
Post by: mrk3cobra on February 03, 2010, 07:18:15 AM
I just bought the truck, the last owner installed the quadrajet trying to fix the idle problem. Not sure were he purchased the carb, I removed the carb and it looks new.
Title: Re: No ported vacuum???
Post by: mrk3cobra on February 03, 2010, 10:18:25 PM
Cliff any suggestion on what I can check?

Does having manifold vacuum at the ported tubes suggest a problem with the carb?
Title: Re: No ported vacuum???
Post by: Toronado on February 04, 2010, 12:41:09 AM
If it were me I would get one of cliffs rebuild
kits and book and rebuild it your self. Even if it
looks new who know who built it before you got it,
and most important if they did it right. Trying to solve the
carb. problem with out rebuilding it will takes
more time then rebuilding it, short of the cost to rebuilt
it only takes two days, remove, take apart, clean, rebuild,
done. Then you know for the rest of the time you own your car/truck
the carb. has been built right!
Title: Re: No ported vacuum???
Post by: mrk3cobra on February 05, 2010, 09:08:22 PM
I removed the carb. and tried to pressurize the intake manifold with 3psi of air and the manifold will not hold any pressure.
I blocked off the EGR, but I cannot tell were the air is going. The air is not leaking out of the manifold is it possible that it is leaking internally.
Title: Re: No ported vacuum???
Post by: mrk3cobra on February 05, 2010, 11:05:06 PM
Could the air be leaking into the lifter valley.
Title: Re: No ported vacuum???
Post by: Marx3 on February 06, 2010, 01:35:42 AM
Since the engine is in a truck, I guess it does not have the biggest cam ever installed, but still, how can you rule out overlap as the factor that lets pressure out of the manifold? I can't imagine that the intake will hold 100% tight at any time, no-matter how little overlap the cam has...

But I dont know for sure... :)
Title: Re: No ported vacuum???
Post by: mrk3cobra on February 06, 2010, 09:05:16 AM
Not sure if there is some overlap on the cam.

I read that you can pressurize the intake to try and find leaks, when I pressurize the intake it will not hold 3psi. I hook up a compressor shoot in 3psi and it drops to 0 as soon as I turn off the compressor.
Title: Re: No ported vacuum???
Post by: omaha on February 06, 2010, 10:07:55 AM
you should back off the rockers. Invariably, there has got to be at least one exhaust valve that is open.
I have never heard of pressurizing the intake to check for leaks though. It woud seem that it would be difficult to keep  the pressure from escaping out of the various vacuum ports. Even withh caps on them it would seem that the pressure would just "blow" them  off.
          You want to hear a crazy one?, I did this back in the old days. I taped up the opening of the carb throut with tape except for a straw sticking up through. Then I blocked open the throttle linkage just a bit. Then light a cigarette and blew some smoke through the straw. The smoke will find the vacuum leak is what I was told by an old timer mechanic (back in 1983).   Guess what, it actually worked. The cigarette smoke magically appeared coming from a crack in my vacuum advance canister (on my '67 GTO). I thought it was great but looking back maybe it was kinda dangerous.
Title: Re: No ported vacuum???
Post by: chewy on January 02, 2014, 11:01:41 PM
      Perhaps this will shed some light on your mysterious lack of ported vacuum. My  Canadian built  1986 Cutlass  came from the factory  with an LG4 305 chevy that has the m4med quadrajet (probably the same carb as in your truck). Oddly enough, this carb does NOT have ported vacuum. The "ported" vacuum source on the passenger side front is actually an EGR signal ("J" on Carburetor), not a true ported source and is therefore not suitable for ported spark advance. The port on the front of the driver's side which is typically ported is in fact full manifold vacuum ("H" on Carburetor).  The signal that gets to the distributor can comes from a tvs that is connected to the EGR "J" and Manifold "H" vacuum sources.
      In addition, the M4med has a dual capacity accelerator pump which varies accelerator pump shot with temperature. Strange carburetor indeed!
Jason