Author Topic: No ported vacuum???  (Read 2612 times)

Offline mrk3cobra

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No ported vacuum???
« 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 ???

Offline Schurkey

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Re: No ported vacuum???
« Reply #1 on: February 01, 2010, 10:26:34 AM »
Throttle open too far?

Incorrectly rebuilt using wrong gaskets or mis-matched components?

Offline mrk3cobra

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Re: No ported vacuum???
« Reply #2 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?

Offline 69ttop

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Re: No ported vacuum???
« Reply #3 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.

Offline 69ttop

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Re: No ported vacuum???
« Reply #4 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.

Offline mrk3cobra

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Re: No ported vacuum???
« Reply #5 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.

Offline Schurkey

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Re: No ported vacuum???
« Reply #6 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.

Offline mrk3cobra

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Re: No ported vacuum???
« Reply #7 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.

Offline Cliff Ruggles

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Re: No ported vacuum???
« Reply #8 on: February 03, 2010, 04:38:07 AM »
"The Quadrajet is a new carb."

"New" from what source?....Cliff

Offline mrk3cobra

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Re: No ported vacuum???
« Reply #9 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.

Offline mrk3cobra

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Re: No ported vacuum???
« Reply #10 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?

Offline Toronado

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Re: No ported vacuum???
« Reply #11 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!

Offline mrk3cobra

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Re: No ported vacuum???
« Reply #12 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.

Offline mrk3cobra

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Re: No ported vacuum???
« Reply #13 on: February 05, 2010, 11:05:06 PM »
Could the air be leaking into the lifter valley.

Offline Marx3

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Re: No ported vacuum???
« Reply #14 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... :)