tfuno

Hey Everyone,
Need to advice. I have a 1977 L-48 with a 1970 300hp engine. Just had the Q-jet rebuilt.

Symptom: Idles to fast, somewhere around 1200 rpm. No adjust left on the high speed choke or throttle linkage.

Check timing which is about 12 BTDC at 750 rpm. I have to remove the vacuum advance to get the engine to 750 rpms.

Question: I notice the ported vacuum is running 15 inches at idle. I was under the impression that vacuum advance was ported. I remove the carb and to check the throttle plate and it appears that the port in the metering block exits below the throttle plates. So my question is do I have the correct base plate or does the carb on a 77 Vette use manifold vacuum for the vacuum advance?

Tfuno

mds3013

You are running the original '77 carb.? My '77 L82 has the vac. advance hooked up to the upper drivers side port. It is manifold vac. even though some here have said it is not. I get full manifold vac. at idle at this port.



Are you sure that the pass. side linkage on the carb. has been installed properly? Are you still using the '77 hot air choke?



Lars is the real expert here. I am just trying to help. mike...

tfuno

Mine is the same. The vacuum advance goes to the same port and is manifold vacuum instead of ported vacuum. I have my timing at 11 degrees BTDC. What are you running your timing at? What is your idle when warm?

I am running the hot air choke.

garygnu

did you use a new gasket to mount the carb to the intake ?mite have a vacuum leak .

lars

If you have verified that your throttle blades (both primary and secondary) are fully closed, and the carb is still idling high, you have a vacuum leak: The engine cannot idle without pulling air from someplace, and if you're sure the blades are closed (especially the secondary side), you have left a port uncapped or you have a leaking major component giving the engine air. It doesn't matter where you have your vacuum advance hooked up in terms of idle speed controllability: You should be able to kill the engine with the idle speed screw no matter where your timing is set - the engine cannot idle without air.

Lars

tfuno

Ok, no vacuum leaks. I disconnected all vacuum lines and capped each vent. Then one by one reattached the vacuum lines. No change in vacuum still at 19 inches at idle. Idle still high around 1100-1200 rpms. I am running just short of 12 degrees BTDC timing. Sprayed carb cleaner and no leaks. I am able to kill the engine through the idle mixture screws.

I did find this crack on the base plate but only see the crack on the bottom side. The crack does not appear to be on the top side of the base plate, just the bottom.

Here is a pic:
[IMG][/IMG]

Thoughts, ideas???

garygnu

check the ends of the shafts bores ,where they come out of the base plate .you mite be able to cap them off ,or seal them .

scottyp99

In Lars last post, he said you should be able to kill the engine with the idle speed screw, not the idle mixture screws. If you can screw the idle speed screw all the way out, and the engine is still running, it's getting air from somewhere.

Scott

micks69

The only way that cracks is someone torqued the heck out of the 4 mounting bolts that base plate and body cant be flat anymore.

tfuno

Yeah, I can't kill the engine through the idle speed screw. After backing out all the way, it idles at 1100-1200 RPMs. Realizing that the PO prob over torque the Q-jet which caused the crack. When I spray carb cleaner around the base, there are no leaks.

At this point, I am beginning to think the carb has issues. Thoughts?

micks69

Well if you cant kill the engine with the idle screw and the carb has a big crack in it, and your idle is way high I would surmise the carb has issues.

Maybe better to start over with something else? Go to a bone yard and see if you can score a non electrically controlled Qjet from just about anything just take a 1/2 inch socket and go hunting.

tfuno

Thanks, I did talk with another Q-jet rebuilder and he said if I cover the primary side with my hand and the RPM remain unchanged (which it did) then the carb has no internal vacuum leak.

I found a Jet Performance Q-jet for $358 through a friend so it time to bite the bullet.

scottyp99

Hey, man, ya never know. It could be something as simple and easy to fix as the secondaries being open a tiny bit.

Scott

tfuno

I picked up an old holley from my brother so I can see if the prob is carb related. I plan to do the swap today. Hopefully, the issue will be resolved. I'll keep everyone posted.

lars

Why don't you send me that Q-Jet and I'll test it and fix it for you. I might even have a good throttle plate for it.
Lars

scottyp99

Is it one of those Q-jet replacement Holleys?

Scott

lars

You can't bolt a square-bore Holley onto a Q-Jet intake...

Lars

tfuno

I was able to borrow another carb and the car idled fine. I plan to go out an purchase a Jet Performance Q-jet. I appreciate all the suggestions

Thanks everyone for your help. Happy New Year!
Tad