Rebuilt qjet won't idle lower than 700 in gear

[Kyle]

Hi everyone, I bought a rebuilt carb from another Firebird owner who had not yet installed it. It is not one of Cliff's. The carb is a 68 service replacement Pontiac.

I pulled the original 2 barrel manifold and carb from my 350 and put a 68 stock 4 barrel manifold and this carb on. Did not change any timing settings. It was running perfectly before the swap. The engine is a low mile rebuild, 30 over, a xe256 cam, running 9.2 compression.

Idle adjustment screw backs right out still wont get it lower than 700 in gear. 1000 in neutral. 15 in vac at best. Choke is not connected, but assembly is there. All vac orifices plugged.

Also runs hotter than before.

People behind me smell gas.

Any ideas?
Thanks!

[Kyle]

Oh yeah, I can stall it with my hand over the choke horn.

[omaha]

sort of sounds like the ported source is not being used for the vac advance. check timing at idle, it should not "move around" and you will not get a steady idle if it is moving around. Mke sure you are using the ported source for the distributer and check the timing at idle. Did you set your mixture screws with a vacuum gauge? (this is another good source of information to tell where your adjustment is at). Start with the basics. Just because it ran fine with the 2 bbl does not mean that the 4 bbl will be automatically set right for the engine.  good luck!

[Kyle]

sort of sounds like the ported source is not being used for the vac advance. check timing at idle, it should not "move around" and you will not get a steady idle if it is moving around. Mke sure you are using the ported source for the distributer and check the timing at idle. Did you set your mixture screws with a vacuum gauge? (this is another good source of information to tell where your adjustment is at). Start with the basics. Just because it ran fine with the 2 bbl does not mean that the 4 bbl will be automatically set right for the engine.  good luck!

The vac advance is connected to a T in line with the power brake line that is connected to the rear baseplate of the carb. The idle is steady, I just can't get it to go lower than 700 in gear or 1000 in neutral. The screws were set with a vac gauge, 15 in is the best I get.

Thanks!

[Kyle]

Any ideas? Please? 

[Kyle]

Ok, I will reply to myself.

After reading up a bit more on the web and Cliff's book, I realize that the vac advance should be ported with no vac reading at idle. I also pulled my carb this morning and realized that the 4 bolts holding it down were barely tight. SO I'll fix all that today and see where we're at.

[Kyle]

SO....

I tightened the carb down, unplugged the advance that was not ported. Got 18 in vac instead of 15 before. idle was easy to control and the carb behaved normally. Checked initial timing it is set at 12 degrees, not sure if I should lower that. Reconnected the vac advance to the ported source that is at zero when idling.

[omaha]

ok sounds good . I am not sure what the spec was for idle rpm on that engine but im guessin 600 in gear (automatic trans). sounds about right to me. 

[Kyle]

Yes the spec is 600 in gear, and 9 degrees mechanical at idle. The engine was rebuilt with a XE 256 cam and is 30 over. I guess I need to figure out wHat it is all in.

[StillLearning]

It sounds as though you are making progress. Remember that timing will affect carburetion more than the other way around.  The ported vacuum slot is in Q-jets for primarily emissions reasons, a late fire cleans up the unburned fuel mixture. If emissions are not a big concern, tune to a normal vacuum source. IMHO
Regarding timing, tune to the best total timing. Aim for around 34 degrees total at around 2800-3000 rpm with just the mechanical advance (vacuum advance off and port plugged).  At 34 or so degrees total let it drop to an idle and see where your low idle timing is. If its close to spec and not rattling at the high rpm setting with the vacuum advance hooked up you are close to ideal timing.  If it "rattles"at high rpm, back off the timing a degree or two at a time until it doesn't rattle any more. You may have to adjust or replace the vacuum advance unit to better control when and how much it advances timing.  Once your timing is set right you will be in a much better position to tweak the Q-jet. It IS worth the extra time and hassle.