Hi all,

my non cc Q-jet has some very weird idle problems.

Until today I had the problem, that the curb idle screw wasn't touching the primary throttle tang. I had a stable idle of about 750rpm. After thinning down the throttle shafts and modifiying the boosters (after the book "how to build horsepower 2") the car developed a small bog off-idle.
I have then enlarged the transition orifices slightly (also after recommendation of this book) which cured the bog.
I also could let the engine die when I screwed the idle screws completely in. It ran good with the screws 2.5 turns out.

Today I bought some carb cleaner to check the carb for vacuum leaks.
When I sprayed it in the primary throttle bores the engine stumbled, so I guess its been rich at idle.
I sprayed everywhere, but found nothing, exept when I spray on the closed secondary air valve it also bogs.

So I assumed the secondary throttle plates aren't sealing right.
I changed the lower part of the carb out for another identical part(same holes and channels).

Now the carb behaves totally different. I can use the curb idle screw for the first time, but the engine runs best with the idle screws turned all the way in.
The engine still bogs when spray with carb cleaner.

My question after this long story is:
Do the drilled transition orifices enrich the idle so much that the idle mixture screws are useless?
What could be the reason for this unusual behaviour?
How do I lean out the idle circuit even more?

Thanks
Alex

 

what engine is below it, and what is the cams duration?

 

the engine is a 327 SBC
the cam is a Lunati Voodoo 262 (219°/227°@0.50")

Vacuum at idle is about 16-17" steady, in gear its 15"

The engine has headers, performer intake and ported double humps. Compression is 9.4:1

 

You are either not idling on the idle circuit, or you have extra fuel getting in somewhere. A Qjet at idle should stall when both mixture screws are turned in. Most of mine will run roughly with one screw all the way in and stall approximately 3 turns from seating the other one.

My guess is you are running either partly on the transition circuit, or you have leaky well plugs. How far is the throttle open to get it to idle? Do you have any nozzle drip at idle (indicating partial operation of the main fuel circuit)?

 

99Hawk120, thanks for the answer.

I just checked for nozzle drip at idle. It doesn't drip. Also the well plugs are epoxied.

The curb idle screw is about 3/4 to 1 turn from closed to get it to idle well.

It idles kind of rough. When I open one of the plugged vacuum tubes, the additional air clears the idle up very nicely.

So I guess the idle mixture is either to rich or it doesn't get enough air.

Iterestingly though that it worked before, although the other way around. (closed throttle, open idle mixture screws)


How do I fix my problem. I would prefer not to drill any orifices.

Thanks
Alex

 

that's not an overly large cam for that engine, so you shouldn't have to drill the throttle plates...

-are you running a pvc? that is an extra source of air at idle...

-what is your base timing at? that could help if you bumped it up a bit

-at idle, you have enough vacuum that you should be drawing your power piston shut, but you can check this, when idling, push a screwdriver down the vent tube, if you can push on the power piston, and make it idle different, then you aren't making enough vacuum to fully seat it, or it's adjusted wrong.....

https://webdisk.ucalgary.ca/~jmknopp/public_html/
snoop through the q-jet troubleshooting articles there, very good stuff.

 

I use a pcv valve, which is working good.
The timing is at 12° intitial advance.

I also checked the power piston. It gets sucked in, like it should.

Thanks for the link.
Alex