Mixture Screws have no effect

[BillCT]

My good friend has a Q-Jet on his 68 396 Camaro that likes to run rich, I went to see him this past weekend to try to help but couldn’t uncover anything.  He’s had this carb rebuilt by someone who specializes in restoring them to factory original condition but I don’t know who.  The main issue is with the mixture screws, in order to get the motor to idle and run decent one screw has to be pretty much seated, unscrewing it just a little and it starts to run rich.  The other one can be screwed in and out and nothing happens.  With them just a crack off fully seated, the motor runs really pretty good, it cruises smooth and has very good throttle response.  The motor will idle right down almost to a stall. His timing is all in at 36 deg.  We set the float levels to specs, I don’t know what size the idle tubes are but I checked them for obstructions with a fine wire and they’re free, the idle channel restrictors are free and clear,  I don’t see any nozzle drip when its running.  This is a 68 carb correct for this motor and according to the GM specs it should have .071 jets and 44B rods which it does.

Seems like there’s something amiss someplace but I can’t locate it.  No vacuum leaks that I could find either.

Any thoughts?

[Cliff Ruggles]

Could be the wrong gasket between the carb body and baseplate, I see that a lot with early Chevy q-jets, they take a different gasket than Olds/Buick/Pontiac/Cadillac's.

Not having any control with one side and WAY rich on the other side could also be an obstruction someplace in the idle system, or the carb simply isn't set up correctly for the application.

A carb number, engine specs, and size of all the idle fuel related items would certainly help?.....Cliff

[BillCT]

Carb ser no. is 7028210  EE.  I asked him to find out what size the rebuilder drilled the idle tubes if at all.  I'm going to see the car again this weekend so I'll find out more info and pass it on.  We'll also check the base gasket as suggested.

Thanks allot for replying, I'll post more info Monday.

[BillCT]

We tried to get info from the rebuilder on what size he made the idle tubes but so far no response so I guess we'll have to use some gage pins and measure them.  The carb is on a 396 325hp motor if that helps any.  The car owner is going to remove it from the motor and see if the gasket is obstructing anything as you suggested.  I'm in CT and the car is in Maine, that's why this sketchy detail.

Thanks

[Cliff Ruggles]

Difficult to assist without knowing all the details.  Most likely the "builder" doesn't know what they are anyhow, so may not be much help.

In almost all cases the idle tubes do NOT get removed during rebuilding, and most folks don't even know where they are, or what they do, even folks who have been "rebuilding" them for years.

I had to chuckle a few months ago a well known "rebuilder" hired us to plate some parts and castings.  We removed the idle tubes from the main castings and sent them back with the parts and did not put them back in.  We always do this in case the "builder" wants to make some changes, etc.

Anyhow, I got a quick call from the "builder", asking me what the tubes were and where they went.  I explained their location and purpose, and he said that he had been "building" these carbs since Moby Dick was a minnow and never pulled or cleaned them for any reason.  He also didn't even own a bushing installation tool, and told me they NEVER need bushings in the throttle body.......Cliff