[JMB36TA]

I’m trying to get the fuel and air adjusted on my Q-jet. Judging by the burning eye sensation I’m getting a lean mixture. I’m going to try it with a vacuum gauge instead of by ear. My question is where is the best place to plug it in, ported or manifold vacuum? Or does it really matter?


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[steve25]

When at idle with the primary throttle blades open the small about they should be it would be Manifold vacuum you want to monitor.

You really don't need a vacuum gauge as you want to set the idle mixture screws for the fastest rpm you can get ( as in lean ) with auto Trans as when you drop it in gear it will go just a tad rich.

Just remember that with a Carb there will be tweaking that you can idle wise between Summer and winter temps if you want to get things Dead nuts on even where you live!

Is this a stock Cam that the Carb was calibrated for , or a bigger Cam?

Also if you have stock Exh Manifolds make sure that the motor is fully warmed up and confirm that the Exh heat riser flap is open in that one Exh Manifold.
Please post the details of your motor if you would.

[JMB36TA]

Specs are...
Pontiac 400 .030 over @ 9.4:1
Voodoo cam 227/233
Q-jet 7029253 rebuilt with Cliff’s kit. 71 jets, 44b metering rods, .039 idle tubes, .059 down tubes, .110 idle bypass mod.

I think it just needs fine tuning.



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[JMB36TA]

Well this is discouraging. Seems like the F/A adjustment has no effect. I turned them all the way in, and up to over 5 turns out and there's no difference in idle of vacuum. The vacuum bounces between 13 & 15 no matter how I adjust.

[george kujanski]

first thought is you are getting fuel from other than the idle system.

George

[gtofreek]

Joe, does the engine try to slow down when you run the A/F screws all the way in? The engine should try to die, or die. If it doesn't, it sounds to me like it is getting idle fuel from the transition slot from the butterflies being open too far at idle to get enough air to idle. You may need to put a small hole in the front butterflies in order to get the butterflies closed enough to not pull fuel from the transition slot.. Cliff or someone else more proficient with Q-jets can correct me if I'm wrong, but that's what it sounds like to me.

[JMB36TA]

Hey Paul. I don't notice a difference in the RPMs when I turn the screws all the way in or if I back them way out. There may be a slight difference but I don't have a tach to be certain. I tried to adjust it with the engine at operating temp so the butterflies were open. One thing I read was to cover the front opening with your hand to see if the engine races, mine dies.

[455GRIN]

Turn the idle down low, and try the mixture screws again

[Schurkey]

Originally Posted by JMB36TA

I’m trying to get the fuel and air adjusted on my Q-jet. Judging by the burning eye sensation I’m getting a lean mixture.

Burning-eye exhaust is typically from misfire. Could be lean misfire, could be bad ignition, could be anything that causes misfire.

Originally Posted by steve25

You really don't need a vacuum gauge as you want to set the idle mixture screws for the fastest rpm you can get ( as in lean ) with auto Trans as when you drop it in gear it will go just a tad rich.

I run the engine until it's fully warm. Chock the wheels, engage park brake, put it in gear, and then adjust for highest vacuum. I find a vacuum gauge to be more sensitive than a tachometer.

Originally Posted by gtofreek

You may need to put a small hole in the front butterflies in order to get the butterflies closed enough to not pull fuel from the transition slot.

I have a Q-jet with holes in the primaries, apparently done by Rochester. That said, I'd drill holes in the casting to route some idle air around the throttle blades, rather than drilling the blades themselves if possible. Many Q-jets are drill-able for idle air bypass.

Originally Posted by 455GRIN

Turn the idle down low, and try the mixture screws again

Yup.

[Greg Reid]

Originally Posted by JMB36TA

One thing I read was to cover the front opening with your hand to see if the engine races, mine dies.

You may know this but just to be clear, you only partially cover the primary side of the air-horn with your hand. Don't choke it out completely. You're just interrupting the air flow a bit to see if it's rich or lean in the idle circuit.

[steve25]

It sounds like the issue is what Paul said with your throttle blades open too far to get your idle speed so that now your out of stage 1, that being running on the idle circuit and your now running on stage 2, the transistion circuit.

[JMB36TA]

Originally Posted by Schurkey

Burning-eye exhaust is typically from misfire. Could be lean misfire, could be bad ignition, could be anything that causes misfire.

I never thought of misfire. I'll have to look into that.

Originally Posted by Schurkey

I have a Q-jet with holes in the primaries, apparently done by Rochester. That said, I'd drill holes in the casting to route some idle air around the throttle blades, rather than drilling the blades themselves if possible. Many Q-jets are drill-able for idle air bypass.

It has idle bypass. Cliff recommended opening it up to .110 which is where it's at now. Not sure where else I would have to drill to add more.

["QUICK-SILVER"]

Originally Posted by JMB36TA

One thing I read was to cover the front opening with your hand to see if the engine races, mine dies.

Originally Posted by Greg Reid

You may know this but just to be clear, you only partially cover the primary side of the air-horn with your hand. Don't choke it out completely. You're just interrupting the air flow a bit to see if it's rich or lean in the idle circuit.

Need to try this again

Like Greg said, don't choke the engine out. Just start restricitng air flow to see if the engine idles up or idles down.

Also might need to check for more vaccum leaks.

This car got the hot idle compensator on back?

Clay

[JMB36TA]

Originally Posted by gtofreek

Joe, does the engine try to slow down when you run the A/F screws all the way in? The engine should try to die, or die. If it doesn't, it sounds to me like it is getting idle fuel from the transition slot from the butterflies being open too far at idle to get enough air to idle. You may need to put a small hole in the front butterflies in order to get the butterflies closed enough to not pull fuel from the transition slot.. Cliff or someone else more proficient with Q-jets can correct me if I'm wrong, but that's what it sounds like to me.

I got a little confused on your post, you were referring to the throttle blades not the idle blades. I can't see down in there to see if they're open all the way or not, but there are already holes in the blades. I don't see transition holes though. Wouldn't they be above and below the plate in line with the idle screw holes?

[JMB36TA]

Originally Posted by "QUICK-SILVER"

Need to try this again

Like Greg said, don't choke the engine out. Just start restricitng air flow to see if the engine idles up or idles down.

Also might need to check for more vaccum leaks.

This car got the hot idle compensator on back?

Clay

I choked it out but as I slowly covered it I didn't notice any change in idle until I got close enough for it to die.

I've been checking for vacuum leaks over and over and haven't found anymore. All vacuum lines are new. I haven't checked around the carb since I put it back on Monday night so I'll check that again. But I had this problem before I took it off that night.

Yes it has the hot idle compensator. It's off an Olds so it has this little spring and rod sticking out the back. I sealed around it so there's no leaks according to Cliff's instruction. Maybe I should find a new compensator and cover?

["QUICK-SILVER"]

Was there parts under the cover? The cover holds the compensator in place.
To run without, the holes in the carb body need plugged.

Also need to find out if those are the right mixture screws. I was thinking 69 used the blunt nose mixture screws, where yours appear to be the long tappered nose. Can't say for sure on that but Cliff or some one else might know.

Clay

[JMB36TA]

Originally Posted by "QUICK-SILVER"

Was there parts under the cover? The cover holds the compensator in place.
To run without, the holes in the carb body need plugged.

Also need to find out if those are the right mixture screws. I was thinking 69 used the blunt nose mixture screws, where yours appear to be the long tappered nose. Can't say for sure on that but Cliff or some one else might know.

Clay

Yes the part was under the cover. I'm 99% sure that I put it back in but now that you mention it I'm going to have to check. There would be no reason that I left it out other than I over looked it.

Don't know about the screws but would they really make a difference?

[richardk]

It looks like to me you have idle bypass holes drilled in the baseplate and holes in the butterflies. That's a lot of extra air, maybe that's part of the problem?

[steve25]

What is that idle bypass mod, the holes in each throttle blade, or .110" holes in the base plate?

If you have both then that's the issue I would say!

["QUICK-SILVER"]

Originally Posted by JMB36TA

Don't know about the screws but would they really make a difference?

I can't remember when the nose shape changed on the screws.

Pretty sure the long tapper screws would bottom out on the spring before cutting idle fuel off. That's if your base plate is suppose to have the blunt nose mixture screws.

Might try taking a spring off to see if the screw will go in further and kill the engine at idle...???

Even with lots of bypass air, should still be able to cut off idle fuel.

Clay