idle/mixture????

[marcd]

Can someone explain to me the proper technique to set mixture and idle on a 2150? I installed a rebuilt carb and it ran okay and passed smog. But the idle was a little rough and seemed low (I don't have a tach). Making it richer made it worse so I leaned it out about a quarter turn and it seemed to help. But then the idle was too low, so I turned the idle screw in a little bit. I think I am on the right track, but would like some guidance. The Haynes manual says something about taking it to a dealer so they can use a infrared thermonuclear plasma exhaust analyzer - which i don't have.

And please, keep it simple . . . I'm real slooow.

[brielly]

Without the proper tools you have to tune by ear. Get the idle as smooth as possible by adjusting the mixture first. Then set the idle speed. This method works for me on my weber and should work on your 2150. I'm sure others will chime in with more advice.

[marcd]

Any other carb gurus out there?

[turtlejoe]

Here's some information about tuning with a vacuum gauge that I found interesting and helpful:

http://www.centuryperformance.com/vacuum.asp

Good luck! smile.gif

[marcd]

Thanks for the link. Anyone else with info??

[Chevelleguy]

All good information.

[79_Limited]

Turtle Joe I could not open the link.

Marcd sounds like you are on the right track. I have just been adjusting my mixture a 1/4 at a time then drive it a bit and keep adjusting.

My idle is smooth but a little high so I will back it down a bit with a tachometer this weekend.

My carb is still stumbling when I hit the gas at take off. I am still adjusting and it it getting better.

It stumbles mostly when it is cold.

[turtlejoe]

yumyum - it looks like the whole site is currently down, but it was working yesterday when I posted. Hopefully it's just a temporary thing.

A cold stumble could be the choke setting, but stumble at any temp could also be a bad accelerator pump. Just a thought.

;)

[79_Limited]

Turtle Joe,

Yeah the stumble is most likely choke related cause it only does it when it is cold.

Marcd have you had a similar issue?

[marcd]

Mine seems more prevalent when it's cold, but I think it's more than that. The carb is newly rebuilt (the old accelerator pump was bad). I don't have a tach - is there a cheap one you can buy for diagnostic purposes (as opposed to one you would put in the car)?? I am also missing the little pipe that goes from the manifold to the electric choke but I don't think that would have much of an impact - at least initially.

Mine is almost like a miss until I get some rpms. The mixture is having an effect - I'm just trying to find a good methodology to the whole thing.

P.S. The link was working when I tried it and I do have a vacuum guage. I guess I just need a tach.

[Chevelleguy]

Disconnect and plug the vacuum hose to your EGR valve then see if the stumble goes away. If it does, your EGR valve is opening too soon, maybe the delay valve is missing.

[marcd]

Thanks I'll try that. I replaced the EGR and can't recall if one of those delay valves was in the line. Of course, the EGR I replaced it with had a built in transducer, the old one had a seperate transducer. Doesn't the transducer pretty much act as a delay? The stumble pre-existed my EGR relacement.

[Chevelleguy]

Not sure on the later model EGR system. My 83 has a delay valve in the front of the intake after the thermal switch. If yours is like the 90's XJ's then yes, it comes as one unit. But they are opened by the computer.

[ February 10, 2004, 06:04 PM: Message edited by: chevelleguy ]

[nograin]

Yes there are diagnostic tachs and thats what you want. After you buy one, someone will give you one for free 'cause they are often found in home garages and stuff but no one uses them these days.

A vacuum gage is harder to come by for free. I found a non chinese made one at one of the retail chains last year. smile.gif

Your idle speed should go up as the mixture gets closer to ideal. What were your emmission results at idle (if they give you them)? That can give you some clues as to which way to go. But the easiest thing to do is start turning the mix screws in, when the idle speed starts to drop, (or vacuum goes down), back off a 1/4 to 1/2 turn. You can do the same backing them out (richer) but the drop off in rpm/vacuum is more subtle.

When you are done, adjust the rpm with the throttle position (adjusment is on the idle stop solenoid IIRC)

I just read something else in your post. Did you adjust both sides or just one? The carb has an idle mixture adjustment on each barrel. You need to adjust both. Neither is easy to get at with all the stuff there, but hardly impossible. Keep looking for different angles to slide the screw driver in while looking at the screw. I think the last time I used a 3prong pick-up tool instead of a screw driver and just left it attached to the more difficult to access screw until I was done.

[ February 10, 2004, 06:41 PM: Message edited by: nograin ]

[marcd]

I did adjust both mixture screws equally. Of course, this assumes they were set equally when i bought the carb. Is there an initial setting, like all the way in, then back them out X turns?

In terms of setting the idle, I was turning the screw that was attached to the lever that the accelerator cable pulls. i assume that was to set the idle.

Also, what does the "fast idle" setting do? Could that be my problem and, if so, how do you adjust it? That being said, i really think it is a combination of the timing and mixture. . .based upon my vast knowledge of carburetion ;)

[Chevelleguy]

Base mixture screw setting would be about 1 1/2 turns. The fast idle setting is for the choke. It adjust the idle speed when your choke is set, like initial start up when cold.

[79_Limited]

Marc

I picked up an old Sears Dwell/Tachometer/diagnostic meter on E-bay for $10.

[nograin]

Marc,
Fast Idle is a part of the choke settings. Leave it alone. If your engine is warmed up when you are adjusting it, the fast idle should have no effect.

Idle mix screws. Adjust each to best rpm and/or vacuum. It is likely they will not be exactly the same amount of turns in or out. Do not be afraid to turn them in until the engine stumbles. If the engine dies, turn it back a half turn and restart engine and resume adjustment procedure.

Aha! That's probably the idle stop screw, not the idle speed screw. Yes. I will explain. Part of the pollution control tricks that were applied starting in the 1970s resulted in a sitauation where the engine wants to diesel (or run on) after the ignition has been turned off. To prevent that, the carburator throttle is closed more when the ignition is turned off than when at idle.

Its done very simply. When your key is in 'on' position, electricity flows to a solenoid that pushes the throttle open very slightly. When its 'off', the solenoid retracts and the throttle rests at a more closed position. Its this closed position the screw on the carb adjusts.

The open position is adjusted by the solenoid position. On the motorcraft carbs the curb idle solenoid is moved in or out with a long scew that runs next to it on the same bracket.

You should think about picking up a Haynes Wagoneer Manual (50029). They are are actually pretty decent and will show you much of this, or at least help you figure it out. (covers too many years to be specific to what you always need)

The choke adjustment procedure is a royal PIA. Avoid if you can help it. OK, maybe not that bad, but do leave it 'till last. Get the warm adjustments all done first.

[marcd]

Thanks No Grain. I do have the Haynes manual but it was not helpful about mixture adjustment. However, I should have read it more carefully so I would have turned the right screw for idle speed adjustment. Thanks for the info. There is a rebuild article in the tech section that also gave me some ideas. I'm going to try to mess with it next weekend.

[graham]

Originally posted by marcd:
I am also missing the little pipe that goes from the manifold to the electric choke but I don't think that would have much of an impact - at least initially.
Mine is almost like a miss until I get some rpms. If this pipe is not blocked of, it could give a vaccum leak resulting in the symptoms your experiencing.

[marcd]

Graham -
The PO filled the hole with silicone and doesn't appear to be leaking. However, you bring up a good point. Maybe I'll squirt a little starter fluid in that area just to double check . . .

[Ernzo]

Originally posted by turtlejoe:
Here's some information about tuning with a vacuum gauge that I found interesting and helpful:

http://www.centuryperformance.com/vacuum.asp

Good luck! smile.gif This is the absolutely best way, get the guage, hook it up.

Otherwise it is the old;

Make sure the unit is warmed up, choke all the way open. Set idle.

Carefully screw in one side until the unit starts to stumble. Screw out until it runs smooth. Do it once again, and return to *just* where it smooths out. Go another 1/4 turn. be carefully not to screw *in* to where the screw seats. That is too far and can damage the setting

Do the other side.

Old timers, isn't that about right? Don't want to give out bad info, but that is how I have been doing it for about 40 years or so.......