Mixture and Idle Adjust 79 FJ40 (1 Viewer)

[Tool Boy]

Hello to All. . . New member and hoping someone can clarify. Had a new shop do extensive work on my 79FJ40 recently. Part of work included new exhaust manifold gasket and now carburetor and linkages seem not right. Carb was rebuilt less than a year ago by a guy that's done hundreds of cruiser carbs. I'm attempting to check the mixture and Idle adjustments and I am currently making it worse. The book I have says to put a tach on it, at approx 650 rpm adjust the mixture screw back and forth until max rpm is achieved. When I do that and test the throttle there is a lag before the engine revs. I was told long ago a way to set mixture was to turn it in until almost dies and then back off a number of turns but I don't remember how many turns?? I am also unsure of having the adjustment screws identified as my stock Calif 79 carb doesn't match the book I have. I'm including photos of what I think are the correct screws in hope that a member can shed some light. Thanks, Andy

 

[TheHardWay]

First picture - mixture screw, also known as idle mixture adjusting screw. Second picture - idle adjust screw is actually the idle speed adjusting screw. With engine idling, turn the idle mixture adjusting screw in til almost stalls, then back off 1 and a half turns. From there, back off to maximum idle speed using tach. Turn idle speed adjusting screw until idle speed is 690 rpm. Then back off idle mixture adjust again to maximum idle. Keep doing this until idle no longer rises, then turn idle speed adjusting screw til idle speed is 650.

 

[Brian/99uzj100]

It also helps if you have a vacuum gauge connected to an intake manifold source. Once you get the idle and mixture close, you can fine tune the mixture by trying to get the highest Hg on the gauge as possible.

 

[Tool Boy]

First picture - mixture screw, also known as idle mixture adjusting screw. Second picture - idle adjust screw is actually the idle speed adjusting screw. With engine idling, turn the idle mixture adjusting screw in til almost stalls, then back off 1 and a half turns. From there, back off to maximum idle speed using tach. Turn idle speed adjusting screw until idle speed is 690 rpm. Then back off idle mixture adjust again to maximum idle. Keep doing this until idle no longer rises, then turn idle speed adjusting screw til idle speed is 650.

Hey LasCrucer, Thanks for the reply. I lived in Corrales, in river valley north of Albuquerque for 10 years. After I posted the request I did a youtube search ( as I needed the truck tomorrow) and found I followed his instructions but he did something different in the end. Your final step agrees with the manual I have. This guy brings idle down to 650 by turning the mixture screw back in rather than the Idle screw adjustment screw out? The engine runs so much better now following the video's instructions but I'm thinking your way is probably the right way and it agrees with the book. I'll be back under the hood in a few days and I'll repeat the process but change the last step to idle adjustment screw and see how it goes. Thanks for the help.
Andy

 

[Tool Boy]

It also helps if you have a vacuum gauge connected to an intake manifold source. Once you get the idle and mixture close, you can fine tune the mixture by trying to get the highest Hg on the gauge as possible.

Yeah, I think I noticed on the other guys video that the vacuum dropped when he turned the mixture screw in in the end. Thanks for that!

 

[TheHardWay]

You're welcome. I've watched that video a couple times already, it's pretty good, and I've learned from numerous Mud threads there are slightly different ways to do the lean drop method of setting the idle. I've been working on my '78 since rebuilding the carb, and located 3 small vacuum leaks that are messing with obtaining a smooth idle. Live and learn!

 

[Tool Boy]

You're welcome. I've watched that video a couple times already, it's pretty good, and I've learned from numerous Mud threads there are slightly different ways to do the lean drop method of setting the idle. I've been working on my '78 since rebuilding the carb, and located 3 small vacuum leaks that are messing with obtaining a smooth idle. Live and learn!

I've been driving my '79 for 35 years now. Coming up on 400K miles. I had a pro rebuild the carb a year ago but didn't realize until today that he never did the final setup and mixture adjust when he put it back in the truck. I've been driving it underpowered for almost a year and tonight when I set the mixture myself and adjusted the timing it ran like a young cruiser again. Very excited.

 

[TheHardWay]

BOOYAH More power!

 

[Tool Boy]

Yup!