Schrade

I'm trying to tune the carb on a buddy's '86 Toyota here.

I reduced the fuel with the mixture screw, and idle went up accordingly, til it started to cut out. Problem is, the idle stop screw won't allow reducing the RPM's. The screw is bottomed out.

I think he has either a vacuum leak (it doesn't lope idle tho'), or he has a missing hardware in the idle stop bracket.

Any ideas here?

I found a carb schematic here, but it's difficult to make out labeling...

wireguy

One thing you might try is running around the engine compartment pinching off hoses with a pair of needle nose pliers. If your RPM's suddenly drop you know you just stopped a vacuum leak.

Schrade

Thanks wireguy - might be my only option here...

Wouldn't it be loping idle tho', if it does have a leak?

Schrade

Found a schematic; anybody know which is fuel / idle mixture control?

wireguy

By no means do all vacuum leaks result in anything other than high idle RPM's. Another possibility is you are backing the idle screw all the way out but the throttle plate isn't following it all the way. Wear on plate rods and linkages can cause that. Another possibility is a vacuum controlled smog part or dashpot or whatever that is leaking at it's diaphragm, including the distributor vacuum advance..

Tdub12

I have had similar problems with my truck having the vacuum advance diaphragm go out.. This caused a vacuum leak as well as changed the timing. The timing being to far advanced made the idle to high at the point I couldnt bring it down with the screw. I bought a new vacuum advance diaphragm on LCE for 60 bucks and problem solved.. I would say check the timing and spray carb clean around the vac lines to look for a leak. And maybe check the vacuum switches for emissions they can go out and cause problems.


Good luck

Schrade

Strange tdub - usually when you advance timing - to BTDC BEFORE TOP DEAD CENTER, it makes more resistance in motor rotation, and RPM's drop. Retard timing, closer to TDC, and RPM's increase, with less resistance in motor rotation.

Not sure what you had there, although I might be forgetting something simple.........

My buddy went to see another guy on the way here today, and the buddy's dad looked at it. Supposedly, it was a bad spark plug. I'm not convinced, although the dad did say to check vac timing advance too.

Tdub12

My bad you are correct I had it backwards. Have you had any luck looking over it?

Schrade

No - he hasn't come by to break down the carb setup yet. We might do it soon here and photo-doc it, if he wants to go that way.

Tdub12

Maybe check the mixture too. If you have a vacuum leak that is not detectable you may have too rich a mixture to compensate for that which would give you a high idle

slow-mo

That's not an accurate schematic of the 1986 Aisan carburetor. It does look like that is for an Aisan, but not what is used on the trucks. That one looks like it's for the Corolla, judging from the round-shape air-horn.

Here is the correct parts fiche:




To answer your question, part number 21271 is the fuel mixture screw. Part number 21273 is the idle speed control screw.

And here the various carb screws:

Schrade

Thank you very much SIR!!!

Appreciate that!