Idle Mixture and Engine Performance

[Tham]

Does adjusting the idle mixture on the carburetor
affect the engine's overall performance ? I was
under the impression all along that it only affected
idle characteristics.

This was posted in one of the Malaysian auto forums,
about how one guy's mechanic tunes the Aisan carb
on his local Proton Saga :

My old-timer mechanic does this when tuning
my carburetor :

If you want pick-up, he'll lean out the air/fuel
mixture (turn the screw in) until the engine
idles rough, before screwing back (richen it) a bit.

If you want top speed, he'll richen the air/fuel
mixture by screwing out more.

[CrUZida]

Are you sure there isn't an idle screw AND a mixture screw?

[Tham]

Are you sure there isn't an idle screw AND a mixture screw?

I'm referring to the mixture screw at the base
of the Aisan carb. The only other two screws
are one for the idle speed at the right, and
one more at the extreme right for the manual
choke (on the 4K's Aisan).

[Rodger]

Yes it is possible to do this as the old guy is saying. Mind you I would be hard pressed to notice any performance change.

Basically what the old guy is doing is richening up the mixture at higher revs. The idle circuit is always in the total fuel/air path with the main and secondary OEM fuel jets, which are set to run a little on the lean side under full throttle and high revs. So this idle mixture trick will add just that tiny tiny bit more fuel.

Better off drilling out the main and secondary jets a couple of sizes bigger and dropping the idle mixture back.

Regards

Rodger

[Tham]

Better off drilling out the main and secondary jets a couple of sizes bigger and dropping the idle mixture back.

Regards

Rodger

Thanks, Rodger, that was informative.

You mean kinda similar to fitting bigger jets,
and then backing (richening) out the idle
mixture ?

I found this standard mixture adjustment guide
for the Aisan carburetor on the 993 cc CB23
engine on the Daihatsu Charade. (The
CB61 is the turbocharged version).

I'm not sure if this might be close to the
Aisan on theToyota's 4K. My idle mixture
screw is turned out 5 and a half turns from
the fully closed position.

[oldcorollas]

it's slightly true, but it is not so much difference...

i think you need to stop listening to your mechanics and learn to do it yourself.

what does the manual for your car say abotu the mixture screw...

i still think (like i did when you posted about your first problems) that you have a vacuum leak, which means you need more turns on the idle screw, as the vacuum is not as good.. due to the air leak... and makes your car run badly etc....

fix it properly, then start playing with tuning...