Dieseling / very high idle on 72 FJ40 w/ Weber 38/38

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Jul 11, 2017
Woodlands, TX

I have what I believe is a Weber 38 on my rig and I can't manage to get the RPMs down below 1600 at curb idle. On startup, the fast idle RPMs are about 1100 but as soon as the thing heats up the speed climbs to about 2000 and stays there.

I've been trying to figure out what changes I need to make to the carb, fast idle was initially higher (~1800) but I brought it down to the 1100 value above and it's starting fine. The issue I have is with curb idle.

The carb looks just like this. Is the screw I need to turn in this image or is it a matter of changing the size of the idle jet?

Thanks for your help.
Have you adjusted your distributor timing with a timing light yet? If so, what did you set it at? That can impact idle rpm’s.
Black Arrow is your idle mix screw, you’ll have another on the opposing side. I’d CAREFULLY screw both of them in all the way until they barely bottom out. Then back them out 1 turn each(360 degrees).

Your red arrow is pointing to your idle speed screw. Back it out until the screw no longer cracks open your throttle plates. Now screw it in until it barely causes your throttle plate lever to open.

Make sure your dang choke linkage (green arrow) isn’t keeping your throttle plate open while you’re tuning. You could just pull the pin and unhook it while you are trying to get your idle down to 650ish.

Thanks bigredrocker

Doesn't the red arrow on my picture control the fast idle speed? Seems adjusting this hasn't impacted my idle once I'm up to temperature but it has gotten my fast idle speed down to ~1100.
I believe you should be able to back that idle speed screw out and screw in both idle mix screws until your idle gets so low your engine will stumble and die. If it won’t, then you may have an issue where your choke mechanism is holding your butterflies open.

@JohnnyC knows alot about Webers
...or...your accel cable may just be too tight. Occam's razor.
X2 Make sure your throttle plates are closed

If your idle speed screw is backed out and not touching and the idle mixture screws are all the way in then your idle jets are too big

What size jets do you have...
Idle jets?
Air correction?

Pics of your weber ... is it a Chinese knockoff? Or crappy Solex weber 36/36 that looks kinda like a 38/38... pics please

You need to open the carb to see the mains (in bottom of bowl)... air corrrctions are on top... idle jets are outside the carb

You say high altitude... what height?
JohnnyC / 65swb45

Thanks for all your help - turns out the accelerator cable was too tight and was placing load on the engine constantly. Loosened it and my settings to try to back down the RPMs killed my engine. Followed Weber's tuning instructions and she's sitting pretty at 700 RPM and idling (relatively) smoothly.

I'll probably spend a few minutes with the mix screws next time I take it out, but for now the RPMs are solved.

Thanks 65swb45, you nailed it.
That's what the 'like' button is for.:)

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