Idle up and down

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Mar 13, 2017
I’m hoping some mudsters can help me diagnosis a weird idle I’m getting.

Here’s the story...

I just rebuilt my 2F, the whole thing! Stabbed her back in and got the carb tuned by a pro.

I’ve noticed that when the engine is at idle is revs down and then back up. This especially happens when it isn’t completely warm yet. Sometimes it keeps doing it after the engine is warmed up and sometimes it doesn’t. This video is when the engine was not warmed up yet. You’ll see the RPM’s drop and come back up.

It also does it when I’m driving and take my foot of the gas. It drops way down and then comes back up. Any thoughts on where to start looking?


It sounds like the engine is in a Deceleration Fuel Cut feedback loop with a disconnected vacuum switch.

The idle is set too high and the vacuum switch is either not connected to its wire or the vacuum switch is broken. Test it.
If your vacuum switch is gone, ground the wire that connected to it.
(for a newly rebuilt 2F, that thing is making a racket)
Wow that’s super weird. I’d definitely follow @OSS advice. It is way loud too, have you done a valve adjustment recently, very clackety clack!
Thanks for the advice everyone! The valves were adjusted by a guy that had a cruiser shop for years so I think they are good. The rough idle seems to be more attributed to it going up and down and almost skipping when it happens. Once I’m driving it it sounds like a sewing machine. I agree the idle is set too high. Once I get this idle down thing fix I can adjust.

@OSS is there a specific way to test the vacuum switch? Thanks!
Ohm the terminal to the housing and apply vacuum; the state of continuity will change
Here's the testing procedure


  • 2F Decel Fuel Cut.pdf
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Very helpful @OSS ! Thank you.

I’m trying to get my hands on a Ohm meter but in the meantime I look closer and it looks like I’m missing the o ring. Am I seeing this right? I couldn’t find a picture online of a vacuum switch with the o rig but it sure doesn’t seem present here.

Toyota says these are NLA. Any guidance on where to get an aftermarket o ring? Thanks!

That picture is the vacuum switch and that brass nipple needs to connect to a vacuum source. No O-ring needed. A wire should be plugged into the switch.

In the spaghetti piping, the vacuum pipe it should connect to is no. 7 in the drawing

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Thanks @OSS! Clearly I had my eyes on the wrong thing. I had that vacuum line hoked up correctly. I just pulled the vacuum for the sake of the picture.

So here’s the latest. Ive been working through the testing procedures. I unplugged the solenoid valve at the carb and the engine did die. So that makes me think it’s working right?

Just to be clear this picture is the solenoid that I need to rest further right.

Yes the fuel cut solenoid is working (that's what may be causing the idle oscillation).
What I [think] is happening (I could be wrong) is if the vacuum switch is dead or wire not connected, the emissions computer thinks you're decelerating above 1500 rpm because it isn't getting a vacuum signal from the vacuum switch, so it turns off the fuel cut solenoid to reduce the fuel going into the carb (which is good during deceleration). Then when the RPMs drop below 1100 or so (cuz the engine is starved of fuel), the emissions computer turns the fuel cut solenoid back on.
With a high idle set, it'll continue to oscillate like that until kingdom come if the vacuum switch is dead.
@OSS I think you are exactly right. Last night I adjusted the idle speed down to around 900 and it hasn’t been doing it since. It does seem to go completely silent when I’m braking although the RPM’s don’t drop like they did before. I think that’s what suppose to happen if the fuel cut solenoid is working so maybe this was all because of the really high idle set. Makes total sense now! Thanks for all of your help!

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