Fast Idle

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SLOonTheGo

SLOonTheGo

10 Year 15 Year
Joined
Nov 29, 2006
Threads
16
Messages
362
Location
Bay Area, Ca
Hey guys.

I have a 74 40 with all stock running gear. f155, 4spd and a 3spd transfer. So far I have rebuilt the carb, napa rebuild kit, changed the intake manifold gasket and water pump. The engine runs much better than it did but it is still idling pretty high. Around 1000 rpms when the engines warm. I am just wondering what you guys might think it is since I am at wits end.

Thanks
Jon
 
Turn the idle speed screw down? If that doesn't work, turn the fast idle speed screw down.
 
you can also adjust your timing, or if your timing is already set then there should be an air/fuel mixture screw that you could play with til it idles correctly.
 
I do have the fsm, its saved me a few times now. Unfortunately I don't have timing light here.

I was planning on doing the valves this weekend, when I did my compression test all the cylinders were between 140 and 160, except #3 at 110.

I just went through my carb linkages and realized they were set up incorrectly :doh:. Fixed that and adjust fast and slow idle screws and mixture, now it idles at around 800. Better but still not great. So it looks like I will have to tear into it and get a timing light this weekend.

Thanks a ton everyone.
Jon
 
I believe that I found both the fast and slow idle screws. I matched my linkage up to the photo I found on the sor website. Adjusted the bottom screw, Slow?, all the way out and it was still at 800 when the engine was hot, adjusted the top screw, fast?, both ways but saw no effect. I changed the mixture as well and that brought rpm's up and down, mixture all the way in with both screws all the way out is the lowest rpms I got at 750 to 800.
042-01G-CA-1.webp
IMG_0633.webp
 
I have the exact truck as you, even freeborn red and it took me a while to get the idle correct also. Embarrassingly enough, there was so much goop on the side of the carb that I couldn't see the idle screw on the lower side of the carb, on the cab side of the carb. at the bottom. That was the one that affected my idle, not the one up higher that you had labeled (I had adjusted that one to no avail). Of course I found it after I replaced my head gasket and rebuilt the carb and blasted the intake. Now she's really clean under the hood! If you have any specific questions, I'd be glad to try and help.

Crazy, I also bought my truck in SLO about a year ago....nice area.

edit: Also might want to just replace all the vacuum lines, just to eliminate that as an issue. The ones in the pic look pretty rotten.
 
edit: OK, ignore what I am saying here and pay attention to what FJ40Jim says below...

I'm pretty sure that the upper screw (the one you have labeled as fast idle) is actually your main idle speed screw, and the other (lower) one is the fast idle.

I could be wrong, but it sure looks like it's contacting the throttle lever and could be backed out more.

Another thought: my carbs have cable systems and not linkage connecting the gas pedal to the carb. One thing that's happened to me is that that cable can be mis-installed (both at the carb and at the pedal) and therefore shortened, which results in de-facto pulling on the throttle lever with increased idle. I'm sure that could happen via bending or whatever with solid linkage, too. So, look closely at that.
 
Last edited:
Also, is your hand choke wire holding things open? It needs to be completely relaxed there...
 
The base idle screw is impossible to see on a 74 cali-spec carb. It is facing out, underneath the secondary throttle shaft.

The high idle screw also faces out. The yellow arrow is pointing at it.

The TP screw is the easy one to see. It is circled in yellow.

The pic shows the CB being pretty badly bent. Straighten it up to get CB and TP function back in the ballpark.

To adjust:
Back out TP screw so it is not touching. Better yet, remove it entirely and discard.

Run engine to operating temp.

Set base idle screw to 500-650RPM (user preference).

Adjust idle mixture.

Adjust idle speed again.

Check ignition timing.

Pull choke knob, adjust hi-idle to 1800RPM.
 
Thank you all for the reply's. I will take a look at all of these things on Saturday, I have a pretty large steel structure design submittal coming up so time is gonna be pretty tight. Thanks for all the help though if any one comes to surf and turf I will buy you a sixer :beer:.

Jon
 
:censor:

So I took a look at my carb and sure enough there was a screw that I had never and would have never seen at the very base of my carb. Impossible to find. I actually had to pull the carb off because I couldn't see it or feel it. :doh:. I did a bit of adjustment on it and got the idle down but still need to go through and follow Jim's instructions, which I greatly appreciate. Things are just a bit too busy lately with midterms, large projects, and the :princess: who hates the cruiser. Shes an environmental engineering student...

So thanks to Jim, Ziff, Eddy, IDave, Miles, and Pin Head for all the help.
 
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