View Full Version : carb adjustment questions

07-26-2007, 08:50 PM
So I've been trying to figure out this whole carb adjustment thing...waiting on an engine manual, probably gonna be another 3-5 weeks, but at least it's on the way :thumb: Figgered out I got a 9/77-12/78 Federal carb per SOR website, so that's only 11 months off my production date :rolleyes:

I'm so confused as to what some of these screws do...once I know then I'll be able to figure it out (I think). The one in the first picture is the idle mixture screw, got that much figgered out...:rolleyes:

In the second pic (looking from the firewall) there are three screws, but all I've read about so far indicates there's only supposed to be two on this side.

I think the one with the question mark is the throttle positioning screw, but I don't know what its purpose is and how it's supposed to interact with the lowest left screw (which you can't see, hidden behind that linkage).

The lower right screw I think is the fast idle adjustment, which is determined by the choke, but is that screw supposed to be touching the linkage when the choke is off so that immediately when you pull out on the choke the idle increases?

I did figure out where the idle mixture likes to be, and for who knows how long the throttle positioning screw (?) hasn't been in contact with anything, it was screwed way out so the spring wasn't even compressed. The fast idle screw (?) wasn't close to anything either, so when I pulled out the choke my idle didn't increase.

07-26-2007, 09:40 PM
Okay so as long as the screws are as I thought then I think Haynes does a pretty good job of explaining how to adjust everything. The only question I have is what is the initial setting of the idle speed screw (hidden screw?), or do you just kinda put it wherever?

Man Jerk
07-27-2007, 09:29 AM
Treeroot has an awesome post on 'mud about setting up your carb/timing etc. I stumbled upon it while searching about setting idle mixture. Might even be in the 40 series FAQ.

07-27-2007, 09:32 AM
I don't think the initial setting is all that critical like the mixture is for a baseline. Just turn it to have it idle where you want. 750 is stock iirc. When mine run good and nice and quiet I usually can set the idle around 400.

07-27-2007, 11:15 AM
Get a vacuum gage and plug it into manifold vacuum, adjust mixture screw for maximum manifold vacuum. Set motor at 650rpm with linkage adjustment to dial timing. Make sure to pull and plug what ever vacuum lines you have running to the distributor. Timing should be set around ~ 10 btdc at our altitude.

Jetting is another whole can of worms.

07-27-2007, 11:57 AM
You could probably run even more than 10. In va at 250ft desmogged I was running 14. It started to ping at about 16. Advance it to where it pings and then back it off a tad.

07-27-2007, 12:05 PM
That brings up something I was asked a couple days ago. How do you know how many degrees you are advanced when there's only two tick marks on the flywheel? I've never timed an engine (need to do the red truck soon) so maybe it will be obvious once I do that. I read Treeroot's writeup, just still trying to understand how the idle speed screw and throttle positioning screw are supposed to work together, and for that matter the fast idle screw too. I've got everything set where the truck seems to be happy, so maybe I'll just wait for the FSM to come and hopefully it'll be more clear then.

07-27-2007, 12:21 PM
timing light with advance feature, or back in the day you advanced to where it pinged and backed it off a tad where it ran best and didn't worry about exactly what it was set at.

The fsm isn't going to tell you how they work together. It tells you how to set it to factory specs which is probably best suited for passing smog and not optimal performance.

Man Jerk
07-27-2007, 12:21 PM
That brings up something I was asked a couple days ago. How do you know how many degrees you are advanced when there's only two tick marks on the flywheel?

This is where a timing light with an advance/retard dial comes in handy. I really wish I had paid the extra money when I bought mine. You can turn the dial and it will advance or retard the light so you can find out exactly where you timing is in regard to TDC.

That's the only way I know of to check it.