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See the yellow thing tucked in between the choke housing and carb body? That’s the fast idle cam and it has steps on it that act as a throttle stop to keep the throttle open and the idle up while it’s warming up. When the choke is closed it’s supposed to rotate so the throttle steps are in position, then when it warms up and the choke opens it rotates back out of the way.
Common problem with them is the arm gets broken off when somebody tries to adjust the little Allen stop screw sticking out of the arm. If yours is broken off, it won’t work and you’ll need a new one.
Nice diagram Kultulz. Not trying to show you up, but I just happen to have an old 4180 laying around so here’s a few shots of the adjustment screw from the underside of the carb. Thank you for the inspiration!
Notice in this shot the adjustment screw (which is part of the lever arm that’s attached to the throttle shaft) is on the highest high idle step.
If that arm arm on the fast idle cam I mentioned previously is okay then it could be jammed up with dirt or maybe it wasn’t put back together correctly. The way the springs go is tricky.
I was trying to figure which screw you were describing. The small one in the cam itself is for the initial choke opening/unloader. The screw in the throttle shaft end actually controls fast idle RPM. Are we on the same page?
I just wanted the OP to choose the right one.
I also have a 4180C boxed up somewhere. I hope we aren't going to play show me yours and I will show you mine. I usually lose on that ...
You's guys are great, I went out this morning and couldn't find any of that! Came back in to find these pics and advice. I will go out and look again after lunch, the temps have dropped around here (30 degrees overnight) so while I was looking for the adjustment screw (10 mins) the engine never did warm up all the way and I never heard it drop down. The idle is so rough the front end rocks side to side, def. seems too slow... I did not have to mind it tho, it did idle on its own while I was poking around under the hood, before correcting the float level I needed to keep my foot on the gas to warm it up on these cold mornings.
I think I have found the adjustment screw, I can't see it , think I put a finger on it but haven't been able to get a wrench or screwdriver in there. Seems a screwdriver needs to get at behind the secondary vacuum diaphragm actuator rod or a wrench slipped into a very small space in front of it! Haven't found the magic technique yet...
There are a lot of adjustments on these carbs. So you need to divide your problems up. First thing I would do is go on a long drive, get the engine fully warmed up. Go back to the driveway and let it idle, and then adjust those screws were you took the caps off. Turn one of them in till it runs worse, then turn it out till it runs the best. Then go to the next one and turn it in till it runs worse, turn it out till it runs the best. I have never adjusted a carb with 4 screws (did you say yours had some in the rear?) but if it does I would do the same for the rear screws, adjust them in and then out till it runs the best. Then I would start at the first adjustment screw and do it again, screw it in till it runs worse, screw it out till it runs the best. It should be running pretty smooth at this time. You have adjusted it when the engine is warm.
Then you have when the engine is cold. That fast idle thing the other guys were talking about is only in effect when the engine is cold and the choke is on. So you will have to wait till the next morning, start it after pumping the pedal a couple of times, and see how it idles and runs. It should run a little fast and on it's own. If it doesn't check to see if the choke door is shut. If it is not, you need to adjust the choke in the "richer" direction. As you do this, the screw they were telling you to adjust should come into play, you might not have to adjust it.
The choke can be very difficult to get correct, and it will take several mornings of cold starts to get it working correctly if everything is functioning with it.
I went out this morning and couldn't find any of that! Came back in to find these pics and advice.
Those photos are great.
I will go out and look again after lunch, the temps have dropped around here (30 degrees overnight) so while I was looking for the adjustment screw (10 mins) the engine never did warm up all the way and I never heard it drop down. The idle is so rough the front end rocks side to side, def. seems too slow... I did not have to mind it tho, it did idle on its own while I was poking around under the hood, before correcting the float level I needed to keep my foot on the gas to warm it up on these cold mornings.
The choke feature (along with fast idle) may be completely out of adjustment. If she shakes on start-up, the choke mechanism may be adjusted too tightly and it will run very rich until the choke valve completely opens (if it does).
There are a lot of adjustments on these carbs. So you need to divide your problems up. First thing I would do is go on a long drive, get the engine fully warmed up. Go back to the driveway and let it idle, and then adjust those screws were you took the caps off. Turn one of them in till it runs worse, then turn it out till it runs the best. Then go to the next one and turn it in till it runs worse, turn it out till it runs the best. I have never adjusted a carb with 4 screws (did you say yours had some in the rear?) but if it does I would do the same for the rear screws, adjust them in and then out till it runs the best. Then I would start at the first adjustment screw and do it again, screw it in till it runs worse, screw it out till it runs the best. It should be running pretty smooth at this time. You have adjusted it when the engine is warm.
Then you have when the engine is cold. That fast idle thing the other guys were talking about is only in effect when the engine is cold and the choke is on. So you will have to wait till the next morning, start it after pumping the pedal a couple of times, and see how it idles and runs. It should run a little fast and on it's own. If it doesn't check to see if the choke door is shut. If it is not, you need to adjust the choke in the "richer" direction. As you do this, the screw they were telling you to adjust should come into play, you might not have to adjust it.
The choke can be very difficult to get correct, and it will take several mornings of cold starts to get it working correctly if everything is functioning with it.
Dave is right on. The choke needs to be fairly well dialed in before there is any point in trying to make any adjustments to the high idle cam.
Then you need to verify the fast idle cam is moving correctly with the opening and closing of the choke. If it’s not moving correctly that needs to get fixed first. Looking down from the top you can see how it should look with the choke open and closed. It doesn’t move a whole lot, not quite a quarter turn.
The motor doesn’t have to be warm to check the movement of the fast idle cam. Just remove the air cleaner so you can see it, bump the throttle so the choke will snap shut (if the motor is cold and the choke does not close there is problem #1) then use your fingers to hold the choke flap wide open, and bump the throttle again to release the fast idle cam so it can rotate into position. If the fast idle cam doesn’t move, there’s a problem.
The later model 4180 carbs had anti-tamper chokes and it may be necessary to take apart the choke and grind off the little plastic tab circled in green in order to rotate the choke cover and adjust the choke. As far as I know the late model carbs didn’t have mixture adjustment screws on all four corners, only two up front on the primaries, so that seems curious.
I was trying to figure which screw you were describing. The small one in the cam itself is for the initial choke opening/unloader. The screw in the throttle shaft end actually controls fast idle RPM. Are we on the same page?
I just wanted the OP to choose the right one.
I also have a 4180C boxed up somewhere. I hope we aren't going to play show me yours and I will show you mine. I usually lose on that ...
LOL, I just enjoy photography and helping when I might actually know something, which isn’t all that often, so I have to make the best of it when I can!
Agree, the correct adjustment screw to set the fast idle is the inaccessible one on the arm attached to the throttle shaft. Pretty much need to remove the carb to get at it.
As for that tiny little screw that’s in the fast idle cam itself, my advice is don’t touch it because that how they get broken!
Update (and a little history)... went out to look at this choke & fast idle cam on cold engine, the choke plate was totally closed at first (at some point in mounting the rebuilt carb I set the choke plate at 'a pencil width of space at the rear edge'). I observed the end of the fast idle cam arm (no set screw in this one) near or at the top of its range, I pushed on the throttle and the cam arm didn't move, then manually opened the choke plate and pushed on the throttle and the cam arm dropped with no resistance to the bottom of its range, while releasing both the throttle and the choke plate the plate returned to the pencil width position and the cam arm back to original position (or pretty close), then pushing on the throttle again made the choke plate close up completely. after that I started it up normally and it held the same slow clunky idle for 2mins then stalled. then I started it again while minding the idle with my foot lightly on accelerator and got it past the 'danger zone' where it would want to stall. next I let it idle and watched the choke plate and cam arm, after about 3mins the choke plate opened just a bit, in another minute it was open to the point of a 'pencil's width' position, then in over the next minute it smoothly opened up completely and the idle dropped just enough that I noticed a change in the sound.
questions: does this sound like a normal operating choke? does that 'pencil width' position sound correct? should I continue with fast idle adjustment? do I really have to remove carb to turn that fast idle screw?
also: the fast idle cam arm I have is blue, if that tells us anything...
The measurement you are describing is the MANUAL CHOKE UNLOADER. When the choke is set, the linkage brings the opening to the required opening distance. You need to find the rebuild sheet and go through the choke mechanism completely using the specs according to the CARB ID. Also hit the linkages with choke cleaner until all moves freely.
There is no way to get an open end wrench to the fast idle adjusting screw?
I think your choke pull-off is not working or not set correctly. It sounds like everything is working correctly, when you hit the throttle cold the plate shuts completely, this is correct. When the engine starts, vacuum builds in the carb area and they have what they call a "choke qualifier" or "choke pull-off". This thing cracks the choke plate open a set amount so the engine doesn't flood out from the closed choke plate. Don't try and adjust the choke plate so it's partially open on the cold engine, it will never work correctly that way. All you are doing when you adjust the round black piece is adjusting the tension on the choke plate, it will always close completely when the throttle is first kicked when cold. I told you the choke is complicated and hard to get correct.
The arrow is pointing to the choke pull-off in this picture. But they say these type were only used on California models. I believe other models had it built into the choke housing.
11-10-2018, 02:23 AM
