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Carter YF 2100s Issues
#1424013 Tue Sep 14 2021 07:44 PM
Joined: Sep 2004
Posts: 218
MarkT Offline OP
I'm having an issue with my Carter YF 2100S running rich. I am also having a problem with keeping the idle at the set RPM. I will set the carburetor idle at around 550 to 600 RPM, adjust the air/fuel at around 2.25 turns outward and get a good vacuum reading at close to 18 or 20 inches. I'll drive the truck for around 100 miles or so and the idle will drop to 350 or so. I'm puzzled.

Jon G had a great post awhile back regarding the diaphragm housing being warped and leaking gas. I need to pull the carb to check that. The timing seems fine. My question is could the drop in idle and the carb running rich be related? Could it be a small vacuum leak at the throttle body or bowl?


1956 Chevy 3200, 1/2 Ton, 235 CI
Re: Carter YF 2100s Issues
MarkT #1424014 Tue Sep 14 2021 07:57 PM
Joined: Jun 2004
Posts: 8,421
The inconsistant idle might be caused by the throttle arm loose on the throttle shaft or the butterfly loose in the shaft.
The diaphragm on those are very floppy. It may not be installed right. Check the float for fuel absorption & adjustment. Make sure the metering rod is installed correctly.

Last edited by Wrenchbender Ret.; Tue Sep 14 2021 07:58 PM.

They say money can't buy happiness. It can buy old Chevy trucks though. Same thing.

1972 Chevy c10 Cheyenne Super
Re: Carter YF 2100s Issues
MarkT #1424015 Tue Sep 14 2021 08:00 PM
Joined: Mar 2010
Posts: 608
I'm a novice so grain of salt. The throttle shaft usually wears in the idle to partly open position. Wide open throttle, less wear. I remove return spring and linkage and try to move the shaft up and down by hand. A little bit of movement is tolerable.

Re: Carter YF 2100s Issues
MarkT #1424019 Tue Sep 14 2021 08:30 PM
Joined: Sep 2004
Posts: 218
MarkT Offline OP
At any rate, I'll need to pull the carb. I'll check the throttle shaft and butterfly. That didn't come to mind. Looks like I'll be using my leaky Rochester for a while until I can resolve this.

1956 Chevy 3200, 1/2 Ton, 235 CI
Re: Carter YF 2100s Issues
MarkT #1424033 Tue Sep 14 2021 10:48 PM
Joined: Mar 2014
Posts: 2,478
Since I posted about the warped pump diaphragm housing (bonnet, call it what you wish), I've seen approximately 8 or 9 Carter YF carburetors...various models...and at least 6 had some warpage. In 3 it was enough there's no doubt suction at the bottom of the diaphragm would have brought raw gas in to the manifold. Now every time I go through one, I automatically use the little gizmo I made to see if warpage exists and it is simple to do. Lay the sandpaper on the wood, press the bonnet down and while pressing down rotate for a few turns. Remove it and if you see bright spots where the 4 screws go and dull spots in the center, it is warped. If it is, fix it by rotating more until it looks all nice and even and then stop. Be certain to check the adjustment of the metering rod after doing that. You'll lower the height of the housing a bit and that forms the stopping point for most of the YF models (that and the clip on the diaphragm rod).

Running rich usually just causes the engine to run rough once it warms up and you will smell unburned gas, feel more moisture coming out of your tailpipe and your plugs will be black and sooty looking.

You have a manual choke. I'm assuming you're warming the engine up and opening the choke fully when you set the idle. But let's make certain the fast idle arm is adjusted correctly. On the 2100S, the fast idle arm is designed to rest hard against a stop cast into the throttle body when the choke is fully open. Check to see that when the choke is open fully there is no space at all between that fast idle arm and the stop. If any space is present, bend the rod to adjust and remove it. There is a possibility that if there's some clearance there when you set the idle then once everything gets nice and hot that fast idle arm might move enough to change your idle.

Also your throttle spring...take a look at that. It is a fine balance, I believe. You don't want a spring that's too strong and you don't want one that's too loose. If that spring is too strong I think it is possible when everything gets all warmed up, it could pull hard enough to change your idle a bit...and the trip from 550/600 to 350 won't take much movement. This would tie in with the fast idle arm/stop above. Good luck! Let us know what you find.


1952 1/2 ton with 1959 235
T5 with 3.07 rear end
Re: Carter YF 2100s Issues
MarkT #1424052 Wed Sep 15 2021 12:27 AM
Joined: Nov 2002
Posts: 2,226
Carburetion specialist
While there may be other issues, why "borrow trouble"? Some adjustment costs nothing.

The idle circuit is WIDE OPEN at 1.5 turns. 2.25 turns is going to be pig rich, which could cause an erratic idle.

Unless the engine is totally worn out, 1 MAYBE 1.25 turns should be oceans.

Maximum vacuum will always be obtained when the throttle plate is completely closed, as any opening of the throttle plate will be a "vacuum leak".

Suggest start the engine and drive maybe 10 minutes until totally warm. Then set both the idle mixture screw and throttle positioner screw.

Next time you start a cold engine, use the choke as needed until complete warm-up.


Good carburetion is fuelish hot air
The most expensive carburetor is the wrong one you attempt to modify
If you truly believe "one size fits all" try walking a mile in your spouse's shoes!
Re: Carter YF 2100s Issues
MarkT #1424074 Wed Sep 15 2021 02:39 AM
Joined: Sep 2004
Posts: 218
MarkT Offline OP
Thanks everyone. I've been blessed with a good reliable 235 engine that's in good shape. It is rare to find one of these old carburetors that is pristine. They all need tinkering from time to time. I'll be working on it Saturday.

Last edited by MarkT; Wed Sep 15 2021 02:44 AM.

1956 Chevy 3200, 1/2 Ton, 235 CI

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