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I have a fully rebuilt Carter 3211S on my 57 235 engine. The idle adjustment screw does absolutely nothing in regards to the vacuum reading (at manifold). Engine is running at 500 RPM idle, timing at 5 degrees advanced. Each time I turn the screw a quarter turn, I bump the throttle but still nothing. I would have thought that the idle mixture screw would at least have an effect on engine RPM but it does nothing throughout its travel (until it comes all the way out at which time the engine dies, or turned all the way in and it runs terrible). I left the screw at 3 turns out from bottom. Anybody have any ideas on what would cause the idle mixture screw to have no effect?

If the throttle positioner screw is turned in too far; the idle mixture screw will make very little change from all the way in to removed and in your pocket.

Two and one half turns is wide open. Anything past two and one half turns should effect no change.

Suggest:

Start engine and warm to normal temperature
Turn idle mixture screw in until thumb-tight
Back out 1 turn
Adjust idle speed with throttle positioner screw

There are other possibilities.

Jon.

One thing I discovered earlier this summer was that some of these YF models will have a warped diaphragm housing (could be called a diaphragm bonnet, also). If this happens, it is a very good possibility your carburetor will run rich and this could partially or largely or possibly totally override the effect your mixture adjustment screw has. Please let me explain: There is manifold vacuum constantly under the diaphragm and there is a spring which regulates the movement of it on the diaphragm rod. The diaphragm should seal completely around the diaphragm housing. There is fuel constantly above the diaphragm (in the float bowl). There is fuel inside the bonnet (which provides fuel for your accelerator pump--when the diaphragm is moved upward against the spring pressure, it forces fuel up through the passage, past the check needle and into the throat through the little nozzle). Now, if the diaphragm housing has become warped (and out of 5 I have checked lately, 3 have been warped), then there is a high possibility the constant vacuum will suck gas out of the float bowl and straight into your manifold. It sucks gas right underneath the diaphragm where it fails to seal to the float bowl surface. Obviously this will make your engine run richer. It could obviously also play havoc on adjusting using the idle mixture screw depending on how messed up the surface of the housing is...and I hope this makes sense to you. I've brought back to the top the post where I explain one way to straighten the diaphragm housing. The title of the post was "Carter running rich? A possible surprise for you" or something like that.

In a for sale post of two Carter YF 4145 carburetors, I mentioned they had been rebuilt. I bought 3 of these last spring/early summer. The housing in all 3 was warped and the housing i another YF I purchased was also. This has now become standard procedure for me to check using sandpaper and the little wood piece I made. It only takes a minute to see low/high spots and correcting them is easy.

Jon -
This seems to be my issue, the throttle position screw is all the way out and it won't fall lower than 500 RPM. I can press down on the throttle lever to get it to fall below 500 but it is taking a bit more force than I am comfortable with. So, I have a problem somewhere with the internals causing the throttle butterfly not to close fully. Probably some adjustment I either overlooked or performed incorrectly. I'm going to take it apart again and try to see what the problem is.



Jon G.-
Since I'll have it apart again, I'm going to check the diaphragm housing for flat like I should have done when I had it apart to put the part in that you sent me (which worked wonderfully by the way!)

Jon G. and Carb King, It is a shock to my pride but you need to know. I replaced the old carb on my 54-3100, 235. In that this is near the last of my AD restoration's I wanted it to be perfect. The new old stock carb which I have had for about 50 years was bought from my fathers store. Of course I rebuilt it (one of very many), but I cannot get it to stop running rich. This is the third set of new AC45's that I have put in it and after a 1/2 hour they foul out. Compression is great, valves need to be adjusted as soon as I can get it to run for a half hour. I flattered myself as being an expert but this old bugger has shown me otherwise. I always do the 600 grit sand paper on everything (air horn and fuel bowl), that I have rebuilt. Many satisfied customer's in the past but, I cannot get my own to run. HELP! Doc.

What kind of carburetor, Doc? I'm certain the other Jon (Carb King) will chime in, but knowing what you have would help. If you have a Rochester B, then there are a couple of notorious problems with that one. If you have a Carter YF, there are a couple of reasons it might be running rich, too. But let's start at the beginning and go from there. We'll await your answer...

Glad the part I sent worked...that was the little metal piece which sits atop the anti-slosh seal. I have a 1/32" thick sheet of 40A Viton and will be figuring out how to make those little seals out of Viton as soon as time allows.

It is a Carter B with manual choke. The tag was not on the core, rebuilt it twice and just left another thread to you and Jon. The truck was bought new in Memphis in 1953. Its a 54-3100. I'm out of idea's. Compression and valves are both up to ideal. Ed

Ed if it is a Rochester B, there is a good chance the problem lies with the power piston/power valve. Now having said that, it is a VERY good chance you have warped castings and therefore you can't have enough internal vacuum to lift the power piston off of the power valve ball. Please don't try to sand the castings flat. There were little ridges cast into the float bowl and air horn that were meant to help seal the gasket. There are only 4 screws holding the top of this carburetor on, and once those ridges are gone, the likelihood your gasket will ever seal the float bowl to the air horn becomes notably lower. Please also don't try to solve this problem with 2 gaskets...it will only make it worse. If you used one of the cheap kits rebuilding it and if you followed the instructions in that kit, this could be contributing to your trouble in ways you might not expect. The fuel inlet orifice in the seat is possibly too large...maybe as large as .110" in diameter. I found 3 different sizes in the ones I dissected and they ranged from around .075" to around .110". Too large and your fuel pump's output will outrun your float/needle/seat's ability to regulate things. In plain talk, your pump will push gas into the float bowl faster than your carburetor can regulate it. You'll run very rich, because you'll have too much fuel in the bowl all the time. Power piston not working or don't have enough vacuum to keep it off the power valve ball? In that case, the power valve will be open and you'll be running too rich. How about the float setting? There were something like 140 different models of this carburetor made. From what I've learned there could be numerous different float setting measurements and the difference between some of them could be as little as 1/32 inch. The instructions in your cheap kit will tell you to use something like 1 9/16" no matter which model you have.

About 3 years ago I showed everyone here how to straighten the Rochester B or BC castings using one small and simple piece of 3/8" steel and a gas BBQ grill. I don't think I could have made that task any easier. I also studied (using a handheld vacuum pump and gauge) the power piston springs. There were 4 different strengths of springs I found in the guinea pig carburetors I purchased for the study. Some of them would allow the piston to lift at around 9 inches of vacuum and a couple required more like 14 inches. If you are losing vacuum due to warped castings, there's little hope you will have 14 inches of vacuum at the top of that piston, so your carburetor will never run as it was designed. That study is still here. I expected it might have become a tech tip, but for whatever reason it didn't.

Having added these hints, have you ever considered purchasing a Carter YF carburetor? Or maybe a Zenith 228? I know some people think a Rochester B is a miraculous design and I tried very hard to keep an open and optimistic mind during the study, but a sow's ear is at the end of the day still a sow's ear.

Also, do not install a gasket between the carb and the bakelite insulator on the manifold.

Gord, what is the result or issue with the gasket installed between the carb and insulator. I just reinstalled my Rochester B and removed and scraped off the old gasket from the carb base. I read in an article that you don't use a gasket if there is an insulator. Not knowing "why" but just doing what was instructed I'm curious what problem that can cause. Thanks!

Since the factory did not install a gasket, there is a good possibility that any gasket installed later will block the vacuum passage and the power piston will not work. If a rebuilding kit has a gasket that doesn't mean that it should be used.

Gord, good point. Thanks!

I installed home made gasket (1/32” paper) on both sides of the insulator. My logic was that both sides had paper gaskets when I removed the carb for rebuilding, so I reinstalled as found. I made sure I didn’t block any of the ports and it seems to run fine. I’m not saying that it’s correct, but it does work.

After setting the valves if I can get it warmed up enough, I will put a YF on. I want to thank all of you for your input. Doc.

Jon G, Thank you for the great informative information. I'm out of my huge back brace so I'm going to lean over the fender and let you know what I find. I would be willing to bet the carburator and maybe the whole truck that you are right, Ed