The Stovebolt Site Forums The Shop Area Engine Shop Carb problems

1952 GMC 228 six cylinder. HEI ignition, a PCV system and Carter 964S carburetor, otherwise stock. The Carter carb was on the truck when I bought it 13 years ago. I rebuilt it once about 5 years ago, using a kit from Mike's Carburetor Parts (sorry Jon) to correct a seeping problem and played around with jetting sizes using an AFR sensor, but ended up back at the 0.099" jet that was in the carb when I got it.

Lately I've only been doing short trips and hadn't been keeping track of gas mileage, which is normally in the 13.5 to 14.5 MPG range. A few nights ago I loaded the trash can in the back for the 600' trip to the road. I left the engine running with the choke off, set the parking brake, turned on the bed/backup lights and got out to unload the garbage can.

There was no wind blowing. The smell of raw gas was coming from the exhaust pipe, really thick. The exhaust fumes were very visible in the beam of the lights. The idle was rough, hunting up and down by 100 RPM or so.

I filled the gas tank drove it to town and back twice over two days. Gas mileage was about 8 MPG.

I looked down the carb throat while the engine was idling. No "liquid" visible.

I adjusted the idle mixture screw all the way closed and it made no difference in idle quality, still real bad. So something is leaking somewhere, probably below the throttle plate.

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So for right now, I have removed the Carter carb and replaced it with a Zenith 28. My 228 would have left the factory with a Zenith 28, but the GMC Master Parts Book does not show this particular carb, which has a tag numbered 10163C.


Jon (CarbKing): Are you interested in looking at the Carter?

Accelerator pump is probably leaking.

I'd also check your oil to make sure the fuel pump isn't leaking fuel into the crankcase.

Look down the throat with the throttle open when its turned off or after it's stat for a while and see if there's a pool of gas on the intake runner.

If the diaphragm is cracked or split even a little bit, raw gas will be sucked into the chamber below the diaphragm (it is constantly under vacuum) and from there right into your manifold. Another problem (which in your case I am less willing to suspect) is a warped bonnet...the one that holds the diaphragm assembly down. If this is overtightened, the corners will pull down warping the surface of the bonnet. I've seen these with between .006" to .010" of warpage and that will let fuel be sucked past and into the diaphragm chamber...where it goes into the manifold. I showed how to make a simple tool to fix this with a flat piece of wood and some sandpaper a few years ago. Jon has some diaphragms which are supposed to be resistant to ethanol laced fuel. They're orange. I've had good luck with one.

Another vote for a probable cracked diaphragm.

So far, the supposedly ethanol-resistant orange diaphragms appear to actually be ethanol-resistant. What a nice change when something actually works as advertised !

Bill - thanks, but no longer have the necessary time to rebuild carbs.

Jon

Jon,

I've had one of your "regular" black diaphragms installed in my Carter carb for the last 3 1/2 years and have put over 10,000 miles on it.

The truck has operated exclusively on 87 octane, E-10 ethanol in that time and I have absolutely no problems with it.

I just remedied this issue with my Carter YF788 on a 235 engine in ’53 truck. All summer, its been running way too rich, lots of black smoke on startup and idling, flooding, hard to start after driving, idle fluctuating, large sooty deposits out the exhaust, occasional hiccup, terrible mileage, etc.

After disassembly, I found the needle valve assembly did not leak, the float was ok, and the accelerator pump did not have cracks or holes. After comparing the new kit parts to the old, the big difference I found was the pliability of the rubber of the accelerator pump. Obviously the years and occasional ethanol use had hardened the rubber enough to cause it to malfunction. I meticulously cleaned every part and orifice of the carb, installed the new kit, re-set the float to spec, and changed the fuel filter. The truck fired right up, no more smoke, idled perfectly, runs and performs better than it ever has.

BTW, Mike’s Carburetor on YouTube has an excellent series on Carter YF carburetors, troubleshooting, disassembly, adjustment, repairs, etc.

I'd suspect that the lack of pliability of your rubber diaphragm is due to the fuel evaporating from the bowl after the truck sits too long. I think if the rubber stays wet, it's less likely to dry out and become stiff or cracked.

I drive my truck hundreds of miles each week/month which might be why I have had no problems with the diaphragm failing.

I'd suggest driving your car every few days or at least turning the engine over to keep the fuel bowl full and the accelerator pump submerged in fuel.

It has just been the last couple of months that the truck didn't get driven much. And even then it made at least one 25 mile round trip to town each week. I've never had problems with a "dry carb" after sitting for a week. It always starts within 3 seconds or so.

I just ordered a kit and added a new float just because it was cheap and wouldn't bump the cost of shipping.

Took the truck out for an "around the block" (that is about 11.5 miles here in the boondocks) test today and the Zenith 28 seems to be working well.

I watched the segment on the diaphragm on Mike's website. I'll take in the others before doing the rebuild.

Sorry Bill, I was responding to RLT wherein he described the dried out rubber diaphragm. So many people with carb problems I can't keep up with who's carb is doing what. Hahaha.

Just as a data point, I had rich condition stuff, mostly on hot restart. I replaced the accelerator pump in my YF based on feedback I got here. It cured the condition. There was no visible tear in the old diaphragm, nor did it “feel” notably rough. I also checked the pump “bonnet” as per Jon G’s procedure while doing this jump. It was not warped.

All I can figure is that the old pump had a pinhole that I couldn’t see.

If the new float is new old stock, and 50 years old, then great If the float is brand new, made in China, be aware that what is used there for solder is incompatible with ethanol. Your original float, if is doesn't leak, is more reliable than one of the new ones.

Jon

JW, your accelerator pump, while not being cracked or visibly pin-holed, may have swelled up / stretched from exposure to ethanol.

When ethanol was introduced to the fuel supply in Atlanta in 2007, it killed the original fuel pump in my Cadillac within 2 months. It stretched the diaphragm enough to where it would only pump half the required amount of fuel. No pin holes or tearing at all.

Since then, no problems in any of the carbs or fuel pumps made with the ethanol resistant rubber.

That’s a good theory

I've noticed if your diaphragm is leaking for any of the two or three reasons it may be happening, you will see the gasket shown below get wet while your engine is running and it will remain wet when you stop running for a while. This seems to me to be a very clear barometer. Also there is shown with a red arrow one of the lower screws. You may also notice this is wet with fuel and in one case I noticed one where fuel was actually dripping off the bottom of it. If anyone wants me to resurrect the instructions on making the "bonnet" flat and to see some of what I found there, please let me know.