I'm new, so this may be something obvious. My T runs pretty well-- I can get up and down hills, cruise at 35 on flats. But the spark plugs and exhaust show it's running very rich. Carbon everywhere. I've tried adjusting the carburetor. I open the adjust screw until it stops gasping and the RPM increases, and there's maybe a quarter turn from there before it starts gasping and the RPM drops. When I operate at the lean end of that range, I sometimes have trouble getting up hills (power drops off). When I turn the adjustment richer I have no trouble with the hills, but I'm coughing out carbon.
Any theories about what is happening? So far I've replaced the choke plate and spring, cleaned out the sediment bowl and replaced the screen.
I'm guessing it's time to service the carburetor. I've read up on "how to" but I don't really have a theory about "why" the carburetor could lead to these symptoms. Can someone help me understand, or suggest somewhere else I should look? I don't mind diving in and trying stuff, but I prefer to start with a working theory. This is my first car with a carburetor, so I have a steep learning curve.
I should have mentioned my carburetor is a Holley NH center drain, 1926.
Arthur, if you haven't checked the mixture needle now is a good time to do so. It often develops a shoulder where it should have a consistent taper down to a point, which can be fixed in a simple jig.
My guess would be the float level is set too low causing the engine to run lean when you are climbing hills. Then when you are idling or using less gas, it runs rich because the float bowl fills all the way up. It could also be that the float needle leaks allowing the level of fuel to rise when idling or running slowly or downhill. It then drops as you pull a hill. One way you can tell if the float valve is leaking is when you park the car with the valve in the fuel sediment open the carburetor drips. Under the above circumstances it is impossible to adjust the needle so it will run smoothly at all speeds and driving conditions.
I agree with Norm that the float level is not correct. Time to take it off, check the needle and the float level. From memory with a metal float it should be 15/64”, just a shade less than 1/4”.
The MTFCA carburetor booklet tells you all you need to know.
OK, finally took the carburetor off and opened it up. Here's what I found:
1) The float was crazy wrong: it was hyperextended, > 1" above the seal (about 45 degrees off horizontal. So much that I'm worried I will break the tab when I bend it back into shape. No clue why or how it got this way.
2) The needle may have also been stuck, but it's hard to say because the float seems to have been pushing it up always anyway. How did any fuel get into the engine??? I've been driving around town with only minor problems, as I described above.
3) The mix needle had a bit of a groove, and also a nick on one side. Not crazy worn, but definitely not smooth.
4) Very little varnish or gunk-- instead there is a light coat of rust on all the steel inside. Is steel wool/ wire brushing recommended for rust removal, or do you recommend something else? I have some of those brass toothbrushes, and guitar strings for passageways.
I will replace the float needle, mix needle, seat, gaskets, and clean the rust, reset the float level. Probably paint it while it's apart. Looks like the carburetor was barely functioning, so I expect I'll see some very different behavior when it's back together. I think it's a testimony to T-era engineering that even with so much wrong the engine could push the car.
Time to get out the torch to finish disassembly. I'll report back when it's back on the manifold. This is fun! Much easier than wrestling with the hogshead!