At least three insidious things that can go wrong with the N.H. Carburetor.
By Tom Carnegie
The N.H. carburetor is the carb of choice for most model T people. It is a very simple, yet effective unit. In its swayback form, it is the only one allowed on the Montana 500. When you have a good tight throttle shaft and have properly adjusted the float level, you will likely have a carb that is smooth over the entire R.P.M. range of the model T, once the mixture has been set right. This is assuming that all the internal passages are flowing freely. These include the two idle ports, the air passage (it exits under the name tag) and the bowl vent. Lately N.H. carbs have been getting harder to find at swap meets. For this reason I suppose, the N.H. carb is now being reproduced. Unfortunately, not all of these reproductions will pass the Montana 500 carburetor inspections. Some of them in their unaltered state have too large of bore. This appears to be the result of a core shift at the time of casting. You can make a simple gauge to check your carb. (see ill 1.)
Just round the ends of a piece of 3/8" or so round stock until it is .710" long, then attach a wire for a handle. For all of its simplicity, there are a few things that can go wrong with these carbs and if you’ve never encountered these problems, it could be quite frustrating to track them down. All three of these problems involve the high speed adjusting needle. As the needle is adjusted it lets more or less gas past the spray nozzle. The spray nozzle is supposed to be screwed solidly into the carb body. It is located right above the valve that holds the float bowl on. It is screwed in from the bottom, and has a gasket between it and the carb. If this nozzle is allowed to get loose, the adjustment can spontaneously change whilst driving. Besides making sure this is tight, a drop of Loctite or similar product is not a bad idea. Problem number two is caused by worn threads on the needle. I have seen the situation where you think you are adjusting the needle, but it is really just going round and round. Of course, simply replace the needle and the part that it threads into to fix this. The last problem is perhaps the most insidious of all.
The steel part of the needle is swedged into its holder. It is possible for this part to become loose. The T will run, but it will be impossible to hold a fine adjustment on the mixture. Even if your carb seems to be working well, these are things you might consider checking.