Tillotson carb on mcculloch 650

My first time to post. Not a Karter, well was back in the 60s, but using a McCulloch 650 on a home brew motor bike. I installed a Tillotson hl232a on the engine as the original flat back primer pump isn’t any good. My problem is that it will not run in the mid range throttle setting.

I have two hl’s, one that I have had since the 60’s and a cheap china made that I bought to see if my problem was just my old carb. Both react exactly the same. Both will tune to a great idle and both will tune to run at on or near full throttle, but both fall on there face in the midrange. No matter how I try to set the mixture, it will not run in the midrange.

I have read the setting the pop off of the carb article, but the article does not indicate what the effect is of a wrong setting. I would go to a slide type carb if I had the space, but I designed the set up with the small Tillotson in in mind, IE space is limited. Any help would be most appreciated. I have lots of experience with float type carbs.

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Welcome @jpob. First question I have for you is… has the engine been checked for vacuum leaks?

Popoff wise…
Too low of a setting means that the inlet needle (Like the needle controlled by the floats in a traditional carb) gets lifted off the seat more easily (at a lower signal/vacuum). So you will draw more fuel with a lower popoff.

Too high of a setting will cause it to be lean.

Generally 8psi is a safe starting point, but on the 232a I’m not certain.

@ScottElkes, @Racing_Psychosis can you guys offer any pointers on this motor/carb combo?

Regarding vacuum leaks, all new gaskets, so I think I am ok on this point. My experience, limited as it is, is with auto and motorcycle carbs and find that vacuum leaks affect the idle mostly. As I said, I’m not an expert, this is just what I have experienced. I’m running the carb off of the bottom of the crank case with the original location blanked off. We did this all the time in the 60’s with McCulloch and West Bend kart engines, so I don’t see a big problem there. Also, I am running the hl232a with the diaphragm pack up. The 232a has a 1" bore vs 13/16" bore of the original flat back carb. Also, this carb is a replacement part for some Stihl chain saws. The McCulloch 650 engine is stock and is 87 cc’s. Reflecting on James’s reply, I am thinking that my pop off pressure must be way to high, which, I think, would affect my mid range. I guess my next step is to make myself a pop off tester. Thanks again for your help, JP

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