66 Monza 250 carb/idle issues

[TedsDucs]

I have a 1966 Monza that has the Dellorto UBF24 carb. In my ownership, it has never run great. Searched through posts and FAQs and found lots of info about air leaks. My rubber mounting gasket is aged, but OK. I did find some warping of carb mounts, and have been able to machine them flat without loosing too much metal. The body seems OK, and the slide moves fairly smoothly. The carb has been cleaned, and I'm pretty sure I have any crap out of it.

It is doing something really weird now. I can get it to idle after a short warmup, and accelerate normally. When I lift if to vertical from the side stand, it starts to race, and if I let it back down it comes back to idle. My first guess was the float. I pulled it all apart and checked and the float seems fine - not leaking, float needle seated, etc. There is no issue with gas flow to carb - both petcocks are new, with plenty of gas both side of tank. What else could cause something like that? I also suspected that I was pinching or pulling a cable, but this happens without any movement of bars. Could it be electrical? Most of the wiring in this bike has been replaced. Very weird. I still suspect the float somehow, but can't figure out how or why?

Any suggestions?

TIA, Ted

[Jordan]

That behaviour is normal with any carburettor that has its float chamber to one side of the jets.

[Nick]

Modern bikes with their modern carbs have given many riders of old bikes unrealistic expectations.

MY MkIII has the stock carb and the Sebring has a Mikuni. They both work well, but I'd never expect a 50-year-old carb with remote float to perform like a modern unit.

[amartina75]

There is a lot of info on here about carb swaps, some favor using a Mikuni. I have a VHB27AD carb on my 250 and it runs well. It seems to me most of the people on here complaining about carbs have a UBF24. The VHB swaps right on no modifications needed. I've found them cheap on EBay they came on some Aermacchi models.

[motoital]

I have had issues in the past with air leaks coming from the rubber insulator so now I use a gasket shellac on both sides and haven't had any issues since. The nice thing about the product is that it's gasoline resistant and comes apart easily if needed.

[TedsDucs]

That behaviour is normal with any carburettor that has its float chamber to one side of the jets.

Thanks, Jordan...I just didn't think it would be that pronounced. Played around some more with the idle and air/fuel screws and got it to run pretty well straight and level. Not sure how long I will keep this bike, as I barely fit on it, but may check out Mikunis if I do...

Thanks, guys...Ted