76 250 MC5 jetting

[Doofus]

Can anyone let me know their successful jetting spec for a 1976 250 MC5.

I'm having really bad over fuelling problems in the mid range. I'm going to be experimenting with a new needle jet at 2.74 down from 2.85 and a brand new needle. Just curious to know what other owners have settled on. The bike is not run at altitude. Thanks in advance.

[Larry Perkins]

That is a big drop in needle jet and pretty lean for a 250.  If your old needle jet has been in the carb awhile it is probably ovalized and running rich because of that.  Most of the time when I see a Bing blubbering or loading in the middle it is because of two things.  A worn needle jet and a worn slide.  If the slide has worn a smiley face into it then you should also replace it.  Most 250 Pentons I have had ran a 280 or 283 Needle jet.

Larry P

[Mike Korgan]

I have a 77 250. My notes show 180 main, 2H1 needle and a 316 needle jet. That's for sea level and I've been as high as 4 to 5 thousand feet without any issues. Call Charlie at Bing, he can tell you the stock settings. He will want the carb number.

Mike

[Larry Perkins]

Before you start chasing jet size to make it run right, remember it ran right once with what it has in it.  So something changed.

Larry P

[Mike Korgan]

I agree. Those were the stock sizes that were in the bike, with that carb number, when it was new, according to Bing.

Mike

[Doofus]

Thanks All. I didn't know the stock needle jet size for my baseline so I'll follow advice and try a new 285. I have to say the one I removed looks OK and not ovaled, but almost impossible to detect I guess using 56 year old eyes!. Slide looks OK. I'll let you know how I get on.