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VM34 tuning help

1571 Views 17 Replies 5 Participants Last post by  makenzie71
Alright I need some VM34 help. I've had this set of VM34's in my shop for ages that had originally been set up for an EX500 race bike. After screwing around with jetting I got a mix that worked really well with the CX500 until a couple weeks ago. Started up and revved through the whole range without any problem...and then all of a sudden she's like NOPE! The bike currently starts right up no problem with the choke on, but the very instant any throttle is given it stalls. I can cup my hand over the intake to restrict flow and get it to rev up but it's certainly underpowered. If I take the choke off it stalls. If I mess around with it and get RPM up above 5k where it's drawing out of the main it'll pull the rest of the way pretty hard and smooth but once it falls back down it's a struggle to save it before it stalls.

Ideas? My first thought was the most simple and obvious...fuel starvation...but the jets are pristine. I've had them both apart multiple times and can't find any issues. I even tried swapping from the 145 mains to a NEW set of 150's thinking maybe the colder air lately was just making it ask for more fuel and there was no change.
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If it was working with those carbs and then suddenly wasn't changing the jets is probably a waste of time unless one of the ones you removed was blocked.

If it isn't making funny mechanical sounds and it has good spark I'd check the simple things like valve clearances & maybe compression but only to procrastinate about taking the carbs out & apart to find out what is blocked.
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