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VM34 tuning help

1574 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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Air leak at the head? Another thread on a bike with similar behavior is leaning that way.
This is just an idea and I hate commenting on this as I know little about your carbs, but running with choke on, or hand choking it (blocking the intake) to get it to run right, to me indicates the same kind of vacuum/compression loss. In chainsaw stuff I'm more familiar with, it'd be an indicator that there's a compression issue, like rings are bad leading to no vacuum through the carb and you're opening that butterfly up and there's now less vacuum being created through the carb and it dies. Your bikes behavior is identical to my last weedeater that I had to rebuild with a new piston and ring set. Compression or leakdown test maybe if it's not terribly inconvenient?

A sudden change means something suddenly changed; I don't imagine this was a simple jet size problem. Although I could see trash in the carb doing it too.
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