Honda CX 500 Forum banner

VM34 tuning help

1573 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.
1 - 4 of 18 Posts
Why don't you test the spark strength?

What do the plugs look like after running? Black and sooty?

It does sound like it could be a low speed coil issue.

Your jetting sounds normal enough based on my limited experience with these.
Scratch that on reading the plug. If you've been running on 'choke' {not actually a choke} they'll be sooty regardless.

Perhaps the plugs themselves have failed and I would at least try another set though.

These may help with the miks.

Not being a smartarse but you do know the mik mixture screw works backwards? In for rich, out to lean as it works on an air circuit not fuel.?
See less See more
The raesan hall replaces the whole lot. Given that you already have coilovers none of the original ignition system will remain in use. The hall replaces the pickups, the ignition head unit takes over the advance functions.

It was previously running OK with those coilovers?
If you have the factory coils it would be 20 minutes work to refit them ... just to see ... and remove them again if the coils aren't the issue. You don't even need to mount them, just ground them to the bike with jumper leads.

I don't expect they will be but if it were me I'd do it just to strike it off of the list of possible issues. I've had a couple of coil failures through incompatibility with regards to resistance.
1 - 4 of 18 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.