When bike starts, the revs go instantly to 5000rpm

[Sami Laakso]

So, i got fuel starvation issues fixed on the XS650 but now the problem is this: immediately I start her the revs shoot up skyhigh! Doesn't matter is the choke on or not, immediately when she starts the revs go up and stay there until I cut the power. What the hell? Air leak? Throttle cable looked ok, both slides hit nicely on the bottom and the idle screw. The revs go so high that I can't keep the bike running, I have to cut it immediately.

Carbs are Mikuni VM34 racing specials, pods are off.

 

[Skubik]

Mine did the same thing here and there. It would sky rocket to 4K upon starting. I would check for air leaks but had none. I was having fuel starvation issues also that I was getting fixed. Can't tell you what is causing it exactly but what I would do is immediately after starting the bike I would lower the rpm with the idle screw and then adjust the idle using the dead cylinder method. Instructions for this are different depending on the year of the bike I think or wether or not your carb manifolds have the barbs on them. Make sure your carbs are synced. I took mine out and did a bench sync as a good base. Sometimes your air leak is on the carb shafts themselves. I noticed a few times before I took mine apart that if I pushed on the screw located on the throttlebody shafts the high rev would either get better or worse.

But yeah could be air leaks also (with engine running spray carb cleaner on manifolds to check that)
Have you checked your fuel float adjustment? Should be at .22mm or .27mm depending on year of bike and float type.
Just throwing some stuff at ya I'm new also but these guys have answered these same questions for me in the past. Do a search for each of these things, there's a good bit of information already out there. I'm sure someone way more helpful will chime in. Good luck to you

 

[5twins]

With the VM carbs, idle speed is controlled by raising or lowering the slide slightly with the idle speed adjusting screw. There is an angled notch cut into the side of the slide for the screw to act against. Some things to check - make sure the slides are installed in the correct carb if your set has a right and left handed idle screw arrangement. The angled notch in the slide needs to be on the same side as the adjuster screw. Make sure the slides aren't installed backwards. The angled cutaway on the bottom of the slide should face the rear. If they're in backwards, they won't drop all the way down. Make sure you have play in the throttle cables. If they're too tight, they'll hold the slides open.

 

[peanut]

One thing to try.
When you start it up.................. try pressing the throttles down hard on their stops with your thumbs to see if it has any effect on the revs. check that you haven't set the slide stops too high by lowering the idle adjust screws if it makes no difference then as 5T says you need to look at your slides and air leaks .
I would inspect all around the carbs and make sure there are no holes open thatt should be blocked or anything missing . Do your inlet rubbers have vacuum barbs ? if so do they have their rubber seals fitted to block them off ?

 

[Sami Laakso]

Ahhh problem solved It was a throttle cable getting too tight because of the new throttle assembly. I loosened the cable, slides dropped and off I went for a test ride. Life is good again!

Thanks for the tips guys!

 

[Skubik]

Woohoo!
Man I can't believe I didn't mention that. Same thing was happening to me also when I turned my handle bars in one direction it would rev up due to a tight cable. Well glad you found the issue