91 kdx 250 tach's out upon starting

[brandonvick83]

rod bearing was shot when i bought the bike, rebuilt the motor seems to run fine except it takes 4 to 5 kicks to start when cold and tachs out for 3 to 5 seconds before the idle starts coming down....once warm starts first kick at normal idle...what could be causing this?

[rbates9]

Sound like an air leak. Check the carb boot, reed block, crank seals, cylinder base gasket.

[brandonvick83]

so i started the bike and sprayed starting fluid around the base gasket and all around the carb no change in idle. how do you check the crank seals? also my cousin was visiting a few days ago and wanted to ride the bike around the backyard he was wearing sandals...needless to say he tripped while pushing the bike, bike fell on its side not hard and very briefly he jumped on it pulled the choke and the bike started first kick at normal idle...started it this morning still taching out...any other ideas?

[rbates9]

You van remove the flywheel cover and see if there is fuel in there. That will say that the seal is leaking, but it could still be leaking air in and not fuel out so you could pull the flywheel and get a good look at the seal and see if there is anything that doesn't look right. For the other side you can check the transmission oil level and see if you are using any.

[MikNY]

Mine does this as well, tried the same things with the starting fluid and found no fuel under flywheel cover or loss of trans fluid. Is it possible it sucks air in through where the exhaust mounts into the head then quickly seals as it heats up?? I notice a fair bit of exhaust leak by the header pipe for the first few seconds after cold startup. I pulled the pipe and the o-rings seemed soft and plyable.
Mike

Edit: just did some google-ing, seems this is very common to KX/KDX and many other brands/models with no clear consensus as to why(many possible causes). Lots of tricks to avoid the high rpm cold start, will have to try some of them.