My bro in law brought over his 98 sp 500 the other day.Said it wasnt running right.Sounded like trash in carb.Cleaned everything(pilot was blocked) and its still running like crap.It starts and idles beautiful but about 1/4 to 1/3 throttle it becomes sluggish and doesnt want to rev.If you hold it 1/3 throttle to wide open it acts as if its starving for fuel.Almost as if the fuel bowl isnt filling fast enuff.I havent checked fuel pump pressure(dont know how) but fuel is flowing from the tank to carb.Ive adjusted float both high and low and still same thing.It is blowing a gasoline mist back into the air filter.Occassionally Ill get a 5 inch ball of fire out the carb.Ive played with this thing and I cant figure it out.Ive also checked the ETC and it seems to be fine.Everything on his 500 is factory with 2600 miles.When he brought it in you NEVER,NEVER had to use choke no matter what the temp.Now you have to use the choke to get it started after cleaning.Installed new fuel filter,air filter and plug.No difference.Thought maybe stator might be getting weak but was replaced a year ago along with the cdi(not sure if this is what its called) Im lost and need help.Thanks.Scott Sanger.

 

Can you do a compression test? May have a vavle sticking, but it does sound more like it is running lean. Check and see if there is a blockage on the fuel lines and no trash in the tank. I will have to check in the repair book on checking fuel psi, but check the pulse hose that makes the fuel pump work that its not busted or blocked.

 

There is an adjustment for the throttle cable inside the housing on the handle bar. if this adjustment comes loose (it has happened to me) it will cause backfire on deacceleration. Just a thought.

jclaude

 

If its spitting fuel back out the carb, into the filter...the intake valve is being held open. Check the clearance.

 

My buddies 400 is doing the samething, did it slowy start to run like crap ,we found the cam shaft was wore out . it sounds like you have the samething going on .

 

Just wanted to let everybody know the scoop.Bad camshaft.Seems the 98 and 99 use the exact same cam and they were real bad about going out.My bro in law is ordering all the parts needed to fix.Ill let you know how it turns out.Thanks for the comments.Scott Sanger.