85 250R HIGH Idle Troubleshooting Help Needed

[FW1]

Hey All,
Purchased a beautiful 85 250R over the weekend. Added fuel (40:1) and fired her up to unfortunately experience the motor screaming full throttle. I turned the kill switch from run to off and luckily the motor turned off. Fired it up a second time and the motor started to idle normal for about 20 seconds then again went into a high rpm scream, tried to turn it off again but required several swipes of the off/run switch before it killed the spark. I read that this issue comes from "dieseling" in which air leaks from the motor or reed valve gaskets and passes into the cylinder. I've also heard this issue is caused from a bad CDI, Stator, or ignition wiring. I have removed the airbox to inspect for hair line cracks in addition to making sure the carb slide was not stuck open and both were fine. Based on the second startup where the idle was normal for the first 20 seconds what would any of you recommend I troubleshoot first (air leaks or electrical)? I appreciate any support!

[gimmeamidget]

My guess is the needle in the carb is sticking closed and not allowing enough fuel into the bowl. It will idle fine till its low on fuel and as it runs lean it revs up like stated. I would clean the carb. If it has sat for even a few months without being ran the needle can stick and cause problems.

[shortline10]

Start at the petcock then carburetor and intake boot , if it’s still doing death revs after that then you either have a crank cavity leak or bad crank seals .
Those symptoms are not electrical related .

[Tazz]

Could be the float sticking in the carb as well

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[yaegerb]

Like shortline10 said, it’s likely an air leak. Start it up and spray carb cleaner around intake, airbox and see if it hesitates. If that doesn’t work you will need to pressure test the crank case. If you lose any pressure within 6 minutes pump back to 6psi and start spraying gaskets and crank seals while looking for bubbles. It sounds like a stator seal is going to me.

[BOB MARLIN]

Make sure the carb slide isn't in backwards also.

[FW1]

Thank you all for the insight. I finally got time to spend a few hours on the carb which was in immaculate condition along with the intake boot, still no luck. At the time I only had 1/3 gallon of fuel in the tank and decided to add another full gallon to ensure fuel would not be an issue. Fired it up and it quickly went into a calm idle. Several restarts with no death revs at this point! Based on Shortline10's previous post I assume I have a petcock malfunction?? The last thing I want to experience is a full throttle get away when I'm riding low on gas. Do I need a new petcock? How would more gas in the tank prevent the "dieseling" I experienced prior. Thoughts??

[yaegerb]

Adding extra fuel to your tank wasn’t the issue. Better keep investigating. You can purchase a new petcock for that machine. Have you checked for air leaks or pressure tested the motor?

[FW1]

Would a faulty petcock combined with a low amount of gas in the tank cause the high rev scenario? Since the tank has been full I have had no issues. Thanks!

[shortline10]

Short answer is yes , a plugged up petcock will cause death revs . Also make sure the gas cap is venting properly .

Would a faulty petcock combined with a low amount of gas in the tank cause the high rev scenario? Since the tank has been full I have had no issues. Thanks!

[FW1]

Update, before pulling the petcock I decided to fire it up. With the petcock valve in the "on" position it started right up and idled fine. Then while idling I turned it over to the "off" position. Idled normal for about 15 seconds and then went into full rev mode. Weird!! I guess I will remove petcock to further inspect.

[gimmeamidget]

That is what it should do when you turn the fuel off. The engine isnt getting fuel and so it revs high due to being lean from no fuel.

[FW1]

I was able to replaced the petcock with the same factory OEM. In the process found some flakey debris at the bottom of the tank. Flushed and cleaned several times and visually inspected before reassembling all components. Also was able to blow air from the bottom of the gas cap out through the vent tub which tells me the vent tube it good. Added fresh fuel, fired it up, and it ran well until I turned the petcock to the off position (once off after 30 seconds of idling it again goes into full rev mode for 45 seconds until running out of fuel). This can't be a normal shutdown. I guess my next area of elimination will be the carb. Should I start by spraying carb cleaner when idling in the general carb area to see if it hesitates? Or does the carb cleaner spray test only apply to the air box and boot? Thanks for the support!

[yaegerb]

I was able to replaced the petcock with the same factory OEM. In the process found some flakey debris at the bottom of the tank. Flushed and cleaned several times and visually inspected before reassembling all components. Also was able to blow air from the bottom of the gas cap out through the vent tub which tells me the vent tube it good. Added fresh fuel, fired it up, and it ran well until I turned the petcock to the off position (once off after 30 seconds of idling it again goes into full rev mode for 45 seconds until running out of fuel). This can't be a normal shutdown. I guess my next area of elimination will be the carb. Should I start by spraying carb cleaner when idling in the general carb area to see if it hesitates? Or does the carb cleaner spray test only apply to the air box and boot? Thanks for the support!

Wait, if I get this right, you are turning OFF the petcock and after a bit of time it is revving to the moon? If that’s the case then all is well. By turning the petcock off no more gas is entering the carb and you are literally dieseling the motor by running on restricted fuel and a ton of air. Don’t do that. Quick way to a scorched sleeve.

[350for350]

The problem could have been the flaky debris in the tank the whole time. It could have been plugging the petcock and starving the carb for gas. As for a possible full throttle rev while riding, if it ever happens, don't try to ride it out, just pull in the clutch.