`81 KDX 250 stalls at full throttle

[NV Rider]

I just purchased a KDX 1981 250 and took it out to the local trails for the first time yesterday. It starts up fine, although a little start gas/ether is necessary, and runs fine as long as I stay in low RPMs or slowly increase the RPMs. As soon as I try to "floor it", it stalls. The idle is also all over the place, meaning up and down RPM wise.
Are there any obvious solutions to this? Or, maybe a few things I need to look into? FYI- I have a brand new air filter installed.

Maybe it`s just the universe`s way of telling me I need a little more time on the bike before I go faster? ;-) I`m in my early 50`s and literally have not been on a dirt bike since this bike was brand new!

[KDXGarage]

"It starts up fine, although a little start gas/ether is necessary"

No. It doesn't.

I am not seeing anything about carb cleaning. What have you done or not done??

[NV Rider]

You`re absolutely right, Jason. It needs help to start so I suppose that doesn`t qualify as "starts up fine"... he he...

I have opened the carburetor to make sure the parts move, but I have not cleaned anything. Wouldn`t know how really. Any pointers?

[KDXGarage]

Some of the vents and jets may have gummed up old gasoline clogging them. Since you have a new filter, keep working forward, just one area at a time.

[NV Rider]

Thanks, man! I`ll let you know how it goes.

[KDXGarage]

After the carb is clean and float level set correctly, etc., check reeds.

Do check the plug as well. Wiring will be checked at some point too.

IT'S NOT THE WATER PUMP OR THE POWER VALVES! I can at least guarantee that.

[mtbfree]

IT'S NOT THE WATER PUMP OR THE POWER VALVES! I can at least guarantee that.

Hahaha... that's for sure!

Hopefully a good carb cleaning will take care of it.

[Tyl3r]

Carb cleaning for sure. May want to check floats to be sure they are adjusted properly. Maybe it is getting too much gas, causing it to stall when you get on it. Or maybe a jet is clogged, and it can't get enough gas when you open the throttle? As mentioned above, just need to start with one area and move forward once you can rule it out as the problem.

Also, I don't know about the 250's, do they have the jet block O-ring in the carb? Didn't know if this carb could be dipped? Figured that may help the OP if he has never done a carb cleaning (but hey, that's something everyone should know how to do! lol).

Good luck! You'll get it figured out, no worries!

[JL4049]

The symptoms you describe are very similar to what I experienced with a main jet, partial blockage. As others have said, probably time to strip the carb and clean it out. It you search this site you will find a number of threads describing how to do this, or try YouTube for PWK carb clean videos. One tip though, do not soak in carb cleaner as it will destroy the jet block, o-ring.....

[KDXGarage]

I doubt a PWK video will help him with his 1981 Mikuni carburetor.

Someone correct me if I am wrong, but I don't think there is a jet bloack gasket on the Mikuni equipped KDX's.

[NV Rider]

I finally got around to dipping and cleaning the carb. As mentioned, no O-rings inside the `81 Mikuni. The engine is now running much better at full throttle. The idle is a little uneven so I`ll check the reeds next.

[KDXGarage]

Awesome. Someone did one repair, then saw how it ran. You are balancing out others who do nine things, then wonder what fixed it or made it worse.

Thanks for reporting back on the dipping results. At least you now know that whatever is wrong, it is not because of a dirty carb.

[JL4049]

Cheers for the update NV, didn't realise the old 250s use a milk uni carb. Lucky you being able to throw the carb in a bucket of carbosol lol. After I had this issue with a blocked jet, I checked the petcock strainers in the tank and found them to be missing! Fitted an in line filter and have not had the same trouble again in 40hrs ride time.