DT250 compression

[zillaguy89]

OK most of you probably remember my problem.runs fine under 1/2 throttle, over 1/2 it falls on its face. I've done everything possible to the carb short of replacing it altogether, checked float level, jetted up jetted down, changed needle positions, tore it apart and boiled it, reset it back to the way it originally was and it runs best there. OK I got off my ass tonight and checked the compression. OK, I have never checked the compression on a bike before, just cars, so I didn't know if you read after 1 kick or after the needle stops going up after 4 or 5 kicks. after 1 kick it reads 65 PSI, but if you kick it 4 or 5 times it goes up to 110-120 PSI. so now I'm starting to think that my top end isn't the problem. the engine was also stone cold cause its about out of gas and I have no premix oil, haha.

any thoughts? besides get rid of it?

[Bandit]

hmmm, This maybe WAYYY off but on my old 250 bigred 3 wheeler a long time ago did this... and it was the pet cock, it was stoped up so that it could feed enough fuel to run upto alittle over 1/2 throttle and anything further it would cut out etc., pull it out and check it, like i said, may not be the reason but its worth checking

[zillaguy89]

You may be on to something there...when I first got it i couldn't get any gas thru the valve at all, so I took it off the tank, took it apart and cleaned it, and I tried to clean the tank out as well, but maybe its not as clean as I thought it was. I'll check on availiability of a petcock for it and just replace it if I can.

it was plugged so badly I had to clamp the main body of it in a vise and drill all the old nasty crud out if it. it was horrid, i'd never seen anything like it. they guy said it sat for close to 10 years though

[370banshee]

well it's a 30 year old dirtbike it probably needs a top end anyway

[Kane]

There are a couple things that can cause that. The obvious are float level being too low, main metering jet tube having the little holes plugged, and wrong needle or needle setting. Check your choke performance. Does it run better choked even if hot? Change the mag-side seal and check for intake leaks. Does the choke hurt performance when hot? Its in the carb and air intake system. And last but certainly not least, if you're sportin' points, clean them. They could have old dried lube on the pivot shaft preventing the action at higher rpm's. Your compression is reasonable. You kick it till the needle stops moving. Check it 2-3 times, then add a teaspoon of 2-stroke oil in the plug hole, wait a couple secs then test again. If the difference is more than 10-15%, change the rings.

[nickxc]

you are supposed to kick it till its stops going up. and from the sounds of it I think you have a good top end...

[zillaguy89]

Its got brand new points, I changed them while I was re-wiring the magneto system cause some idiot chopped the coils out of it. If I remember correctly running it with the choke on didn't have any effect on it

[Kane]

If running with the choke on didnt stall it out your problem still lies within the carb area. From point A to end.........Reeds, could be failing to open completely due to far too many things to list, just check em again, maybe flip them over. Intake seal, gasket leak or tear in the boot itself or improper fit at the carb body. Carb, if you have the book check your needle specs (online sites can help here as well) clean the air jet from the airbox side of the carb opening(will WD-40 flow through it?) is the main jet the CORRECT size regardless of how many others you've tried? Airbox, check the airbox to carb tube...any holes? the tube to box seal...ripped or leaking? airfilter...is it a stock replacement? does it make a good seal to the mating surface? are there any holes in it? is it lubricated? has the box itself been modified with perhaps too many added holes? is the original airbox cover still attached?

[KamakazieX]

...I was already there on mine..


..REEDS!..reeds...reeds...reeds.... and check the intake boot (carb to reed cage) for dry rotting.

Ya, and make sure you get the boyesen reeds. At 40 bucks, u gain midrange torque and give it the power to idle all day.. it was worth it to me!

Rule of thumb, change them once a year or two.

That will get you closer. Now the just as important thing, check the timing i think its 3.2mm BTDC (i had to do it over so many times i memorized it..) Get a plug feeler and set it right. The points at 3.2mm BTDC should just begin to open. Use a multimeter and check resistance frame to that. It will run like a yz after that. (or sound like one at the least)