happysnapper Posted March 13, 2010 Share Posted March 13, 2010 Hi All After months (bought last year as a no-runner) rebuilding my DT250MX (1980) and with the sun finally out, I thought now would be a good time to try and start it up! So fresh petrol battery charged sort of, kicked it over after a few kicks it burst in to life, straight to 8000 rpm and stayed there, flick the throttle and nothing happened. So took carb off checked rubbers for leaks and adjusted float height (16mm) put it back on and same problem, any and all ideas welcome. Cheers sandy Link to comment Share on other sites More sharing options...
wizzer Posted March 13, 2010 Share Posted March 13, 2010 I had the same problem with my Dt125, make sure when you put the carb back together the throttle cable isn't snaged anywhere; like at the tank causeing the bike to rev up Link to comment Share on other sites More sharing options...
Moderator Airhead Posted March 13, 2010 Moderator Share Posted March 13, 2010 Yeah like wizzer said, you should hear a clunk as the carb slide bottoms out when the throttle is closed, if you dont the cable is pulling or badly adjusted. Link to comment Share on other sites More sharing options...
happysnapper Posted March 14, 2010 Author Share Posted March 14, 2010 Just checked that and the slider does bottom out.......with about a finger space at the bottom. Link to comment Share on other sites More sharing options...
Moderator Airhead Posted March 14, 2010 Moderator Share Posted March 14, 2010 Just checked that and the slider does bottom out.......with about a finger space at the bottom. Hmmm i assume the finger space is just the slide cutaway, there should be hardly any space at all at the back of the slide. Are you sure that the throttle stop screw has not been screwed in too far? Link to comment Share on other sites More sharing options...
happysnapper Posted March 14, 2010 Author Share Posted March 14, 2010 Hmmm i assume the finger space is just the slide cutaway, there should be hardly any space at all at the back of the slide. Are you sure that the throttle stop screw has not been screwed in too far? Just my small fingers......have tried adjusting the throttle stop screw when running it has no affect, also just noticed that it is not taking in any oil from the autolube but maybe it wouldn't as the throttle is in the closed position? Link to comment Share on other sites More sharing options...
Moderator Airhead Posted March 14, 2010 Moderator Share Posted March 14, 2010 Just my small fingers......have tried adjusting the throttle stop screw when running it has no affect, also just noticed that it is not taking in any oil from the autolube but maybe it wouldn't as the throttle is in the closed position? Not much oil will be fed with a closed throttle, only when the pulley turns as the throttle opens does it allow the pump to stroke further. As for the engine racing, its an air leak Either from throttle not fully closed, or from perished or split intake manifold Leaks around joints each side of intake manifold. Gaskets missing or broken at cylinder base or reed block to cylinder Crank case seals worn. There is a rough and ready way to test for crank seals Link to comment Share on other sites More sharing options...
happysnapper Posted March 14, 2010 Author Share Posted March 14, 2010 Thanks......was thinking it might be the crank seals as everything else checks out new gasket on read valve & cylinder. What's the rough and read way to check the crank seals? Link to comment Share on other sites More sharing options...
Moderator Airhead Posted March 14, 2010 Moderator Share Posted March 14, 2010 Thanks......was thinking it might be the crank seals as everything else checks out new gasket on read valve & cylinder. What's the rough and read way to check the crank seals? Magneto side, remove cover start engine and spray WD40 through slots in flywheel aiming the jet toward the seal, listen for a change in tone or engine speed. Clutch side, place the transmission case breather pipe in a jar with some white spirit in, Start the engine and look for bubbles or for the fluid being drawn up the pipe. Link to comment Share on other sites More sharing options...
happysnapper Posted March 14, 2010 Author Share Posted March 14, 2010 Great thanks will give it a try, this is now uncharted territory for me is it a big job to change the crank seals? Also with it reving without any oil going in should I put some in with the petrol to be on the safe side. Thanks for all your help, most appreciated. Link to comment Share on other sites More sharing options...
Moderator Airhead Posted March 14, 2010 Moderator Share Posted March 14, 2010 Great thanks will give it a try, this is now uncharted territory for me is it a big job to change the crank seals? Also with it reving without any oil going in should I put some in with the petrol to be on the safe side. Thanks for all your help, most appreciated. 2 stroke oil pups work in two ways 1: Throttle position (engine load), with no load or a closed throttle the pumps stroke is minimal, with high load or open throttle the pump stroke is maximum. Between these two is proportional. 2: Engine speed, with low speed ie idle the pumping frequency is low. With high speed the pumping frequency is high. In your case you have a closed throttle but high speed engine, this means small pump stroke but frequent pumping frequency, all is well. However I dont know how familiar with your bike you are, did you bleed the pump? do you know the pump works? You can force the issue if worried by removing the pump cover and turning the pulley to max by hand for a minute while the engine is running, as I said this will allow the pump to stroke fully. Link to comment Share on other sites More sharing options...
Recommended Posts