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| MT250 strange throttle problem https://vintagedirtbikeforums.alp-sys.com/viewtopic.php?f=21&t=684 |
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| Author: | newfie [ Sun Jun 12, 2005 10:34 am ] |
| Post subject: | MT250 strange throttle problem |
Hello again, i sparked up the 74 MT250. I think she is running ok, but I have a strange problem with the throttle. First, I took off the carb opened the bowl and let it soak overnight in cleaner. It did have a fair amount of varnish inside. This seemed to clear everything up. I hoped for the best and put it back on the bike. When I fired her up she revs up real high until i lean the bike a little (10degrees) to the right then the revs drop off to idle. straighted it back up and it revs right back up to the redline. I have (what i think is) the idle screw on the left of the carb tightened all the way in and the mixture screw on the right of the carb tightened all the way in. I should add that when I open the throttle while leaning to the right she bogging out. I should also add that I have the airbox disconnected for now so the carb is open on the rear end. Any ideas? Thanks Newfie |
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| Author: | ricortes [ Sun Jun 12, 2005 12:13 pm ] |
| Post subject: | |
Got to be a float problem if leaning the bike makes a difference. Could be anything from bad float needle to wrong height to sticking. After you go through the floats: The left side is idle speed and the right is idle mixture. Good starting point is idle mix screw out ~1 1/2 turns. Warm the bike up and shut off the choke/enrichment, lever in up position. Reduce your idle speed with the idle screw then fiddle with idle mixture screw to get a faster idle. Repeat that until you can no longer get a rev increase by adjusting the mix screw and you are done. Mix screw should be between 3/4 and 2 turns out or you have the wrong/bad idle jet. FWIW: Just read a post by someone who said they started a fresh barn bike w/o checking the carb and couldn't get it to take throttle. They opened the carb and the main jet was missing, gone, not there! In my experience 95% of the old bikes had a ham fisted mechanic or poor maintainence take them out of commission like this. Very seldom will you find only one thing wrong. There is always a group of things that were neglected like bad oil injector/no mix, worn sprockets, siezed rings, etcetera that caused the bike to be parked. |
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| Author: | newfie [ Sun Jun 12, 2005 12:34 pm ] |
| Post subject: | |
Thanks for the insight. If the float or needle is bad would that cause the bike to rev high? I thought that that send out fuel through the overflow. I am new to this type of bike. I will take the carb apart again and give it a check over. I noticed that the needle valve did have some wear on it where it contacts the float. Any thoughts? Thanks Again Newfie |
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| Author: | newfie [ Sun Jun 12, 2005 4:53 pm ] |
| Post subject: | |
I found out what the problem was and I created a new one. The reason why the bike was reving unpredictably was because there was a gap between the flang coming from the carb to the cylinder. I placed a couple of washers on the bolts and she ran great. so then I began fiddling with the mixture and idle screws. the bike was singing so I took her for a short run. Then I found that she was wanting to rev up as I was in neutral and as i was idling along. When I got back to the garage I notice that the mixture screw had fallen out!!!! "Gone like the wind," you could say. Anyway I dont have another. How can I get one? Is it a common part? Where can I start looking? Thanks, Newfie. |
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| Author: | ricortes [ Sun Jun 12, 2005 8:10 pm ] |
| Post subject: | |
Needles are often in carb rebuild kits. If you happen to have a spare carb around you may get lucky too. Just fooling around to see if it worked: I checked the idle screw for Keihin off a CR250, a Keihin off a 125SL, and a ~32mm Mikuni and they all interchanged OK. The tapered needle part was the same length on all of them, they just had different shaft lengths from the shoulder to where the screwdriver fits. Just means the adjustment screw may be below the surface of the carb or sticking out. Make sure you get a spring to go under the needle too. That is what is supposed to keep it from backing out. Congrats on your test drive anyway. I have driven/own two MTs and I really like the handling. For such a long wheelbase they really turn tight. One of the sweetest dual sport bikes I have ever ridden. |
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| Author: | newfie [ Mon Jun 13, 2005 5:07 am ] |
| Post subject: | |
Thanks for the info. I am going to start some hunting for that idle screw today. The bike was singing when I had the screw in. I can't wait to work out the bugs. I put up some pic of the bike you can see below. Thanks Newfie |
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| Author: | ricortes [ Mon Jun 13, 2005 9:52 am ] |
| Post subject: | |
Looks pretty much complete, nice find! Even has the spark arrestor and the magneto cover is intact. Just about every one I have seen has a broken magnetor cover from the chain coming off and the spark arrestors have been taken off and discarded. The only thing I noticed missing was the oil tank/side cover. Is there a reason why your shocks are mounted upside down? |
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| Author: | newfie [ Mon Jun 13, 2005 10:22 am ] |
| Post subject: | |
I have the oil cover taken off to repair. The plastic holes for the tongues to hook into are broken off. I am going to try to get a new one to replace it or repair. As for the shocks being upsidedown...that is the reason why I took the pics of those. I was going to ask that question later. I am the 3rd owner and I know the last owner very well. The first owner (who is a good friend of the second owner) used the bike for some trials competitions back in the mid 70's. That is why there is a different front sproket on the bike. I am wondering if that my have something to do with the upsidedown shocks. any ideas? Newfie |
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| Author: | Rick [ Mon Jun 20, 2005 10:57 pm ] |
| Post subject: | |
Upside shocks lower the center of the unsprung weight of the shocks. It was also beleived that turning these upside down would re-vive old leaking shocks. It only changed the point that they had no damping or re-bound. |
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