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'74 Honda MT125 running poorly-anybody think they might know why

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I would like some input from those who know more than me about bikes. My '74 MT125 is struggling and I need some help with the diagnosis. It will start fairly easily although it helps to give it some throttle and it shouldn't need any throttle. After it begins to warm up it begins to struggle to keep running and you have to give it more throttle to keep it running. You may end up having to hold the throttle at 1/3 to 1/2 open to keep it running and it is miss firing all the while. You can't ride it because it will not make enough power to keep going and it will bog down and die. I have changed the gas and spark plug. Taken  the carb off and taken it apart and cleaned it. I have a new air filter and there seems to be plenty of exhaust coming out of the pipe. I have noticed and thought it strange that you can take the slide out of the carb while it is running and it only runs up the rpms a little. I thought if you removed the slide while it was running it would run wide open. Could the reed valve be restricted and that somehow be the culprit. I can not get the cover off to check the points because the screw heads are messed up so I haven't checked the points.

 

I hope I haven't left anything out and I hope someone knows what might be going on here.

 

Thanks in advance for any help.

 

 

P11007133.jpg

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Points and the timing advancer (probably behind the points)  I think I've got a nice clean carb on the shelf for that bike.

 

Does yours look like this one ? This might be a 175 carb I'm not sure.  It measures about 24mm 

 

IMG_8584_zpsbae9cc8c.jpg

 

IMG_8582_zpsb52605cf.jpg

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Hey guys thanks for the feedback. This does look like my carb. I think 24mm is correct. Am I correct in thinking that if you pull the slide out of the carb it should take off wide open? Would you drill the bolts out of the points cover?  Funny thing is the bike had been running very nicely, for what it is, before this happened.

 

I'll try to get into the housing this weekend and see if that's it.

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yes it should rew wildly if you pull the slid out. if the crews holding the cover on are philips head you should be able to drill the heads off if you can't get vise grips on first ? if none of this pans out it could be a bad crank seal .

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I couldn't wait till the weekend to fix this so I got the points cover off and found them to be set perfectly and clean as could be so I pulled the carb again broke it down and cleaned it again. When I put it back together it ran much better with the only real complaint being that you have to hold the throttle wide open to start it and then back off the throttle. I suppose this may end up being the starter jet. I suppose I just wasn't thorough enough the first time cleaning the carb. Anyway thanks for every ones input. Always grateful at the help here on the forum.

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Pull the choke/enricher  plunger out and have a look at the rubber on the bottom of it.  If this is damaged or gone (as I've seen) It will act like the choke is on all the time. 

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If your carb is stamped 021 behind the choke lever like the one in BruceD's photo, it's the stock MT125 24mm carb.  I also second his idea of a choke problem - I think it's the only thing that could explain the motor still running about the same with the slide removed.

 

When you pull the slide out of a properly-functioning MT125, the motor should scream its brains out until the slide is fully out - then it should die pretty quickly of lean starvation. since there will be little vacuum at any of the fuel outlets except one - the enrichener (choke) outlet on the right side of the carb throat, which will keep pulling about the same amount of fuel it did at WOT.

 

 

Ray

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Bruce and Rayivers you guys are dead on with with the choke diagnosis. When I pulled out the choke lever there was almost nothing left of the rubber at the bottom of the plunger. I wonder if I destroyed it using carb cleaner? I am also now looking for that rubber seal if anyone knows where to get one please let me know. 

 

Thanks again for the help.

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If you have a paper hole-puncher and an old inner tube or similar piece of 1/8" thick (or whatever) rubber, you should be able to punch out a small rubber disk and epoxy it into the end of the plunger.  This will at least get you up and running while you look for a replacement plunger.

 

The same 16046-361-670 choke assembly is used on most all the '78-and-earlier Honda 2-stroke motorcycles, so any of them should work (new ones are rare and expensive).  It might be easier and cheaper to find a whole carb.

 

Ray

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