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Furious E 01-24-2012 11:24 PM

The Woes of an Ancient Two Stroke
After lurking on this forum for quite a while, I think the time has finally come for me to post something, seeing as my two stroke project seems to be coming to completion right around the time ADV has created a two stroke forum.

I have a 74 Honda MT125 that I am attempting to fire up after more than two years working on it. The cylinder has been honed and re-ringed as the bore and piston both measured within spec. Compression seems good enough as far as I can tell from putting my thumb over the spark plug hole. The bike has spark and I've gotten it to start on carb cleaner and run for 10-20 seconds a few times. However, once it shuts down, I can never get it to restart. The plug will be wet with gas and the plug still has spark, it just wont fire The idle is fine while it runs, steady and at a decent rpm, and if I blip the throttle it responds just fine. The bike origionally was oil injected but I am currently running premix at a 32:1 ratio and the oil is a high quality synthetic. I am running the bike withut a battery, which the internets told me at some point was fine on this simple old machine. I'm rather stumped at this point, however, and would like some advice on trouble shooting the old beast.

Shocktower 01-24-2012 11:47 PM

Ok how long has the bike been sitting and if for more than 10 years you need new main seals, and how clean are the carbs, the pilot jet is probably dirty, clean the carbs, make sure the floats float and reconnect the injector, and try some fresh ethinol free gas, also you might want to check the strength of the spark, you may see it but it may not be enough to run the bike.

I hope these things get you somewhere, Oh BTW welcome to here

anotherguy 01-25-2012 08:52 AM

Having spark doesn't mean enough energy. Sounds like the CDI is tired. I would leak down test for the main seals on a bike that age. Typically they won't stop a bike from starting but leak and cause seizure. The wet plug tells me to look at the spark. Do you have a service manual?

Furious E 01-25-2012 09:51 AM

The carb is sparkling clean and the crank seals were both replaced while the engine was apart. Also, it has points ignition rather than CDI. Could the lack of a battery be the culplit? Has anyone on here run an old enduro like this sans battery?

anotherguy 01-25-2012 10:13 AM

Yes that should indeed be the problem. The '74-'76 MT 125 was spec'd with a 6n6-3B battery. Even if it's a magneto style system it needs a battery in the system.

YamaGeek 02-01-2012 05:24 PM

The battery is for the brake light, horn, and turn signals, and has little to do with the magneto ignition.

Mr. Carts 02-14-2012 09:07 PM

Clean the points with Emory cloth. I bought a mint MR 175 that would not start the guy replaced the points (but they had a coating on them). He couldnt get it to run and got bummed out and sold it for a song. I cleaned the pionts and it fired right up.

rackdaddy 01-09-2013 05:23 PM

Mt 125/mr175
Just purchased a mr175 and having similiar problems. Will run well for 2 minutes (until warm) then has a hard time starting back up. I have noticed it acts like it is flooding out. after sitting for a few minutes it fires up first kick. After cooling or drained??? Also would like a line on seals i know it needs them. Good luck any ideas??

Man i will look at the date on a post next time geeze.

Twin-shocker 01-10-2013 01:51 PM

First thing to do is replace points/condensor, plug and plug cap with new. If it still wont run then its almost certainly to do with the carb. Sometimes bikes that have been standing for many years suffer from carb blockages in the very small drillings of the pilot circuit, that may not be visible, and will make bikes very hard to start. If it does look like the carb, then its worth trying ultrasonic cleaning, but in some cases replacement will be the only way to deal with the problem.

mortimer 10-24-2013 04:35 PM

I just bought a 75 mt125, been having a blast with it.

It runs without the battery, ignition is entirely independent of the battery - so is the headlight.

My bike did the same thing, hard to start, wet plugs, good spark, compression, also premixed oil and fuel - the only way I could start it was to bump start on the big hill I live on. Rebuilt the carb, found leaky needle and seat and a pinched bowl overflow hose, which was dumping excess fuel into other areas of the carb.

The fact that it idles well, and you can blip the throttle without hiccups suggests that your jets (idle and main) are OK, though they are easy to remove and clean.

Starts great now, the insole of my right foot is almost not sore at all anymore. :clap

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