Vintage Dirt Bike Q & A

I have a '73 MT125 and I have just changed the air filter housing to a CR pancake type set up (but I am still using the original "L" shape rubber air feed hose of the MT).

Since then I have been unable to reduce the idle below 2000 RPM.

Mix screw is at 1.5 turns from full in - as per recommendations and the throttle stop is wound right out.

I suspect that I am getting too much air from somewhere but can't find any air leaks.

The bike starts first kick, revs freely and runs well.

If I remove the air filter it makes no difference to the idle and if I restrict the air at the filter (air intake side) with my hand the engine dies.

The throttle slide seems ok but doesn't close off completely when the throttle is shut (There is a 1mm gap).

I have tried different filter medium ( some fine and restricting the airflow and some course and allowing more airlow) but this makes no difference.

I have a suspicion the problem lies within the carb.

Does anyone have any ideas as to what the problem is? or what steps to take to identify possible causes?

Ray

SOP would probably be a leak down test. The gasket that keeps the transmission oil out of the crankcase goes out on the 250's so it could be a source of air. The seals on the ends of the crank i.e. under the flywheel, go out too. If the engine seems to smoke a lot then its probably a case seal or the crank clutch side seal.

Sometimes a sticky float will cause an overfill and extra gas leaks into the engine. I would make sure the floats are adjusted properly before I did anything serious like spliting the cases.

You really have to check the idle jet too IMHO. Sometimes they are damaged during cleaning or replaced by previous owners with the wrong part.

Two strokes are kind of like a saxaphone in that often even the intake system is tuned. Slightest change can cause a dramatic change in the engine. It may end up you have to replace your idle jet.

Thanks Ricortes,

I take your point about a tuned system - maybe I have upset the balance, as the problem only appeared once I installed the new air filter system.
I still have the original which was off the bike for refurbishing, so this weekend I'll refit it and see what happens.
cheers & beers.

Ray

Make sure you have slack in your cable from the throttle grip to the carb. Check the clamp between you carb and the reed valves is tight. Spray carb. cleaner around reed valve and different parts of your engine to see if the idle speed changes. Sometime you can find air leaks using this method!

Remove the shop towell from the air box/carb.

When my MT does this it is usually because I have pulled the cable out of the Y where the Oil Pump cable splits off. Make sure the cable is seated properly on all 3 ends of the Y. Also if the cable adjuster jamb nut on the top of the carb isn't tight the adjuster has a tendency to move out while you screw slide cap back on. The slack adjuster is typically hidden under a rubber cap

Jake,

You were right on the money!

The cables had popped out at the Y junction.

I fitted them back in and alls well

many thanks for the input

Ray

Ray:

Glad to hear it was a simple fix.

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74 MT-250
06 Husky TE-450