Thread: finished project?

After enjoying 6 months of rebuilding a 1970 CT70, yesterday was the trial run. I didn't tear into the motor itself, but replaced or shined and painted every part. It started on the second kick, and seemed to just need a carb.adjustment. Anyway, it immediately started smoking down where the muffler attaches to the motor (bad smelling smoke). Also the muffler got burning hot--immediately. It was running for about a minute, and you couldn't touch the heat shields on the muffler, and it actually started melting a fuel line that was close to the muffler.
What do you think is wrong?

maybe it's running uber lean?

If it's running lean, that means not enough gas---How exactly does that cause it to overheat?

Perhaps a plugged muffler causing the exhaust to stay in muffler and leaking out where it attatches to the head. Maybe some type of foreign material inside muffler? The muffler will get extremely hot but not that quick. I do not know for sure. Never heard of that problem.

If it's an old muffler, you could have a mouse nest inside. When the material starts to burn...voila! secondary combustion chamber. If the motor is pumping oil through the combustion chamber and running massively rich, all you'd need is a small air leak ahead of the muffler to get things ignited inside the muffler.

Interesting bit of trivia: a leak near the exhaust port can actually cause vacuum in the headpipe and draw air into the exhaust stream. This is why a leaky exhaust gasket can cause popping on deceleration.

Hi.You should put your hand by the muffler and see how its flowing.there is a baffle inside the muffler about 8 inchs or so and they get pretty clogged over time.When u look in there u will see that even a mouse coulnt get past that sucker.u could take the muffler off and drill it out a little.DONT DO IT ON THE BIKE,U might suck some metal shavings in the combustion chamber. Or stick a long flat head screwdriver and hammer it open slightly with it on the bike.It really helped the flow problem.And helped performance with my bigger carb.

Actually, I just put a new baffle on it. I wonder if it had a partial block in it, and when the new baffle went on, it closed it down even more. I'll check that.

Jeff Just FYI Fuel is also a lubricant just like oil, just like water, just like any liquid. Dean

Originally Posted by jeff b

If it's running lean, that means not enough gas---How exactly does that cause it to overheat?

Gasoline, like virtualy any other liquid, absorbs a LOT of heat energy when going from liquid to a gaseous (vapor) state. This is know as latent heat of vaporization; it's also what make air conditioners possible. A certain percentage of unburnt gasoline is desireable under most conditions. When cold starting, only about 20-30% of the gas vaporizes, that's why you need extra fuel for a cold start. The air/fuel mixture is functionally lean, even with raw gas flowing through the engine. On the other end of the scale, latent heat of vaporization is an essential element of keeping combustion chamber temps under control. 14.7:1 is stoichiometrically correct (ideal air fuel ratio) for efficient steady state operation. At WOT 12.7:1 is the tuning target; 14.7:1 at WOT would cause meltdown. Lean mixtures, up to a point, actually produce more power since less combsution chamber heat energy is used to vaporize liquid fuel. The tradeoff is higher peak temperatures. Running lean, up to point, can make slightly more power, but it's a risky proposition. Now consider that these little engines spend most of their time at 50% of maximum, or better, and you'll have some idea of why lean mixture would be suspect when overheating is present.

That makes sense. Thanks for ALL your help. Now to correct the fact that it may be running lean. Is there any trick to this (besides what the owners manual says about adjusting the carb.)