Troubleshooting Fun

[BSOrion]

All was well on my RS200 until last week when I was nearly home from a 30-minute ride.  I was running it normally through some streets and all was great.  I jumped on the interstate and took her to the limit for about 10-minutes... turned around (all still good) and another 10-15 minutes back on the interstate.  About 1-mile from home I noticed the engine was dying out except while on the throttle.  Got her home o.k. but had to keep goosing the throttle.  I noticed more smoke than usual and could not get it to idle and starting was difficult.  So those are the symptoms I've observed.

Now, I've decided to post this so that anyone can chime in and also so that the very few people who know less about trouble-shooting can see how this progresses and maybe even so that if someone ever experiences the same symptoms, it just might point them in the right direction.

Here's what I've observed so far in the troubleshooting:

1.  Good fuel supply from the tank through the petcock and to the carbs... no obstructions and jets, needles, floats, etc. all look good.

2.  Battery fully charged and both cylinders are firing.  I pulled each plug wire while it was running to be sure.

3.  Upon starting the bike later on, I noticed excessive smoke and right plug was a bit more oily than normal.

4.  Good cylinder compression (110 lbs. on both sides)

Per Steve Reed's guidance, I'm going to pull the right clutch cover and inspect the oil seals to see if one blew or something, soon as I find a garage to do it in.

I'll post more later.

Cheers, Orion

[Toystoretom]

This is a crude test but is something you can try.

Pull the covers off so you can get to the carb mouths. Open the throttle all the way and have someone kick the bike over while you put your hand over the carb mouth. You should feel a fairly decent suction. If you feel almost nothing you MAY have some seal problems on that cylinder and further testing may be in order. You can compare both cylinders.

I have several Suzuki two strokes and a test we run on those is to see if it will hold 5 or 6 PSI for 5 minutes. You have to make a testing device that you insert into the the intake rubber (after removing the carbs) and pull the exhaust and seal the exhaust port. You have now isolated that crankcase chamber. Your device needs a inlet tube so you can use something like a MightyVac (with a gauge) to pump it up. If the seals are good it will pump up. If the seals are bad it won't pump up at all. Don't use compressed air from a compressor to do this, if you shoot 100 psi in there you may blow an otherwise good seal out.... you don't want to do that.

You would have to design a fixture to mount on the side cover for the rotary intake valve but it could be done.

[BSOrion]

Thanks Tom.  I've since checked and reinforced the clutch side rotary seal and have replaced spark plugs just to be sure.  I'm currently trying to round-up some replacement condensers to see if that could be it.  I don't have a shop to do serious trouble-shooting.  I am interested in doing the crankcase pressure test.  My gut says I've blown a seal but would love to find out otherwise.  Orion

[Toystoretom]

You can try the hand over the carb mouth suction test  without too much trouble, and both sides should feel about the same.

As I am finding out, two strokes are funny things. If the seals are blown out it will still show good compression, because the pistons are simply compressing the air that already exists in the cylinder. It won't move any new air in or out.

The way that air is forced into the cylinder is that when the piston is traveling down it pressurizes the crankcase (air UNDER the piston is compressed). There is a port that allows this compressed air to flow into the cylinder. Bridgestone uses a rotary valve to keep the air from flowing back out the carb and therefore forces it into the intake port.

When the piston moves up it does two things. It compresses the air in the cylinder for combustion (because the ports have been closed by the piston) and it also creates a vacuum in the crankcase under the piston which sucks in air through the carb. This is the vacuum you are trying to feel with your hand. If the seals are bad it will suck air through there instead of through the carb.

The problem is the seals can be bad to a varying degree, which is why you may need to do a leakdown test. That will show it to be 100% good or bad to a certain degree.

I hate to say this but 40 year old two strokes probably have petrified seals. If you like the bike it might be the best course of action to just replace them. I pulled the crank out of my Suzuki GT 550 and there was almost nothing left of the seals. I replaced them and it runs great now.