Here's my latest project. Getting this sled ready for a Christmas present this year and it's giving me fits. It hasn't run in about 8 years and I don't know the history on it so I can't give any specifics really. All I know is what my dad (old small engines technician) and I have done to test it.
1979 Yamaha ET300C
two cylinder in-line
2-stroke
single carb
PROBLEM: Has trouble starting, sometimes not at all. Once you take the plugs out and dry them, it will give a good kick the first time and then nothing after that. Doesn't seem to matter whether you choke it or hold the throttle open, or a combination of both. While we had it running, we left the throttle at idle and held the choke open. It would not run unless the choke was open, but would not rev up at all. Cracking the throttle would cause it to die, as would letting the choke close. When the fuel supply ran out, it did rev up and jump on the pipe without cracking the throttle, which immediately makes me think that it's just pig rich and needs to be leaned out.
FUEL TANK
We have bypassed the fuel tank and fuel pump. I am running an upside down soda bottle with a hole in the cap that doesn't leak. It's held above the fuel bowls to allow gravity to feed the bowls to eliminate any possible pump and tank issues.
CYLINDERS
Compression test yielded 85psi on the first pull, 95psi on the second pull. I can't find anyone that will tell me if this is good or not but it is equal for both cylinders. Plugs in, plugs out, it doesn't make a difference. Same reading every time. What we could see of the pistons looks ok. THOUGH, the belt cover does have a Wiseco sticker on it, we are unsure if they are stock pistons or not.
CARBURETOR
The carb is a unique piece. It does not have a slide at all. In fact, it is just a butterfly and that's it. There are no reed valves or anything. First thing dad and I did was clean the carb and check the floats. It appears to be getting more than adequate fuel. The plugs are always wet after one or two pulls. We had to replace the choke cable spring and needle tipped piece that regulates fuel flow. The needle-tip end was completely missing, we're not sure how or why, but that's besides the point. Yamaha lists anywhere from a 200-240 main jet that comes stock. When we pulled the bowl off a 210 main jet was installed. We have no other reason not to believe that this was the stock jet.
ELECTRICAL
The handlebars are a rats nest of wiring due to the old grip warmers, but it does get adequate spark. When placed on a ground, the plugs spark well and bright. When we had it idling it didn't seem to have any hiccups.
EXHAUST
Exhaust is original, but is free flowing and does not have any restrictions. The pipe appears clear and in decent shape for the age. We've ran it with and without the exhaust to prove that it wasn't the cause.
AIR BOX
The air box is a maze of tubes and passages, but appears to be in good shape, we've taken it apart and it does not seem to have any problems with air flow. We've ran it with and without the air box to prove that wasn't the cause.
I suppose it comes down to timing if compression isn't the issue, but I'm not sure how these 2-strokes adjust timing. Obviously it's getting good spark so I have no reason to believe that the other ignition parts are working too. Plus we had it running, which makes me think that's not the case. Thoughts?