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Discussion Starter · #1 ·
I have been working on my Yamaha ls2 100cc 1972 two stroke motorcycle for a long time.

There is excellent spark. When i use startspray it starts running for a second or two before it dies. I cleaned both the carburetors with no luck. And it looks like the sparkplugs
are wet meaning that the fuel is sprayed into the engine. I really don't understand what the problem is. When i get it running (starts prefectly sometimes) it runs perfectly, very smooth, no problems. It even starts with one kick. But when I can't get it started, it stays that way and I really dont get why.

To make it even weirder, the bike started and i drove it on the road and parked it as it was the day before. The next day, nothing, except with startspray.

Any help, literally anything is well appreciated. I have tried everything. Cleaned the carbs twice, checking timing, spark 10 times. Thank you!:)
 

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2 things have stumped me on 2 strokes and i always look at them first. Crank seals have confused the heck out of me, as well as needle valves in the carb. On a old rz the rubber on a needle valve started flapping around, it would seal some times and let unregulated fuel thru at other times. If the bike sat for a very long time i would change all of the above. Another side effect of the needle valves going bad is a exhaust full of fuel.
 

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Discussion Starter · #3 ·
2 things have stumped me on 2 strokes and i always look at them first. Crank seals have confused the heck out of me, as well as needle valves in the carb. On a old rz the rubber on a needle valve started flapping around, it would seal some times and let unregulated fuel thru at other times. If the bike sat for a very long time i would change all of the above. Another side effect of the needle valves going bad is a exhaust full of fuel.
I have changed the needle valve on one of the carbs because it was leaking. I think I need to change the crank seals. I didnt get it started at first. But I sprayed some starter fluid into the carbs, and surprisingly it ran as long as I wanted. It seems to just have problem starting, but when it starts it runs well. Does the crank seal affect some oil/gas leaking that prevent the bike from starting or something? Thanks for your help!

Btw: this is the needle valve that is used in my Yamaha, i changed on one because it had ring form on the tip.
11957
 

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Hello
I was going to ask you about the quality of the gas in the tank since you said it's been a long while you have been working on it. But, then you said if it starts it runs real well.

Probably a dumb question but Is this an engine that you mix oil and gas? Is there a coil that may be weak in keeping it from starting? You said it has good spark. A grounding issue possibly. Sorry I'm not much help but maybe I can help stumble on an idea.

Sometimes when plugs get fuel fouled they are ruined.
 

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Discussion Starter · #5 ·
Hello
I was going to ask you about the quality of the gas in the tank since you said it's been a long while you have been working on it. But, then you said if it starts it runs real well.

Probably a dumb question but Is this an engine that you mix oil and gas? Is there a coil that may be weak in keeping it from starting? You said it has good spark. A grounding issue possibly. Sorry I'm not much help but maybe I can help stumble on an idea.

Sometimes when plugs get fuel fouled they are ruined.
Yes it's a two stroke engine where I mix gas and oil myself. I have 4 ml oil for every 1 liter gas. 4% oil. And this seems to be good. I don't think its the gas. Maybe something about the coil like you said? But the plugs are giving a nice blue spark when i pull them out to check. But the coils and all of the electric components are very old i assume. But I dont know if the coil may giving starting problems.

Fuel fouled? I dont know, but the plugs seems fine and like i said, the bike runs very well when it first starts.

Thank you for help, it really helps me get to the bottom of this! :)
 

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Discussion Starter · #6 ·
2 things have stumped me on 2 strokes and i always look at them first. Crank seals have confused the heck out of me, as well as needle valves in the carb. On a old rz the rubber on a needle valve started flapping around, it would seal some times and let unregulated fuel thru at other times. If the bike sat for a very long time i would change all of the above. Another side effect of the needle valves going bad is a exhaust full of fuel.
I started the bike today with startspray and I kept getting fuel and air. But it dies if i dont rev it, which is weird considering it idle very nicely yesterday. There is oil spitting from the exhaust too, and its leaking oil from somewhere on the engine, even when I changed the gasket on the clutch cover side. Maybe I do need to change the seal on the crank? Maybe the bad seal leaks and the air fuel mixture gets messed up or something inside the crank? Not too sure, but seems like the only thing I could check.
Thanks for reply!
 

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Possibly a short in the ignition. Or grounding problem. I assume it's a keyed ignition where as sometimes it catches and sometimes not causing hard starting.

With 2 carbs I assume it's a two cylinder setup. Maybe the carbs aren't in sync with each other.

You cleaned the carbs twice so we can rule out a float bowl issue getting stuck.

Again, I'm probably off in left field but sometimes ...
 

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Discussion Starter · #8 ·
Possibly a short in the ignition. Or grounding problem. I assume it's a keyed ignition where as sometimes it catches and sometimes not causing hard starting.

With 2 carbs I assume it's a two cylinder setup. Maybe the carbs aren't in sync with each other.

You cleaned the carbs twice so we can rule out a float bowl issue getting stuck.

Again, I'm probably off in left field but sometimes ...
Yes well, I have cleaned the carbs, adjusted the float and changed gasket in both and changed the needle valve in one, so I dont think its the carbs. There could maybe be a coil that is bad? But again, when it runs it runs well on idle as well, so heck this bike's really giving me a hard time. How can I fix grounding problem or short in the ignition? Im not familiar with these problems.
I appreciate your help Slider!
 

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There's likely a grounding wire for the coil. It may be loose or rusted and not making good ground. Then again I might be wrong but here is an image for a Yamaha coil that may be different than yours. But there are connections that are worth checking.

Hope it helps
 

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Discussion Starter · #11 ·
There's likely a grounding wire for the coil. It may be loose or rusted and not making good ground. Then again I might be wrong but here is an image for a Yamaha coil that may be different than yours. But there are connections that are worth checking.

Hope it helps
Thank you for the help, I tried to rubb of rust and make a good metal to metal connection, but still have the same problem. Gonna see if changing the seals at the crankshaft works when I get the seals.
 

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I started the bike today with startspray and I kept getting fuel and air. But it dies if i dont rev it, which is weird considering it idle very nicely yesterday. There is oil spitting from the exhaust too, and its leaking oil from somewhere on the engine, even when I changed the gasket on the clutch cover side. Maybe I do need to change the seal on the crank? Maybe the bad seal leaks and the air fuel mixture gets messed up or something inside the crank? Not too sure, but seems like the only thing I could check.
Thanks for reply!
Well, as the oil spitting out could mean that your rings are getting stuck, I have an old husky that sat for a while and the rings got stuck leading to odd running scenarios, but it still had compression and spark, and would also run, so maybe it's time for some new rings or a new piston assembly all together. Cause the oil could mean blow by from the case.
 
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