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Discussion Starter · #1 ·
I have a 1979 yamaha 340 ex it is in very nice shape but when I take it out it will run good for 30 mins or so and it will just start boggin and die I changed all the fuel lines and cleaned the carb it seems like when it gets hot it stops and I will have to sit there for 10 mins and it will start up and go again? I tried everything does anyone have any thoughts?
 

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When it dies does it still have spark?
 

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is it f/c or l/c? if its fan cooled perhaps the fan wouldnt be working because the motor would get hot and doesnt want to work ?
 

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after it dies check your spark. your ignition coil might be bad. my friend John's pro xr 440 did the same thing last year. Turned out the coil was bad. we talked to a guy at the local dealership and he said when the coils heat up it will kill your spark and the motor dies or runs like crap. That's where i would start if i where you.
 

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check your primer, sounds silly but, it happend to my old sled, usually after an hour, as I let off the gas, the vacuum pressure would pull extra fuel into the carb and flood itself out, let it sit for a while, ran fine. Replaced the primer since, also check all your ignition components.
 

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Discussion Starter · #6 ·
i checked the spark and it still has it. it seems like when i changed the sparkplug it will start up again sometimes and the next time not? ill be sure to check the primer as well. do you think it could be the seals?
 

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how about compression? alot of times pistons and rings and cylinders will expand and contract when hot. get it hot to the point were it dies and check comp
 

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Discussion Starter · #8 ·
They are all good. I just rebuilt It not that long ago.
 

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i know this sounds strange, but did you take off your gas tank and clean it as well? some times there are floaties in there and they get sucked up against the pickup and it starves for fuel...[?]
 

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Discussion Starter · #11 ·
i toke the fuel tank off and cleaned it. but ill check all the lines again just to be safe
 

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Crank seals could be your problem. Start your engine and spray carb cleaner around the pto seal and see if the rpms change. If it does your seals are bad.
 

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If you are still running the stock carb? If so put a 36mm round carb on and call it a day.
Bruce
 

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Had a 4-wheeler that did the same thing, check your plugs and make sure they aren't a step or 2 hotter, that will heat a engine up real fast. Make sure its getting oil also.
 

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check if the head bolt are torqued properly, my old man told me this story one time about how his buddies sled was doing the same thing and they couldnt figure it out, turned out the head bolts were not tight enough, i guess the bolts expand more than the head when they heat up, and the engine would loose compression, dont no how true it is , but would be an easy thing to try
 

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Discussion Starter · #16 ·
i double checked the fuel line after talking to you guys i found a little small crack and it was sealed up but i am thinking when it gets hot it splits apart in till it cools down then reseals itself. but i think i am going to redo the seals anyways.
 

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Discussion Starter · #17 ·
well i finished redoing the seals and put it all back together. now the compression is only at 70 psi! so i guess i am going get new pistons and rigs! if that's not it i am out of idea's. [Disappointed]
 
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