1976 340 Help!

[9500blizzard]

I have a 340 olyimpic that has had almost everything rebuilt on it. It starts up first pull and well run great for about 15mins and is then very boggy before it dies. It well start up again in an hour or so. It has good spark on both cylinders. I have put a mikuni 34mm on it that has all new parts in it. It has all new iginition except the stator, new crank seals, new pistons. I'm stumped, any help would be greatly appreciated thanks jordon.

 

[yardsale_oem]

is the carb vaporlocking?

 

[wwtbuick]

Does it have spark when it dies? You have to make sure the spark is good not discolored or different looking irratic. pop your fuel cap lose a little bit to to see if it trying to pull a vacuum on the tank. Is that a cdi or points, fan cooled? They will also die if they are too rich on the carb and it drowns out when it gets hotter, but it ought to be wet in the cyl if its doing that.

 

[Red-eye]

Try changing coil some time they used to brake down with heat. Hows the timing and or points.(Ithink they had points)

 

[DOODOG]

I had an old Yamaha that sat around for a couple of years. When I started working on it it would do about the same thing your describing. What I found was that mice had gotten in to the exhaust and built a nest and partially plugged the exhaust pipe. I cleaned it out and it ran fine. Drove me nuts for awhile trying to figure it out.

 

[longtrack156]

I had an old Yamaha that sat around for a couple of years. When I started working on it it would do about the same thing your describing. What I found was that mice had gotten in to the exhaust and built a nest and partially plugged the exhaust pipe. I cleaned it out and it ran fine. Drove me nuts for awhile trying to figure it out.

Had a similar thing, baffle in the muffler came loose and would plug the exhaust.

 

[9500blizzard]

I worked on the sled today and discovered a few things wrong. The primer line was spliced in between the feul pump and the carb and was pushing fuel into the motor. The sled still shuts off after 15 or 20 minutes, it has good spark but is not pushing fuel. The fuel pump is rebuilt and the crank seals are new what would cause this?

 

[brushwolf]

When it dies off loosen the gas cap and see if you hear the rush of air into the tank or if that changes things. If it does then you should look at your tank venting. Either there will be a small nipple with a hose attached somewhere near the top of the tank and the hose is either plugged or pinched off or you need to have a vented cap because yours is not working properly.