1975 JD 400 runs fine for a while, then bogs out and dies

[Buffalorun]

Hi all. Newbie to the sledding world. I recently bought a very clean low mileage 1975 JD 400. It has a KEC 340 engine. I have rebuilt and cleaned carb. It fires right up, runs like a top for about 10 to 15 minutes, then without warning just bogs out and dies when throttling . It will fire right back up and idle well, but will not out again when I throttle up. I am stumped and my mechanical abilities are not the greatest. Could it be the CDI?

[Matt]

It very well could be ignition related as when electronics can be fine when cold but fail when they heat up. Your sled does not have a CDI, it has points & condensers. It has both internal (under the flywheel) and external ignition coils. I would guess you might have a bad internal coil. Check your spark when you first start it, and then check it again after you have the issue, and it may look weaker. I'm guessing you will still see it since it will idle.

[75522]

Oils seals... Sled is 43 years old. I would check/replace them... As the engine winds up, it draws more vacuum. That vacuum is what runs the fuel pump. Given the age of the oil seals, they dry out and leak air into the system. This leans out the gas and cause the engine to bog down and die out. Let me guess, while idling it has a normal amount of exhaust... When throttling, the exhaust gets super thick and very white?

[Buffalorun]

Yes. You are correct about the white thick smoke. Never would have thought about that. Now I gotta order more parts. Just ordered cdi , regulator and coil, but I guess it won't hurt to have extras. Although I am not a mechanic, and appreciate the advice, wouldn't the leaky seal cause it to more sputter and at least be attempting to continue running during the bog out? When the sled dies, she dies with no attempt at recessitation , not a cough, not a sputter...just flat out dies.

[Buffalorun]

Oh...on a side note, the sled is a 1975 year model. Not a 74 as originally posted, my bad.