I just bought a 1988 yamaha xt600 it runs great except sometimes out of the blue it will just start cutting out and backfiring and i scary rattling noise comes from the motor area then after i turn it off and start it again it sometimes runs just fine again. would a leaky intake manifold cause this?

Hi and welcome to fixya,

Hi and welcome to fixya. The intake manifold should be taken as the last option. Basing from your description of the problem, the most probable case is poor firing. You will need to do a full check on timing, fuel mixture and spark.

Cutting out can be caused by poor fuel. In this case, there might be water on the carburetor reservoir. Drain the carbs. Until the muddy or rusty residue becomes clear. This procedure doesn't require you to open the carbs but if you want to be sure. You can do full carburetor cleaning meaning you clean the jets as well.

The rattling noise can be caused by timing. The timing chain maybe loose or there's a failure of one of cylinders to fire. Since you mentioned it runs fine again, im guessing there's a problem with the spark plug. Make sure you have good plugs. I recommend replacing them all at once to narrow down the problem. Even if the spark plug seem to be fine, the copper core could have been corroded. I recommend replacing it if its more than a year old and have been extensively used.

Make sure you have good fuel and air mixture. It usually is 2.5 to 3.5 turns out from a full close. but if you haven't made changes to it yet, let it be. This will save your plugs from wearing out too soon.

The leaky intake manifold can cause lose of power, and will definitely lean out the mixture. Physically check it to make sure there isn't any cracks or big holes that can let air in.

I hope this info helps.
levibit