Only two of the 4 cylinders of my motorcycles exhaust pipes are heating up when it is running.?

Question

This is a 1981 Yamaha Maxim 650 that has been sitting for a few years. Is this just a sign that two of the spark plugs are not firing or fouled or something more? Will new plugs correct this problem?

Answer 1

4 cylinder bikes have 2 ignition coils, one per 2 spark plugs. I'm guessing you either have a bad ignition coil or a bad wire harness leading to one of the coils. Swap coils one side to the other and see if the exhaust pipes that get hot change with them... if they do, you need to get a new ignition coil, or might as well replace both of them while you're at it.

Answer 2

Which two pipes are cold will be a good clue to the problem, Assuming your engine has the typical Japanese 1,2,4,3 firing order, if the cold ones are the two outside (1&4) or the inside two (2&3), then it's likely the problem is ignition. One ignition coil will fire each pair. Instead of the coil, I'd be checking the magnetic pickups (the eleectronic replacement for points) on the end of the crankshaft. Here's a quick way to determine if one of them has a problem. Physically swap them side for side without cutting the wires and then swap the small wires feeding the ignition coils. If the previously hot pipes are now cold and the cold hot, it's the pickup that's the problem. Forget checking them for resistance as the manual suggests because I've found from experience they can check out good but be dead. If the pickup is the problem, you should be able to get a replacement "breaker plate" from a salvage yard for a reasonable price. By the way, save the good pickup from the orginal plate because you never know when you might need it and it'll save buying another plate. If the two dead cylinders are the two on the left, then it's likely the bike was left setting on the sidestand for a long time and the two left carbs are gummed up and need to be removed and throughly cleaned. Do not use the high powered carb soak parts houses sell because if you soak the carbs overnight, it'll erode any brass parts not removed. (Found out the hard way.) Also do not use wire to poke through the jets, if anything, use a toothpick, straw from a broom, etc to prevent damaging the jet.

Answer 3

Which two pipes are cold will be a good clue to the problem, Assuming your engine has the typical Japanese 1,2,4,3 firing order, if the cold ones are the two outside (1&4) or the inside two (2&3), then it's likely the problem is ignition. One ignition coil will fire each pair. Instead of the coil, I'd be checking the magnetic pickups (the eleectronic replacement for points) on the end of the crankshaft. Here's a quick way to determine if one of them has a problem. Physically swap them side for side without cutting the wires and then swap the small wires feeding the ignition coils. If the previously hot pipes are now cold and the cold hot, it's the pickup that's the problem. Forget checking them for resistance as the manual suggests because I've found from experience they can check out good but be dead. If the pickup is the problem, you should be able to get a replacement "breaker plate" from a salvage yard for a reasonable price. By the way, save the good pickup from the orginal plate because you never know when you might need it and it'll save buying another plate.

If the two dead cylinders are the two on the left, then it's likely the bike was left setting on the sidestand for a long time and the two left carbs are gummed up and need to be removed and throughly cleaned. Do not use the high powered carb soak parts houses sell because if you soak the carbs overnight, it'll erode any brass parts not removed. (Found out the hard way.) Also do not use wire to poke through the jets, if anything, use a toothpick, straw from a broom, etc to prevent damaging the jet.

Answer 4

If it has been sitting for a while the carb or carbs may have been gummed up by any old fuel in it. Just clean it out or bring the carb to a dealership for them to professionally clean and tune the carb. Also changing the plugs wouldn't be a bad idea depending on the amount of time it has been sitting.

Answer 5

I'd say you have 2 dead cylinders. Either they're not firing, or they're not getting fuel. Clean the plugs and clean out the carbs and fuel system if it's been sitting a while.

Answer 6

New plugs won't fix it, but they surely wouldn't hurt. It's the best first step. Then I would swap the coils as Mick suggests, or at least the wiring to them. After that I would follow bikinkawboy's instructions.

Answer 7

def seems like 2 dead cylinders. also check your spark plugs(and gaps) and fuel lines.

Answer 8

Ide think so