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Discussion Starter #1
What would make one of four cylinders run consistently hotter than the rest?

The engine is running and it seems to be missing a little, I check the cylinders and # 2 and #4 are running hotter.(special temp gun from large unnamable department store, had to try it!) They are about 189f hot and the rest are running about 89f, that is a big difference. I think the others are missing...
 

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Retired twice: Navy and as a govt contractor
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It is an older 4 cylinder Yamaha Royale.
 

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wonders if one carb controls the same two hot cylinders

And what is the correct temp, 89 fully warm seems very cool and 189 seems about right,
 

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Discussion Starter #5
is 4 carb, 4 stroke, 4 coil, Water cooled V. opposing fire(two coils fire at one time, duh) maybe the water passages are plugged up on the other two cylinders(AWHH!!)
 

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Troublemaker
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Carbs probably need adjusted, two cylinders running lean. Could also be water not circulating good enough if it's an older bike that has never had it changed out.
 

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ZAMM Fanatic
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An air bubble in the cooling system could easily cause affected cylinders to read a LOT hotter. Have you tried burping or vacuum evacuating the cooling system? I'd suggest looking for a factory "how to"
 

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[If the bike has electronic ignition] How many spark generators does the bike have? How many spark [timing] boxes does it have?

With my Magna V-4, one of the two spark generators was out part of the time, and then it ran on two cylinders. Changed it out and it runs very well. It was an expensive part so testing is indicated before replacement.Switching coils and other parts around will not detect the problem if it is the spark generator. You can , however switch the spark boxes around.

I recommend a cheap timing light from Harbor Freight as the way to really see how your spark plugs are firing. That light will detect each and every miss of the spark event and can even detect weak vs. strong sparks. Much easier than grounding the plug against the engine case. I doubt that I ever would have found the exact cause of the problem without the manual and that light.

Also, the spark generators do have resistance [values are given in the shop manual] that can be measured while the bike is not running. On Hondas, it is pretty easy to do right at a 4 prong snap wiring connector without dis-assembly. Hope this helps.
 
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