One cylinder cooler than the other

rohalloran

Active Member
I am having an issue with my CB750. One cylinder is always about 50 degrees Fahrenheit cooler than the others. I have swapped plug leads around, checked float heights, cleaned carbs and checked shim clearances and all is OK. It idles smooth and revs OK. Anyone any thoughts? Would I need to adjust the air screw perhaps? Maybe its running a bit rich?
 

JerryAssburger

Been Around the Block
Cooler than the rest? Maybe its parents bought it nicer clothes for school?!?! (Sorry! Couldn't resist!)

Seriously, though. If all plugs look the same, your mixture(s) should be close. Maybe compression a slight bit down (rings) in that cylinder? Don't hold me to it, but I think the outer two cylinders tend to run cooler, but that doesn't really explain why just one is affected....

Exhaust blockage..?
 

rohalloran

Active Member
Cooler than the rest? Maybe its parents bought it nicer clothes for school?!?! (Sorry! Couldn't resist!)

Seriously, though. If all plugs look the same, your mixture(s) should be close. Maybe compression a slight bit down (rings) in that cylinder? Don't hold me to it, but I think the outer two cylinders tend to run cooler, but that doesn't really explain why just one is affected....

Exhaust blockage..?
Good joke :p

Compression check done and all are good. 170. Engine was completely rebuilt recently. It is one of the inner cylinders that is cooler, #2.
 

doc_rot

Oh the usual... I bowl, I drive around...
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How are you measuring the cylinder temp?

Its possible that the compression ratio is slightly different on that cylinder because of differences in the piston/combustion chamber volumes. Its not uncommon to have a 1-2cc delta on either the factory piston or the combustion chamber.

But beyond that, if it runs well, and there are no issues, then I wouldnt worry about the temperatures.
 
Last edited:

rohalloran

Active Member
How are you measuring the cylinder temp?

Its possible that the compression ratio is slightly different on that cylinder because of differences in the piston/combustion chamber volumes. Its not uncommon to have a 1-2cc delta on either the factory piston or the combustion chamber.

But beyond that, if it runs well, and there are no issues, then I wouldnt worry about the temperatures.
I am measuring using an inferred thermometer and my hand on the exhaust. It is way cooler to the point I can hold the header pipe (which is wrapped in exhaust wrap)
 

gt alex

Been Around the Block
First though was spark plug Second thought is intake air leak on that cylinder third thought is coil.
 

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