A little Hot on the Right

[Vince Carucci]

I have a 45hp cut case with Mikuni carbs. My right cylinder is running hot according the CHT gauge. There is a thermal couple sender mounted under each spark plug. I swapped the wires on the back of the gauge and the problem followed the wires [gauge must be good]. I swapped carbs and the problem stayed with the right cylinder [carbs must be feeding fuel and air equally]. Both plugs are gapped the same. I'm hoping the problem is a faulty thermal couple. Any ideas on what can make one cylinder burn hotter than the other?
Can I use an IR thermometer to read CHT and get an accurate reading?



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[Kamcoman77]

Do both spark plugs show the same mixture? If one plug shows leaner than the other, you could have an air leak in the intake. I've also heard of some 1/2 VW users having to run a different main jet in each carb to get the same mixture & CHT. If mixture looks the same what about trying a small airscoop on the hotter cylinder? Remember that airflow from the prop is downward on the left cylinder and upward on the right.

[LSaupe]

Maybe already covered, but forget IR on the surface, wont be accurate enough without a calibration and really good proper set up.  Easiest thing to do is move is to swap the TC's and see if the problem follows the TC (not just the back of the gauge, but swap the wire sets).  One quick expedient is to place the TC sensing ends (both at the same time) into a boiling pot of water and make sure that they both read the same and that it is the boiling point of water for your given atmospheric pressure of the day (like 210F ish).  Also look for kinks or sharp bends in the TC cables as they go from the gauge to the cylinder as that can activate those areas to respond to temp gradients nearby (either go off scale high or low).  Is your gauge temp compensated (i.e. corrects for ambient temp in the final reading)?  Ensure you do not have intermediate (spiced in wires or jumpers) in any areas that might see a temperature gradient.    Just some thoughts.

[LSaupe]

When you swapped the carbs did you keep everything intact for each or just swap the bodies and maintained the original slides and cables on each side?  Just looking for possible things that would not transition in that case such as the slides and needles etc. that would affect the CHT.

[Vince Carucci]

I swapped out the carbs fully intact. That way if the problem was in the carb assembly, it would transfer to the other side. No such thing occurred. The problem stayed with the cylinder and not with the carb.

[Chuck in Indiana]

FWIW, what do the plugs look like?
Also, I've had problems with westach thermocouple wires, and all I had to do was take them apart at the connection, spray them with Caig DeOxit (accept no other) reassemble, etc. Same deal at the instrument itself.