750 Twin cylinder temp

I resurrected a '78 Twin that sat for 12 years with a locked up motor. I have it started now but noticed a difference in the cylinder temperatures. After 2-4 minutes of warm up the right cylinder is very warm (I can only keep my palm on the fins for 3-5 seconds) and the left is much cooler (more than 10 seconds with my palm). Carbs have been sonic cleaned and rebuilt, floats adjusted, 1 1/2 turns out on idle screws, and bench (not vacuum) sync'd. Two odd things 1) the right plug looks good and the left plug looks sooty 2) I get a little exhaust lifter noise after a couple minutes of warm up.

So the questions 1) Inadequate oil return on the right side? 2) Still need to vacuum sync the carbs? 3) The right side temp is actually correct and the left is running cooler because it is running a little richer?? 4) Other thoughts?? Thanks.

Sounds like the LH cylinder isn't firing properly. Have you checked the valve adjustment? Performed a compression check?

You may want to (carefully) check the header pipes for heat after a cold start. It's not uncommon for one to be cool (not firing) and the other scorching hot (firing.)

When you cleaned the carbs did you remove the pilot screws to clean those passages? 1.5 turns open is on the shy side too. Try 2.5 turns and then tweek/tune to the highest idle (assuming both cylinders are firing of course.)

How did you break it free? WD40 in the cylinders? If that's the case I'd be looking at compression first. Possibly scored cylinder walls and rings.

Blow by out the vent?

You can also do a power balance test. Short the spark to ground at one cylinder at a time and compare the drop in RPM. Poor compression, poor combustion, lower cylinder temp.

Aside from the seat, looks pretty clean!

I will check the compression and do a balance test. I tweaked the idle screws last night (out on Right, in on Left) and the temp difference seemed smaller. It does not smoke at all so I am hoping the rings and walls are good. I broke it free with a 50/50 mix of Marvel Mystery Oil and used oil (used oil has some acid in it). I kept topping it off over 5 weeks and tugged (gently) on the crankshaft bolt once a week. Went to tug on the crankshaft on week 5 and it just started moving. This is the second motor (see the Yamaha I got for free attached) that I used this method on and it worked both times. Both had sat under a tarp in the backyard for more than 10 years so they did not look this good when I got them!! Thanks for the feedback.

Yeah, that seat foam is pretty ugly. Just got the seat finished up last night so I took a pic with its "brothers". The frame and seat pan have been shortened 4" but the seat still locks and flips up like the original.

What about valve adjustment?

I guess I assumed that they would all wear about the same and wouldn't contribute to a temp difference between the cylinders. However, I do have checking the clearances on my To Do list.

Valve clearance goes down with mileage and when it's all gone the valves and seats in the head burn. The laziness of many owners leads to serious damage to these parts which is why it's absolutely critical to perform a valve adjustment on any old bike you purchase. Damaged valves will cause poor compression which first manifests itself as a poor idle and hot running/starting issues.