I have a set of temperature guages under my spark plugs, I look at them mostly when warming the bike up but I do look at them occasionally on track!
Last year they got up to around a max 230C/210C, so the left/right, so the left is up to 20C hotter than the right most of the time, both warming up and riding. This is something that occurs on small throttle openings, not just wide open. Shut the motor down and they quickly settle at the same figures cooling down, to the degree, so I'm pretty sure the different indication isn't gauges.
This year, with a set of Maney alloy barrels they get to around 195C/175C, so up to 40C cooler but similar difference in temp. This is a great indication of the improved heat dissipation of the alloy barrels, and also a clue not to suspect barrels and pistons!. Pistons are similar JSMs, cam, timing, head, carbs etc are the same. I used fresh head and manifold gaskets in the build, which should eliminate a leak there.
The difference isn't huge, and doesn't show signs of damage due to the heat. But I would like to close the offset up.
It probably isn't ignition timing because I have a wasted spark Pazon set up, so they really must be the same each side. (I had a coil fail at my last race meeting, so it now has a new one.)
Carbs and air leaks could contribute, but there is nothing obvious, and will have to have remained the same after the top end strip, which I would have thought is unlikely.
My wife notices that on start up on the rollers there is always a puff of smoke from the right (cooler) exhaust, I'm not sure if that is oil or fuel, but it clears right away. These are Mikuni VM carbs with fuel enrichment, one might flow a little more fuel on choke than the other. But the 'chokes' get switched off quite quickly in 25C plus! (yes it was cooler today than it has been, 36C plus last time it was raced!)
Mentioning the exhaust, I had a couple of cracks show up at the pipe end, and have just had them welded. First run today after that work, no difference, just a little quieter according to those whose hearing works.
So, where am I going? Is the 20C differential a problem or a characteristic? I read a couple of techy papers that suggest that 50C to 60C is a problem, but 10C isn't. They didn't say if about 20C is a problem?
I think that a Norton twin may naturally run hotter on the left. And I'm pretty sure Jim Comstock's contributions here have noted that in the past, though I think that related more to cam wear patterns.
I understood it's because the oil is fed into the right side and the oil that arrives at the left is already a little warmer. But how much?
As short term action I'm going to (re) check slide opening sync anyway. I am also thinking that I might put a shim under the left needle clip to make it a little richer on lower throttle openings upward, principle being richen the potentially lean one, not weaken the potentially rich one. I suppose I should recheck the float heights, but I have checked them previously though probably before though prior to fitting the gauges.
I'm going to try and reduce the differential by getting a better balance and a bit more fuel in at lower throttle settings, but am I chasing a non problem?
