Temperature differential, left cylinder to right cylinder.

SteveA · 2026-07-31T16:22:41-0600

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?

BERT · 2026-07-31T17:07:11-0600

SteveA said:
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?

Is the 10% heat differential evident in a plug chop?

Triton Thrasher · 2026-08-01T00:38:50-0600

Have you tried swapping the sensors over, left to right?

Fast Eddie · 2026-08-01T03:12:44-0600

Great thread Steve.

IIRC people like Comnoz (and others) have suggested for a long time that the left cylinder benefits from and extra half thou or so of clearance when boring.

I think it’s also ‘well known’ that the majority of seizures occur on the left side (conjecture / opinion only here).

So, I think it’s fair to say that there is a trend of evidence that suggests there is a left to right difference of some kind.

I’ll follow this thread with interest to see what folk suggest the root cause is.

But in the meantime, maybe just richen up the left side a tad to cool it ?

Also very interesting to see the big difference using alloy barrels ! I assume you’re using a copper head gasket ?

robs ss · 2026-08-01T03:45:45-0600

Crazy thought, but just maybe....
The left cylinder works a tad harder because it is the most directly coupled to the drive pinion. The other, to some extent, goes along for the ride, having a more flexible connection.
Okay, okay... I'll stop now and go back to drinking.
Cheers

SteveA · 2026-08-01T03:46:29-0600

Fast Eddie said:
Great thread Steve.

IIRC people like Comnoz (and others) have suggested for a long time that the left cylinder benefits from and extra half thou or so of clearance when boring.

I think it’s also ‘well known’ that the majority of seizures occur on the left side (conjecture / opinion only here).

So, I think it’s fair to say that there is a trend of evidence that suggests there is a left to right difference of some kind.

I’ll follow this thread with interest to see what folk suggest the root cause is.

But in the meantime, maybe just richen up the left side a tad to cool it ?

Also very interesting to see the big difference using alloy barrels ! I assume you’re using a copper head gasket ?

Yes, Copper head gasket. A 21 thou one from JSM. Same as I used with the iron barrels.

Definitely going to richen a little on the needle.

SteveA · 2026-08-01T03:53:18-0600

Triton Thrasher said:
Have you tried swapping the sensors over, left to right?

No, I thought about it, but as soon as the engine stops, the readings from both sides rapidly align which seems to me to eliminate the gauge.

When stood in the workshop the gauges are a pretty good measure of ambient temperature and read identical.

SteveA · 2026-08-01T03:55:11-0600

robs ss said:
Crazy thought, but just maybe....
The left cylinder works a tad harder because it is the most directly coupled to the drive pinion. The other, to some extent, goes along for the ride, having a more flexible connection.
Okay, okay... I'll stop now and go back to drinking.
Cheers

Hmmm, I understand your thinking, but since it shows up under the no load condition of running in neutral, I don't think so.

SteveA · 2026-08-01T04:11:51-0600

BERT said:
Is the 10% heat differential evident in a plug chop?

I confess I don't do traditional plug chops.

Why? Apart from the inconvenience of having to push the bike at some point, and the risk of sitting up unexpectedly and shutting off on a live race track, I find it very hard to read plugs with modern unleaded fuels.

Most times they show some black around the outside and white in the middle. The difference line in colour on the electrode tends to be in the middle. IW27 Densos are the only plugs I have used in this engine and they seem to be the right heat range.

I have used a Tetra ethyl lead additive (Tetraboost) and have seen plugs from engines that use 50/50 Avgas, and they are more readable, the Avgas particularly so.

But apart from spending more money, but as a 72 year old running around at the back, I don't think I need the benefits of these fuels.

I prefer the 'simplicity' of just having to seek out Total Energies stations to buy the 98 that is supposed to be the best available pump fuel in France.

MexicoMike · 2026-08-01T04:12:50-0600

Consider that you are worrying about this because you CAN. If you didn't have temp gauges you would base everything on how the bike/engine RUNS and have no concern re cyl temp.. Does it run well? If so, I'd forget about it.

SteveA · 2026-08-01T04:25:09-0600

As far as this statement is concerned: 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?

It isn't about the oil temperature is it, its the fact that the right big end has probably sprayed more oil out and there is a flow/pressure reduction on the left compared to right. That came back to me overnight!

And for avoidance of another potential difference left to right, the rods are JSM Carrillo, so no oil holes.

kommando · 2026-08-01T04:37:24-0600

Considering all the Brit twins with end fed cranks suffer more wear on the driveside then it must be related to the oil flow. Never done this but if this was modelled on a PC today then the timing side hole in the crank would be reduced in size so that the flow out of both crank pins was equal. With equal flows of oil then the oil film thickness stand a higher chance of being equal both sides.

SteveA · 2026-08-01T04:38:37-0600

MexicoMike said:
Consider that you are worrying about this because you CAN. If you didn't have temp gauges you would base everything on how the bike/engine RUNS and have no concern re cyl temp.. Does it run well? If so, I'd forget about it.

Mike, I think this may be the final answer, but I will richen the left needle, apart from anything else, to satisfy my thirst for knowledge! I would like to see if the gauges pick up a difference in setting, and engine longevity! If no difference shows, I would certainly question their value. One thing to be aware of is that we tend to spend more time at race meetings sitting in collection areas with bikes getting hotter than we used to back in the day, so it's nice to know if the bike is potentially overheating.

I had a misfire at my last race meeting, but that came to a head when the bike wouldn't start for the first race. That is when I replaced the coil and it fired first turn on the rollers and ran fine.

No, the difference does not show in poor running, and the little ole 750 short stroke was pulling away from the 850 Seeley also running at the back of the field in the second race.

Though I could claim that as a more experienced racer I was getting on the throttle earlier

.

The fact that the rider of the 850 Seeley came over after the race to ask me again what size my engine was may indicate what he thought.

Fast Eddie · 2026-08-01T05:30:48-0600

kommando said:
Considering all the Brit twins with end fed cranks suffer more wear on the driveside then it must be related to the oil flow. Never done this but if this was modelled on a PC today then the timing side hole in the crank would be reduced in size so that the flow out of both crank pins was equal. With equal flows of oil then the oil film thickness stand a higher chance of being equal both sides.

Yeah… but…

A Norton cranks oil feed isn’t like a traditional crank in a 4 cyl motor with oil ways etc.

In a Norton, the main oil feed feeds a big chamber, and both big ends are fed from the same chamber…

velo54 · 2026-08-01T05:38:25-0600

I'd swap sensors and gauges. Ignition timing probably has 2 separate pickup magnets, 180 degrees apart on the cam, so the timing from one cylinder compared to other on compression may be slightly different. Try swapping carbs left to right before changing mixture. Graham

Triton Thrasher · 2026-08-01T05:46:12-0600

If you’re raising the needle on the hotter side, you may well find that one notch is too big a change.

Carl H · 2026-08-01T06:35:31-0600

To really do Amals justice, you need to "Blue Print" the carbs (my terms). You need to measure all the parts and make sure they are the same. That goes for slide cut aways, needle diameters , needle jet hole size, how far the needle hangs down from the slide ,pilot jet diameters and If they are plugged up somewhat, and all the hole sizes that feed gas into the air stream as well as fuel bowl level. And several other things that can be screwed up.

Also, the carbs need to be synced with a mercury stick. No other way is accurate enough. And you get to see the whole range of slide motion in the mercury columns, which is very informative.

Maybe a quick test like exchanging the slide and needles or needle jet from the LH to RH side would help to test those parts? One other thing was wrong on my Combat that was on the OEM carbs I was bringing back to life, I brought as a non runner, was one of the aluminum plugs in the mouth of one carb was leaking air into the pilot jet system and luckily I found it and made screw plugs to seal up both carbs. Apparently, the carbs had been that way for many years. Today's Amals are nowhere near Japanese quality. Sadly.

kommando · 2026-08-01T06:43:57-0600

Fast Eddie said:
In a Norton, the main oil feed feeds a big chamber, and both big ends are fed from the same chamber…

The chamber is central and its oil still comes from the timing side, a chamber is just a very large diameter pipe and the timing side journal takes the first oil and then what's left get passed to the driveside. Its very complex modelling program used to make sure the engine parameters create equal oil film thickness at each journal and would probably also point out more flow of oil into the crank is needed. I saw a Subaru crank a few months ago, oil pressure had been compromised and the further away from the oil entry point into the crank you got the more wear on the bearings.

Fast Eddie · 2026-08-01T08:10:35-0600

kommando said:
The chamber is central and its oil still comes from the timing side, a chamber is just a very large diameter pipe and the timing side journal takes the first oil and then what's left get passed to the driveside. Its very complex modelling program used to make sure the engine parameters create equal oil film thickness at each journal and would probably also point out more flow of oil into the crank is needed. I saw a Subaru crank a few months ago, oil pressure had been compromised and the further away from the oil entry point into the crank you got the more wear on the bearings.

I understand pressure loss along a pipe as a concept (Box’s formula IIRC).

But in this case, the ID of the pipe (ie the central chamber) and the close proximity of the relevant holes, would mean an almost incalculable loss between the first and second hole I imagine.

We also have the added complexity of centrifugal force acting on the oil. Which, assuming neither hole is starved, would surely be equal on both holes?

Fast Eddie · 2026-08-01T08:24:01-0600

SteveA said:
Yes, Copper head gasket. A 21 thou one from JSM. Same as I used with the iron barrels.

Definitely going to richen a little on the needle.

Thought so

I’m of the belief that a copper HG helps as it conducts more heat from the head to the barrel, which being alloy is better at getting rid of it. Conversely, a composite HG acts somewhat as an insulator.

Temperature differential, left cylinder to right cylinder.

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