Opinions on exhaust temperature measurement?

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1950 ES2

I was noodling with the pilot air adjustment today, as part of investigations into the engine stumbling to a halt when hot & idling, and measuring the exhaust temperature with one of those handheld infrared thingamajigs.

I could pretty much "dial in" temperatures (right up close to the rose) between 370 and 440C with the air adjustment, which sounds right as it was changing the air fuel mixture.

So I'm wondering if this has any diagnostic value. Does anyone have a smooth-running similar machine that they've taken this number from which can help with whether a particular temperature is in the right sort of area?

(I've also been trying a few ways of measuring the RPM, since my ears don't know what they're listening for, and I understand that seeking a higher idle speed with minimum throttle input is how the air adjustment should *really* be done. But I'd like to see if this correlates with a "correct-ish" temperature)

(ps I was interested to notice the exhaust rose unscrewing while i was investigating. That seems like a Bad Thing)

Bad idle is often due to…

Bad idle is often due to wear and air not flowing correctly under the slide ,try some thick grease to see if it changes. Also a low fuel level makes for variable fuel pick up.You are right to not try to go too slowly .My properly refurbished carb will idle very slow ,but can then hiccup on starting off.

Got a good fuel level in the…

Got a good fuel level in the tank, so we should be good there.

On the grease, would that be for general lubrication of the slide? I did note that after bipping the throttle the engine doesn't return reliably and quickly back to the previous idle, so perhaps there's some stickinedd there?

Measuring temperature at the…

Measuring temperature at the same distance from the head on each of the pipes on a twin ought to help carburettor balance. It's difficult to see how a particular temperature should be targeted because it would change so quickly as you move further from the head.
If engine is slow to return to idle, it's worth checking if the carb slide drops freely. On one of my monoblocs I had to file about 10 or 20 thou off the flat face of the air slide because it was too fat and jammed the throttle slide between carb body and jet block

Yes, the temperature…

Yes, the temperature thoughts make sense. Perhaps it's a matter of noting what the temperature is when things are running well, and the variance with distance to see if that makes the whole thing unreliable, and referencing that in the future as a quick way of checking for possible issues.

I think it's time to stick a little camera in to investigate the slide. As a side effect that would also helps determine when the throttle adjustment screw comes into contact with it, I expect.

Thanks for the help

You should be able to see…

You should be able to see the slide quite easily. It might help if you remove seat and toolbox tray. Just open the throttle wide and snap it shut and see if it drops with a click. Do it with air slide fully up and also down.
The air pressure difference each side of the slide can make the slide a buy sticky. Modern chromed or hard anodised slides are much better than the original soft alloy ones, which wear out quickly and mess up the air/fuel ratio. Although Monoblocs aren't as bad as Concentrics for this because their design supports the slide better.

Thanks – I think the…

Thanks – I think the previous owner put a chromed slide in there, but I guess I'm going to find out. I wonder if checking for stickiness is worth doing when the engine's cold and when it's hot? It doesn't sound like too much trouble to do both.

I was referring to the fuel…

I was referring to the fuel level in the float chamber that can vary depending on float needles ,floats, and float needle jet seating .

Oh of course ... duh. Would…

Oh of course ... duh.

Would checking that be a matter of dismantling and inspecting, to see what level the float rises to and checking for cleanliness, that sort of thing? I have access to an ultrasonic cleaner, which could be handy I reckon.

Completely ignoring your…

Completely ignoring your temperature question (though I like your out of the box thinking), but hoping to help you in your investigation nevertheless:

My Model 50 would also stumble to a halt when idling hot, while also having a hickup when trying to catch it in time with the throttle or when driving off. Setting idle was also never very definite. It all came down to a worn carb slide, which I had then sleeved. Now the bike starts first kick, is very reactive to adjustment of the air screw and will actually idle when standing still at a crossing.

Regarding the exhaust rose; mine has a tendency to unscrew itself too. Normally, you notice it in time as the exhaust starts popping on the overrun,. But one time after a long stretch, the exhaust came off and rotated away from the bike while still sticking into the silencer. It's a good way to remove chrome, that's for sure...

I now lock wire the rose. Double!

Hope this might help you.

The slide seems to be firmly…

The slide seems to be firmly in frame for this job. I disassembled, and as I think the previous owner had mentioned the original had been replaced with a chromed version, which seems to move very cleanly but had quite a bit of gunk on outside towards the engine-side. I've put the whole lot through an ultrasonic clean, which has smartened it up, given the float valve a going at with jewellers rouge, and ordered a repair kit (including jets) so I'm hopeful of a result like yours - sounds like a dream!

The rose sounds like a nightmare. I was planning on some wire as a marker so i could tell if it's unscrewing, but that sounds like a more solid plan.

Cheers!

Just Sayin.

My ES2 had similar idle problems and it had a quite old Concentric, it was cured by replacing the insulation washers in the magneto.

True!

BUT my M20 BSA idle and starting was much improved with a brand new standard Concentic.

Don Anson
MMelbourne Australia.

So many things to go wrong :…

So many things to go wrong :(

I believe the magneto was overhauled recently, but if the carb clean and rebuild doesn't do the business, magneto's next on the list

Cheers Don!

Norton Owners Club