Modern Vespa

I've just watched my CHT slowly climb from 310F to 345F I was looking for the point where it might settle but I didn't have the nerve to risk letting it go higher. Sensor is in the head. I've been staying off dual carriageways so I hadn't held it over 60mph for any length of time until now.

It was interesting to see how it responded to the throttle, I didn't have to roll off much for it to slow and then stop climbing, then a bit more for it to start falling. I might need to go back up one size on the main.

Anyway, at what temp do the rest of you start to get concerned?

300f under the plug.

In the head probably 350-375°

Under the plug Scott is right at 300°

With an aluminum cylinder, I'd say 350 under plug, 375-400 in-head.

On my Lammy, I've seen close to 450 in-head, but I drilled that one myself and it's in there pretty deep, so closer to the heat. Of course, I also have an EGT and it was was both steady and well within range, so once the CHT settled in, I quit worrying about it. Rode for a 25 miles or so that way without issue.

If you're really concerned, get an EGT, IMO. An under-plug CHT takes close to a minute to register the actual cylinder head temperature.

A good example of this is to run the motor hard and then shut it off. You'll see the temp continue to rise a good 10-15 degrees over the next 30-45 second. Some of that is the lack of air cooling, but it lets you see the way that it takes time for the heat to radiate out.

The reason it rises when you shut off is because its the fuel that's providing the cooling to the piston. Shut off the fuel and the heat builds rapidly. It will then slowly come back down after that 30-45 second rise - that's when you can see the heat dissipating.
Another good example is if you have a slightly too rich main jet, and are cruising along on half throttle. The temp will actually fall as you open to full throttle even though you are accelerating. Its a good technique to regulate the temp on long runs.

I agree as well, EGT is so much more responsive. I use one from a microlight airplane that works off a thermocouple, so no need for power source. I bought it from the US some 15-20 years ago for about £200. Unfortunately with todays tariffs the same gauge is over £1000....

I bottled it well early compared to the last couple of replies. I'll give it another 60+ run and let it go higher.
It came down pretty quick when I rolled off, maybe 5 to 10 seconds before it started dropping, the text is small and the road wasn't straight so it's hard to be precise.
BGM 177 so the head has a fitting for the CHT right in the centre.

Do you have a picture?

Do EGT's work on AC current?

Normally they're either freestanding like dunf's (I have one like that on my Lammy) or 12V DC.

You have to weld a bung onto the exhaust for them, though, so they're not as plug n' play as a CHT.

This is not quite as easy as one size fits all. Depends on the top end material and the level of tune. And the capacity.
The minimum when near stock needs to be hot enough to at least attempt to burn the 2 stroke, >200F (oil specific). And hottest for large cc, nicasil, air cooled with higher tune, something like 400F but ideally cooler, if the jetting has capacity.

With the BGM sensor ports on mine the temp drops in seconds when shut off or past a hill.
Shut off, no fuel but also no combustion.

Under the plug/bolt sensors will continue in both cases to rise exactly as you describe.
Just my experiences.

I found on my 187 Super G engine that the in head CHT's were actually lower then with the under the plug sensor. This might be due to the center mounted plug being at a hotter location or even that the threads I tapped for the sensor weren't deep enough or otherwise botched.

I started to worry when temps started getting close to 330F and continuing to increase after 5 miles at 50mph+. By this time though, I was getting a hanging idle from an air leak which only became apparent after that particular ride.

Do you have the M10x1.0 sensor? I have a BGM 177 and I could not find the listed Koso parts in the datasheet for reasonable prices stateside. I ended up using a M10x1.0 bolt to put my existing Trailtech sparkplug ring sensor on the center temp port instead of the sparkplug, but I assume I am getting the same decreased responsiveness as a result.

https://kosonorthamerica.com/product/temperature-sensor-with-white-jst-connector-0-250c-%e2%88%bc-32-482f-50-k%cf%89/

^^^FTW

I had 1 under plug & 1 on head bolt... about 50f difference between them, & when plug got hot (& head got hot) you could cool plug quickly with a few blips of throttle, but head took 5-10min to dissipate heat

I do.
And yeah you are getting decreased responsiveness and readings, absolutely.
I snagged my Koso to Trailtech gauge spliced connection with the cowl pin on one of mine a while back.
Not wanting to repeat the micro surgery, I did just as you did.
It works as an insurance policy.

Yeah I wasn't keen on splicing in case there would be a calibration difference between the probe and the gauge. I had an even harder time finding the appropriate Koso gauge with a black plug (the ones with a white plug only read the temp range for a liquid cooled engine).

Edit: I checked the website again and I found a couple new additions that fit the bill! I was wanting one of the black/red small gauges but this one seems like a good package: https://kosonorthamerica.com/product/pro-1-rpm-and-temperature-meter/

I was impressed by this test. Jetted well still it stays cool with low airflow. Something like 260 to 300F.

Would you go 11.5 on your personal scooter? Not a jab at you, honest question. Cause I'd say a bit too rich, but I don't have much Vespa experience.

11.5 what?

On head on my VMC for long duration (3-5 min) incline, weighted with sidecar, highway speed (65-70mph) WOT:
When jetted for abuse, 373 degrees is settling point.
When jetted for fuel efficiency, it will creep up over 390 and I will back off.
It will go well beyond 400 safely, but probably not wisely.

As others note, the absolute value is different by motor and measurement source (plug, head, stud…)

In my view, it's also about steady state.
If it just seems to want to climb if you continue to hammer it, at some point it will seize.

OTOH, If it will plateau, it's probably safe.

There is at least race jetting, touring jetting and road jetting. All work in their own ways. That video, I thought was a good example of how the jetting/timing/head behave when set up for the road. Do what you like and doesn't go over 300F (plug). Will do tens of thousands of miles until you fancy something else. You decide when to rebuild, not the engine.

Wow.

Notice how the temp readings change at different targets on the cylinder?
Of course, the top center of the head is much hotter than anywhere else.

That really was impressive. Not sure if it would go higher in a real world situation but it certainly instills confidence. I wonder if he'll sell less high-flow fans as a result?
Would love to see the same test with his iron cylinder.

AFR

If you mean the 115 on the laptop, I'm pretty sure that's the speed in Km/h.

However, if it were, AFR meters readings vary, 11.5 on one can be 12.5 on another. +/- 1.0 is generally accepted as the wideband accuracy. More however, when the engine is hot, 2 stroke and under heavy load the accuracy can be +/- 2.0 possibly more.

Yep, it's speed. I had to zoom in to see. 😁

I ran 2 CHT sensors on my VMC Super G, one under the plug and another that screwed into the head.

The under plug sensor typically gave higher readings, though not always. Sometimes the screw in sensor did, other times they were pretty close. I chased high CHT's for a long time. Cruising steadily at speed made temps continue to climb compared to varied riding even if it pushed the engine harder. Jetting considerations seem to vary a lot with type of use.

For getting nervous, I start to pay attention at 320F and definitely by 350F.
400F is kind of the safe limit for air cooled automotive and aircraft engines with brief climbs to 420 considered ok if the engine is pushed for short periods. The considerations for 2 stroke engines are different though.

Related question...

i have a head plug CHT.

I have been tuning a mallosi 210 (sold as 223, but guessing 210) i bought as someone else's project, now mine.

I have been targeting ~100 degrees or under at steady state running. I have no reason other than it's what you'd run a water cooled engine at or below.

i didn't quite get a definitive answer of "how hot is too hot" from this thread, other than "it depends".

Don't have EGT

Hoping for advice, as if i can go hotter, i will (tiny bit rich, bit want a tourer so would rather be rich and stress free for miles than lean and stressed, bit if too rich, then obvs bad. plug is dark, bit not wet or black).

maintains ~100 deg on the flat at 100kph with moderate ambient temps (Vic winter in Melb) over about a 40k freeway run (my usual route to a regular activity)

love to hear anyone's opinion in addition to the responses in this thread for the described conditions.

Cheers

3-350°F is ideal for max temps it seems. So 150°C for you weirdos in the entirety of the rest of the world.

100C or 212F seems quite low. Air cooled engines run at hotter temps than water cooled and temps vary a lot more depending on engine loads and conditions. Water cooling works to keep the engine at a steady temp while air cooling allows the temps to vary within certain limits. Air cooled automotive engines often use an oil cooler to help with engine cooling and to maintain sufficient oil pressure.

Get the 100C riding up to 130C and it will be good to go for a long time.

If your in-head temp climbs above 150°C and won't drop with throttle blips, I'd shut it down.

thanks all. Good to know i can likely drop a main jet size.

Cheers

With my BGM 177 and head screwed in Temp measurement I can get up to 180 celsius if im thrashing it.
No issues as of yet.
I always use Motul oil.....

Another thing I forgot to mention is I have a nice Polini rev counter and temp display, that is programmed for max revs and temp.
An led blinks when the set temp is reached.
But the difficult part is knowing at what temp is too hot and it seizes.
For me at 180 celcius i have had no issues, and maybe it can handle hotter temps?

A modern alloy cylinder will likely not seize over 200C. Problem is that high temperature slowly wears out the entire top end including rod bearings and seals. If rotary an over heated crank can damage the pad too. The temperature for longest scooter engine life is around 130C at cruise.

130 celsius is about where Im at when cruising at 90 kmh or 55 mph.

Jet that thrash temperature down to 150C, while keeping the cruise temperature at 130C.