Maximum Cylinder Head Temperature?

[paydem]

I'll be buying CHT gauge for my Minarelli 70cc AC engine. I wonder, at what temperature should I stop and turn off the engine? Sorry if this was posted earlier, can't seem to find it or I haven't searched that much.

[ThaiGyro]

errmkk, there are some variable factors and opinions based on tuning...compression and such.

In my air cooled TA125, RD350/400 days on race engines, we worked well at 215-220 degrees F. (100-104+C) Max for the fuels and usual atmospheric conditions, at sea level, we ran 230F at the head. (110 C)

None of those temps got us in trouble, however any quick rise after warm up on a WOT run should be a warning. For example: running on a hot day in traffic say...225. Then you jump a green light and soon see 235 and climbing! Time to evaluate...stay calm and gradually back down on the throttle, allowing extra cooling and rich decel.

We did often soft seize, meaning we got tuning wrong for the conditions. But that was racing. For the street you should be tuned for slightly rich at WOT, so decel is always helping. (I tune lean in the mid range, but that is my preference)

There are other opinions. Explain what you are tuning for and others can easily shove you the right way. Street hooligan? Commuter? Top end crazy? No judging, just better to assist.

[90GTVert]

The best advice I can give you on max temp is to ignore most of the numbers we say and pay attention instead to what your particular setup does while you tune and ride. I've only had good results with a CHT from my own experience with each setup. I've owned 3-4 different brands/types of CHTs and they all ran a little different temps. I had one that read 330-350F WOT, and then switched gauge setups and it ran 280-300F. No way the cylinder just wanted to run 30-70F colder all of the sudden. Exactly how they're in or on the head and airflow and other variables will throw it off at least to some degree, aside from differences in the actual engine combo.

So mainly if I know mine ran x temperature WOT when tuned and stuck around there after, and then one day I start seeing it hotter; then I would start to worry. If I run 300 and ThaiGyro runs 230, I'm not that worried. That said, there are limits. With any sort of gauge setup and most engine combos, most don't want to see over ~350F WOT.

Be real careful with part throttle. The CHT is great there and helps you see where it runs a little richer or leaner.

[gsx600racer]

I try to keep temps in the low 300's range. Iv rode in the 350's arena, and have visited high 370-380 range but not for long.

There are so many variables that can control/effect engine temps.

Carb size
Carb jetting/tuning
Compression
Squish
Ignition timing
Spark plug heat range
Quality and type of fuel
Quality and type of 2t oil
Engines ability to breathe(intake and exhaust restriction)
Engine cooling fan, restrictions of air flow

[jack667]

I'm trying to keep my zumas under 350 CHT. But they seem to run really well at 350 give or take. I know I need to spend more time experimenting and just changing one thing.

I'm almost there with my Red '05 Zuma. The latest is that I can do most of my 5.5 mile test ride at WOT, but towards the end, I need to back off and pull over to let it cool a bit. I'm going to try a slightly larger main jet and hope that helps. I also want to swap out the oil from the crap that came with it to some better synth stuff. I have Malossi 7.1 in the other scoot and will probably stick with that for simplicity.

[repherence2]

A great minarelli tuner here in Hawaii told me not to exceed 350. So I tune to that. I have not had any problems with soft seizing a bore since then. One of that tuner's apprentice recommended that I tune for Wide Open Throttle, "because if your racing someone on the streets, most times you are going to be holding WOT during the race". So I blend the 2 pieces of advice together, and tune for WOT at 350° or less. That way, I can hold the throttle wide open (all day long) and it stays at 350. In my experience this has worked on cast iron as well as aluminium/nicasil bores, 47mm, 50mm, and 54mm.
Be aware that I use a fan shroud air scoop. I also use jog90 cooling fan on my 47mm bore engine, it required cutting and modifying the stock 50cc fan shroud. The 54mm bore block I have is virtually a jog90 so it has the large fan already.
Like a previous post stated, it depends on different factors and carb size is a factor. I was not able to tune the 47mm bore to stay below 350. That was with the stock chinarelli carb as well as with a Polini CP 24 carb. So I modified the shrowd, added a scoop, and the jog90 fan. On my 54mm bore block, the Polini CP 24 would run on the hotter side. So I ended up running an OKO 28. It guzzles a lot of gas, but it can supply more than enough fuel to cool the cylinder down.

[ThaiGyro]

Hahaha! My Big oops! The 62 year old brain...minus 100 degrees. The same story however. That would be Three hundred range. Just checked our Kawasaki 150 thing-a ding, ding, ding. It runs 340+/- at full throttle in the hot and humid Thai weather.