Jetting by engine temperature?

[ereksonj]

I used to own a nitro R/C car (before I realized it was as expensive as owning a blaster . The standard jetting procedure for a nitro car was to drive it hard and then take the temperature at the head of the engine to see how hot it was. I would shoot for about 220 degrees Fahrenheit.

Has anyone ever thought of using the same method to jet a normal sized engine? It makes sense to me... too lean = melted piston (too hot). Too rich is the opposite. I think the engine temp would have to be measured right at the top of the cylinder.

Anyone ever compared engine temps to plug chop readings? That would be interesting to see...

 

[Wifesblaster]

Yes. We do not jet off of temperature readings, but we do shoot them with the infrared gun as the weather changes.

 

[MrTwoJay]

So what head temperatures do you guys get?
I used to use exhast temps on the turbo conversion (cars) as a guide when things are starting to get lean, also an alarm set to warn you when things get too hot. A failing fuel pump/filter/regulator/injector can wreck a motor in seconds.

 

[Wifesblaster]

Our temp was right around 260. Cant remember if we took it from the head or cylinder. I will be running an exhaust gas temp gauge on my new build.

 

[MrTwoJay]

WOW, thats reasonably cool, was expecting to read higher temps.
I want to design a warning device when the cylinder gets to hot, maby even a few LED's, that indicate when the motor is warm enough to ride hard, normal, and too hot.
I will probably do that in a microchip, then who ever wants the code can let me know and ill mail it to them. (along with pcb layout).
My original idea was to build a tacho, with speedo in the middle, then add temp, but now im thinking it might be a waste, and too much effort (and bulky).

 

[ereksonj]

I was thinking of getting this Trail tech GPS computer with an engine temp gauge built into it and thought it would be a bonus if it could help me with my jetting, too.

They also sell a computer that is strictly a temperature gauge: Trail tech temperature gauge

The temp is taken between the spark plug and the head - about as close to the piston as you can get...

 

[ereksonj]

So what head temperatures do you guys get?
I used to use exhast temps on the turbo conversion (cars) as a guide when things are starting to get lean, also an alarm set to warn you when things get too hot. A failing fuel pump/filter/regulator/injector can wreck a motor in seconds.

I just took my infrared sensor and checked the temperature on my Yamaha XT250 after it had been ridden for about a half hour and the head was around 220-230, while the exhaust header was beyond it's measuring capability (430+ Fahrenheit).

 

[Wifesblaster]

We went through all 6 gears then did some doughnuts before we tested ours. Wanted to show the highest temps the bike will ever run.

 

[Herpderplol]

I have the trail tech with tempature gauge.

My readings are about 420. once hit 500 but i didnt like that....

 

[Wifesblaster]

That's pretty hot herp. Might want to bump up the main 1 or 2 sizes.

 

[Herpderplol]

I suspect new airleak since i checked it after the rebuild. I will fix it this winter before i ride again I:I

 

[Blaaster]

I may be old school, but a plug chop is the best way to determine jetting.

Detonation caused by a slightly lean AFR will not nessessarily show up as a significant increase in head temperature, but may change the temperature at and around when the exhaust port opens.

Jetting by temperature is a very risky business.

 

[Wifesblaster]

I like to plug chop too. But I also like to monitor the temp from time to time. I will be running an egt gauge on my bike and look forward to having piece of mind.