Has anybody heard of jetting by exhaust temperature? A fellow who was working with me a couple of weeks ago used to be involved with racing motorcycles. He suggested installing a thermocouple in the exhaust pipe 8" from the cyl. head and monitor the temperature at full throttle for the main jet and 1/2 throttle for setting the needle. The desired temperature is 1425 to 1450 so if the temp is higher 1450 this means the mixture is too lean and if the temp is lower than 1425 means the mixture is too rich.
This makes a whole lot of sense to me so I have ordered a couple of 1/16" type K thermocouples from my supplier and will give it a go.
I would sure appreciate some comments, am I loco or would this really work?[img]i/expressions/face-icon-small-smile.gif[/img]

 

Is that in Fahrenheit or Celsius?

 

The 1425 to 1450 is in deg F

 

What about a fuel/air ratio gauge? I think k&n makes one?

 

I know quite a few people that use the exhaust gas thermocouple to monitor their engine. They mainly use it to help see what the motor is doing and so that they don't burn it up but it can also be used to control the jetting. I don't personally have one so I don't know what to look for but I do know that the distance from the engine that you mount it means a whole lot, there is a right and a wrong way to mount it. I would do some checking with HPD before I mounted one. They sell one and they are very knowlegable on the subject. Just my .02 cents.

Balzak
2000 modded up scrambler that is getting better all the time

 

We used to do this for the race cars. It makes sense if you think about it, and if you dont want to do the thermocouple thing, you can monitor the sparkplug(s) dark is rich, light brown almost white would be lean. Then you have to decide what to change though, timing? mixture?, jets sizes? spark plug temp range? From experience trial and error and alot of practice runs give you the answer it needs. Other factors are; altitude, humidity, air temp to name a few. So the thermocouple gives you some good information, but you can read into this alot more if you want to.

 

I used to have EGT gauges on a snowmobile, they are quite handy. Those temps seem way high to me. I had mine jetted to run about 1250. Maybe you can run a 4-stroke hotter, but I think you will melt a piston at 1400.

 

i have a digital EGT meter with a memory. it remembers the highest temp reached and you bring it up after the run by just pushing a button.you do not have to look while testing. 1450 is for a 4 cycle 1250 for a 2 cycle because the raw fuel coming out the pipe causes the temp to be lower. find a steep hill and mark your throttle for 1/4,1/2,3/4,and full and run up the steep hill at each throttle setting and get your readings.

 

Thanks for the responses. It looks like there is something to this method of jetting although the temperatures given to me may be on the high side. As one member mentioned there may be a difference between 2 and 4 stroke. If there are any more comments I would sure like to hear them. My thermocouple has not arrived yet so I have time to dig a little deeper.

 

This really is the most accurate way to jet your motor. In addition it will let you know (by temperature change) if you develope a problem so you can shut down before damage occurs. Even simple things like dirt in the needle seat or a vacuum leak will be reflected by a rise in temperature.
The key thing though is placement of the temperature probe. Make sure to follow the manufacturers directions carefully. A little distance in mounting of the probe one way or the other can change the accuracy of the reading.
The closer you run to the maximum temps the better it will run but it is riskier. Leave yourself a little fudge factor for the extreme times. You will be glad you did.......