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- EGT-

Start To Believe

The question - I just got a Digitron EGT gauge for 98 Polaris 400 sport. I put a 38 mm carb,RCR pipe, V-Force reed cage, clutch kit on. I need to know what temp to run and how jet changes will affect temp. I have tried a 300 in and my temp goes up over 1350 in 1/8 mile at full throttle so I shut down. I went to a 320 and it still wants to get up there but not as fast. It is so blubbery from 1/4 to half throttle 'til you get it wound out. Need help don't know where to go with jets. Also my tach want to read 4-5000 rpm at idle.

The response - Wow - you added a lot to it at once! How's your bank account now?!? How far is the probe from the piston anyway? Running at a true 1350 (f) for very long will melt aluminum very fast - depending on the probe location. You have to get that down to about to 1200 (for the brave) 1150 (safer), perhaps even 1100 wouldn't be a bad choice. You also have to be careful not to lean seize the motor in the midrange - so keep an eye on the gauge there too. Going over the safe limits there is a no-no.

I have an old (1988) Digitron unit. At that time the installation called for placing the probe at 6 to 7 inches from the piston face. It also instructed to let the probe enter the exhaust pipe 1/2 way in - that would let about 1" of probe sticking into the pipe and leaves quite a bit of probe sticking out. I don't know what the newer models recommend but what ever it is - place it exactly in that place. I run mine at 1150-1200 max at WOT for sand drags and 1050 to 1100 in the midrange while just cruising around. It almost never goes lower than 900 (f) no matter how low he RPM is.

Is that carb a 38 mm TMX Mikuni? Don't be afraid to go too rich with the main jet, (go up 3 to 4 sizes or more), then work your way back down.

Many times reed cage changes effect the jetting quite a bit - and not only on the top end. It is not uncommon for the Boyesen RAD valve (for example) to make a motor run very rich in the lower RPM, therefore requiring a needle clip change or two (raise the clip to lean it) and perhaps a different needle if it won't adjust out. The idle mixture screw will need adjustment and perhaps the slow jet will need a change too depending on whether the air screw lets it idle right.

I have no experience with the V-Force cage, but with its double reed block set up it must use light tension reeds in order for the motor to open them all at low RPM (I'm assuming they're all the same tension), this could be creating that over rich condition at low RPM.

Which RCR Pipe did you get? The pipe makes a BIG difference to the over all jetting. Did they supply you with jetting recommendations?

Which clutch kit did you install? Clutch kits generally increase the engagement PRM of the motor - and some of them increase the shift RPM too. You're dealing with additional mid range and top end jetting considerations either way. It sounds like the clutch kit may be over revving your motor before shifting.

Additionally - Did you tell the person you got the Digitron from you were using it on a 2 stroke? If you have a 4 stroke unit, your tach readings would be reading double.

Rick

My instructions say to install sensor 2" from the face of the exhaust port. Its also centered. My carb is the 38 mm TMX Mikuni. My pipe is RCR trail pipe. I called Rick at RCR Racing and he told me I should be running a 380 main, so I'm going to try that. Hot Seat Performance said I should be running my temp up to 1300. Don't know if I should believe him. Can you please explain what temps I should have at idle, half,and WOT. Do I just jet to get the right temp? My instruction say a too lean condition will be indicated by a drop in exhaust gas temp and an increase in cylinder head temp. just don't seem right. Also I'm from the froze up part of N.D. and my wife is still screaming about the money, but heck you can't take it with you when you go.

Cole

I put up a scan of an article about this on my web page a few days ago. It's not for your model gauge but it should help.

www.blastercentral.com

click.

/Enter

/Library

/Non-Blaster

/High-Tech Tuning

YFS200

Thanks for putting that on your site, YFS200 - a lot of people could use that info. I only take issue with the way the author describes the placement of the probe in the exhaust. He says his readings were taken with the probe 6 to 8 inches from the cylinder. That's a little too vague for me. Perhaps in the instructions with the unit they have more a more accurate placement procedure.

My probe is 6 1/2 inches from the face of the piston, which (here is the vagueness) is anywhere from 3 1/2 inches to 5 1/2 inches from the cylinder depending on where you start measuring that distance. I think it may be more accurate for them to specify exactly where to place the probe since bringing it closer or moving it away from the piston changes the temperature recorded dramatically. If the probe is placed in the exhaust and the plug is read properly the end user will be able to establish the temperature which works for his setup. Perhaps this is most important. On - MY MOTOR - Running to 1300(f) at WOT under racing conditions would be too high. I have seen that temp and a little higher, but I get better power running at 1200(f). The hills I race on are VERY steep and VERY long. Keep in mind that with - MY MOTOR - a change in one size of the main jet will change the WOT temperature about 50 degrees. So, if (with my 39mm PWK) I have a 190 main jet and I'm running at 1050 (f) at WOT, dropping to a 188 will bring me up to about 1100 (f). Dropping to a 185 will bring me to about 1150 (f) and so on.

I wrote yesterday :

"I run mine at 1150-1200 max at WOT for sand drags and 1050 to 1100 in the midrange while just cruising around. It almost never goes lower than 900 (f) no matter how low he RPM is."

I might add to that - Average dune RPM's would be around 5000 - 6000. At idle the temp is about 300 (f).

To sum it up - Start big with the main jet (the 380 sounds good) and make several passes at WOT under load and average the max temp you get. Pick a temperature you want to shoot for. I still think 1300 is too high for most applications. Maybe shoot for 1150 to 1200, then see how it feels.

Once the main is where you want it, do some midrange cruising. Keep the motor in the normal cruise range RPM - perhaps at a point where the motor just starts coming on the pipe - and note the temperature. If it's too high, you'll need to richen up the midrange by lowering the clip on the needle (to raise the needle). Shoot for a temperature about 100 to 150 degrees cooler than your main WOT temp. Change the idle mixture screw to provide the highest RPM at idle. If it increases too much, turn the idle down. Make a note of your idle temp.

With a motor like - MINE - it is not uncommon for me to have to change the main jet 3 to 7 times a day and make 4 or 5 needle clip adjustments. This is because of air temperature (RAD) changes and so on. It is not necessary to do this amount of work unless you want your motor at peak operating temperature all the time. I make changes when there is a change of 30 to 50 degrees in the exhaust gas temp while racing. For pleasure riding I'll let it go to 100 degrees before changing it.

You CAN jet-it-and-forget-it, many people do. But since you have invested so much money into your motor you might find it worthwhile to make changes when there is a change of more than 100 - 150 degrees.

Rick

Thanks for the help. I am planning to get a Digitron EGT gauge as soon as my engine gets back. I think the plan is to use the stock carb with 92 pump as a "jet-it-and-forget-it" not THAT worried about performance.(need better gas mileage). I am looking at a larger carb for 105 race that I will fine tune for Banshee beating performance. (ya right:) Best of both worlds I guess. Thanks for the help,

YFS200

Many TRX owners have their stock 34 mm Keihin flat slide carbs on a shelf or sitting of a box somewhere. You should see if you can find one. It'll be cheaper and you can still get parts for it very easily. Otherwise the 35 mm PWK works very well on that motor.

Rick

Good idea. I was looking at a 34 mm. I don't really want the screaming top end ONLY of a 35 mm. Most of my riding will be in the mid range, with peaks in the red line. I am not sure if I should get a Mikuni or Keihin. Kenihin seems to be the choice of Duncan(who has my top end for porting). But Mikuni is more familiar. Choices, Choices.

YFS200

I'm not as familiar with Mikuni products as I should be - I haven't kept up on them that much. We had a 38 mm Mikuni on an Odyssey 350 a long time ago. The last one I had was a 40 or 41 mm (I forget) Pro series that I used on a Quad 500. Do they still require you to use a screwdriver to remove 2 screws on the carb top to access the slide/needle? Do they still put small Phillips screws inside the slide to hold the throttle cable in place there too? I remember wishing they had changed that when I had to mess with all those small screws. It was so tedious.

The Keihin has a screw-off top to access the slide. The throttle cable removes easily and the needle access is fast since you use a jet wrench to turn it (you can use a screwdriver if you want). Needle changes are quick - usually about 3-4 minutes tops and I'm back on my way.

Rick

No, it's a screw top, but there is a screw holding a bracket to prevent the top from unscrewing during that all important jump off the bed of the truck. (Don't need no stinken ramps.:)

You wrote:

"Do they still put small Phillips screws inside the slide to hold the throttle cable in place there too?"
Yep. But they just hold the needle in. A pain.

I can change the main jet in about 4 minutes. With the rebuild of my Blaster, I am making a lot of the nuts and bolts hand removable for quicker service. Should help.

You made a lot of good points. I have a Keihin on the Ody. Had it apart more times then I can remember until I got the float set right. No problems now. I guess when I get the money, I will make a decision. Thanks.

YFS200

I went to a 400 main jet,down to 5 groove on needle. At WOT temp around 1220 no matter how long I hold it on. Mid range temp wants to go past 1300 if left on. Just tearing around on and off throttle temps in 1100. Nice and crisp,no blubbering in bottom end. But sometimes bogs when I let off and get on throttle on bottom end. How can I get mid range temp down and fix bog? By the way when I was testing temp in North Dakota was 10 below zero (brrrrrrr dam cold). Thanks

Cole

That sub zero temp makes jetting even more difficult. The motor's packing more punch with that cold dense air. The fuel is more dense too. You have to run a richer setup because of it. Your WOT sounds about right, though you may need to go to the next richer needle if your clip is already at the bottom. Needles are cheap. When you get the new one put the clip in the middle position and try it out. Running too lean here can make a motor sluggish. It seems like you're getting very close to where you want to be.

Rick

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Start To Believe
The question - I just got a Digitron EGT gauge for 98 Polaris 400 sport. I put a 38 mm carb,RCR pipe, V-Force reed cage, clutch kit on. I need to know what temp to run and how jet changes will affect temp. I have tried a 300 in and my temp goes up over 1350 in 1/8 mile at full throttle so I shut down. I went to a 320 and it still wants to get up there but not as fast. It is so blubbery from 1/4 to half throttle 'til you get it wound out. Need help don't know where to go with jets. Also my tach want to read 4-5000 rpm at idle.
The response - Wow - you added a lot to it at once! How's your bank account now?!? How far is the probe from the piston anyway? Running at a true 1350 (f) for very long will melt aluminum very fast - depending on the probe location. You have to get that down to about to 1200 (for the brave) 1150 (safer), perhaps even 1100 wouldn't be a bad choice. You also have to be careful not to lean seize the motor in the midrange - so keep an eye on the gauge there too. Going over the safe limits there is a no-no. I have an old (1988) Digitron unit. At that time the installation called for placing the probe at 6 to 7 inches from the piston face. It also instructed to let the probe enter the exhaust pipe 1/2 way in - that would let about 1" of probe sticking into the pipe and leaves quite a bit of probe sticking out. I don't know what the newer models recommend but what ever it is - place it exactly in that place. I run mine at 1150-1200 max at WOT for sand drags and 1050 to 1100 in the midrange while just cruising around. It almost never goes lower than 900 (f) no matter how low he RPM is. Is that carb a 38 mm TMX Mikuni? Don't be afraid to go too rich with the main jet, (go up 3 to 4 sizes or more), then work your way back down. Many times reed cage changes effect the jetting quite a bit - and not only on the top end. It is not uncommon for the Boyesen RAD valve (for example) to make a motor run very rich in the lower RPM, therefore requiring a needle clip change or two (raise the clip to lean it) and perhaps a different needle if it won't adjust out. The idle mixture screw will need adjustment and perhaps the slow jet will need a change too depending on whether the air screw lets it idle right. I have no experience with the V-Force cage, but with its double reed block set up it must use light tension reeds in order for the motor to open them all at low RPM (I'm assuming they're all the same tension), this could be creating that over rich condition at low RPM. Which RCR Pipe did you get? The pipe makes a BIG difference to the over all jetting. Did they supply you with jetting recommendations? Which clutch kit did you install? Clutch kits generally increase the engagement PRM of the motor - and some of them increase the shift RPM too. You're dealing with additional mid range and top end jetting considerations either way. It sounds like the clutch kit may be over revving your motor before shifting. Additionally - Did you tell the person you got the Digitron from you were using it on a 2 stroke? If you have a 4 stroke unit, your tach readings would be reading double. Rick My instructions say to install sensor 2" from the face of the exhaust port. Its also centered. My carb is the 38 mm TMX Mikuni. My pipe is RCR trail pipe. I called Rick at RCR Racing and he told me I should be running a 380 main, so I'm going to try that. Hot Seat Performance said I should be running my temp up to 1300. Don't know if I should believe him. Can you please explain what temps I should have at idle, half,and WOT. Do I just jet to get the right temp? My instruction say a too lean condition will be indicated by a drop in exhaust gas temp and an increase in cylinder head temp. just don't seem right. Also I'm from the froze up part of N.D. and my wife is still screaming about the money, but heck you can't take it with you when you go. Cole I put up a scan of an article about this on my web page a few days ago. It's not for your model gauge but it should help. www.blastercentral.com click. /Enter /Library /Non-Blaster /High-Tech Tuning YFS200 Thanks for putting that on your site, YFS200 - a lot of people could use that info. I only take issue with the way the author describes the placement of the probe in the exhaust. He says his readings were taken with the probe 6 to 8 inches from the cylinder. That's a little too vague for me. Perhaps in the instructions with the unit they have more a more accurate placement procedure. My probe is 6 1/2 inches from the face of the piston, which (here is the vagueness) is anywhere from 3 1/2 inches to 5 1/2 inches from the cylinder depending on where you start measuring that distance. I think it may be more accurate for them to specify exactly where to place the probe since bringing it closer or moving it away from the piston changes the temperature recorded dramatically. If the probe is placed in the exhaust and the plug is read properly the end user will be able to establish the temperature which works for his setup. Perhaps this is most important. On - MY MOTOR - Running to 1300(f) at WOT under racing conditions would be too high. I have seen that temp and a little higher, but I get better power running at 1200(f). The hills I race on are VERY steep and VERY long. Keep in mind that with - MY MOTOR - a change in one size of the main jet will change the WOT temperature about 50 degrees. So, if (with my 39mm PWK) I have a 190 main jet and I'm running at 1050 (f) at WOT, dropping to a 188 will bring me up to about 1100 (f). Dropping to a 185 will bring me to about 1150 (f) and so on. I wrote yesterday : "I run mine at 1150-1200 max at WOT for sand drags and 1050 to 1100 in the midrange while just cruising around. It almost never goes lower than 900 (f) no matter how low he RPM is." I might add to that - Average dune RPM's would be around 5000 - 6000. At idle the temp is about 300 (f). To sum it up - Start big with the main jet (the 380 sounds good) and make several passes at WOT under load and average the max temp you get. Pick a temperature you want to shoot for. I still think 1300 is too high for most applications. Maybe shoot for 1150 to 1200, then see how it feels. Once the main is where you want it, do some midrange cruising. Keep the motor in the normal cruise range RPM - perhaps at a point where the motor just starts coming on the pipe - and note the temperature. If it's too high, you'll need to richen up the midrange by lowering the clip on the needle (to raise the needle). Shoot for a temperature about 100 to 150 degrees cooler than your main WOT temp. Change the idle mixture screw to provide the highest RPM at idle. If it increases too much, turn the idle down. Make a note of your idle temp. With a motor like - MINE - it is not uncommon for me to have to change the main jet 3 to 7 times a day and make 4 or 5 needle clip adjustments. This is because of air temperature (RAD) changes and so on. It is not necessary to do this amount of work unless you want your motor at peak operating temperature all the time. I make changes when there is a change of 30 to 50 degrees in the exhaust gas temp while racing. For pleasure riding I'll let it go to 100 degrees before changing it. You CAN jet-it-and-forget-it, many people do. But since you have invested so much money into your motor you might find it worthwhile to make changes when there is a change of more than 100 - 150 degrees. Rick Thanks for the help. I am planning to get a Digitron EGT gauge as soon as my engine gets back. I think the plan is to use the stock carb with 92 pump as a "jet-it-and-forget-it" not THAT worried about performance.(need better gas mileage). I am looking at a larger carb for 105 race that I will fine tune for Banshee beating performance. (ya right:) Best of both worlds I guess. Thanks for the help, YFS200 Many TRX owners have their stock 34 mm Keihin flat slide carbs on a shelf or sitting of a box somewhere. You should see if you can find one. It'll be cheaper and you can still get parts for it very easily. Otherwise the 35 mm PWK works very well on that motor. Rick Good idea. I was looking at a 34 mm. I don't really want the screaming top end ONLY of a 35 mm. Most of my riding will be in the mid range, with peaks in the red line. I am not sure if I should get a Mikuni or Keihin. Kenihin seems to be the choice of Duncan(who has my top end for porting). But Mikuni is more familiar. Choices, Choices. YFS200 I'm not as familiar with Mikuni products as I should be - I haven't kept up on them that much. We had a 38 mm Mikuni on an Odyssey 350 a long time ago. The last one I had was a 40 or 41 mm (I forget) Pro series that I used on a Quad 500. Do they still require you to use a screwdriver to remove 2 screws on the carb top to access the slide/needle? Do they still put small Phillips screws inside the slide to hold the throttle cable in place there too? I remember wishing they had changed that when I had to mess with all those small screws. It was so tedious. The Keihin has a screw-off top to access the slide. The throttle cable removes easily and the needle access is fast since you use a jet wrench to turn it (you can use a screwdriver if you want). Needle changes are quick - usually about 3-4 minutes tops and I'm back on my way. Rick No, it's a screw top, but there is a screw holding a bracket to prevent the top from unscrewing during that all important jump off the bed of the truck. (Don't need no stinken ramps.:) You wrote: "Do they still put small Phillips screws inside the slide to hold the throttle cable in place there too?" Yep. But they just hold the needle in. A pain. I can change the main jet in about 4 minutes. With the rebuild of my Blaster, I am making a lot of the nuts and bolts hand removable for quicker service. Should help. You made a lot of good points. I have a Keihin on the Ody. Had it apart more times then I can remember until I got the float set right. No problems now. I guess when I get the money, I will make a decision. Thanks. YFS200 I went to a 400 main jet,down to 5 groove on needle. At WOT temp around 1220 no matter how long I hold it on. Mid range temp wants to go past 1300 if left on. Just tearing around on and off throttle temps in 1100. Nice and crisp,no blubbering in bottom end. But sometimes bogs when I let off and get on throttle on bottom end. How can I get mid range temp down and fix bog? By the way when I was testing temp in North Dakota was 10 below zero (brrrrrrr dam cold). Thanks Cole That sub zero temp makes jetting even more difficult. The motor's packing more punch with that cold dense air. The fuel is more dense too. You have to run a richer setup because of it. Your WOT sounds about right, though you may need to go to the next richer needle if your clip is already at the bottom. Needles are cheap. When you get the new one put the clip in the middle position and try it out. Running too lean here can make a motor sluggish. It seems like you're getting very close to where you want to be. Rick