KOSO EGT calibration

[Joe]

I have a KOSO dual EGT gauge that is reading 1400 to 1500 degrees F on both cylinders when running in the mid range and 1250 to 1300 at WOT. The plugs come out a Hershey mild chocolate color. I have ridden it over 150 miles like this. If the gauge was telling the truth, would I have melted something by now?

[tyler440]

hmmm yeah thats seems pretty hot... my setup reads just barely over 1200 at WOT for 1500+ feet its going to depend alot on where you have the probes in the pipes... mine are in the middle of the first bend (6 inches from the piston i believe)

[Vmax540]

From the Articles Section (Dyno Tech)

"For those of you who monitor exhaust temperature, our experience on the dyno and in the field shows that the V- Max shouldn’t be run on 92 octane gas at much over 1150 degrees."

[Bob Vehring]

Alot of things come into play here, the distance from the piston, type of probe, if the gauge was calibrated and so on. Until your comfortable balance what the gauge says against piston top and plugs.
With the numbers you posted I would say raising the needles one or two notches would lower both midrange and top end back into the safe zone.
Often people don't understand that 90% of the time the needle and Needle jet control overall jetting. The needle moves up and down in the N Jet with the throttle, they each have a given taper. At any given point, they can only flow "X" amount of fuel. You can increase the main all day but still only "X" amount will flow past that point You may make WOT fatter because the needle is now up out of the NJ, but the mid or part throttle will remain the same temp until you move or change the needle setting.
Very good info is available by reading here;
http://www.sudco.com/Carburetor/SudcoMikuniCatalog.pdf
If it doesn't take you to the right page, start around page 40

[Joe]

If I drop the needle one notch, it blubbers in the mid range and does not come up on the pipe until I go over 90% throttle. It is horrible to ride like that. That was why the clip is in its present position.

I'm not brave enough to raise the clip one notch based on the EGT reading.

Why does the midrange run so much hotter than WOT? Is it because the flame front is further down the pipe at WOT? Would that justify moving the probes further down the pipe. They are currently 6 inches from the piston.

[Bob Vehring]

Moving the probes doesn't solve the problem, it will give you a cooler reading because the temp has cooled as it rolled down the pipe, its still the same temp coming out of the piston.
Have you changed jetting and it suddenly got that hot, or, did you just buy the gauge and have been running those same jets for a long time? If its the latter and hasn't burned down before, I would assume its the gauge or the probes.
A company named Digitron, makes a test unit to check a gauge, altho I'm not sure it would work with the Koso unit

The reason it is hotter in the midrange is because the needle jet and the need control the jetting in that range and the main has little to do with it. A Mickuni carb is a very good choice for trail riding simply because they offer adjustable jetting in any throttle position. From the bottom (low speed) up, the idle jet and the cut away on the slide control the idle and very low end, their also is an air correction jet on some sizes. From their the combination of NJ and Needle control the mid range. Each has its own taper, picture a cone within a cone. As the needle is moved up inside the NJ as the throttle is opened, it allows more fuel to flow in that range. That flow is still less then what the main will flow. Its not until WOT when the needle is pulled up almost out of the NJ that you are running off the main alone or in the case of the VM carbs, the main and the Power jets ( which really, I see no use for)

If you looked at the chart in the Sudco catalog I linked, you will see that both NJ and needles are listed and speced out so you can change jetting in any throttle position. I prefer to change needles because you can simply do it by un-screwing the cap rather then taking the entire carb apart to change the NJ. It took me about 10 needles to get what I wanted on my SRV, but now my temps stay 1000-1200 across the board and I simply raise or lower the needle as the ambient air temp changes day to day.

I had just started to dial in my VM last week when some one on here picked up from a picture, that my sled came with the wrong pipes. If the new pipes show up today, I will get back on that this weekend.
Every engine if other then stock will be different because of air flow. I don't run an air box, I have taper bored carbs and the engine is ported so I doubt what works for me will be right for you

I would find someone else with EGT on their sled and swap gauges, determine if your gauge is right, then if still hot, play with some needles to drop it down in the mid, with the right needle, you can pin point any range and adjust it.

[Vmax540]

Remember you can raise/lower the needles 1/2 position by moving the thin white nylon washers from below & above the needle clip.

[Bob Vehring]

Joe, I was just thinking, I have read that unlike analog gauges, with digital, they should be either right or wrong across their entire range. I can turn on any of the 3 set set of gauges I have on sleds when cold, and they will read the same. If the pipes are cooled down and not holding any heat, they will read the ambient air temp, same as the thermometer in my trailer, what the radio says or anything else. I know these work the same both in winter at 10 deg. or on karts in the summer at 90. Should give you an idea if their close.

[Joe]

The KOSO gauges read "LOW" instead of a number below 250F. That makes it very annoying that I can't check ambient nor place them in boiling water to check calibration.

Yes, the main jets have been in there a couple years. The sled developed a bad bog when E10 was mandated for all gas pumps in Washington State two years ago. I raised the main jet two sizes and got WOT running right. The mid range did not respond correctly when I lowered the clip. That is when I bought the gauges and noticed the high temperatures.

I took the clutch spring out for inspection last May and noticed it was no longer square on the end. It looked like a leaning tower. I replaced the spring last June and both mid range and WOT are now running correctly. The EGT temperatures are still bothering me.

There are no washers in my carbs. They have not been there since I have owned them.