2015 TM MX 125 engine in a 125 cm^3 kart

[ptuomov]

Do you guys use a lambda meter with two strokes? Where do place the probe in the exhaust? What’s a good target lambda for shifter kart?

[David Redszus]

Do you guys use a lambda meter with two strokes? Where do place the probe in the exhaust? What’s a good target lambda for shifter kart?

We have been using lambda sensors on karts for about 20 years. On the TM125 Enduro SuperKart, the sensor was placed in the divergent cone of the tuned pipe; not in the header to avoid flow blockage. The EGT was located in the mid-section and the exhaust pressure sensor in the header about 4'" from the port.

Typical lambda values would range from .81 to .89. The numbers would often drift with rising EGTs down long straights.

Two stroke lambda values are very difficult to relate due to the ever present possibility of short-stopping and the absolute probability of inconsistent partial throttle lambda values.

[ptuomov]

Do you guys use a lambda meter with two strokes? Where do place the probe in the exhaust? What’s a good target lambda for shifter kart?

We have been using lambda sensors on karts for about 20 years. On the TM125 Enduro SuperKart, the sensor was placed in the divergent cone of the tuned pipe; not in the header to avoid flow blockage. The EGT was located in the mid-section and the exhaust pressure sensor in the header about 4'" from the port.

Typical lambda values would range from .81 to .89. The numbers would often drift with rising EGTs down long straights.

Two stroke lambda values are very difficult to relate due to the ever present possibility of short-stopping and the absolute probability of inconsistent partial throttle lambda values.

Is that an unlimited class as in no rev limiter and open engine? What was the last model year you tested?

What sorts of rpms did the driver use? We at the pits don’t know what we are doing with the car but the driver is very good for his age and uses 9500-14000 rpm range with the current setup.

In terms of EGTs, with that sensor location, what EGTs did you see at top speed rpm and at 10000 rpm?

In terms of the pressure sensor, are you using a slow pressure sensor for cycle average pressures or a fast pressure sensor to read pressures with one crank degree resolution?

I am having great fun but there’s no question that I’m out to lunch with this stuff. But it’s fun!

[David Redszus]

Is that an unlimited class as in no rev limiter and open engine?

Super Karts USA (SKUSA) was an unlimited professional class of kart racing, Displacement had to be stock but any porting, compression or pipe could be used. Fuel was limited to race gas. The field usually consisted of Yamaha, Honda, and TM engines with a number of Italian chassis like Tony Karts.

What was the last model year you tested?

We were involved at the origination of SKUSA which, if memory serves would be approx 1996 -2000.

What sorts of rpms did the driver use? We at the pits don’t know what we are doing with the car but the driver is very good for his age and uses 9500-14000 rpm range with the current setup.

That is a very wide rpm range for a 125 with a race tuned pipe. The usual operating range was about 11,000 to 14,000.

In terms of EGTs, with that sensor location, what EGTs did you see at top speed rpm and at 10000 rpm?

EGTs were located in the center section of the pipe in an attempt to obtain an average pipe temperature to determine pipe tuning. We found that EGTs were quite unstable and would change rapidly with throttle position, rpm, and time at wide open throttle. Pipe temps usually ranged from 1150F to 1225F depending on the track, pipe and ignition box being used. But, as you know, EGT readings do not reflect actual exhaust gas temperature but show a time weighted average of actual gas temps from 650F to 2000F.

In terms of the pressure sensor, are you using a slow pressure sensor for cycle average pressures or a fast pressure sensor to read pressures with one crank degree resolution?

We used a fast response pressure sensor. The sampling rate was 1000 Hz which was limited by the old Pi System data logger. But it did provide max and min values and a histogram of the data points and was very useful.

Partial throttle carburation was a source of problems; engine builders (including myself) did not test partial throttle on the dyno. It was only after we incorporated the AIM data system with a great many more sensors did we realize its importance.

The use if infra-red tire temperature sensors at all four corners provided useful information regarding chassis set up.
At one point we developed a FEA analysis of the tube chassis to determine its flex points and proper chassis tuning.
The belly pan proved to be the most important tuning element.

Another area that needed significant improvement was the braking system; they are all poorly engineered.

But, as with many open classes, a very well engineered and developed engine and chassis coupled with a professional driver, destroyed the rest of the field and led to over-regulation. Pussyies.

[brentry]

Just look up TM125 kzr1 or kz10
Homlogation , those are the actual shifter engines. Mx engine is way different.
I use to dyno 2 strokes
I checked the pipe temp in belly as well.
And used vannicks sim. .

[GRTfast]

Is that an unlimited class as in no rev limiter and open engine?

Super Karts USA (SKUSA) was an unlimited professional class of kart racing, Displacement had to be stock but any porting, compression or pipe could be used. Fuel was limited to race gas. The field usually consisted of Yamaha, Honda, and TM engines with a number of Italian chassis like Tony Karts.

What was the last model year you tested?

We were involved at the origination of SKUSA which, if memory serves would be approx 1996 -2000.

What sorts of rpms did the driver use? We at the pits don’t know what we are doing with the car but the driver is very good for his age and uses 9500-14000 rpm range with the current setup.

That is a very wide rpm range for a 125 with a race tuned pipe. The usual operating range was about 11,000 to 14,000.

In terms of EGTs, with that sensor location, what EGTs did you see at top speed rpm and at 10000 rpm?

EGTs were located in the center section of the pipe in an attempt to obtain an average pipe temperature to determine pipe tuning. We found that EGTs were quite unstable and would change rapidly with throttle position, rpm, and time at wide open throttle. Pipe temps usually ranged from 1150F to 1225F depending on the track, pipe and ignition box being used. But, as you know, EGT readings do not reflect actual exhaust gas temperature but show a time weighted average of actual gas temps from 650F to 2000F.

In terms of the pressure sensor, are you using a slow pressure sensor for cycle average pressures or a fast pressure sensor to read pressures with one crank degree resolution?

We used a fast response pressure sensor. The sampling rate was 1000 Hz which was limited by the old Pi System data logger. But it did provide max and min values and a histogram of the data points and was very useful.

Partial throttle carburation was a source of problems; engine builders (including myself) did not test partial throttle on the dyno. It was only after we incorporated the AIM data system with a great many more sensors did we realize its importance.

The use if infra-red tire temperature sensors at all four corners provided useful information regarding chassis set up.
At one point we developed a FEA analysis of the tube chassis to determine its flex points and proper chassis tuning.
The belly pan proved to be the most important tuning element.

Another area that needed significant improvement was the braking system; they are all poorly engineered.

But, as with many open classes, a very well engineered and developed engine and chassis coupled with a professional driver, destroyed the rest of the field and led to over-regulation. Pussyies.

Who were your drivers? Probably some of my friends.

[ptuomov]

Just look up TM125 kzr1 or kz10
Homlogation , those are the actual shifter engines. Mx engine is way different.
I use to dyno 2 strokes
I checked the pipe temp in belly as well.
And used vannicks sim. .

I think this is an MX engine not one of those KZ engines. It has two coolant ports in the head and it has a variable-geometry valve in the exhaust port. But what do I know, I’m on a vacation and an opportunity arose to hang around a small team.

I haven’t even looked at a carburetor for 30 years, so that’s also a learning experience. How do people solve the fundamental problem with carbs that WOT fueling at 9000 rpm has to come out a same size jet as WOT fueling at 14000 rpm? How’s that one size jet going to work for both?

By the way, as a side point, I found this technical documentation on Keihin carburetors excellent, given my starting point level of total ignorance:

https://www.keihin-na.com/assets/1/7/slide_valve.pdf

[GRTfast]

Just look up TM125 kzr1 or kz10
Homlogation , those are the actual shifter engines. Mx engine is way different.
I use to dyno 2 strokes
I checked the pipe temp in belly as well.
And used vannicks sim. .

I think this is an MX engine not one of those KZ engines. It has two coolant ports in the head and it has a variable-geometry valve in the exhaust port. But what do I know, I’m on a vacation and an opportunity arose to hang around a small team.

I haven’t even looked at a carburetor for 30 years, so that’s also a learning experience. How do people solve the fundamental problem with carbs that WOT fueling at 9000 rpm has to come out a same size jet as WOT fueling at 14000 rpm? How’s that one size jet going to work for both?

By the way, as a side point, I found this technical documentation on Keihin carburetors excellent, given my starting point level of total ignorance:

https://www.keihin-na.com/assets/1/7/slide_valve.pdf

The jet is metering fuel per unit air. More air getting sucked in at higher RPM = more fuel per unit time (and more air per unit time), but very close to the same amount of fuel (and air) per combustion event. Obviously this is a simplified view, but it makes the point.

From there you can tune the EGT curve with the ignition curve provided you have a programmable ignition system. As I (and others) have said before, getting the EGT curve right allows you to maximize the effect of the expansion chamber exhaust because the speed of the exhaust pulse is a function of the EGT. Finding that balance between the best advance to time the cylinder pressure for max work out, and to get the pipe optimized at any given RPM requires some talent.

[David Redszus]

Just look up TM125 kzr1 or kz10
Homlogation , those are the actual shifter engines. Mx engine is way different.
I use to dyno 2 strokes
I checked the pipe temp in belly as well.
And used vannicks sim. .

I think this is an MX engine not one of those KZ engines. It has two coolant ports in the head and it has a variable-geometry valve in the exhaust port. But what do I know, I’m on a vacation and an opportunity arose to hang around a small team.

I haven’t even looked at a carburetor for 30 years, so that’s also a learning experience. How do people solve the fundamental problem with carbs that WOT fueling at 9000 rpm has to come out a same size jet as WOT fueling at 14000 rpm? How’s that one size jet going to work for both?

By the way, as a side point, I found this technical documentation on Keihin carburetors excellent, given my starting point level of total ignorance:

https://www.keihin-na.com/assets/1/7/slide_valve.pdf

The jet is metering fuel per unit air. More air getting sucked in at higher RPM = more fuel per unit time (and more air per unit time), but very close to the same amount of fuel (and air) per revolution. Obviously this is a simplified view, but it makes the point.

From there you can tune the EGT curve with the ignition curve provided you have a programmable ignition system. As I (and others) have said before, getting the EGT curve right allows you to maximize the effect of the expansion chamber exhaust because the speed of the exhaust pulse is a function of the EGT. Finding that balance between the best advance to time the cylinder pressure for max work out, and to get the pipe optimized at any given RPM requires some talent.

The problem we ran into was running out of fuel at the end of long straights, i.e. Road America (road course, not kart track). The needle and seat size was inadequate to replenish the fuel drawn from the float bowl.

Your comment regarding fuel delivery is right on. While the carb meters fuel based on air speed (not air mass), they get richer with increasing rpm, an air correction jet was used on the Mikuni carbs along with a bowl extension to increase bowl fuel volume.

EGTs were another problem. Theoretically, based on port mechanics, rpm, pipe design, there is an optimum EGT that will cause the exhaust plugging pulse arrival angle to be correct. But EGT is a function of timing, throttle or load, and time at full load. Coupled with possible combustion in the header section and air cooling of the pipe at speed, it did not take long to learn just how formidable the tuning task actually was. Thankfully, we had comprehensive data collection to show us what we did wrong. And we did a lot that was wrong; primarily due to listening to other racers.

Fuels were another problem. A two stroke needs a fuel that is completely different than that needed by a four stroke.Once we developed such a fuel blend, life got much easier, engine destruction was minimized (none), and the piston crowns looked beautiful. Until the foolish sanctioning bodies began to write poorly understood fuel rules.
The problem persists today.

[GRTfast]

I think this is an MX engine not one of those KZ engines. It has two coolant ports in the head and it has a variable-geometry valve in the exhaust port. But what do I know, I’m on a vacation and an opportunity arose to hang around a small team.

I haven’t even looked at a carburetor for 30 years, so that’s also a learning experience. How do people solve the fundamental problem with carbs that WOT fueling at 9000 rpm has to come out a same size jet as WOT fueling at 14000 rpm? How’s that one size jet going to work for both?

By the way, as a side point, I found this technical documentation on Keihin carburetors excellent, given my starting point level of total ignorance:

https://www.keihin-na.com/assets/1/7/slide_valve.pdf

The jet is metering fuel per unit air. More air getting sucked in at higher RPM = more fuel per unit time (and more air per unit time), but very close to the same amount of fuel (and air) per revolution. Obviously this is a simplified view, but it makes the point.

From there you can tune the EGT curve with the ignition curve provided you have a programmable ignition system. As I (and others) have said before, getting the EGT curve right allows you to maximize the effect of the expansion chamber exhaust because the speed of the exhaust pulse is a function of the EGT. Finding that balance between the best advance to time the cylinder pressure for max work out, and to get the pipe optimized at any given RPM requires some talent.

The problem we ran into was running out of fuel at the end of long straights, i.e. Road America (road course, not kart track). The needle and seat size was inadequate to replenish the fuel drawn from the float bowl.

Your comment regarding fuel delivery is right on. While the carb meters fuel based on air speed (not air mass), they get richer with increasing rpm, an air correction jet was used on the Mikuni carbs along with a bowl extension to increase bowl fuel volume.

EGTs were another problem. Theoretically, based on port mechanics, rpm, pipe design, there is an optimum EGT that will cause the exhaust plugging pulse arrival angle to be correct. But EGT is a function of timing, throttle or load, and time at full load. Coupled with possible combustion in the header section and air cooling of the pipe at speed, it did not take long to learn just how formidable the tuning task actually was. Thankfully, we had comprehensive data collection to show us what we did wrong. And we did a lot that was wrong; primarily due to listening to other racers.

Fuels were another problem. A two stroke needs a fuel that is completely different than that needed by a four stroke.Once we developed such a fuel blend, life got much easier, engine destruction was minimized (none), and the piston crowns looked beautiful. Until the foolish sanctioning bodies began to write poorly understood fuel rules.
The problem persists today.

Great post. My dad always joked that getting all the parameters working together on the kart enginers was like calculating the muscle force for each face muscle and your diaphragm in order to produce a whistle of a certain pitch.

[ptuomov]

The jet is metering fuel per unit air. More air getting sucked in at higher RPM = more fuel per unit time (and more air per unit time), but very close to the same amount of fuel (and air) per combustion event. Obviously this is a simplified view, but it makes the point.

Here’s what I was thinking. I may be completely out to lunch.

The mass flow rate of air is proportional to air density times air velocity. The dynamic pressure of air flow (and thus the “suction” thru the jet) is one half times air density times velocity squared. Fuel is not compressible so the fuel flow rate is proportional to the square root of the static pressure differential between the bowl and throat. So the fuel flow rate is proportional to sqrt(density)*velocity. Mass air flow is proportional to density*velocity as noted above. The air-fuel ratio that the jet produces is therefore proportional to square root of air density. If the density stays constant, fuel-air stays constant, which is good. The concern is that the static density of air doesn’t stay constant as the velocity changes because air is compressible. Velocities close to the speed of sound will produce leaner mixture than velocities far slower than the speed of sound.

This is either carb 101 or wrong, I’m just trying to understand how a carburetor is supposed to work not having held on or thought about one in three decades plus before this vacation.

[GRTfast]

The jet is metering fuel per unit air. More air getting sucked in at higher RPM = more fuel per unit time (and more air per unit time), but very close to the same amount of fuel (and air) per combustion event. Obviously this is a simplified view, but it makes the point.

Here’s what I was thinking. I may be completely out to lunch.

The mass flow rate of air is proportional to air density times air velocity. The dynamic pressure of air flow (and thus the “suction” thru the jet) is one half times air density times velocity squared. Fuel is not compressible so the fuel flow rate is proportional to the square root of the static pressure differential between the bowl and throat. So the fuel flow rate is proportional to sqrt(density)*velocity. Mass air flow is proportional to density*velocity as noted above. The air-fuel ratio that the jet produces is therefore proportional to square root of air density. If the density stays constant, fuel-air stays constant, which is good. The concern is that the static density of air doesn’t stay constant as the velocity changes because air is compressible. Velocities close to the speed of sound will produce leaner mixture than velocities far slower than the speed of sound.

This is either carb 101 or wrong, I’m just trying to understand how a carburetor is supposed to work not having held on or thought about one in three decades plus before this vacation.

Need to think on that a little, it has been a while since I have done any serious fluid calculations. David probably has a reply closer to the front of his brain.

[brentry]

St probably isnt the place to talk about 2 strokes. But i use to believe a different fuel is needed. As well. Now i dont. To a extent. Depends. I have had to deal with hot pipes as well . But things have come a long ways in last 10 years.

[ptuomov]

St probably isnt the place to talk about 2 strokes. But i use to believe a different fuel is needed. As well. Now i dont. To a extent. Depends. I have had to deal with hot pipes as well . But things have come a long ways in last 10 years.

What is the right place to talk about two strokes?

[vannik]

St probably isnt the place to talk about 2 strokes. But i use to believe a different fuel is needed. As well. Now i dont. To a extent. Depends. I have had to deal with hot pipes as well . But things have come a long ways in last 10 years.

What is the right place to talk about two strokes?

https://www.kiwibiker.co.nz/forums/show ... 1131167672

Most serious 2T guys goes through the complete thread but that takes weeks!
Wobbly that posts there has been doing the development work for TM's latest engines.