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KTM 300 38mm carb swap


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Some interesting pics of a carb swap from 36 to 38mm on an '07 300 XC-W.

 

The two intake bells, the stock 36 and the 38 that I cleaned up.

20150109_134715.jpg

 

Looking backwards through the intake boot towards the carb.  Apparently all 300s have this mismatch.

20150109_132657.jpg

 

 

The same view with the 38 after some minimal grinding and polishing to make it as smooth as possible.

20150109_135622.jpg

 

It is amazing how well the bike runs with such obvious intake shortcomings.  I'm looking forward to fine tuning this carb, which seems to be tending towards the same jetting as the 36.

Edited by Lehiguy
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very nice... im really interested what changes you notice...will be keeping an eye out..

 

did you just do this????  assume both in and out bells are same size and straight up bolt on????

Both the inlet and outlet of the two carbs have identical OD.  All other exterior dimensions are likewise identical.

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Some interesting pics of a carb swap from 36 to 38mm on an '07 300 XC-W.

 

The two intake bells, the stock 36 and the 38 that I cleaned up.

📎20150109_134715.jpg

 

Looking backwards through the intake boot towards the carb.  Apparently all 300s have this mismatch.

📎20150109_132657.jpg

 

 

The same view with the 38 after some minimal grinding and polishing to make it as smooth as possible.

📎20150109_135622.jpg

 

It is amazing how well the bike runs with such obvious intake shortcomings.  I'm looking forward to fine tuning this carb, which seems to be tending towards the same jetting as the 36.

 

Why did you want to go to the 38mm carb? Just curious, that is the carb that came stock on my 300.

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Why did you want to go to the 38mm carb? Just curious, that is the carb that came stock on my 300.

Just looking to see if it would run even stronger than it does.  I have ridden around the block on the pavement, but not off road, so the jury is still out.

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Just looking to see if it would run even stronger than it does.  I have ridden around the block on the pavement, but not off road, so the jury is still out.

 

Keep us posted, I am interested to find out how you like it. I've always wondered why KTM reduced the size of the carburators on the newer 300s

Edited by Fetona
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Keep us posted, I am interested to find out how you like it. I've always wondered why KTM reduced the size of the carburators on the newer 300s

 

Especially since my 125 can run the 38 or 39 carb too. 

These big carbs work surprisingly well on these small motors. 

I was thinking of swapping the other way so I am watching with interest.

 

Steve

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This what I have found out so far;  I smoothed out the intake bell of my 36mm like I had done to the 38mm, removing the flat face area around the bell, making one continuous radius.  It runs worse, less low end and mid-range, feels like I have a rubber throttle cable, very poor response.  I find it interesting that Dick's Racing offers this service when it clearly has a negative effect.  Apparently, the engineers at Keihin know what they're doing.  I will test the 38mm next in an off road situation.  I get the feeling that I'm going to end up with the stock 36mm back on the bike.  Again, giving credit to the engineers, this time at KTM.

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Especially since my 125 can run the 38 or 39 carb too. 

These big carbs work surprisingly well on these small motors. 

I was thinking of swapping the other way so I am watching with interest.

 

Steve

The higher the VE, the bigger the carb.  With larger displacement, VE actually goes down, but they have a much broader torque curve and require higher intake charge velocity which is most easily accomplished with a smaller carb.  Anyway, I'll post the details once I have them.

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  • 2 years later...

Ok, resurrecting my own thread with GOOD NEWS!  I am to blame for the first effort failing.  I tried to "improve" the intake bell and ruined the aerodynamics.  But I am not too stupid to learn from my mistakes.  This time I took a stock 38mm Keihin from my son's RM250, disabled the power jet and TPS, and went for a 30-mile test drive in the desert.  There are no downsides to the 38mm, NONE.  The torque increase is substantial, I climbed some crazy huge hills in third that I would have had to do in second before.  I used exactly the same jetting as I had in my 36mm.  I am very impressed and, frankly, wondering why it wasn't built this way to begin with.  For the record, the top end of this machine is a 2014, the pipe is a PC, the silencer an FMF shorty, red power valve spring, V-Force 4 reeds, ports are matched and polished, but not reshaped.  The machine is a beast with this set up, just huge torque and more top end than ever before.  Needless to say, I'll be buying a 38mm for permanent use on the 300, I may even have the 36 bored to 39mm.

Edited by Lehiguy
left out import facts. Duh.
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  • 3 weeks later...
On 3/24/2017 at 8:08 PM, Lehiguy said:

Ok, resurrecting my own thread with GOOD NEWS!  I am to blame for the first effort failing.  I tried to "improve" the intake bell and ruined the aerodynamics.  But I am not too stupid to learn from my mistakes.  This time I took a stock 38mm Keihin from my son's RM250, disabled the power jet and TPS, and went for a 30-mile test drive in the desert.  There are no downsides to the 38mm, NONE.  The torque increase is substantial, I climbed some crazy huge hills in third that I would have had to do in second before.  I used exactly the same jetting as I had in my 36mm.  I am very impressed and, frankly, wondering why it wasn't built this way to begin with.  For the record, the top end of this machine is a 2014, the pipe is a PC, the silencer an FMF shorty, red power valve spring, V-Force 4 reeds, ports are matched and polished, but not reshaped.  The machine is a beast with this set up, just huge torque and more top end than ever before.  Needless to say, I'll be buying a 38mm for permanent use on the 300, I may even have the 36 bored to 39mm.

Can you tell me your jetting specs right now with the 38 PWK on the 300?

Main:

Needle:

Needle clip position:

Pilot:

Altitude:

Temperature:

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2 minutes ago, redrider144 said:

Can you tell me your jetting specs right now with the 38 PWK on the 300?

There are oddities in KTM jetting specs, but I have found that the 250SX specs are an excellent place to start.  Do a search for the MXA article on their 300SX project bike.  They started with stock 250SX jetting and ended up going leaner even at sea level when doing their dyno runs.  This is what I use and it's perfect.  

Main: 152

Needle: N1EI (250SX)

Needle clip position: 2nd

Pilot: 38

Altitude: 5000+

Temperature: 70 ish.

At 2500 feet I go one up one size on the main and one groove lower on the clip position and it works great.

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29 minutes ago, Lehiguy said:

There are oddities in KTM jetting specs, but I have found that the 250SX specs are an excellent place to start.  Do a search for the MXA article on their 300SX project bike.  They started with stock 250SX jetting and ended up going leaner even at sea level when doing their dyno runs.  This is what I use and it's perfect.  

Main: 152

Needle: N1EI (250SX)

Needle clip position: 2nd

Pilot: 38

Altitude: 5000+

Temperature: 70 ish.

At 2500 feet I go one up one size on the main and one groove lower on the clip position and it works great.

A 300 should have slightly leaner jetting than a 250 with the same carburetor.  MXA's article doesn't really help me though as they were running a PWK 36, for which there is already a shitload of jetting information.

Your numbers help, as I can compare to some of the others running a 36 at high altitude.  

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1 minute ago, redrider144 said:

A 300 should have slightly leaner jetting than a 250 with the same carburetor.  MXA's article doesn't really help me though as they were running a PWK 36, for which there is already a shitload of jetting information.

Your numbers help, as I can compare to some of the others running a 36 at high altitude.  

Pretty much everyone reports that going from a 36mm to a 38mm requires no jetting changes.  If your 36 is dialed in, your work is already done.  

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