flexpipe length help...

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spincycle

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I'm running a 2 cycle for the first time this year, so I'm in learning mode....got a superstock kt100 with an RLV L4 pipe on it, question is, what length of flex pipe would be a good starting point? I read that Buller recommends 9 5/8" to start with their pipe on a Yamaha.....is that length of the actual piece of flex pipe, or total length from the back inside edge of the header to the back inside edge of the pipe? Thx!
 
I used to measure from the piston skirt thru the ex. port to the edge of the flange, and then use that as a fixed number and add to it for whatever I needed for flex.... meas. along centerline of exhaust tract...
 
Racer32 said:
That would be the total measurement from the piston skirt to the start of the first cone. L4 pipe uses about 8-1/2” plus/minus roughly a 1/4”
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Thanks for the help, got it set up at 8.5 from the Piston skirt.....getting close to firing it up....?
 
lastminuteracin said:
I may be wrong but I'm pretty sure the A and L series pipes run at around 7 1/2 from the back of the piston to the end of the flex.
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You may want to confirm with some one , as I believe this is right.
the flex length adjusts the power up and down the rpm range .
 
finally got to run the kt100 for this first time this past weekend...started with the flexpipe at 7 3/4 from piston skirt, engine pulled awesomely going for starts, but seemed maybe a tad bit like it was running out of that real hard pull about 3/4 down the chutes, max rpm was about 12,500, maybe try going to 8 or maybe 8 1/4' long flex? or is that a tad too much top end rpm for a superstock? no egt on the motor yet to tell what that's doing....
 
finally got to run the kt100 for this first time this past weekend...started with the flexpipe at 7 3/4 from piston skirt, engine pulled awesomely going for starts, but seemed maybe a tad bit like it was running out of that real hard pull about 3/4 down the chutes


Go shorter on the flex, no longer. Try 7 1/2". If you're always above clutch lock-up, have a 7 1/4" length to try.
 
Are you using the A or L series header? The A header will work but has the extra joint. The L header is the improvement and uses 1/4" spacers to adjust length. If you don't have one of those then the pipe's not going to work the way it was designed. I always ran 7-1/2" with 9,900 clutch and 15K max.

If you have a delta or G series (big) header you can adjust the flex clutch and gear to get good bottom end but you'll go off the pipe in the upper rpm.
 
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sundog said:
Are you using the A or L series header? The A header will work but has the extra joint. The L header is the improvement and uses 1/4" spacers to adjust length. If you don't have one of those then the pipe's not going to work the way it was designed. I always ran 7-1/2" with 9,900 clutch and 15K max.

If you have a delta or G series (big) header you can adjust the flex clutch and gear to get good bottom end but you'll go off the pipe in the upper rpm.
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RLV L4. i bought some muffler exhaust pipe tubing at the local auto parts store and have cut sections into different lengths.....
 
You can also cut a slot in the end of a ruler to go over the exhaust bridge. Makes it easier to hold the ruler on the piston next to the exhaust bridge. Or you can get a paint stir stick from home depot. Cut it in half lengthwise, slot the end for the brigde then make your own ruler marks on it.
 
I've spent a lot of years designing and building 2 cycle expansion chambers, just in case you wonder.
The best place to find out where to set the header length is at the Manufacturers. RLV has a website, I'm sure they have recommendations there. When they say what header length to run, they're talking about the distance from the face of the piston, to the weld on the 1st cone. The lengths from the piston face to the header flange is hard to measure with the pipe on the kart. Find out what it is and subtract that number from what RLV says and then you can measure from the header flange to the 1st cone.
 
I usually measure from the spark plug to the first cone Take off an inch( or half of piston size. It’s a Quick way to measure
 
I found this and saved it maybe it can help you?
 

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12,500? A stock KT 100 can easily turn 15,000+ RPM. After you get the correct header length from RLV, work on your tuning. Years ago, with their pipes, we most often ended up setting the carburetor at 1/4 top and 2 1/2 low. One advantage we had was the EGT. Nothing better for tuning in my opinion.
 
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