Well I am going to give the sewer pipe a try, just to see what happens. If need be I'll have the bottom of the intake to manifold protrusion welded up a bit thicker so I can open that entrance to the engine's port up something closer to 40mm

If I can't open up a stock intake manifold larger, I'll have to get one made from metal using rubber to connect both ends. But then again the 44mm OD of the HS40 will fit into the stock manifold, so I can at least start with that important step and it'll all bolt on just like my TM36.

For as much apparent low end as my KLX/TM36 still has (14/48), it'd sure seems like there's plenty of low end power available to trade for a higher rpm power boost. I probably won't put it on until my a/f meter is here to save a lot of guesswork while doing the jetting set up.

I have very small gas tank coming so I can ride it then check the plug and make adjustment without a lengthy disassembly process, along with a fistful of needle jets, jet needles, main jets, and air jets

There are kits that offer 4 40mm carbs for 4cyl highly tuned engines in the 1200cc range afterall So how it works on both the 250 and the 331 is still to be determined.

It's just an experiment. If if doesn't work I can sell the hardly used Mikuni 40mm pumper to someone with a 650 or maybe even a Harley back in the USA.

Motoman wants to roughen the floor to help his epoxy hold is all remember about that part of the article, but I wasn't taking it in word by word,

 

You really should consult some people who have the experience, like the guy who did the development on the 351 kit, they have done the work. Then consider what the riders do with the 650s, they don't use a 44 or 46, they use a 40 or 41 and usually just because they're slide carbs, not the size. the stock carbs are usually 40s. If that doesn't shine enough light consider what the actual high performance MX bikes have - 250s run 37s, 450s run 41s.

But if you decide to do the adventure, get some kind of baseline performance to compare. If you don't have access to a dyno, do some kind of up hill acceleration rate or the like to test the mid-range. You might feel a kick in the pants that may just be representing a loss of mid-range and a burst into the upper range that may not represent any real gain. We saw that on two strokes, guys sticking 36 or 38mm carbs on 125s. When they hit they hit fairly hard, but narrowed the power band and pushed it up higher in rpm. A 32mm worked best, even on the flat track. Power isn't always represented well by the seat of the pants, something to demonstrate increased performance is needed.

Have fun, good luck, and keep us up if you go with the 40.

 

But why is bill using the 34 if the 36 is often talked about being better (can run with airbox lid off). Maybe the 40 will be better than the 36 delivery (and tunability) is also important, not just throat size. Maybe the 40 has a better design that allows easier tuning without loss of hp?

 

Thanks for the input. Using this formula, i have settle on the 15/42 ratio for now. After 351... back to the 39t in back!

 

It's already at my shipper in AZ waiting to be sent to me here in the Philippines along with a variety of jetting and the all-important a/f meter to get it dialed in correctly.

I'll try it at various levels: 250 with/without the cam mod; 331 with/without the cam mod

Seat of the pants is just too vague unless there's been 10% change or more and a bit hard on parts as you repeatedly go thru the gears trying to sense changes in jetting/power

Hopefully I can convince someone with a larger bike to do some comparisons; most bikes here are sub 250cc.

 

can the kls250s fit 15/39 gearing without modifications? is know the sf model uses the 39T as the standard rear, i need higher gearing for more highway, i will try the 15/42 first then see how

 

39 will fit, im not sure if you need a smaller chain or the adjusters have enough room to be moved back

 

From what I've read and learned from a number of 36 users, the throttle linkage is on the proper side for easy bolt on. It is also less costly than Bill's 33 bored to a 34mm Keihin after his modifications, at $288 versus the BB carb at around $350. And the fact is the 36 is only 2mm bigger than OEM 34mm CV carb.

Contrary to your comment, venturi area is the primary concern - proper air flow, not just volume, but velocity. Velocity creates the draw to syphon fuel from the float bowl. Too slow, low draw.

Now knowing venturi area will be Pi times the radius squared it is time for some numbers - a 34mm carb has a throat area of 908 square mm, a 36 has 1018 square mm, and a 40 has 1257 square mm. The 36 is 12% bigger, the 40 is over 38% bigger!

You have to understand fluid flow. For the same volume of flow the bigger the intake venturi the slower the flow, the lower the vacuum, the less draw on the needle jet (the tube through which the fuel is drawn). That not only can present jetting headaches, but with slower flow the charge entering the intake has less ram effect (momentum of flow).

The 40 is too big for the volume of flow for a 250cc cylinder (and borderline for a 400cc cylinder since the trick on a DRz is to install a 39mm FCR or a 40mm TM, not a 41 or 42) to keep good velocity to draw fuel and efficiently fill the cylinder. It takes a certain amount of draw to use specific sizes of carbs.

To throw in a bit of history, on two strokes the tuners usually stepped up two millimeters in size until the factories got really competitive and started putting them on as original equipment. My KLX has a 40mm CV carb and the trick is to replace it with a Keihin FCR41 - one millimeter bigger, but the key is that it has a cable slide throttle, not vacuum.

The air flow issues are the same with cars. There is a HUGE four barrel carb, the Holly Dominator, that is used on big inch high rpm racers. Looks good, big and all, but will drown even a high performance street vehicle, flowing 1040 cfm versus the more common four barrels used, that flow 780 and 850 cfm. Flow at the rate required by a street car makes the Dominator a problem and waste to run. Then there was the Ford Boss Mustang project where they had a set of heads with HUGE round ports (nicknamed sewer port head, where I got my name for the huge carb). It didn't flow as good as smaller square port heads - too big, low velocity, less ram effect, lower efficiency.

You think if a carb could add more horsepower and be easier to tune that the factory wouldn't have a 40mm or bigger carb on their 250 MX bikes? There is a reason why they use a 37mm on the 250 MX bikes and 41mm on the 450 MX bikes. If they could pull more USEABLE power from a 44mm they WOULD put it on.

In my questioning about putting a Keihin FCR37mm carb on the KLX, because they could be had for half the price of the Mikuni it seems no one has gotten the jetting down pat, plus linkage issues. There is the fact that the 37 has a venturi area of 1075 square mm, which is 18% bigger - the venturi area grows exponentially. Based on what I've read and seen I'm guessing the break point is around 15%, over 15% is too much increase to keep good velocity in the intake. Odds are the 40 will present similar issues and the gains would be really questionable due to the fluid dynamics of the huge venturi.

Summary:

• 36mm has a 12% increase in venturi area.
• 37mm has an 18% increase in venturi area
• 40mm has a 38% increase in venturi area

Kinda like why you always buy the large pizza, it may only be a few inches bigger, but the actual area increases exponentially, a 12" pizza is more than twice as big as an 8" pizza (Pi radius squared - or in this case Pie radius squared - pun intended).

But hey, if someone has the money why not play... I just did it on paper first.

 

The TM36 is probably going to require some jets to get it right. I think I spend $50 or more. Bill's 34 may not need additional jetting, but I'm not sure of that.

 

Now another question for you since you have much more technical knowledge than I do, for a 351cc klx, would the gain from air flow volume of a larger 40mm carb, even with a slower velocity, coupled with perhaps a ported head or longer duration camshaft be possibly still more beneficial to that with less air flow but a higher velocity of a let's say, 34mm carb?

I guess what I'm asking is, if you were to stick a 40mm carbutetor on a 351 pot, what changes, if any, to the top end could be made to capitalize on the increase in CFM?

Some already have a 351 with a tm36-68 as well as a ported head work and webcams 101 grind I'm sure, would they see a benefit going to the larger carb?