Well, after remembering how well the 40mm seemed to run when I first put it on, and re-reading my initial posts on it.. I ran it today without any intake trickery pieces-just the 40mm carb alone.



Two short timed sprints netted identical 5.25 sec readings-just .13 sec behind the 36mm now, (including a slight AP bog that's probably adding at least .10 sec) and...

Checking a/f ratio, I see mid range is a little rich while WOT is 13s and 14s with the 147.5; which is a bit lean with 10% ethanol - so there's some power left to uncover once I rejet it to 150 or 152.5 and eliminate the AP bog by shortening it's stroke time or leaning the nozzle size.

There's a major typhoon heading towards the islands to our north, so the weather's bound to be a bit wet for the next few days. I'll have it rejetted and ready to go when the weather clears up. We're on the boot shaped island about 3/4" below the pp in the word Philippines, and out of the storm's direct path.(whew)


Eyeballing the intake port flange while putting the 40mm's ported intake back on, I see there's lots of Al there and it'll be no problem opening up the hole in the flange 1 mm all the way around to match the enlarged stock manifold when I get the urge. I wouldn't be surprise if opening up the intake flange hole a bit lets the 40mm work a little better and move past the 36mm in performance.



(Timing certified by: The Anaheim, Azusa & Cucamonga Sewing Circle, Book Review and Timing Association)

 

A change in main jet from 147.5 to 150 netted a drop from the previous time of 5.25 down to 5.16. Just .04 sec behind the 36s time with more jetting experiments to follow. I'll bet it's still a little lean because the small increase in jetting probably wasn't enough to counter the 13's and 14's a/f ratio I saw last time. I'll put a 152.5 in it today and check the a/f ratio.

Also helping was my reset of the AP to kick in after the slide was out of the way; producing a nice power wheelie today as I quickly rolled into WOT from 17kph. That alone could explain the ET reduction as it got rid of the slight bog I encountered last time. I may reduce the AP nozzle size down from 45 a little at some point.


- Just for fun, I pulled the carb boot to see if performance would improve...the bike slowed by .6 of a sec! I don't know if it was too lean or just the shorter intake tract was the problem, but it really had an effect on performance. (though the engine sounded okay).

 

Great thread!! Great tech!! Can't wait until you get into the MCM, 331 and beyond...

 

Ya I'm anxious to at least get at the MCM; just a couple more carb checks and it'll finally be time.

Normally I'd do the 331 right away and just start experimenting from that point, but stretching the mods out gives me bitty things to fiddle around with for a long time in this land of endless Summer.


I think next I'll open up that intake port flange hole a little after the next couple carb/jetting check sprints. I'm betting the 40mm performance will perk up if I "port match" the flange hole to the lightly larger intake

 

Yehey, the 152.2 main jet netted a 5.13 after a perfect 17 kph start. All but equal to the 36's 5.12 performance.

That 152.5 mj was reading low 12s and high 11s for a/f the other day and I was thinking it might be a little rich, but it seems to be just right-beating the 150 mj's time by just a little.


I guess it's time to make a more substantial change if I want to realize any more power from the engine with either carb. I think I'll port the intake flange a bit first and see how well that works with each carb before moving on to the MCM. A little intake & flange porting may turn out to be the thing to do if you have a 36mm carb.

 

In summary then, the 40mm ended up being exactly as quick as the 36mm carb in timed WOT sprints over a distance that basically took 5.10 sec for either carb. Which kind of correlates with Marcelino's comment that the cam mod doesn't work well on larger bore KLXs due to performance restrictions in the port or valves etc.

Venturi flow boosters didn't increase either carb's performance results, evidently because there hasn't been a loss of velocity with either carb. There may be more WOT power to be realized at any displacement once someone figures out what's holding it back.

 

Richard, the premise is that the MCM enhances TRQ and moves its' curve lower in the RPM range compared to standard timing. Talking Heads have spewed guesses that the mod makes no power above some RPM - usually they pull 8-8.5k outta their a$$e$$....
While my charts, to date, do not yet have valid numbers with a properly rich AFR, they do prove that the MCM does not drop power at all. In fact, the tendency to go lean occurs at the lower and upper RPM ranges with the stock CVK carb - this causes a "drop" in power in the upper ranges and a drop in TRQ in the lower ranges. The more you get and keep the WOT AFR closer to 12:1 , the more lower and upper power you get - you mainly notice that the top end wakes up through your seat-o-pants.

Uninformed dogma assumes the MCM would cause the BB to loose top end - I say the maximizing of volumetric efficiency with the MCM timings should occur regardless of the size of the piston in your KLX and there is no data supporting any kind of "drop off" of power with the MCM. Probably just a lot of peeps running around with 14+:1 AFRs @ WOT ....

 

Marcelino hisself suggested in that TT post I quoted that those wanting power above 8000rpm or so would be better off with a bigger carb etc rather than the MCM.

As (apparently) seen with the 40mm testing, there may be a limit to flow via the intake port and/or valves for both carb size and MCM results as the bigger bore demands more? Maybe the limiting factor starts with the size of the intake manifold hole?

I think 250cc will even handle more CFM in the upper rpm range, and am prepared to open up the intake flange hole aprx 1mm all the way around to see how the engine responds to the slightly enlarged intake & port. As soon as I get around to pulling the engine to do it..

If it doesn't work, the stock size intake manifold/port mismatch with aid in slowing any intake tract reversion during valve overlap.

I've got the 34 CVK installed, but we've had tropical rains over the last few days so things will have to dry out before I start fooling around with it and doing the MCM.