Are these two members still running the MCM on their 351s? I recall someone saying the gain was so little that it wasn't worth the risk. I would need some long term testing before I would consider. When I talked to Bill about it he mentioned someone had done the timing advance and damaged their motor. Maybe they didn't time it correctly? We can theorize about why there should be no issues doing the MCM on the 351 but I wouldn't want to be the guy that finds out "oh, that's what happen when you do it".

 

No, they are not. David R finally decided he did not like the combo after initially praising it . His analysis posts remind me of my Fraternity meetings - drunken incoherent inconsistent babble. Also, he ran "personalized" mods on the exhaust and induction systems that were quite "outside the envelope" (very restrictive) from the norm.

Jhoffy22 did the MCM, 351, and I don't remember what all else, all at once. After 5+ hours of running, his engine failed - all the pictures etc etc concerning his engine failure have been removed from the MCM thread. While it seems it was head gasket failure due to a flawed install - this event ended the MCM+351 testing.. Blame for this event was a shotgun blast whereby anything and everything was considered suspect - so the MCM got its' share..

I know we are a collection of folks with vastly different capabilities, knowledge, and beliefs - I understand any of us being fearful of MCM+351 as data and experience with the combo is minimal. Someone is going to have to "break the chain of fear" by running and testing it. I have almost zero fear of 351+MCM as it actually HAS been run, and run hard, by David R. If it causes engine damage, he and Jhoffy22 would not have been able to run it. I would measure stock vs MCM valve clearances to verify MCM compatibility with Bills piston before running it - as doing that is just prudent as well as good information...

Note that the webcams 101 for our bikes has more lift and more duration than stock and runs quite well with the 351 - GBAUTO is one member running such a combo..

 

I also ran the MCM as my first mod on a totally stock bike. So you and I know that the MCM should be the first mod done to a stock bike.

However, it is also certain that a bike fully modded for only top end power (6.5k-10.5k RPM), should keep stock timings. (Full exhaust system, lidless airbox, carb setup for proper top-end AFR..)

You are correct in that there is no long term data available for MCM+351. That is a frontier we've yet to fully explore.

 

If the Webcam is what u say, then the MCM mod should be ok. But with the 351, you really don't need any more power down low, IMO.

 

Well, as far as the webcams 101 grind, that's easily verified by comparing the specs - it has been done in here somewhere - Web Cam Inc. - Performance and Racing Camshafts / Kawasaki KLX 250 SF (09-10) DOHC 4v


Point is, while a 351 makes great low end power, the best possible enhancement to a 351 thru cam timings may be with the MCM (vs Stock).. Idea being that it's gonna run outta breath above 6.5k regardless of cam timings due to the small, stock, ports and valves..

 

Personal opinion from what I've read on big bores, I'd leave the cam timing stock. I think the power improvement from the big bore gives enough that the timing mod either is relatively ineffective or is so little that it doesn't really make much change in the "seat of the pants" dyno that it isn't worth dealing with it.

But that is just my opinion. As for the stock 250, it is a definite "DO IT". Besides the price is right.

 

I agree that prevailing opinion/wisdom has always been that 351's don't "need" the MCM..

But then prevailing wisdom toward carb setups never even got close to realizing a lidless, slipon'd 250 requires a DJ140/K152 main jet for a proper performance AFR curve near sea level.. Dogma can kill innovation..

It's quite possible that, like the dyno showing a need for the DJ140, someone will prove the results of MCM vs Stock for the 351.. I think I have a good argument in favor of MCM with 351 - that the stock ports and valves are too small to adequately feed a 100cc increase in bore in the upper RPM range, so why not go ahead with the proven optimization of the MCM and just move your shift point down to 8.5k (or so) RPM's..

 

The short answer, I'm scared. I don't want to be the one to find out if the theory is correct. I had no issue doing the MCM on my 250 because it had been proven to not do harm if done correctly.

 

Well RB, I was speaking rhetorically - not directly to you.. Certainly, if you are fearful, don't do it. If I ever do it, I'll carefully measure stock and MCM valve clearances, with a properly torqued head, and jettison the whole idea if MCM clearances prove less than stock. You see, I am personally quite sure that it is safe - from David R and Jhoffy22's running of the combo, but since I don't know them personally, I'm gonna measure first before running it.. Also, only a before and after dyno session will prove the results - dyno-butt evals are not enough..

 

Point was it just may not be worth the work even if it gives more horsepower.

A friend kind of got that impression when he had his 42,000 mile V-Max with stage 4 jetting/pods/Kerker w/racing baffle dynoed. It only put out 90 hp, my friend was disappointed... until the dyno operator asked him "Who built your engine?" My friend asks "Why? It only put out 90 hp." The guy pointed at the giant plateau of a torque curve that ran from around 2500 to peak torque... it was almost flat, monster torque from 2500 up. That was why my friend thought it had major horsepower, it was the extremely broad range of power.

Of course, y'never know unless you try. Odds are I'd try it if I went big bore, since it would be easy to check the clearance while doing the kit. Use the old head gasket to check, keep the new one to finish the build.