From a Phone Call I made on Jan 29, 2020 . . . (Microsoft Word docx file)

 

From many conversations with Bill over the years concerning his dyno testing procedures, my dyno testing findings, and general KLX250S performance, there are some inconsistencies that should be addressed. In each case of my interaction with Bill, I have posted the information we exchanged in this forum.

Excerpts:
I asked Bill about lidless air intakes that lots of KLX250 riders use. Get ready for a surprise: Bill said that that particular modification does not work well with his Big Bore Kits that also use aftermarket, free flowing exhaust systems! He explained that his extensive rear-wheel dyno testing (his local dyno shop measures "real HP to the ground") indicates that the best TQ / HP vs RPM results with the 351 kit on carbureted bikes are only obtained with either 1) stock air box / KLX250 nozzle or 2) stock airbox and larger KDX200 nozzle or 3) stock airbox and no nozzle. He said he was very surprised at this outcome himself and did not fully understand this result as most hot-rodders expect to get more engine power when the system is as free-flowing as possible – high-flow air intakes and high-flow exhausts. At a much later date, Bill got together with an outside researcher who had a special air-flow measurement probe that could be inserted into the air box and the air intake path to the carburetor of the KLX. Then they tested the three aforementioned air intake systems as well as a lidless system and they discovered that the lidless KLX airbox configuration when used with the [heavy breathing] 351 kit AND a free-flowing exhaust systems, produced unexpectedly high airflow turbulence into the carb intake tract (using either the CVK or Mikuni). I did not think to ask if EFI bikes with 351 kits had the same result.

Bill has told me that he never tested lidless airboxes ( and certainly had never dyno developed proper fueling for lidless applications.). He was quite surprised to learn that I (and now a large number of KLX250S owners) was/are attaining HP numbers that exceed his published "Stock KLX with 351 BB kit" HP value.. TRQ with a lidless KLX does not match up but is still a huge increase over stock.

The Lidless recipes produce extremely strong but absolutely smooth power delivery. This would not be possible if the air entering the CVK was turbulent. The CVK malfunctions with turbulent air entering the intake bell of the carb. Dyno testing chart results (with lidless recipe fueling) , and years of actual field experience, dismisses the idea of turbulent air with lidless airboxes.

Many 351BB owners are running a lidless airbox (and a modified lidless fueling recipe) with great success.

Without a lidless airbox, the 351 loses power extremely quickly as RPM's rise. You end up with a "tractor engine" .. Lidless airbox fueling and complete performance exhaust systems greatly change the power delivery of the 351, allowing full potential to be realized.



Yet I do not recall reading about anyone reporting valves bending or valves damaging pistons as a result of messing with the valve / cam timings, but maybe they were just lucky. Or maybe I missed it. Or maybe they did not reveal it to us!

You are correct. No 250 OR any 351's have ever bent a valve or had piston/valve contact when running the MCM. Bill verified many years ago that the 351 kit maintains stock clearances between the piston and the valves. Two previous members, years ago, ran their 351's with the MCM without any such clearance issues , or MCM issues at all .. MCM will optimize volumetric efficiency (airflow / power) the same exact way regardless of the size of the piston below the valves! IMO, the 351BB kit presents a need for valve retiming that we cannot provide. The MCM makes TRQ below 6k rpm - which is not really the "area of need" for most 351 owners. The stock valve timings attempt to make max power @ approx 8500 rpm, which is too high for the 351 as the 351 becomes "stifled" via the stock ports (intake and exhaust) below that RPM (free flowing airbox and exhaust help tremendously but the ports can only flow so much before limiting power levels).. We need dyno power curve data to show whether the MCM should be run on the 351 or not. At any rate, leaving the valve timings stock and adding a lidless airbox and full exhaust system will give the 351 the best opportunity to provide its' tremendous performance increase across the largest possible RPM range. On the other hand, doing the MCM on a 351 would make an insane TRQ monster of an engine..



Your conversation did produce one "new" bit of information. His admission that his pumper carb does not provide additional power over the CVK. While always suspected (at least by me) , this is confirmation. Use of the TM36-68 provides the same "pumper carb" riding characteristics while empirical evidence indicates it also adds power to the power band. (Bill should have gotten on the TM36-68 bandwagon long ago, with a "plug and play" version and fueling options..)

KLX ownership and performance would not be the same without Bill and his products. His company offers performance potential that we could not obtain otherwise. With a lidless airbox, Barker exhaust system, and a proper CVK fueling setup, 28 hp (up from the stock 18-20) is reachable with huge TRQ as well.

 

Klxster, I definitely agree with you on at least two of those points...the lidless airbox and lack of problematic turbulence...the slightly superior application of the TM36-68.

That fear/assumption many have of how inlet air can negatively affect a CV carb is only true if the inlet air is quite close to the carb throat or the airflow is somewhat pressurized by having the air intake positioned where vehicle speed forces air fairly directly into the CV carb. Now my statement is somewhat general, but it applies to the KLX, and it applies to the "old days" of installing those clamp-on K&N shorty filters to CV carb equipped bikes like a GPZ750 and such. The many, many inline fours of the day would still function with those filters being that close to the carb mouth, but you usually got some funky fueling issues at times.

I also recall Kawasaki doing a CV carbed GPZ or Ninja years ago that had a higher pressure air intake as speed increased, but they had also come up with a huge airbox that somehow cut down turbulence to the CV carbs. Anyway as a general rule, CV carbs and carbs in general like a big, dead air space. The KLX's airbox location and the distance from the carb allow the lidless operation without a downside other than proper jetting to accommodate it...and some water entry and noise as a possible negative side effect.

I also agree with the issue of the TM36-68 being an overall better carb for the bigger bore applications like 300-351 displacements. The 34mm works fine, but the 36mm will still even fuel cleanly with the 250 OEM bore when jetted properly. And as you pointed out, the bigger benefit of the pumper carbs is throttle response rather than big power gains. However, as the displacement goes up, the 36mm carb does start to offer higher rpm performance.

 

I worry for the future of bikes like our beloved KLX250S, CRF250L & Rally, DRZ400, etc.. The Chinese are now selling us KTM 250 EXC (street legal) clones at the same price !
Clones, meaning when a part breaks, you can choose to go to the local KTM dealer and get a KTM replacement ! With "copied" EFI, watercooled, engine(s) that are bulletproof and almost as powerful as KTM's. And carry cloned KTM (WP) suspensions with full adjustability ! Only thing the KLX "has on them" is buddy pegs..

"Made in China" is taking on a new meaning, just as "made in Japan" did when I was growing up ..

Read the video reviews of this thing !
GPX FSE250R $4999.00
https://gpxmoto.com/index.php?route=...&product_id=63

 

I have the FSE (aircooled 250). They're decent bikes. But my KLX is a head above construction-quality-wise. At $5k, I would choose KLX250 over FSE250R every time. GPX and other off-brands with products from the same factory (Motoland, BSE, GR7, etc) have been making the same 3-4 models for the past 4 years. They haven't improved much in that time. It's definitely not what it's cracked up to be. Look at what their MSRP is, cut it in half - that's what the bike feels like in person.
And, no, there is no copied KTM engine. Not even close. They either stuff licensed copies of ancient Hondas in or fairly cheap and very detuned Zhongshen motors. All of which, as far I've seen, have FI issues to date.
I bought mine as a play bike; something I wouldn't feel bad about tossing around. But the lack of durability doesn't lend it as a good option even for that.
Look at SSR bikes as a better alternative (on the old aluminum Honda frames). They're about a grand or 2 more for a comparable model. And none of those Chinese bikes have federal emissions papers to make them street legal in states like OR. So YMMV based on location.

 

Wow, ok Gronk, thanks for the "low down" on your GPX playbike .. I'm going off the reviews of their KTM clone (FSE250R) , which is a totally different bike from yours... They talk about its/the Zhongshen EFI engines being bulletproof from being manufactured for many years (and proven) in the Asian market..

Gonna look at the SSR's now... thanks again..

Oops, SSR does not offer anything like the GPX FSE250R (street legal, EFI, KTM EXC clone).. Closest thing they offer is their 450 dirt bike - and it is still using a carb..

 

It's not a KTM clone. The frame is a 2017 Husky FC rip-off. Otherwise they're all the same bike but different engines: 250cc aircooled/carb, 250cc water-cooled 4T/FI, 250 water-cooled 2T/carb. Other "brands" of these bikes offer 300cc 4T (just a BBK), as well as a 450cc (different engine). There are also carbed versions of the 250cc 4T w/c. There is a FB group for Pitster that covers these bikes. It's ran by one of the importers, so take positive things said there with a grain of salt.

 

Only concern I really have with the Chinese bikes is the lack of a solid distribution network. If the importer quits or goes broke you now have an orphan bike with no supply of anything beyond generic bits. That is the main advantage of the Japanese and most Euro manufacturers - an established solid distribution network, all you do is find a dealer and there are plenty of them. Better than having to rely on the one small operation importing whatever they buy.

 

Yes, the lack of a trustworthy distribution network (dealerships) is what'll keep the established brands most viable. Problem is, I fear this shortcoming is due to inferior product(s). With a superior product lineup, a network may be forthcoming..