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KDX200 Carb problem + Bernoulli Discussion

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Hey all, I have a carb problem with my 1995 KDX200 lean bogging at low RPM speed WOT, the mid and low throttle are excellent, as it is tuned to have no surge at cruise.. like a honda scooter. The idle to mid throttle response in Neutral is excellent, but if I go to Full Throttle, it bogs. I can jet the main up (155) till it chugs and lugs at WOT, but then the overrev is gone, jet to great top-RPM power (142, maybe lower) and she bogs, not talking through the transient either, I mean hold it there for a few seconds and power is very low, never picks up. It is stock, with stock pipe, FMF turbine core 2 silencer, fresh topend, 607 reeds.

I being a geeky engineer, have spent a lot of time analyzing carb design (design industrial fuel injection at my day job), it seems to me that all these 2 stroke carbs are a big compromise, I got a few hours researching this subject and I see old school tuners who believe that a 2t cannot handle WOT transients at high-load-low-RPM! I see post with similar jetting questions, and responses like "use throttle control, 2-strokes can't take full throttle while lugging" or "A 2 stroke can't cruise, it surges, just the nature of the beast". I did dispel this myth by grinding slides and fattening needles till she putts like a honda scooter:thumbsup: I believe these old myths are a by-product of the inherent flaw of a carb... That is as air velocity goes up (increased RPM) the air density goes down at the venturi, so there for the mixture will become richer. My argument follows the reason why OEMs went with the PWK Powerjets, that little solenoid removes fuel when above a certain RPM. I also believe that low/medium throttle circuits are artificcially richened by many tuners to improve transient behavior (kinda like accelerator pump function), but are too rich at steady-state to allow no surging/sputtering/4-cycling cruise (typical factory Motocross tuning). Anybody else analyzed this and have thoughts, additions, oppositions, data, hypothesis, etc?

Anyone have scooter like smooth cruise and jetted for peak RPM power and get crisp lug RPM full throttle power? If so please present the data and formula!

I want my cake and eat it too.... I am thinking about trying a KX125 powerjet PWK on my KDX200 then use a simple 555 timer circuit RPM switch to control the solenoid. The KDX boot is too small for the 36mm PWK, but the KDX reed gasket matches a lot of year KX125s, but a lot of the KX125s boots are unique to a year or 2. Hmm...Which One?

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With a two-stroke, proper engine response depends on more than just the carb. The engine is a part of a complex wave-tuning machine made up of the air box, carb, reeds, porting, and expansion chamber. For the engine to run correctly, all the pieces have to be right.

Kawasaki completely blew the design of the KDX pipe. It's very quiet and very tough (double-walled construction), but it's extremely restrictive and makes the bike run very poorly.

In your case, get rid of the stock pipe, install an FMF K30 (Woods/Torque) or K35 (Desert/Rev), open the air box up (drill holes in the lid or just toss the lid) and jet the bike correctly. Proper tuning of the KDX is well known, as is your particular complaint about poor throttle response. If you uncork it and tune it properly your issue with poor throttle response will disappear.

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If you uncork it and tune it properly your issue with poor throttle response will disappear.

Ah, Chokey, you're always helpful. :bonk:

I know it is common knowledge that the bike wakes up with the FMF woods, but I love the quiet, strong stock pipe in the Rockies.

So my exact symptoms (rich high RPM WOT, lean Low RPM WOT) are typical of the stock pipe and they all go away with the FMF woods pipe?

Chokey, I know you eluded to this in your post, but are you generalizing "poor running" or being specific to the rich high RPM/lean low RPM WOT throttle?

If my exact symtoms are confirmed by Chokey's or some others experiences or data, I'll go get one... I hate throwing money at a problem without conclusive proof:prof:, this is how money pits are created!

Thanks

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What I was getting at is that with the stock pipe it's nearly impossible to jet the KDX correctly across the entire throttle range. It just doesn't evacuate well because it's so restrictive.

If you keep the stock silencer the bike will still be very quiet even with an aftermarket pipe. There will be some increase in noise with a single walled pipe, but it's not as prominent with the heavy gauge Gnarly as it is with a thinner MX pipe like the Fatty. And the stock silencer is actually pretty good, the biggest gain from an aftermarket silencer is weight loss, not performance.

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Just a thought, but when's the last time you inspected the power valves? If they get carboned up the actuator rod will shear the teeth off of the drum valves and they'll quit working. The main valve will still work, but the drums not opening can have you chasing mysterious jetting gremlins and never finding a solution, and a low-end bog is one of the biggest symptoms.

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Just got the bike a few weeks ago, did the top end and meticulously cleaned and lubed the PVs. Watched with the cover off for proper operation:thumbsup:

I don't have the stocker silencer, wish i did. Does one of those aluminum snake spine guards quiet the pipe down, or is like snares on a drum?

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The aluminum guards don't make much difference, unless you let them get loose. Then they develop an irritating buzz all their own...:bonk:

They do help reduce rock damage to the pipe though.

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I wonder what wraping a pipe in carbon fiber cloth and resin would do for sound and toughness.... Hmm, sounds like another thread topic:thinking:

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I put an FMF Woods pipe, Boysen reeds and had RB Designs reshape the head and put a bat wing in the carb in my kids 200.

The bike runs really, really good. Almost imposible to stall and pulls from zero rpms with a nice hit when the power valve opens.

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