Jetting advice please

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thinairflyer
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Jetting advice please

Post by thinairflyer »

I just recently bought a '90 KDX200. The seller stated he thought it needed rejetting for 3/4 to full throttle, he said he knew little about bikes but had cleaned the carb as the bike has been sitting for a couple of years.

It was dripping gas rapidly out the overflow so I took the carb apart, inspected the inlet needle and seat. The needle looked good, the seat looked like it might have some old gum from fuel on it. I made a small ball of very fine steel wool and cleaned the seat by turning the wool with a screwdriver.

The float was way low, about 22mm or more, he probably did that trying to stop the fuel overflow. The needle clip was in the top groove, again I expect an attempt to stop the overly rich condition caused by the too full float bowl. The idle mix screw was 1/2 turn out.

The carb has a R1172N needle, 50 pilot, 130 main. Airbox is stock. Mix is 40:1 dino oil.

The pipe is an FMF Gold Series Fatty with a Turbine Core muffler.

I set the float to 16mm, moved the clip to the 3rd groove, set the idle mix screw to two turns out.

The carb does not overflow now at all.

The bike has a lean spot as throttle is opened about 3/4 or full until it comes on the pipe. It doesn't stumble or blubber, just goes flat with poor acceleration. If I pull the enricher out the flat spot goes away while of course it is rich at lower speeds. With the enricher out it pulls smoothly right on up to on the pipe.

It pulls hard at lower rpms and hard after getting on the pipe with the enricher off.

I live and am riding at 9,300' elevation but the 130 main seems too lean to me, can't imagine who put it in there as the bike came from about 5 to 6,000' elevation.

The engine never four strokes, even wide open in first gear or even in neutral at full throttle. It just screams.

So, since I've not had a two stroke in many years, my thought is to go to about a 140 main and maybe still move the clip on the needle to the fourth slot down.

If the 130 main is not considered too lean at this elevation I can still set the needle higher, to slot 4 or 5.

Advice appreciated from you folks more familiar with the KDX200 than I am. L. D.
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Gotanubike
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Jetting advice please

Post by Gotanubike »

Sounds like you know your stuff! Good work on getting the carb set back up correctly.

As for jets, my '90 had 158 main / 48 pilot and after some searching I found they were the factory jets for that year. Shortly after or during 1990, they lowered main to a 150 because 158 was probably too rich for any altitude. FMF and Fredette both recommend between 152-158 main and 45-48 pilot with R1173N(one step thicker) needle if using FMF pipe.

Whoever jetted that bike was on hard drugs because the pilot certainly doesn't need to be any higher and a 130 main is just unheard of. Then again, 9300ft elevation is pretty high. I'd be afraid of seizing the motor with a main that low. But they say drop one size for ever 2000ft of elevation, so at 9000ft starting at 158 would land you at around 145/142 as a good place to start.

At 300ft elevation I jetted mine down from 158/48 to 155/45 and noticed better overall responsiveness from the motor. But I was experiencing a rich condition. Many go lower to say a 148/45 seems to be a popular one for the late model E model. All depends on riding style, what rpm ranges you utilize, elevation. I do a lot of my riding in the high rpm range, I feel safer being slightly rich rather than too lean.
1990 KDX200
Bike Profile -> http://www.kdxrider.net/forums/viewtopi ... 61#p136315
Suspension Overhaul(Shock+89-92 conventional forks) -> http://www.kdxrider.net/forums/viewtopi ... 15&t=15255
96'-98' RM125 Showa 49mm fork swap -> http://www.kdxrider.net/forums/viewtopi ... 04&t=16994
KDXrider world map! -> https://www.zeemaps.com/map?group=186158
thinairflyer
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Re: Jetting advice please

Post by thinairflyer »

Thanks Gotanubike, your reply is helpful, that 130 just seems way too lean although the bike pulls very hard. I'll order some jets and tune a little more. I may be riding as low as 4,000' some time so could change the main easily on the PWK if I go down hill. Most of my riding will be 6,000' and above so I definitely will up the main into the 140s and will likely drop the pilot to a 45.

Since I posted at first I went back to the bike and raised the needle one more notch to the fourth notch down. That made quite an improvement on the flat spot, it is almost gone. The bike really is close to perfect now, just slightly off still. Oh, and it is too rich off idle, that 50 pilot is too big. Tomorrow I will go on down to the fifth notch on the needle and see if that will eliminate the flat spot. I still will not leave the 130 main in even if it does though. Right now I am not holding full throttle for more than a few seconds at a time.

Yeah I think the last tuner was flying up with the stars.... :-)
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Gotanubike
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Jetting advice please

Post by Gotanubike »

Glad I could help, that's more / less all the info I gathered last month before I rejetted my bike
1990 KDX200
Bike Profile -> http://www.kdxrider.net/forums/viewtopi ... 61#p136315
Suspension Overhaul(Shock+89-92 conventional forks) -> http://www.kdxrider.net/forums/viewtopi ... 15&t=15255
96'-98' RM125 Showa 49mm fork swap -> http://www.kdxrider.net/forums/viewtopi ... 04&t=16994
KDXrider world map! -> https://www.zeemaps.com/map?group=186158
thinairflyer
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Re: Jetting advice please

Post by thinairflyer »

It is rideable but too rich on bottom and pretty lean on top even with the needle clip in notch 5.

I have ordered a 140 and 145 main and 42 pilot jet. Should have them Monday and will see how close I guessed. LOL! L. D.
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bufftester
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Re: Jetting advice please

Post by bufftester »

Sounds like you're getting it sorted. Keep in mind that when running that lean, its the transition from throttle open to throttle closed that puts your engine at greatest risk. On reed inducted 2Ts running it up in RPM then abruptly coming off the throttle effectively starves the motor for fuel....and that all important oil that it is also carrying! That where you see your 4 corner seizures.
thinairflyer
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Re: Jetting advice please

Post by thinairflyer »

Thanks, I'll watch that!
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