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KX300 Jetting/Powervalve Problem

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Hey all, I am having a problem :banghead:.  I have a new 301cc cylinder done by Kustom Kraft for my 1997 KX250 :thumbsup: .  It runs great except that it seems to require two different jetting conditions.... open vs closed PV.  It blubbers a little right before the PV opens then it runs great.  If I jet it (Main) so that is doesn't blubber before the PV opens, then it is too lean when it does.  Another interesting condition is it seems worse when you rip on it a few times.  If then you take it easy for a few minutes then rip on it again it runs a little cleaner the first time then it gets a little worse again...maybe temperature sensitive.  Could this be a ignition timing thing?  I am pretty advanced.  I have had several 2 strokes and am very picky about my jetting, i have been through a few needles and actually like the stock one the best.  I have had the carb off 30 times for jetting changes so I don't feel this is a simple newby problem.  Any ideas???  

 

175 main- 5000ft

45 pilot

stock N3WF needle in mid clip position 

KDX200 flywheel and ignition.

Stock Pipe and silencer with a Legalizer

 

Thanks All!

Edited by TurboStew

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How does that work??

KDX200 flywheel and ignition.

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Honestly, it sounds like your tuner did a poor job with the port layout. With the power valve closed the engine is retaining most of it's fuel/air charge, and with the power valve open the transfers are blowing a substantial portion of it right out the exhaust, hence the Doctor Jekyll/Mr. Hyde jetting profiles.

 

Just a theory though...

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I just put together a 04 KX300. The ports were not altered at all aside from the center valve being cut down. It jets flawlessly throughout the powerband. We run straight race gas though.160 main, 45 pilot, stock needle in the stock position. 110 Cam 2 race gas. No pinging or loading up, Its strong off the bottom with a nice meaty mid and good topend. Lugs nicely in the woods, pulls a tall gear well! It hangs with my 500 in the all important drag race down a dirt road test! Im at sea level though. Was the bottom end gone through, we had a very similar issue with a bad clutch side crank seal. When lugging it like in a rocky or tight technical section it would run ok. When I would open it up in a more open section it would run like it was loaded up and smoke excessively. Sometimes it would clear up some.

Edited by zz3gmc

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I just put together a 04 KX300. The ports were not altered at all aside from the center valve being cut down. It jets flawlessly throughout the powerband. We run straight race gas though.160 main, 45 pilot, stock needle in the stock position. 110 Cam 2 race gas. No pinging or loading up, Its strong off the bottom with a nice meaty mid and good topend. Lugs nicely in the woods, pulls a tall gear well! It hangs with my 500 in the all important drag race down a dirt road test!

 

 

If you went all the way to 300cc, I assure you the ports were altered. Going that big on the bore cuts so much material out of the cylinder wall that it cuts into the curved top of the transfer ports and they end up flowing up at the head instead of into the center of the combustion chamber without being re-worked.

 

Altering the flow direction of the transfers is one of the techniques tuners use to alter the power characteristics of a two-stroke. For example making them flow more towards the intake side of the cylinder will improve low-end, while aiming them more towards the exhaust will improve top-end.

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I am thinking it is a combination the needle and the 200's ignition curve. You may need to fine a different needle that is richer a 1/2 to 3/4 throttle and the same WOT as the one you have now. I know I have ran about 6 different needles in my 300 working on getting them to run the way I wanted. Don't know what is in my I right now but it is running great LOL.

 

zz3gc good to hear you like your 300.......now get some L series parts and crank and put a KDX trans in it :thumbsup:

 

Ed

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OK 295. They were not altered as far as any port work. Im friends with the owner of Power Seal, the co. that did it. When I sent the cylinder to him, actually handed it to him, he said they may port it. When I got it back I noticed no porting was done. I spoke with him about it and he said that changing the ports from stock( other than the obvious cutting into them when bored) may change the power characteristics into a powerband that may not be best suited for our type of terrain that we race/ride in. He is a long time enduro rider, racing in the same series as me and throughout the years the same class so he is well versed in what works well for us. I didnt want  radical, hard hitting powerband. It pretty much mimics the stock 04 power characteristics but more everywhere. If it was set up for moto then he probably would have changed the porting.

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Clyde, I have a couple of 03 and 04 engines under the bench and may just do that!

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I am thinking it is a combination the needle and the 200's ignition curve

 

Ed

 

 

You know, I missed that, and it's an excellent point. It's very possible that the 200 ignition curve is simply all wrong for that engine.

 

By his description of the power valve opening affecting the jetting, I'm not so sure it's a needle issue though.

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Well, I think it is solved. I have had my head wrapped around jetting, jetting, jetting.  I started to think about ignition :thinking:.  The E-Series KDX ignition is speced for a B9ES plug, hence no resistor, so I tried a B8ES instead of a BR8ES and it was better.  I then gapped it to .028in and it seems to run pretty good.  I think the higher compression ratio of the cylinder work with the fact that the compression ratio is higher with closed PV made the old KDX ignition a little weak.  It is crappy windy today but I will get some more time on it next weekend.  Thanks all for the ideas!

  • Like 1

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Good that you are gaining on it. I just reread your first post and t looks like if you go looking for a different needle you need one a little leaner in the 1/4 to 3/4 throttle range maybe to clear things up some.

 

Good luck

 

Ed

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