89 framed Hybrid, air cooled motor

Discussion for swapping a KDX motor in a MX frame...
arjay200
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Re: 89 framed Hybrid, air cooled motor

Post by arjay200 » 03:47 pm Oct 22 2020

My guess would be an early 83-85 KDX engine would fit. Maybe the 86-88 engine. I'd use Ebay crankcase/bottom end pics to get a better sense of it. Compare the 84 KX125 crankcases with 84 onwards KDXs. It's always going to take a bit of work - cutting new front mounts and head stays at the least. Main thing is that the back of the engines are the same layout. Good luck, it would be a cool build.

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Tyl3r
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Re: 89 framed Hybrid, air cooled motor

Post by Tyl3r » 08:59 am Jan 27 2021

Sweet ride, just saw this now! How is she running after the dyno tune?
04/05 KX220 Hybrid all decked out
18 KTM 500 EXC-F Sumo toy

arjay200
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Re: 89 framed Hybrid, air cooled motor

Post by arjay200 » 03:13 am Jan 28 2021

Thanks! The dyno run topped out at 22 hp at the rear wheel. With a mid range lean spike that we couldn't jet out. There's also detonation under load at high revs when riding on the dirt (didn't show up on the dyno bench). I just found out there was a mismatch between the E series manifold and the C series reed block. The two apertures don't line up (with potential sealing issues). The E series manifold mouth is offset, C series reed hole is centred. So I've put in an E series reed block and am hoping this was the issue.

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Tyl3r
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Re: 89 framed Hybrid, air cooled motor

Post by Tyl3r » 10:35 am Jan 28 2021

Nice, that's awesome that you have a dyno at your disposal to help with the fine tuning. How did you find out about the reed block issue? I only have experience with the H series bikes
04/05 KX220 Hybrid all decked out
18 KTM 500 EXC-F Sumo toy

arjay200
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Joined: 12:10 am Nov 26 2019
Country: New Zealand

Re: 89 framed Hybrid, air cooled motor

Post by arjay200 » 05:18 pm Jan 28 2021

This mismatch. Just found it by accident when servicing the top end.
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Tyl3r
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Re: 89 framed Hybrid, air cooled motor

Post by Tyl3r » 09:58 am Jan 29 2021

Ah, gotcha, yea can visually see the difference there.
04/05 KX220 Hybrid all decked out
18 KTM 500 EXC-F Sumo toy

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