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Rob Jack

Air cooled woods bike

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Finally got this project out of the garage,

1987 KDX200 motor (Lectron carb, 04 Flywheel & Ignition)

1989 KX125 chassis (modified to single down tube)

RMX 45mm Showas

I'm getting the bike dialled in. I've just been out for the one shake down ride. The timing and jetting were a fair way out and the back needs more sag to offset the slightly shorter front fork. It's early days... :)

 

 

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Really neat idea, very well done.  Looks cool and fun.

I've got a 200RR which I like a lot, but it has got me wondering about the advantages vs. disadvantages of an air-cooled motor.  I've never had an air-cooled bike.  Could you tell me about that?

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Really sweet bike, would love something like this!

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2 hours ago, WRider995 said:

That thing is cool.  Curious how it is when you get it dialed.  

Thanks. I'm jetting it dyno tuned next week. Starting from scratch really, with the new ignition set up, then onto the fuel. There's a few variables, but I had the motor running well in the past (in an 86 KX chassis)

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1 hour ago, shrubitup said:

install a case saver soon; great looking bike!

There's a skinny one which you can just about see in the last pic. What I'd like to do some time would be weld rail extensions on like the KDX.

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1 hour ago, Jesse Haifley said:

Really neat idea, very well done.  Looks cool and fun.

I've got a 200RR which I like a lot, but it has got me wondering about the advantages vs. disadvantages of an air-cooled motor.  I've never had an air-cooled bike.  Could you tell me about that?

The KDX 86-88 is the only air cooled dirt bike engine with power valve (I think) like a kind of crossover when all other bikes were water cooled. I like the idea that there's fewer things to go wrong with it. Less to damage when bouncing off trees :) The downsides are its noisier (the ringing from the fins) and they get hotter so it needs to run a bit richer on the top end. Changing plugs on the trail needs good gloves. Water cooled engines are smoother, more refined and make more power. This one is less well mannered but fun to ride.

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Weigh it please. Im very curious . 

 

 

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22 minutes ago, seer said:

Weigh it please. Im very curious . 

 

 

Sure, I'll give it a go when the tank is empty. It doesn't feel as light as I was expecting. I'm guessing 220 - 225 lbs. Some heavy duty gussets went in when the frame work was done. And the frame has extra bolt on steel brackets at the rear of the engine for the linkage. It wasn't the lightest 125 of the era. 

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52 minutes ago, Rob Jack said:

There's a skinny one which you can just about see in the last pic.

That's a no see um one then. Looks like you fabricated the fuel tank and the sub frame too? All looks good :ride:

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1 hour ago, Rob Jack said:

The KDX 86-88 is the only air cooled dirt bike engine with power valve (I think) like a kind of crossover when all other bikes were water cooled. I like the idea that there's fewer things to go wrong with it. Less to damage when bouncing off trees 🙂 The downsides are its noisier (the ringing from the fins) and they get hotter so it needs to run a bit richer on the top end. Changing plugs on the trail needs good gloves. Water cooled engines are smoother, more refined and make more power. This one is less well mannered but fun to ride.

Thanks for that, Rob.

Everytime I buy radiator guards I think it would be nice to not have the cooling system to worry about.

I hope you'll share with us the dyno chart once you've completed that stage.

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3 hours ago, Jesse Haifley said:

Really neat idea, very well done.  Looks cool and fun.

I've got a 200RR which I like a lot, but it has got me wondering about the advantages vs. disadvantages of an air-cooled motor.  I've never had an air-cooled bike.  Could you tell me about that?

I raced an air cooled 500R, it was rip your arms out of the sockets fast. lighter than the water cooled 500R's, no leaky water pumps, rust and corrosion in the system to worry about, and a blast to ride when they are dialed in. Disadvantage? They get mighty hot, and ping, foul plugs on some bikes, and a pain to start. Buy one and have a blast!

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14 hours ago, shrubitup said:

That's a no see um one then. Looks like you fabricated the fuel tank and the sub frame too? All looks good :ride:

Ah ok you mean the case saver in front of the chain. Yup next on the list :) Subframe is standard 89 KX, and yes the tank is designed to fit. I made the templates from card and got it fabricated.

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14 hours ago, M E T A L A C I D said:

I raced an air cooled 500R, it was rip your arms out of the sockets fast. lighter than the water cooled 500R's, no leaky water pumps, rust and corrosion in the system to worry about, and a blast to ride when they are dialed in. Disadvantage? They get mighty hot, and ping, foul plugs on some bikes, and a pain to start. Buy one and have a blast!

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That bike would have ripped. Do you think it could have been detuned to trail friendly?

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