Advice needed for possible Hybrid Project

[willomega]

Have an '89 (?) KX 125 (last year before the perimeter frame)
that I would like to bring back to life for my adult daughter to use as a trailbike (she has riding experience). The 125 motor is just not suitable for the tight, technical trails we ride.

The obvious answer is to put a KDX motor into the KX chassis.
Does anyone know if the '89 KDX motor is a direct bolt-in to the KX frame? Looked up the parts lists online and my results were inconclusive: both engines share some, but not all, lower end bearings and seals. Both the KX and KDX lower ends appear identical in the parts fiches, but obviously, can't go by that.

Does anyone have personal experience doing a project on the '89's?
Does anyone have an '89 KDX motor either for sale or that you would be willing to help me get dimensions and measurements for comparison?

Any and all comments welcome.

 

[julien_d]

You'd be looking for an 89 - 94 KDX engine. I would imagine that would be the closest thing to a direct swap you could get, although I would think at least some massaging to the frame will be needed. Particularly motor mounts and such.

Have you considered just doing a big bore on the 125 engine? You might end up with similar results with much less effort spent.

 

[willomega]

Have given thought to doing a big bore on the 125, but am concerned that:
1) big bore parts for such an old bike may be hard to come by.
2) The wide ratio gearbox in the KDX motor should be a little friendlier on the trail.

Would someone like L.A. Sleeve do a big bore mod on this bike, do you think?

 

[dirt bike dave]

If you keep the 125 motor and bore it, I think you would also need to budget for a good porting job to get the most out of the big bore for trail work. You will still have a close ratio box, but the tall first gear should be less of a problem with a big bore tuned for low end.

Personally, if it were my daughter, I'd be inclined to find a decent running '89 - '94 KDX200. Your daughter may love it, as is. If so, then sell the 125 and save yourself some labor.

If she mostly likes the KDX but wants a lighter weight bike, then you can contemplate the swap.

 

[julien_d]

Sound advice, Dave! The chances of finding a serviceable E series engine at a good price are slim anyway, unless a bike comes with it.

 

[craig_enid]

220 with lowering links and an EFM auto clutch.
Power delivery and height should be OK.
TTR 125 is also a good bike, size and power-wise.

 

[glad2ride]

No. It is not a direct swap at all, much work involved, sorry.

Depending on height and skill, KLX140, Honda CRF150F or KDX200/220R

 

[Tedh98]

No. It is not a direct swap at all, much work involved, sorry.

I agree with this and the other comments. I personally wouldn't put that much work into an '89 bike. I don't think anyone has done a hybrid with something that old, so you really don't know what surprises you might be in for.

 

[julien_d]

No. It is not a direct swap at all, much work involved, sorry.

Depending on height and skill, KLX140, Honda CRF150F or KDX200/220R

Done it? The frames are quite similar, as well as the bottom end of the engine. I'd be curious to hear the report if you've worked on a swap like this. Very few things are a "direct swap". How much work it is is kind of relative. I've looked at a couple really cheap kx125's of that era, with the intention of dropping my e series engine in there. Didn't look too difficult at a glance. I think it'd be easier than most the hybrids that get done regularly.

 

[sr5bidder]

I would bet someone has done it but back in the days before the world wide web.

 

[dirt bike dave]

People have been dropping KDX 200 engines into KX125 frames since before the KDX got watercooled.

I like craig enid's idea of a 220 w/ lowering links for tight technical trails. Seems more appealing than marrying two x 20 year old bikes together.

 

[julien_d]

Yeah, sort of personal preference though. If the op is like me, the thought of building something himself is exciting.

 

[Tedh98]

I'm 100% on board with building something myself or building something unique. I just wouldn't do it on a bike that old. A year or so ago I sold an '86 KX 250. That was a great bike, but I spent more time waiting on parts or trying to find parts than riding the bike.

'99-'02 KX's can be found cheap and parts are still readily available.

 

[julien_d]

I'm 100% on board with building something myself or building something unique. I just wouldn't do it on a bike that old. A year or so ago I sold an '86 KX 250. That was a great bike, but I spent more time waiting on parts or trying to find parts than riding the bike.

'99-'02 KX's can be found cheap and parts are still readily available.

Never had a moments trouble finding parts for my 89 kdx. With the bit of work I've done to it, I prefer riding it over any of my buddies late model KTM's. :)

I also have this profound sense of satisfaction when my built by me 21 y/o bike has no trouble getting the best of equal riders on "nicer" machines.

 

[Tedh98]

Yeah, sort of personal preference though.

Now that we can agree upon.

So willomega, what direction do you think you are going to go?

 

[willomega]

Been off the forum for several days. Thanks for all of the great advice and suggestions.
Those that expressed concern about the potential complexity of the engine / rolling chassis swap have voiced many of my own concerns about this project.

The idea of a big bore has occurred to me, but as with the engine swap, it too has it's potential challenges.

As to other possible bikes; I let her try out a TTR 125 my son had outgrown. Even though the TTR is a great bike, it was more than a little cramped for her.
Have thought about trying to find a nice CRF 230 or TTR 230, but was hoping to
offer her a ride that's a little lighter and better suspended than either of those bikes.

At this point, still not decided which way to go. Other thoughts, ideas, comments??

To julien_d: You indicated you have - or had - an '89 kdx. Do you still have it? If I send you dimensions / proportions on my KX lower end, would you be willing to take comparable measurements of your engine? Just let me know. Thanks!

 

[glad2ride]

CRF230F or TT-R230 will be fairly heavier than a KDX. I have a TT-R230 in my garage and know well of its heft. I think picking it up off the ground during a long, hot ride would be just what she deos NOT want.

Let her sit on a KLX140L.

 

[julien_d]

To julien_d: You indicated you have - or had - an '89 kdx. Do you still have it? If I send you dimensions / proportions on my KX lower end, would you be willing to take comparable measurements of your engine? Just let me know. Thanks!

Of course I still have it! IMO the e series KDX is one of the best trail bikes ever made. The engine is incredible, and the bike itself is super comfy.

Anyway, I can get you measurements if you like. I actually have a set of case halves I could send you for next to nothing, if you want to do some test fits with those. They wouldn't be much good for building an engine from, but they'd work great for a test fit.

Just let me know :)

J.

 

[willomega]

Awesome! Give me a few days to get some measurements. I'll send them to you, you can compare with your engine. If it looks like they're really close, I'd like to get the
case halves to do a unit-to-unit comparison.

Thanks for agreeing to help me with this!

Happy holidays!