placelast

Member
Apr 11, 2001
1,298
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It was never my intent to do this trail test, but with the hours I spent adjusting this, replacing that, I felt it was well deserved.

A friend had been complaining about how poorly his custom-made trail bike was running, a KDX-powered KX125. Whenever he’d have it out, it would run shoddy. And I was partly to blame, persuading him to buy it from the fabricator. Therefore I suggested whilst I had all this time off, that I’d determine the source of his woes. My only payback was to let my middle son, Shorty, take it out one day to see if he could be persuaded onto something other than an XR200/250 which he admires deeply; so deeply there was no sense in arguing with him on what would be his next bike in the event I was able to secure one just for him. He was saddened as he saw the $ for a ’99 Husky WR125 go to my wife’s new (to her) four door of late - I thought him riding this would only cheer him up.

The symptoms of the sad state of affairs with this little mean-green machine showed itself as fuel starvation. The float bowl was set way too low, thanks to the help/directions posted on our own Canadian Dave’s JustKDX website. But being the perfectionist I am, that was the last thing I corrected as the levers were set too high (good for sit-down riding only); the shifter and brake pedal too low - can’t have those rocks contact boots, now; a missing bolt here and there; spooge coming from a gap in the stinger/muffler union needing RTV; and on and on it went - I didn’t get to the fix until well after dark; but that’s the way I am - picky about bike setup/prep.

So as I awoke the morning of the ride, thinking: why not enjoy the fruit of my labor? On the one hand, all I needed to do was to swipe a main jet out of my RMX, and take a risk at meeting a ranger - you see, my friend never affixed a green sticker. But on the other, how often does one get to spend time on a custom, one-off machine like this?

Therefore I had Jr. load the greenie. And I was glad I did. Read on:

The previous owner happened upon a ’98 KX chassis, sans forks. Into it he wedged a ’90 KDX engine, and graced the front with RMX conventional forks. This thing has “trail ride me” all over it; all the more tempting to take it for a spin.

Now mind you, I have never ridden a KDX off road, only up and down the street - four of which I was considering buying, and one had fins - not the aquatic type either. Therefore I did not know what to expect, but only what I read here and to a lesser degree elsewhere. I figured it would certainly outpace an XR250 (one of which I briefly owned), but expected something along the lines of my old IT175; perhaps like my RMX but with a little less boost. Don’t get offended by what I am about to say, though I did have high expectations.

The Engine. A KDX mill is certainly one of the most electric-like in delivery of any two stroke I’ve been on. It’s like “where’s the hit?” that could be good or bad. But perhaps it’s best it does not have one (a hit); that is why it makes such a fine, sedate trail engine. It would happily pull skinny-old-me along without a hitch all day if need be, and not go much faster at middle RPMs than the uppers. It wasn’t ‘til I revved it like a 125 - and kept it there - when it got to serious moving; if the RPMs would drop there was a slight delay in getting it back up, even with clutching. This motor could simply use more beans, being stock except custom-singered FMF Fatty. It could certainly be better with Eric Gorr or Jeff Fredette’s massaging in the porting arena. What I wouldn’t want to change about it is the willingness to go and not kill you at any RPM - a good characteristic for survival riding situations.

Transmission. Gear selection was paramount, perhaps not as much as on a 125 (I don’t do MXers); nonetheless one needs to pay more attention than when on a 250. No surprises here - I’ve gotten lazy in my aging. The six-speed pretty much covers any situation.

Chassis. Now here is a winner. What more can I say than what can be inferred? The KX shock is excellent; the RMX fork - very good, but sprung for a 10-lb heavier machine, so this one needs lighter springs, although the action was good, it reamins on the stiff side.

Handling. Being lighter is welcome. The thing turned quite well, and did good in the whoops; if it had more power, it may even master the latter.

In summary, this is one fine trail bike; maybe even a superb tight/tough enduro mount; although having small lungs limits it’s desert prowess. But for my son (and perhaps even for some tought/tight enduros for me) it would make an excellent mount and reason to forgo valves and cams.
 

placelast

Member
Apr 11, 2001
1,298
1
Pix? No, but I will take them for you & have to e-mail them as I do not have pix posting capability. Send me an e-mail millenniumdsj@hotmail.com

Let my son ride it? No; it was my day for fun; he gets to ride it tomorrow. Although he helped wash it two times now, earning his privedge to ride it...I have no doubt he will like it as it has such a reasonable, un-intimidating delivery. I'll post his comments here afterwards.
 

WoodsRider

Sponsoring Member<BR>Club Moderator
Damn Yankees
Oct 13, 1999
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PL - The aluminum power valves on the '89 - '94 KDX engines were notorious for stripped gears. They would run great down low, but "run out of steam" on the top-end.
 

placelast

Member
Apr 11, 2001
1,298
1
Originally posted by WoodsRider
PL - The aluminum power valves on the '89 - '94 KDX engines were notorious for stripped gears. They would run great down low, but "run out of steam" on the top-end.

Yes, so I hear - they were an easily-stripped type, unless kept clean.

Do you know of a common fix for the lack of steam? It wouldn't be a problem with shorty, just me - the one with the greedy right hand. I found the motor to be slow revving and without much meat, but it's an unfair comparison next to my RMX or the old, snappy IT I had.
 

placelast

Member
Apr 11, 2001
1,298
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Originally posted by WoodsRider
PL - The aluminum power valves on the '89 - '94 KDX engines were notorious for stripped gears. They would run great down low, but "run out of steam" on the top-end.

Yes, so I hear - they were an easily-stripped type, unless kept clean. The motor was gone thru lately; top+bottom end and exhaust valve.

Do you know of a common fix for the lack of steam? It wouldn't be a problem with shorty, just me - the one with the greedy right hand. I found the motor to be slow revving and without much meat, but it's an unfair comparison next to my RMX or the old, snappy IT I had.
 

jaguar

~SPONSOR~
Jul 29, 2000
1,502
82
South America
a good running KDX has a nice "hit" on top, so you might want to take the pipe off and look inside while moving with your hand the actuator (after you take the cover off and disconnect the part that moves the actuator being careful not to damage it by not holding onto the lever while turning the nut clockwise). You should be able to see all 3 valves opening and closing while you move the actuator in/out. If the actuator doesn't move freely then everything wasn't cleaned sufficiently. If all valves move freely then the problem lies elsewhere such as main jet, clogged air filter, or maybe even ratty reed valves. maybe even take off the silencer to see if it's clogged and restricting exhaust flow.
 

dirt bike dave

Sponsoring Member
May 3, 2000
5,349
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Placelast - what gearing is on the bike? Dropping a tooth to a 12 tooth countershaft sprokect will help.

I had a '90 with all the usual mods (FMF pipe, rejetted, plus a Rad valve). When the bike is running well it is much stronger than an XR250 but just doesn't have the same power as any good running 250 two stroke.
 

2strok4fun

Member
Apr 6, 2002
1,085
1
Placelast gave me the opportunity to ride the frankenstein bike (albeit a much better looking fab) so I will give my limited expierence.

My first impression on sitting on it was what felt like a lower seat hight (compared to my 98 ktm 250e) and softer wider seat (again note the compared) The little 125 chassis felt tight and very manuverable. The suspension felt balanced, more than I expected coming off of 2 very different machines. In the power department, I dont have alot to compare it to as the last KDX I rode was probably a mid 80 model at the newest. I would say the motor lacked a little beans in the lower rpm range, then a noticeable hit in the midrange then pulls willingly without too much excitement to overrev/ top end. It seemed to respond to some clutching to bring up the rpm's to power (something I am not very good at doing smoothly) I wouldnt suspect powervalve sticking unless stuck open, and the bottom didnt feel that far off as if it was stuck.
Without other modern KDX's to compare it to it is hard for me to tell what would be "normal" but I expected a little more grunt off idle. *If* this is NOT a typical KDX 200 power delivery, I would suspect somthing in the stinger area, length, diameter, as this looks like the only fabricated part that would affect the powerband. I would guess the hit could be mellowed with jetting changes, but that wouldnt give it the beans, just delivery.

I dont think this is a platform to add huge power to and have it still be fun, but a little more wheel lofting tork would make this a real blast to get through the twisties.
 
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WoodsRider

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Damn Yankees
Oct 13, 1999
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My old neightbor bought an early 90's KDX that had been punched out to 214 (?) by Eric Gorr. I'm not sure if the cylinder had been ported, but the bike had an FMF pipe, Answer silencer and 13/49 gearing. This was probably the easiest 2-stroke to ride. It had tremendous low-end power and was so smooth. It felt almost like a trials bike. However the bike had almost no top-end. It could barely make it up some short, steep hills that required a good head of steam to make it up.

When he tore down the top-end the power valves came out in pieces. He replaced the valves, but also installed 13/47 (stock) gearing. After that change the bike definitely had more top-end but it did lose some of it's low-end chugging ability due to the gearing change.
 

Canadian Dave

Super Power AssClown
Apr 28, 1999
1,202
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Placelast do take the time to check out the KIPS valves. If you pull the pipe and look up into the exhaust port you'll be able to see both subport valves. Both should be in the fully closed position. You can then check the operation of the system as others have described to see if the valves are rotating properly.

A snapped or stripped valve is a very common problem on 89 to 94 KDX200s. Luckily new valves are relatively inexpensive. If you find that's the problem get back to us and we'll give you some tips while reassembling the top end to help ensure it doesn't happen again.

I'd have a look a jetting as well to make sure everything is up to snuff. Obviously the bike isn't going to perform like your RMX but the power shouldn't feel flat, certainly not as flat as an XR250.

David
 

2strok4fun

Member
Apr 6, 2002
1,085
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Placelast-

If you want to try some different needles, I have some from what I have read should transfer to the KDX. CEJ, DDJ,DEK, and possibly some others I cant remember now. The CEK is in my KTM now.
 

placelast

Member
Apr 11, 2001
1,298
1
Jag: the hit is there, it’s just much less pronounced than I expected, and was easy to fall off of as it takes more split seconds to get back into than what I am used to. I figure with some Delta Force reeds or porting it would be easier to exploit.

The reed petals were arched and had some minor wear marks, so I flopped them over to get some more life until the owner decides what to do with the bike (he’s contemplating consolidating into one newer bike.)

I replaced the main from a 160 to a 155 per the JustKDX website, and the filter is (was) fresh.

I might spend more time looking into the exhaust valve this weekend before I hand it back to its rightful owner.

DBDave: the rear is a 48; front: looks like a 13. I like the spread of gearing now, as first is an excellent granny and 6th a welcomed overdrive for the desert, so I may not recommend changing that, although you are right: a 12 would tighten up things a bit and make it easier to pull the gaps.

Woods, C’Dave: agree: looking at the valves wouldn’t hurt.

2strok: thanks. I believe the jetting is close on this one. I’d rather ask you to dial-in my RMX’s idle Sunday. I heard a few knocks today, so maybe it’s due for an exhaust valve cleaning.

All: my son said he liked it, though it doesn't appeal to him as much as something with cams and valves (too bad: I'd be buying), although it looks like the deal is dead as my wife doesn't want to part with the $ right now.
 

placelast

Member
Apr 11, 2001
1,298
1
OK guys: I took the pipe off & PV cover to exercise the PVs - everything seems to be working/rotating properly. The two drums on the left & right rotate, and the upper one does also. What is that cast piece which fills in the gap between the upper PV and the cylinder? It appears to just sit there, doing nothing. Is it a spacer? Is it suppose to do something or no? BTW, this is a '90 engine.

If all this is normal as it appears then what my friend has here is in fact a bike that will smoke an XR250, and then some; more $ (RB carb mod, Delta Force reed cage, FRP or EG porting) would certainly help and be his options.

I never intended to mean it does not have a higher-RPM hit; it does, but it's simply not as pronounced nor as sufficient as I thought it would be (for my preference); and it's better than what an XR brings to the table. I'd like to keep it.

Maybe new reeds would be of benefit and make it even livelier at higher RPMs and in the middle. It acts as if there's a bit much flywheel weight, and that's not a bad thing for woods riding. My RMXes have heavy OEM cranks, yet I added even more with Steahly's because they are all too willing to spin the rear tire when negotiating rock gardens, and when brake sliding they lock up too easily (I came off of a few years with a 4-stroke). Oh: I ride mostly in the desert, but for those two things I found it worthwhile to give up some of the quick rev.
 

dirt bike dave

Sponsoring Member
May 3, 2000
5,349
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John/Placelast,
The '89 is the same motor as the '90 but has slightly retarded timing. You might want to try changing the timing to '89 specs - there is a fairly recent thread on it somewhere. My buddy had an '89 when I had a '90 - both bikes had the same mods and his always had a bit more hit and top end than mine.
 

placelast

Member
Apr 11, 2001
1,298
1
The '89 is the same motor as the '90 but has slightly retarded timing. You might want to try changing the timing to '89 specs - there is a fairly recent thread on it somewhere. My buddy had an '89 when I had a '90 - both bikes had the same mods and his always had a bit more hit and top end than mine.

Thanks Dave. I'll try that if I get to buy it in the future.
 
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