'91 KDX250SR - help diagnosing a starting problem

[Tom68]

Well, after taking apart the kickstart mechanism and doing some fiddling, it's now starting acceptably. There's still something not quite right, but I can't figure it out, and neither can my friendly neighborhood bike shop.

Throttle response is good. It doesn't want to idle warm or cold (42 pilot), but I should be able to sort that out with a bit of work on the jetting. It's running a KX gasket, tassinari reed block with carbon reeds and a pro circuit muffler, so a bit of tweakling is to be expected.

This week I was finally, finally able to take it out to our nearest MX track and even with road gearing on it, it really rips out of corners and down the straights. Goes round corners...well, like a KDX. Very easy to ride and forgiving, despite the weight.

It was a bit scary over the jumps, however. Not sure precisely what the problem because I was out there by myself (day off work) and didn't really want to push too hard and end up in an ambulance. Stutters seemed OK with the proper body position, but anything bigger than whoops it's coming down so hard it's just about winding me - even sitting back and keeping my legs bent enough that I'm getting tapped on the ass by the tail.

The forks don't actually seem to be bottoming, surprisingly enough, but maybe they need higher rate springs, or maybe the damping isn't up to big landings?

Is this just normal behaviour for a ~134kg trail bike on a MX track, or does it sound like something I could cure with work on the forks? I've only ever ridden tracks with modern WRs and YZFs but have had no problems jumping those machines, so I doubt it's purely a technique problem. If it just sounds like "your bike is too fat", I guess I'll just have to take it easy over jumps... :(

Any SR owners have an opinion?

My forks bottomed horribly, I thought it had 0.34 springs and I ran air pressure for a while. finally got around to fitting 0.46 springs, turned out the original springs were 0.29's it's good enough now, I'm 165 lbs. Sounds like your springs may have been upgraded if your not bottoming or your oil level is too high.
Do a search using my username if you want more info, I may have added some pics in old posts.

 

[idl1975]

My forks bottomed horribly, I thought it had 0.34 springs and I ran air pressure for a while. finally got around to fitting 0.46 springs, turned out the original springs were 0.29's it's good enough now, I'm 165 lbs. Sounds like your springs may have been upgraded if your not bottoming or your oil level is too high.
Do a search using my username if you want more info, I may have added some pics in old posts.

Thanks again Tom. Talking to the guy who runs my local bike shop, he thinks it's definitely soft forks that are causing the problems in the air. He used the phrase "pallet of bricks", which summed up the joy (i.e. fear) of landing jumps on it better than I was able to.

I already had a set of KX forks and I've now located a KX wheel to match. Will be putting those into the stock yokes and respringing (they're 125 forks, so they would have had 0.3 something springs) with 0.46kg/mm, which was handily enough the Racetech recommendation for me, as well as what you're running.

Once I had that set up I want to ditch the stock fender and subframe/supports, which should save a few pounds. Plus avoid the pro circuit can melting my right rear indicator...

 

[julien_d]

You're putting the kx forks in the kdx triples? What year are the KX forks? I don't think this will work very well, as the offset is likely different, which will mean custom axle/spacers, and the OD of the forks at the clamping area is likely different as well. Most people use the triples (yokes) from the forks they are installing, and swap the stem from the KDX to the KX triples. Definitely a worthy upgrade, whatever it takes!

 

[idl1975]

You're putting the kx forks in the kdx triples? What year are the KX forks? I don't think this will work very well, as the offset is likely different, which will mean custom axle/spacers, and the OD of the forks at the clamping area is likely different as well. Most people use the triples (yokes) from the forks they are installing, and swap the stem from the KDX to the KX triples. Definitely a worthy upgrade, whatever it takes!

KX125 H2 or J1. Irritatingly, it's taken me so long to fix other crap that the e-bay auction has expired and I don't recall what they were sold as. Not sure how reliable that would have been, given they were sold as KX250 but have "X125" stamped on the fork bottoms, which to me says KX125.

I've read about other people who've re-used the KDX yokes, so I assumed they would fit. If not, I'm going to feel mislead by the internets. (e: as you say, even if I can't reuse them, consensus appears to be the KDX items are so horribly damped, especially if you're going to jump the bike, that it's not worth spending money tweaking them.)

The KX USDs and the KDX USDs look like they use the same Kayaba 43mm housings (the bit that isn't the slider), so hopefully it's not an issue. My understanding was that some people prefer the KX lower clamp because the later KXs ran double fork-clamp bolts on the lower. But the current forks have survived jumps with single clamps, so I think it'll be fine as long as I break out the torque wrench and remember the loctite!

If I have to get some KX clamps, it's not a big issue. There are about 10 of them on e-bay at any one time. I just want to avoid the PITA of pressing stems in and out TBH.

As to the offset, I've got a KX wheel, axle and spacers, so it should be a straight swap. It means I lose the speedo drive, but I have a Veypor that came with my TY250, so I'll put that on if I feel the need. Although the clocks are handy when green laning (trail riding), I foresee no circumstances where I'd rather ride the KDX than my Hornet for pure road-riding. :)

 

[julien_d]

You may be right. I didn't realize the kx forks you are using were so old. Both are 43mm, so they might fit the triples. FWIW the kx125 and kx250 shared the same forks.

 

[idl1975]

You may be right. I didn't realize the kx forks you are using were so old. Both are 43mm, so they might fit the triples. FWIW the kx125 and kx250 shared the same forks.

Yes, using old KX forks as they (i) (e: are supposed to...) fit and (ii) despite the age, still have far better internals than the basic KDX items. I also got them for next to nothing, which is handy.

Not trying to make it into a MX bike, but it would be nice to be able to take it round a circuit without praying and crying when a kicker appears on the horizon.

 

[julien_d]

I think you'll be happy with the 43mm kyb's. I am running 43mm KYB's from an early 90's yz250 and like them very much. The 92 kx forks should be very similar. I do find them a bit harsh on the small stuff. I'm going to get around to re-valving them for woods work at some point.

 

[idl1975]

I think you'll be happy with the 43mm kyb's. I am running 43mm KYB's from an early 90's yz250 and like them very much. The 92 kx forks should be very similar. I do find them a bit harsh on the small stuff. I'm going to get around to re-valving them for woods work at some point.

Thanks for the info - if the new forks are only "a bit harsh on the small stuff", it'll be an improvement! :laugh:

Might do some novice enduros on it in the mean time!

 

[julien_d]

Well, more than a bit. In the really tight and nasty single tracks we have around here, I often times miss the stock front forks. That feeling goes away as soon as I can get over 15mph though :). A little work in the shim stack should take care of that though. Just have to get off my lazy behind and do it.

 

[idl1975]

Have finally got around to getting the KX forks on the KDX, having spent a bit of time sorting out a new-to-me CBR600 F2 and doing some track days.

Finding springs for it was proving a bit of a pain, so I opted for having a local spring manufacturer put together a pair in .46kg/mm to match the springs out of the KX. Forks were then rebuilt with new oil, seals and bushes.

As the KDX uses a different axle (for a company so in love with their own parts bin that they still make entire GPz900s, this they had to make different...), I'm using a KX wheel with the KDX disc and spaced out accordingly. To save dicking about with the soft/hard line combination, which is in any case not the right configuration for the longer KX forks, I've spent ~£25 on a Hel braided line, which looks nicer (kwakka green and black banjos) and avoids the horror of dealing with ancient hard-line connectors.

Still waiting for a spacer to be cut to size, but took it for a quick spin up the road (after carefully rechecking my clamp bolts!) spaced out with washers, and the brakes and front end feel much nicer, to the extent you can tell on tarmac anyway. A lot more feel under braking and more controlled action. Going to hopefully take it to the track or a novice enduro on Sunday to see how it goes.

The KX front came with a D752 in 90/90, as opposed to the fatter section road-legal knobbly I have on the KDX rim. Anyone have a view on whether the 752 suits KDXs? And will that size work? The two biggest MX tracks we have locally are (i) soft earth and (ii) hard sand and chalk - basically pretty damn hard. I gather it's a soft to intermediate tyre - will that work at all on the hard surface, or will I get dumped on my ass?

At least it's coming together at last. It even started like fourth kick, despite having sat for two or three months. I have a KX tail from UFO which I'd like to mount up, ditching the stock rear frame rail. If I could get that done, my bike would be a single colour again, but there's quite a bit of cutting and fitting, plus I need to fab up a bracket to carry the number plate and indicators.

 

[julien_d]

I ran a 752 for a while. It was decent. 90/90 is a bit small, but should work fine. Sounds like you put a lot of effort into the details of your fork swap. Good for you! You will really like the new forks, I think :).