'91 KDX250SR - help diagnosing a starting problem

Having read through several threads on starting issues with KDXs, I wanted to see if I could get some suggestions on my new-to-me bike from more experienced KDX owners.

The bike is a 1991 KDX250SR (euro model) in the following spec

Original "SR" carb, with 128 MJ / .38 pilot, needle clip in the middle
OEM exhaust system
Clean foam filter
New B7ES plug (was running BR9 iridium, have tried a BR9 as well)
KX gasket
Tassinari reed block and carbon reeds
New redline shockproof gear oil

The last owner had just rebuilt the top end with new SR piston, rings etc. and sold it to me as having done only a few miles on the new internals. He was running it with the needle clip at the top and with Motul fully synthetic premix at 30:1. He mentioned it was a bit hard to start and that he had resorted to bumping it. When I picked it up, it started 1st or second kick on the choke, and having owned a tired KDX220 previously that required 4 or 5 kicks, it didn't worry me.

A few days later I got a chance to take it for a ride. It started second kick on the choke, warmed up happily enough and seemed fine. I did 3 miles or so on the road and then stopped for a few minutes. It didn't want to start again. After about 15 minutes, I got it to fire up, and it ran for a few minutes. Before I could get my gloves on, it died again (despite blipping the throttle regularly).

Since then, I've not been able to start it, including by push-starting - it will fire and the lights will come on, but then it dies almost instantly. When the problem first occurred, I removed the existing plug, which was wet and dark (flooded or otherwise fouled?). However, there's no difference with replacement plugs (and these have not fouled during starting attempts).

The clutch is not disengaging properly (rolls fine in neutral and selects OK), so this is probably the reason why it won't bump properly - once second is selected, you effectively can't whip in the clutch, so it stalls. New springs and plates are on their way to me. However, that doesn't explain why it won't start in neutral.

Did a bit more diagnostic work on it on Sunday. When kicked from cold, the bike produces a healthy spark. Took out the carb and it is clean as a whistle, as are the jets (I would say that the PO had just cleaned it very thoroughly). Choke circuit was not blocked (checked opening and closing by blowing through it with a bit of hose). It had been running with the needle clip at the top and a .42 pilot, so I put the clip back in the middle and put in a clean .38 pilot (recommended by several KDX250 resources). The tank and float bowl were drained down and replaced with fresh premium petrol and syntethic oil at 2% premix (oil in the jerry can, then pump petrol). Fuel tap is providing normal fuel flow. No obvious wear to carb rubbers on either side. The autolube tank is empty.

It still doesn't start, and doesn't even _think_ about starting unless you bump it.

I did notice that the kickstart doesn't seem to engage consistently, and that it may be failing to crank the engine. However, it has clearly cranked properly at other times (turned the crank over) and failed to start.

Any ideas about what likely potential issues are and where to go from here?

Tom68 said:

38 pilot may be too lean for an SR, more so if you run premix, mines a shade lean with a 40. Starts good with a 45 but gets a bit doughy.
SR compression isn't affected by exh valve as it only has the drum valves controlling volume.
Factory bore plating wasn't very good, have a look up the exhaust port and see if it's through to the aluminium.

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Thanks guys. Good points, and especially that no compression could be the problem. I can't believe it's a fuel or spark problem as I've seen a strong spark and an extremely clean carb.

It was the previous owner who did the rebuild. I take the point that he may well have been dodging a bullet by selling it, but OTOH since he did rebuild it himself, I would have thought if there was a compression problem he'd just tear it down and fix it, since he'd put a lot of work into the bike.

I'll look into checking compression. I have the original reed block, so could swap that back in. What should I be looking for when I inspect the carbon reeds? I'm not a two-stroke expert by any means (have a TY250 twinshock, but it's pretty bulletproof so this sort of thing hasn't come up :) )

Just managed to break my collar bone, so I'll have time to strip it down if necessary... :ahhh:

Tom, thanks for the SR info, it's tough to find. I'm going to assume the KIPS isn't an issue. What carb is yours using? Mine's a euro model and I _think_ the US bikes used the KDX big-ass carb, while the euro bikes got a tiny 32 or 33mm PWK. I'll get a 45 to try as well.

Tom68 said:

Keihin PWK V700FF13, needle clip middle, main 132, pilot was 42 now 40, slide K45, needle HKNO can be hard to start cold after a couple of weeks sitting. I think the throat is 35mm.
Plating was worn through on my bore, as it is a $450 job down here I fitted a Gas Gas300
piston while I was at it making it 285cc jetting stayed the same.

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What year was that GasGas piston from? That sounds worth trying out in the future. Hopefully the bore is OK - the PO did say he had inspected it and no problems, but we'll see.

Just updating this - figured out the problem was that the flywheel hadn't been properly torqued and the woodruff key had sheared. Put a new one in, lapped everything in, torqued up the flywheel nut and she started third kick.

Now to take it to the track and see how bad the suspension is!

Tom68 said:

My sr turned out to have 0.29 springs with no preload, there's a post about it, may be on page 2.

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Thanks Tom - I've actually got hold of some KX forks for it for all of £20, so will be fitting those in due course. They're even the correct blue for the KDX. Am going to finally take it to a track on Satuday to see how comically undersprung it is.

Mine is quite smoky. My 220 was the same, even at 50:1 or whatever with fancy oil. It's also now not starting in our current -2c weather! :bang: I'm waiting on some warmer weather and some new jets to try out. The power isn't too amazing - my 220 was at least as good - but it does cruise happily around town at 30-40mph, which the 220 absolutely hated.

Yes, it's autolube and I'm pretty sure a '91. If yours is stock without autolube, it sounds like ... a non-SR KDX? :joke: I think there were export and domestic SRs as well. We probably have different ones - maybe there were non autolube SRs for one market. Sure yours hasn't just had it removed? It's a common enough thing to do.

As far as the forks, there are threads on here that confirm which years fit. I believe it's 91-95 KX? I now have the forks, and the fork bodies look the same as mine (except for the cool press-bleeders), so they should fit the clamps. The issue is o/c that the KX, being a MX bike, doesn't have provision at the axle end for the speedo drive, and quite possibly has a different axle spacing. It may be people are just running a complete KX front end in KDX triple clamps/yokes with no speedo (or a bicycle speedo or a veypor). My old 220 had a 92 or 93 KX front end on it, which felt great, although it was honestly a bit too hard for our muddy trails, but it didn't have clocks (hour meter removed).

GhostofSilverider said:

I also have a 91 SR, with jetting problems. Strange though that you mention you have autolube on it. Mine doesn't have it, and apparently it comes stock without it. Are you sure it's a 91 ? I also read the euro model (for France at least) had a 152 MJ stock. I don't know what I have on it but the bike keeps fouling plugs (wet and dark) and upon removing the pipe to fix a spooge leak and do the pipe mod (which was already done in fact) I noticed the piston and cylinder are also coated with spooge. I removed the airbox cover and plan to play with air screw and needle , hopefully that will alleviate the rich condition. The bike runs very well otherwise although with too much smoke, top end power is a bit disappointing compared to 250 motocrosser though.
BTW do you know if a 91 KX 250 fork will fit on the 91 KDX 250 SR ?

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Merry Christmas etc.!

Don't have the bike right here, but the autolube reservoir would be located behind the end of the kicker in that picture - just under and to the right of the tank, above the cylinder. e: the reservoir's under the tank and to the right. It appears to feed in between carb and reed block, unless I'm very confused about what the wee pipe is for.

Just got around to changing the jets - it was running a 128mj and a 38 pilot. So that should be stock pilot and leaner on the MJ (135 stock according to sudco). I'm switching to a 42 pilot and 132 main per Tom's comments above, as it has decided it still doesn't want to start in winter temps. :bang: Has spark and there's fuel in the float bowl. Air screw and idle screw were on stock settings (1.5 and 2 turns out from closed). At this point an electric start DR-Z400 sounds increasingly good. :whoa:

ee: IIRC, there are some funny SR pictures on Yahoo japan and other Japanese sites. They had some truly weird and wonderful paint jobs. Purple and shocking pink anyone?

GhostofSilverider said:

Yeah that's the problem with the KX fork, no more speedometer ....

I looked closely at my bike and there are no brackets under the rear side panel to install the autolube. Where does that autolube hook up on the carb ?
It's very strange because I have an old mag with an ad for the KDX 125 and 250 SR (91) and the ad mentions the autolube on both. But then in another issue, they go over the mods for the 250, remove all the extra weight and no mention is made of the autolube.

Here is the 91 model, the bike pictured here is brand new, it's incredible. Mine is in mint condition but not showroom condition like this one. This bike also have autolube, but me thinks mine is a completely different version because I have the large headlight , no auto lube brackets (although on the picture it looks like the autolube tank is bolt on an extra bracket to the rear subframe, so that bracket could have been removed) . Mine never had passenger foot pegs either. Go to bikepics, then type /pictures/2029774 in the address bar

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Tom68 said:

P.S Just changed the Iridium spark plug, (went from an 8 to a 9, just because I had it in stock) was in there nearly 4 years, cold starting is now improved.
Hot start is always one swift kick.

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Rub it in why dontcha.

Have been waiting for a warmer day to try starting it with the larger jets. Not sure it's a carb problem at all though, as it was on the stock pilot (38) and correct air screw settings, so it should have started, even if the MJ was off.

Haven't had a problem with hot starting, as you say, but getting it to start cold is still kicking my ass.

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?

Also, is anyone running a KX rear fender who can tell me what year to get?

I'm going to ditch the stock tail (needs a repaint anyway) and the supports/subframe to save a bit of weight.

I've seen a variety of adaptations on the 'net but am not sure what's going to be easiest out of the box.