Maintenance & Technical > KX500 Original
Keep Breaking Carb Needles
2-Stroke Tom:
Well, I took the old 500 for another ride this last weekend and ran into a little problem. Fired the bike up for the last ride of the day and was hammering pretty fast over some whoops when all of the sudden the bike just bogs once I let off the gas and tried anything over 1/4 throttle. I knew immediately what the problem was since this has happened to me on this bike twice before. A broken carb needle, which I recently figured has happened about 16 months of pretty consistent riding. Not the worst thing since it won't leave you stranded, but if I threw down money to enter a race and got a DNQ on one of the most reliable bikes around, that would suck. The strange thing is that the needle brakes right under the the bottom most groove every time. I have the needle clip in the 3rd groove. I have an older picture (not the best picture, but it can be zoomed in for detail) from when I had the needle and slide out last time here:
http://s1246.beta.photobucket.com/user/rjhansens/media/IMG_0492.jpg.html.
Since this is an Ex Team Green KX500, I have no idea if anything was modified that I'm not aware of (I know all the jetting specs because they write them on their carbs, but was the slide modified, the non-replaceable needle jet modified, etc?), but in addition to a new needle and clip, I'll by the slide this time. The slide seemed fine when I had it out, but is it possible for these thing to wear enough to cause a problem? Could the carb body be worn? Just from the naked eye, everything looks perfect and the bike runs exceptional until the needle breaks. One key thing I feel I should mention is that every time this has happened, the throttle doesn't hang up at all, and this just occurs all of the sudden. So, even though I'm replacing the jet needle, if the needle jet is tweeked I thought this would make it hang up at least once during this much riding.
The first time it happened I wasn't too concerned because I figured maybe it got a little tweeked when someone tried to put the slide assembly on with the fuel tank on the bike. The second time, I inspected everything very well and couldn't find anything wrong, and just figured maybe it was a fluke and threw an extra clip and needle in my camelback in case I got stuck out in the hills. Now, I'm going to chase it until it's fixed, but I'm just confused since I've never heard of this happening before and I was a career mechanic back in the day. Raced thousands of desert miles with a few clubs and still, haven't heard of this.
This is the only problem I've had with this bike after owning it for about 4 years with no rebuild. We're talking a lot of hot, dusty desert miles I know I need to do it soon, but it's just so fun to keep riding! Just keep eating up gas, grips, and tires. Please give me your thoughts.
Twosmokerfan:
Hey Mate,
I have come across this on my 500 when I bought it off the previous owner (still have the broken needle that he included in the stuff he gave me when I bought it).
When I bought the bike, the air filter was falling apart, there was fine dust and larger chunks of sand in the carb and intake, the crank seals were gone and leaking oil into the crankcase from the tranny, and there were scratches on the needle (indicates to me that dirt was stuck to the needle and going back into the seat).
Anyhow, after rebuilding the engine (totally cause the con rod was kaput and the hard face on the big end was missing) I also broke another needle in the same spot you have. I replaced the slide, needle and the female part of the carby where the main jet screws onto (mainly because I couldn't get it to jet cleanly and put it down to wear in the brass female part) and all is well so far. I think there was too much play due to wear over the years in the slide, allowing the needle to fatigue and break in use.
I don't know yet if the problem is fully solved, but all is good so far and when I check the free play on the needle when in the slide, it seems to be a lot tighter than what it was.
Hope this helps.
TSF
2-Stroke Tom:
--- Quote from: Twosmokerfan on October 30, 2012, 11:56:32 PM ---Hey Mate,
I have come across this on my 500 when I bought it off the previous owner (still have the broken needle that he included in the stuff he gave me when I bought it).
When I bought the bike, the air filter was falling apart, there was fine dust and larger chunks of sand in the carb and intake, the crank seals were gone and leaking oil into the crankcase from the tranny, and there were scratches on the needle (indicates to me that dirt was stuck to the needle and going back into the seat).
Anyhow, after rebuilding the engine (totally cause the con rod was kaput and the hard face on the big end was missing) I also broke another needle in the same spot you have. I replaced the slide, needle and the female part of the carby where the main jet screws onto (mainly because I couldn't get it to jet cleanly and put it down to wear in the brass female part) and all is well so far. I think there was too much play due to wear over the years in the slide, allowing the needle to fatigue and break in use.
I don't know yet if the problem is fully solved, but all is good so far and when I check the free play on the needle when in the slide, it seems to be a lot tighter than what it was.
Hope this helps.
TSF
--- End quote ---
Thanks Twosmokerfan. When you say you replaced the brass female part that the needle sits into, do you meen the whole carb body? The jet needle goes into the needle jet, which on my Keihin I believe is non-repaceable and is part of the carb body. This is a 2004 PWK 39mm carburetor.
Anyone else come across this?
Twosmokerfan:
I had a spare 38mm carb and too the security bolts out from part of the carb that the main jet screws into (not quite sure of the name) from the bottom, and the needle goes into the top and swapped them over. It has one of those special torx bits with the stub in the middle of it. Have a look when you pull your carb off. You can't miss it.
2-Stroke Tom:
--- Quote from: Twosmokerfan on November 06, 2012, 12:08:25 AM ---I had a spare 38mm carb and too the security bolts out from part of the carb that the main jet screws into (not quite sure of the name) from the bottom, and the needle goes into the top and swapped them over. It has one of those special torx bits with the stub in the middle of it. Have a look when you pull your carb off. You can't miss it.
--- End quote ---
Thanks Twosmokerfan. It sounds like the bolts you're describing are tamper proof torx. I'll check it out when I pull the carb this week. Parts should be here by Thursday, so I'll keep everyone posted.
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