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]

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

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]

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.

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.

[2-Stroke Tom]

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.

Thanks Twosmokerfan, you were right about this part which I have found out is call the jet block that I can supposedly order from Sudco. Can't find the replaceable needle jet that screws into it anywhere though. I think I may have found a lead to my problem. I just put a new slide, jet needle, clip, and retainer in. I pulled the float bowl and main off to see what the needle looks like from the bottom and in the picture here you can see that the needle isn't centered:



This could be causing it to break, but how odd is it that the bike runs perfect for a year of riding, then breaks a needle. No symptoms of any needle hanging up at all. Now what?? I may just buy a new carb if I have to keep throwing parts at this, some of which may not even be available. Already had to buy float, slide, gaskets, etc, and that was about $100. Plus, this is the third carb needle I've broke at $25/ea. I can get a new carb for around $210 delivered, so this may be the smartest route. I can just swap out the jets.

Also, as usual with these bikes, something was always different and/or modified. Team Green used a notched slide while the replacement oem slide I received is not notched. It's hard to see, but look at the top of the picture for comparison (bottom of the slide):

[Twosmokerfan]

Hi 2S Tom,

Yes, that part comes out and then entire internals of the carb comes out up to the venturi part that is in the throat of the carb.

I might suggest that the movement of the needle over the past year has caused fatigue in the clip area, causing it to snap.

I changed mine out from my spare carb (because I didn't know how many miles the original had, and knew exactly how many hours the replacement had done) after I couldn't get it to jet correctly.  After that, no more jetting issues and (so far touch wood) no broken needles.

It is just a theory worked on seeing I had replaced the needles, slide, and mostly every other component in the carb, and still not able to get it to jet.  Just my theory.....

Hope it works out.

[2-Stroke Tom]

Well, before my trip last weekend, I went to jet my brand new Sudco replacement carb, and I finally got to the bottom of my issue with breaking carburetor jet needles. Believe it or not, it had to do with the jet needle retaining clip where the throttle cable attaches inside the carburetor. A couple dollar part!! As you can see in the picture below, the new Sudco retainer is relieved underneath so that it does not "trap" the jet needle clip. I had both slide and needle assemblies in my hand (both brand new, one was oem and one was Sudco) and it was apparent that the oem retainer caused the clip on the needle to be trapped, and would not allow the needle to rotate and basically self center.


So, everything is brand new from Sudco with the exception of the new OEM jet needle I installed.


And, everything installed with new compression release modification.


The end result was the bike ran great. The bike ran great with the higher compression head gasket and the 50/50 mix of 100 octane VP race fuel/91 pump gas. Thanks to Motorrad's advice, I was confident I wouldn't have any problems. The only think I'm going to do is go to a N82M needle per Team Green Specs (instead of N82P), now that the compression is increased an race gas is being run. I could definitely feel the power difference, so I'm sure some throttle chops to get the jetting spot on would even help more, but the bike pulled hard with no hesitation, and fired first kick all day, so I never messed with it.

[Motorrad]

shouldnt need 50/50    you could get away with just tossin 1.5 gal in a 5gal can.   just to "FIX" the s**tty california fuel problem.