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Yes, Another Jetting Problem Please Help!

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I apologize because this will be a long post but I want to make sure I give all required info.

I have a WR 426 w/the air box lid removed, grey wire cut, a B.D. baffle, YZ throttle stop, and twin air filter.

The bike ran great on stock jetting for 500 miles. A mix of high speed and tight single track trails. It always started on the second kick when cold and w/the hot start if dumped or stalled. The pilot screw has never made a noticable difference so I have always kept it at 1 1/2 turns. When the weater got colder, in the 50s It began to get a bad sputter at constant half throttle. I thought it would be a good time to take it in the basement for the winter. I tore it down and serviced the whole bike. I cleaned the carb and changed the plug. The carb was very clean to start with. The plug and head pipe was full of carbon. I figured that it was running a little rich and over the 500 miles the plug was just starting to foul to the point where I had the above symtoms.

I did the BK mod and changed to YZ timing. I lowered the needle clip to posistion #5 as both mods need richer jetting. I know that both mods need several jetting changes but I wanted to test the bike first. I got everything done quicker then I thought I would and the weater held out. I brought it out of the basement for some more rides.

I did some short street runs first in 50 degree weather and it ran good but seemed to take a long time to warm up. I thought this was because it was colder out and running on the street w/few stops was keeping the bike cool. I let it warm up for several minutes and took it for another short ride and it ran good w/no sputter or bog.

I then, on a 60 degree day went to do some trail riding. I have to street it 12 miles to the trails at my parents home. About 1/2 way there the bike sputtered so bad it died. I changed the plug and limped to my parents w/the hot start out to get it going. Once there I backed the screw out for the BK mod to elimate that and leaned the needle out two positions, one past stock to the #3 posistion.

I then rode about two hours mostly tight single track w/some high speed runs. It ran GREAT, no hesitation or bog and started just as it should. No sputter at all at constant throttle on the 12 mile road run home. I noticed a huge difference w/the YZ timing, the bike just ripped!

Yesterday, I was running late to meet some friends at a sand pit 25 miles away. The air filter was still clean so I didn't change it. I road it hard on the street for the 25 miles and it ran great. As soon as I got in the sand pits at trail speeds it began to sputter bad again. I immediatly left for my parents, about 15 miles away. It ran good all the way there as long as I stayed hard on the gas. If I didn't it would sputter badly.

I know it was rich, I pulled the plug and it was full of carbon. The electrode was fairly clean though. I cleaned the plug w/a wire wheel on a bench grinder and some fine grit sand paper as I didn't have a new one. I leaned the needle out one clip to position #2. It was way too lean and the pilot screw wouldn't make a difference.

I then put the needle back to where I had it in position #3 and played w/the pilot screw again. It was immediatly way too rich and sputtering.

So now it seems that I need to be between clip positions #3 and #2. #2 is way too lean and #3 is way too rich. The pilot screw has never made a difference in the idle and seems to make no difference in the way it runs.

What has me confused is how it ran so good for more then two hours on the first day I had the clip in the #3 posistion w/the BK mod bypassed. It ran awsome and the engine temp seemed right on the money. It got hot , but not too hot like it would if it was lean. On that day I thought I had it all figured out.

One question I have is, on that day I had about three miles on the new plug after I limped it to my parents, where I changed the clip from #5 to #3 posistion. I never changed or cleaned the plug as I only had one spare. Could I have gotten the jetting right but severly shortened the life of the plug on that three mile run? Again, it ran great for over two hours. But yesterday it acted very rich after the 25 mile street run. Can a plug be cleaned and reused like I did yesterday.

Would a bad plug make the bike act rich.

After I cleaned the plug throughly I had rich symtoms immediatly at clip #3. I have no problem throwing a new plug in and doing some testing. I don't want to be stranded in the middle of no where though after two hours of riding.

Again sorry for the long post. I am sure I made several jetting mistakes but I am new at this, especially since the bike ran so good from the start. I would appreciate any advice.

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One thing that would help is to list performance characteristics at various throttle ranges. See "High-Altitude Jetting - CO" for what I consider a good list of tests to perform to give everyone a good idea of where we should focus on your jetting.

Assuming your BK mod was done correctly and you had reduced your spray from 2+ secs to 0.5 secs or so, "undoing" the BK mod would only cause your bike to run richer...which it sounds like is your problem.

Also, I'm assuming that by "stock jetting" on your 426 you mean something like 42PJ/75PAJ/DRR/165 main?

Overall, without knowing more (and considering I'm still a novice at this jetting gig), it sounds like changing the jet needle clip position between 2 and 3 is merely compensating for a jetting problem elsewhere. Thus, the suggestion of doing those tests that I suggested above to help pinpoint where your problem might be. Check out that post and read the Patrick Burns article referenced there to gain some more insight...

Sorry I'm not much help...but providing some addl. info might enable the jetting gurus to help you out a bit easier.


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Short and to the point, #3 clip and a new plug. Clean the carb inlet around the air jets to make sure they aren't clogged. NOBODY runs a DRR#5 (I think) without eventually fouling a plug.

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I took the afternoon off and bought some new plugs. First, I confirmed that with the clip in posistion #3 the bike is very rich. It ran rich around the block and a check of the new plug confirmed it.

I then went w/a 38 from stock 42 pilot jet. I left the needle at position #3 played w/the pilot screw when I started it and could hear it idle up becoming a little lean when I got the screw all the way in. I put the screw at 1 1/4 turns, rode around the block and checked the plug. Still very rich but the electrode looked a little different. It looked grey on the end and the rest of the plug was black. The bike did run better w/just a hint of a pop on decelleration and pulled strong at every throttle position.

What has me confused is how the bike is running so rich all of a sudden. The only change I made ( I undid the BK mod ) was to YZ timing. It is my understanding that this would make it leaner.

If I go smaller on the main jet will this effect the jetting through the whole range?

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Burns recommends getting the main jet sorted before anything else... or just jumpo off a cliff. My understanding is that this operates/has an effect throughout the whole range, but mostly on the upper range.

If you havent gone through the Burns article I suggest you do so, it gives tips on how to ID and change rich and lean conditions in pilot circuit and main circuit.

I think that clip height (should) change upper range acceleration performance more than anything.

If you can wind your fuel screw all the way in and out without much affect to the idle speed you probably need to change your pilot jet to a leaner setting. Fuel screw should end up somewhere in the 1 - 2 turns from seated. If you can wind it in all the way and you are still idling you are way RICH.

Needle diameter affects (amongst other things) the low throttle response. Thicker for leaner, thinner for richer.

Again the best you can do for yourself is to read the burns article and work the problems you have against those and the recomendations from the experts (not me yet, but I'll get there or die trying!!!!)

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I agree with Missle...read the article...read it again...and then once more for good measure. It's really quite a bit to digest. But once you've gone through it a few times and written down some notes for yourself about the effects of the different jetting variables, you finally start to clue in a bit (atleast you think you do anyway!) :)

Good luck!

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just some pointers that i hope help.

header "full of carbon". mine is, yours is, everyone's is so forget it. your exhaust is a reflection of the last what, five seconds or less of running. so if you were pulling your sidestand out......

YZ doesn't change jetting for everyone. certainly not richer i wouldn't have thought. the higher airflow would REDUCE the jetting.


don't go by their colour

certainly not on an old one

the only plug colour worth noting is that of the top end (top speed)

go by the middle of the exhaust (entry to the tailpipe)

if you have a very lean condition at any point and run your bike in that rev range constantly you will BREAK the PLUG DOWN.

these bikes only run an 8 'coz Yamaha got the jetting so wrong. the only rule is to give it enough and never too little.

the trade off? you run an 8 when a 9 is the minimum for a true racing machine.

an example? riding really hard on the road for miles in a lean condition, straight onto green lanes, a different jetting circuit (pilot?) which is possibly the other way etc. plug can't cope.

please don't use a wire wheel. use a soft wire brush made for the job

non new plugs are the best IMHO opinion (no facts on this one) they don't have any nooks or crannies because the carbon has filled it. fuel is absorbed without the electrical discharge being earthed before it's due time.

you can use and reuse a plug. best burn the carbon off over a flame and wait for the blue flame to turn orange as it burns the fuel off.

keep all your old plugs that you 'fouled'. just run the bike and change plugs quickly while the engine's hot. thrash it. put it in a box back on the shelf.

i always copper grease the thread's. check your gap! .0028" for me but .0024"-.0032" is right.


[ November 19, 2001: Message edited by: Taffy ]

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