Hey Clark!!

Thanks for the exhaust info. I am eagerly waiting for all your upcoming info on exhaust systems.

It is now my turn to ask for some jetting advice.

I have done all the YZ mods including removing the octopus. I am running with the insert removed and the entire air box lid gone also. I have used ALL your jetting recommendations including the DVP needle, 4th clip down from the top, I believe. I am still using the 48 pilot, 180 main. I have rotated the TPS full clockwise. When I fired off the bike 6 weeks ago for the first time, it seemed the bike would only start using the hot start bypass. It did not matter whether it was hot or cold. I went 1/2 turn in on the fuel screw. It was 2 turns out from my dealer. It starts now in 1 - 5 kicks, generally. When decelerating w/ closed throttle, it has moderate backfiring and popping. I was riding last Sunday. To avoid problems with the locals, I was running at low throttle settings for extended periods of time. The bike had been running OK when it suddenly started to severely stumble at full throttle. I thought I was fouling my plug. I kept the revs high for 2 miles back to my van and ended my day. When I got home, my 4 year old son, Adam, insisted on a ride. I do wheelies with him in my driveway. The bike started normally & seemed to run OK. I blasted down the road (without Adam) and again the bike would not rev out and severely stumbled. Tonight I pulled the plug. The plug insulator was completely white on one half and carbon black on the other half. I know the plug is no longer representative of my full throttle operation last Sunday. I live at 900 feet but ride at sea level. I do have some minor scuffs in my cyclinder where foreign material entered in (possibly from the head vent-stalled it several times in deep, muddy water and re-started it in that same water). I am pulling the carb tomorrow for a disassemble to look for anything. I will be checking the accelerator pump also. The valves have been adjusted this spring.

Thanks a heap Clark!!



99 WR

Start with a new plug!

My bike with the 48PJ likes the fuel screw betwen 0.75 and 1.25 turns out. I also run with the 45PJ and the fuel screw between 2.0 and 2.5 turns out. If you remeoved the octopus I recommend you leave the 60 start jet in (which feeds the choke start circuit) and not put in the 65 as I previously had suggested. The above should help with the starting.

Also make sure the carb is clean. I'm getting a lot of dirt in around the top of the carb around the gasket between the carb and the carb top. This carb has several places where air leaks can develope and you must make sure there are none. If you remove the octopus make sure you porperly sealed the Port on the carb intake boot from the air box, sealed the port into the left side of the carb where the #2 air start jet is, etc. If you removed the bottom of the accelerator pump cleaned it and put it back together make sure you got the diapharam in right side up. The side with the raised sealing ring goes up and the side with the longer center metal button plunger stop goes down and make damn sure you get both of the "O" rings in place between the accelerator pump housing and the pump bowl on reassembly or you will have a air leak and no pump performance. Make sure the rubber seal on the hot start button is not slighly pulled out causing the hot start button to not close all the way when pushed in. Make sure there are no cracks in the hose that goes from the intake manifold on the head to the hot start button and that both ends are fully engaged on their bosses. Finally make sure the carb top is tight, and all three hose clamps that hold the carb on are tight and that the intake carburetor manifold is aligned porperly on the head and carb. There are alighnmnet tabs on the head, on the rubber intake manifold and carburetor.

As to you top end situation I would put the TPS back to stock position. Make sure of your clip position and the MJ. At 900' with a DVP and 180MJ it should not stumble on top. I should run cleanly right to the rev limiter. With stock gearing you should hit the rev limit at 53 in third 71 in forth and around 87 in fifth. These are approximate MPH's and will vary slighly. Remember the rev limiter is a soft cutout it will start to not fire on every pulse and seem like stumbling, the rev limit is not a instant cut off. With YZ timing things happen faster approaching the rev limit and you may be hitting it thinking the bike is stubling.

If after you check out your carb and are confident of all the jets, reset the TBS and still feel its stumbling on top You may want to jump to a 185MJ and try it and see if the synptoms change. If not then try a 178 MJ and repeat. If you truly have YZ timing you should end up with a 178 to a 182MJ depending on conditions as long as you are riding below 6000ft. If above 6000' then probably and 175MJ.

Its hard to tell exactly whats happening but this carb is pron to air leaks and it does raise hell with and mask other problems.

Hope this helps and keep the forum up to date on your progress so we all can learn from your experience.


Minor poping on decelleration id ok mine does it too. If I run the DVR needle it is worse.

I'll have a go at it today Clark. If you ever get lost and end up in NH, I make an extemely good veggy omelot. I think I'll owe you something like a years worth of free breakfasts just to pay you back for all the help you give!!

Thankyou, Clark!!



One thing I forgot to mention is make sure your spark plug cap is fully engaged on the top of the spark plug. If it is only sitting on the plug it could easiily demonstrate the performance you describe. I recommend using a little electrical silicon grease on the plug cap to ease installation and removal. I wrote something on this earlier I think its in the technical section. Anyway my plug cap boot is a real bear to get on snug.


I checked everything out. All sealing surfaces were fine, all o-rings in place, nothing. I found no foreign material anywhere. I set my fuel screw at 1 turn out. My float heght was slightly low, I fixed that. I put the 60 start jet back in. I set the TPS back to standard. I put a brand new Splitfire plug in it. I have not done anything with the MJ. I fired it off and all the symptoms still remain. I cannot tell you if the starting is any different. I never tried starting it after the initial first time. I guess I could be hitting the rev limiter. Holding it wide open it just stumbles badly. It still backfires and pops moderately when decelerating w/ no throttle. I have no clue what rpm the engine is at when it is stumbling. I do have an auto dwell/tach that is at least 20 years old. I wonder if I can get this to work somehow? I am used to screaming 2 strokes so maybe this is just normal.

A question I do have is will I get an accurate plug reading with the bike wide open (probably 4th gear) for 1 mile? I am not sure if the rev limiter will give me an erroneous reading. I have never dealt with rev limiters other than a U-Haul truck.

[This message has been edited by Kevin in New Hampshire (edited 05-12-2000).]


Does it stumble as soon as you go to WOT or does it stumble after you build up speed and have held WOT for a few seconds?


I open the throttle, the revs build and then peak and the bike stumbles. I'll hopefully have more tomorrow. I'll slap a plate on, bring my tools, and head out for a 8 mile trip with throttle open in 4th. I will kill the engine, coast to halt and pull the plug. I will be going to the Yamaha dealer who carries zero parts.


What is the final drive ratio on your bike 13/50, 14/50??? the lower the gearing the faster you hit the rev limiter. Again with the hit and stronger feel of the YZ timing on top a rider me included tends to flirt with the rev limiter more often than he does when running the WR timing.

Sounds to me like you are hitting the rev limiter and nothings basically wrong or incorrect. But with further testing you may better dial your jetting.

Keep us posted


Clark, I did go to a 13/50 setup. As of today, Sunday 13:39, I have not had the chance to do anything with my jetting. I spent my entire Saturday felling trees in my yard. Today I am cleaning the garage so I can get my lawnmower out. I bought this house last summer and am still dicking with all my stuff crammed into every nook and cranny.

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