Carb. jet tweaking help required

After putting 30 miles on bike I changed stock 150 main jet to a 160 MJ. I only did the main jet at that time because the dealer didn't have pilot or starter jets in stock. What I noticed after MJ was changed was the bike ran better but after shutting bike off I would get a pop from exhaust 80% of the time.Another thing I noticed was when I grabbed a fist full of throttle I'd get a hesitation before she would get up and go. I chalk that up to being a lean pilot jet.

Saturday I pulled the carb off and changed the stock pilot jet ( not quit sure what was in there ) to a #48 Like 70% of what TT'ers with WR450's are using and also changed starter jet to a #72.

After these changes I found hesitation is still there but not quite as bad. Pop is now heard on engine starts and not engine stops. Not a full backfire just a pop. Bike will start on a 50-60 degree day without choke but will stall after 5-10 seconds. I also can restart hot bike without hot start lever but it will fire alittle faster using it( still popping on restart). It seems to me it's a fuel screw adjustment (have to go leaner on fuel screw) but I'm new to all this jetting stuff. I did order the ZIP-TY fuel scew but have not recieved it yet. Seems alot of TT'ers are waiting for theirs also.

I have the same hesitation, but I can get it out most of the time by adjusting the fuelscrew/Zip-ty. I just put in the YZ needle but I haven't had a chance to test it out yet. It's sitting at the dealer getting TSB.

Do you have part # for the YZ needle. I got a part # for it off one of the other posts but it was wrong.

Yep, I do have the part number for the YZ needle (5TA-14916-JN). I also have the part number for the non-U.S. needle if you want it (see your manual). I'm using the non-U.S.(OBDUQ 4th position)and it works great! No hesistation, all European stock jetting specs. :):D :D :D


I just read in one of the posts(there was a link)and it says if you use the YZ needle without doing the YZ timeing(exhaust cam) you will overheat the engine. :)

Lets not get too far off track here, I still need some jetting advice. :)

Foursmoke, wear is the hesitation for full throttle, 1/8, 1/4, 1/3 starting point throttle opening?

Indy, It seems to be when I gun it. Not a very big problem. I am more concerned about the popping on restart. Do you think its the fuel screw or should I go a little leaner on jetting? :)

I think you need to start rich and work your way to leaner. pop in a 165 main needle in clip #5 from the top and ride it. Your baffle removed and fully open pipe is creating a lean condition in my opinion. Leave the pilot circuit alone 1.75 to 2 turns on the air screw and let us know what you feel at different throttle openings starting from different throttle positions:

1. closed throttle, then wack it open

2. 1/4 throttle, then wack it open

3. 1/2 throttle then wack it open.

Let us know what happens. :)

Thanks again Indy! It might take a while, my dealer doesn't seem to stock jets. Will probably have to order. I have 90 or so miles with original plug that has seen jetting changes. I have a new one but would rather save untill jetting is closer. Would inspecting original plug be useless. I should have my Zip-Ty fuel screw next Wed. Thanks for your help.

You can inspect the plug any time. You need to run the engine for about a couple of minutes at the throttle opening where you suspect you are lean then hit the kill switch and inspect the plug. ( I use a flat gravel road to do this) Look at the color and it should be tan brown. light gray is to lean and black to rich. Ignore the outside of the plug area where the threads are. I use this to get close or if I am not sure to go richer or leaner. The rest of the tuning has to played with by riding and testing the throttle response. You can really tell how your main is doing when you are wide open in fifth and see how it pulls. Get a nice 5 mile long road picked out.

Indy, If the plug is already colored due to either rich or lean condition, when you make a change to either jetting or fuel screw are you saying the color will change on the plug? One other question, where can I get jets from quickly? My dealer or any other dealer in this area don't seem to stock jets for thumpers let alone the WR450. If I order them it takes 2 weeks. That being the case should I order other jet sizes? what other jet sizes will I need running an open stock pipe? I want to plan ahead with hot weather ahead.Do I go richer with hotter weather? Sorry for all the questions, I trying to understand jetting and the affects temp and humidity have on it. Thanks for your patiences!

Forget trying to read the plug if you have never done that before. Try pulling in your hot start to different positions and holding it while you are running at the problem area throttle position and then see how it accelerates when wacking the throttle opem. If you are rich the hesitation will be reduced or elimnated with the hot start pulled in to some degree. That will be an indication of being to rich and you will need to go down on jets or up on your needle depending on where the problem area is. :)

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