XR 280 jetting – OLD Bull Trailriders

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March 29, 2009 at 9:04 am #96103

Mike Wyeth
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Just wondering if anyone can help with jetting for a mikuni 34mm flatslide pumper carb?
Have just bought and fitted new carby and although bike runs ok with standard jetting, between idle and half throttle it goes well but between half and wot it stumbles/flatspot. Have changed main jet from 127.5 to 132 and left pilot at 37.5 with air mixture 1 1/2 turns out. Currently now bike ticks over but as soon as throttle is turned bike dies.

March 29, 2009 at 9:24 am #127226

Greg
Member

mike wrote:

Quote:

Just wondering if anyone can help with jetting for a mikuni 34mm flatslide pumper carb?
Have just bought and fitted new carby and although bike runs ok with standard jetting, between idle and half throttle it goes well but between half and wot it stumbles/flatspot. Have changed main jet from 127.5 to 132 and left pilot at 37.5 with air mixture 1 1/2 turns out. Currently now bike ticks over but as soon as throttle is turned bike dies.

If I am reading the above correctly it is now worse, if thats the case I would put it back the way it was check the attached chart, after having just cruised ove the below, you could try the needle before anything and doesnt cost anything also.

1. IDLE: Set idle speed to correct r.p.m. by adjusting the IDLE SPEED SCREW. Turn The AIR SCREW to achieve the highest idle speed and best response. After this adjustment has been made adjust the IDLE SPEED again back to the correct r.p.m.

2. OFF IDLE to 1/4 THROTTLE: The SLOW JET and the AIR SCREW are most effective in this range. When you want a richer mixture use a larger SLOW JET or turn the AIR SCREW in. The opposite holds true for a leaner Mixture.

3. 1/4 to 3/4 THROTTLE: The JET NEEDLE is the most effective component in this range. Raising the needle by lowering the clip position at the top of the needle will richen the mixture. Lowering the needle will lean the mixture.

4. WIDE OPEN THROTTLE (W.O.T.): Changing the MAIN JET effects this range. Select the size which offers the best W.O.T. performance, then install one size larger MAIN JET for ideal engine durability.

NOTE: Not all Carbs have AIR SCREWS. Some have FUEL SCREWS, it is important to differentiate as to which one you have. This depends on the location of the screw on the carb. (looking at the center of the carb) If the screw is in between the cylinder and the carb body , it’s a FUEL SCREW, turn it OUT to get more fuel (richer) if the screw is in between the carb body and the air box, it’s an AIR SCREW, turn IN to richen it. As far as I know this rule of thumb is correct. Check with your dealer or manual if at all unsure. Note: most air/fuel screws should be set somewhere between 0.5 to 3.0 turns from seated, if it is outside this range you should look towards your slow jet.

Thanks to justXR.com

TB

March 29, 2009 at 10:03 am #127228

Dean
Member

What TB said:laugh: if the throttle response is good in the first half it doesn’t need to be altered.leave the pilot/fuel screw alone. you should be able to alter the leanness/richness of your midrange by altering your clip position there is a definant difference with a single movement each way. With the main go as rich as the engine can handle.remember its not what revs the engine is doing its what position your throttle is in that determines which stage of the carby is brought into play. That is why you should put some tape on your throttle housing and divide it into 3 sections helps to easily identify which circuit needs playing with.and lastly always do your checks on an engine that is thoroughly warmed up,ie go for a quick 5 minute squirt around the block. Hope this helps,you have to do a bit of fiddling to get it right.

March 29, 2009 at 10:29 am #127238

Mike Wyeth
Member

Thanks Guys , I will be out in the shed armed with that info tomorrow to try and get this thing set-up and get the boy off my back:laugh: The different carby does give it a helluva lot more go… well worth the money:cheer: will keep you informed on how it goes;)

March 31, 2009 at 6:13 am #127241

Mike Wyeth
Member

Thanks again for everyones help bike is now running really well with a 130 main jet and 37.5 pilot with the fuel screw 1 1/2 turns out. Mikuni flatslide pumper is the way to go cost 400 bucks from US delivered and has increased power dramatically. 😆 😆

March 31, 2009 at 7:30 am #127446

Dean
Member

great stuff mike, also Ballards mechanics are a wealth of knowledge mate pick their brains,good guys as well

Ollie

April 1, 2009 at 2:50 am #127452

Anonymous

So Tommo’s happier now he has a few more ponies to keep up with the WR then

April 1, 2009 at 10:27 pm #127639

Mike Wyeth
Member

yea :laugh: he sure is it does go really well now for a 250 anyway, he still cant keep up with me though.

April 1, 2009 at 10:29 pm #127784

Anonymous

It’ll be good when we can get out for a a rip. I’ve sorted a bike for October to take Dad out when he gets here. I’ll surprise him with it when he gets out

You, Tommo and the crew out this weekend?

April 1, 2009 at 10:41 pm #127786

Mike Wyeth
Member

Yea nobody from the crew seems to be heading out up this way on sun but me, tom and darren and all that are heading out for a ride somewhere anyways. So you surprising jimbo with a ride aye that be good to show him all the tracks and that down there.

April 1, 2009 at 11:22 pm #127788

Anonymous

We’ll only do some easy stuff, I guess I’ll ride the borrowed bike and he can ride mine. If he gets adventurous there’s always soemthing around to bring you back down to the ground (literally) Plus he’s only got little legs :laugh:

I’m riding on Sunday too, will be good. The bike has just been fettled up too and has a nice sticky new rear tyre. This is the 3rd one since you were here
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