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X4r has hesitation when landing. JETTING??? 28mm

1044 Views 11 Replies 6 Participants Last post by  pitster44
I have a 2008 pitster X4r. 150cc I changed the stock molkt carb for a mikuni 28mm round.

As for jetting it runs great if you ride it hard, but seems to surge if you just cruise at a steady speed. This means it is rich right? Would a smaller pilot fix this.

Also just recently i took it to an arenacross track and it seemed to want to hesitate after landing hard from jumps. Like it wants to stall for a second then its fine, this is making it hard to ride because it throws you forward, especially due to the fact that this thing has huge decompression braking.

Any Help ?

What jetting are you guys running? Slides? needles? Clip Position?

Do the mikuni 26 carb kits come with a slide and needle?

My jetting



needle in 2nd position from top


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move needle clip down to the bottom and try it, the bog is from a lean condition.
OK thanks

Are my jet specs close to what they should be ???

I'm only like 250 ' above sea level here.
ya close but the pilot looks a tad big, lower the clip on the needle all the way down and let me know if it fixs it.
Thats what you get with the 28 and a stock engine...thats why many like that pumper carb to get that lean condition out off idle.... Listen to Hack he been ridng these longer than most.
Has your carb got a perforated emulsion tube in it (needle jet) which suits 4 stroke engines better ? Most stock Mikuni carbs are set up for two strokes and are a bitch to sort out to run on 4 strokes ... Also has it got a jet ring over the main jet to remove the effects of fuel slosh ... A jet ring looks like a brass cup with small holes drilled around the top ... It holds fuel around the main jet to maintain a steady flow of fuel ... thus eliminating the hesitation you are talking about ...

Older XR75 tuners and racers discovered that mod since the 22 mm Keihins also suffered from hesitation / bogging over jumps ... they designed jet bell shrouds to fix the problem which Mikuni now calls "jet rings" ....

I've also noticed that some Keihin carbs have plastic jet shrouds over the main jet ie XR400 carb ...
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I'll have to check about the perforated tube. The carb I'm running was most definently off of an older 2 stroke so ya. haha. As for the plastic shroud yes it has that. Hard to jet and ride the bike right now its -12 out with 2 feet of snow. The real Mikuni 26 carbs that tbolt has, do these have the perforations for the 4 strokes?

As for those plastic shrouds in the carbs back in the xr 75 days, I heard Jeff Ward himself was one of the guys who came up with that.
Yes Ward Racing was one of 'em ... We couldn't get the parts here in OZ so I made up my own ...
I had a new '85 Honda XR200, with the RFVC engine, and two carbs, a primary and a secondary one. It would do the same thing, bog off a jump or landing. It had the main jet shroud, but none on the idle jet! Possibly your problem, you let off over a jump then go to gas it at the landing and the fuel is gone from around the idle jet, the main is still covered but that doesen't matter as you are using the low end again. I extended the jet down with a piece of brass tube soldered to it, and cut the shroud to let this in and feed fuel from under the shroud. Worked perfectly. Those twin carbs were not very close on jetting, etc when I bought it and I had to do a lot to them to make it right but they were nice when tuned properly.
This may help.. up_jetting made simple.jpg

also this..

Pitster Pro Forum - View topic - Carb tuning and set up tips.

If not give us a call and we will see what our tech guys can do to help as well.

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Okay I put in a 20 pilot. Still no good. Now a 17.5 pilot and it runs much better no surging when i ride it at a constant speed. Only question now is the air screw seems to do nothing at all I can close it completely and it still runs ok, open it up and it barely makes a difference. Should I go smaller yet on the Pilot ? Because thats what the air srew is leading me to believe. Also the clip is still 2nd from bottom.
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