Half throttle stuttering


I am riding a WR400 with both intake and exhaust Hotcams, a high compression piston, and am running a California WR muffler with a modified sound reducer.

My jettings are 165 main, 42 pilot and 72 starter jet with a Power now and a stock needle off an 04YZ450F.

I am running a motard set up and uses my bike for commuting.

Each time when I slowly pulls to half throttle, the bike felt like choking and stammering bad. But if I rev beyond half throttle, it will be fine.

Anyone can identify the issue? I will try my best to provide more info. I am not sure how to put it, so i hope for your kind patience.

Thank you

Just to add, it means I cannot maintain at half throttling and the bike will keep coughing. I have to go over half throttle in order to bypass it.

Take out the power now

Inspect your AP.

Sounds like mine did when I was running rich on the needle. Try moving your clip up 2 slots (leaner) and see if the stutter moves in the rev range. If it doesnt move or stop the stutter then its not the needle.

Like William said....ditch the powernow to eliminate a big variable. If your AP is tuned and jetting is right then the Powernow is pointless.

New Emulsion tube (Needle jet) and Needle (jet needle) and clip

I removed the Power Now(PN) and my AP seal is fine (My carb is quite new and clean) - For info: My carb has a non-adjustable leak jet, I also did not do any mods for the AP.

Test ride it, and the stutter remains. A short, slow bog was reintroduced when rolling throttle off traffic without the PN.

Continuing without the PN, I shifted the needle to the 2nd clip position from 3rd clip, the stutter did move to a higher range.

Originally at half throttle, the stutter kicks in at around 64-67km/h or higher speed/rev from 78-83km/h

After shifting the clip to 2nd, the stutter kicks in at 71-74km/h or higher speed/rev at 84-88km/h

I am running a NCY R needle.

Is this needle too rich for my settings?

Is there a recommended needle or chart for the Keihin/Yamaha part I can follow?

Thank you guys.

Edited by austinmation

I'm no expert, but it seems that you are too rich.

Rather than a new needle.

Try a 162 or 160 main. Only about $6 to test it and a lot less hassle to swap.

Another thought.

Make sure your float is set correctly.

Height Could be set too low making it rich.

I will do so and feedback later. I hope I do not have to order the needles too because my dealer takes a month to ship the needle in. And if the needle ain't right, I will have to wait for another month to get it.

I liked the NCVS needle / 3rd best

The flow bowl is within 8mm.

The emulsion tube is fine.

I had adjusted the following:

165 drop to 158 MJ

Needle to first clip

Still have a little "cough" or "tripping" during half throttle. More can be felt during quick acceleration at 70km/h and if gradual roll during highway at 82km/h. With or without PN.

Should I continue to lower my MJ?

I have an NCV Q from a friend. Is this a leaner needle compared to NCY R?

The needle width of the main shaft is what desgnates it as to how lean or rich it is up to 1/2 Throttle.

The Taper angle (lower 1/2) of the needle effects how lean or rich it is from 1/2 to Full Throttle. I am running OBEKP needle, E is 1 step leaner on the Taper and P is 3 steps leaner on the Straight portion and than stock OBDTM.

This means in your case the Q/R/S options of the NCV needle are only the straight portions (which is up to 1/2 throttle).

To go leaner at wider throttle openings you need a leaner taper ( 2nd letter) or leaner main.

Keep trying....you'll get it.


MJ Adj.png



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