Intermittent stutter at 3-4,000rpm

[Zanderk]

Hi All,

I moved to the factory 3.0kit, airbox delete and Unipods recently (kit from Hord). I jetted as per the instructions but got a bad flat spot between 4-5,000rpm. I’ve moved through main jets, needle position, air screw setting and improved it no end. It now pulls really well from low down and through the previous flat spot region to the invigorated top end that the kit provides.
My only issue is an intermittent flat stutter that occasionally occurs between 3-4,000rpm. Sometimes it pulls through it, sometimes it doesn’t and sometimes it isn’t there at all.
I’m currently running 165-162, needle clip position 3 and air screws 1 3/4 turns out (after starting at 2 1/2).
I should have tried pulling the choke on when it occurred to see if that changed the behaviour and give me a clue.
so I’m thinking to try experimenting with needle position only at the mo’
Any (polite) suggestions please ?

[Captain 80s]

When you were "done" did you install a set of new plugs? While I didn't change as many components as you, after dialing in the main jets, when I got my bike running sweet I had the exact same issue. Intermittent stumble. It was really aggravating because it ran so good almost all the time. A set of new plugs fixed it.

Same exact thing on one of my VF1000R. I was going crazy chasing jetting and I thought no. No way. Yep. It didn't do it all the time either. Not one issue since.

Probably not your issue, but it sounded very familiar. Disclaimer, I didn't start with brand new plugs on either bike, but they weren't that old and looked perfect.

[Zanderk]

No I didn’t change plugs so that’s something to think about though they were new a year ago. I’d have thought if it were carb-related it would be there all the time but maybe it’s on a certain throttle opening. It’s way better than when I started playing around and doesn’t spoil my riding pleasure but it’s there and therefore needs sorting ! Thanks for the suggestion

[rcthirty]

I have the same setup as you. Intermittent plug fouling could also be the culprit. I started with 165 160 mains and had flat spots and fouled plugs. After a lot of jet purchases and tuning I settled on 160 158 mains. middle position needles and 2.5 out on the air screws. I also had to change the pilot screws though. I believe I still have a half space washer under the front needle. Also I did the 8" spring stretch routine. It now runs like a champ. Here are a couple on charts that might help your tuning.

Pic 1 .jpg Pic 2 .jpg jet_needle.jpg

[Zanderk]

That’s great info, many thanks. I’ve taken all the plugs out and they were pretty black so I’ve cleaned them up, rechecked the gap and refitted them. Beating the above in mind I am going to drop the needle one position to lean it off a tad. I did try 160-158 clip 3-21/2 turns out but the flat spot was worse and it improved when I opened the choke so I figured I was running lean hence the 165-162. I’m nearly there and looking at the great info you have posted I think needle clip should sort it as well as cleaning up the plugs. Thanks for posting

[6]

Did you install the adhudtable tapers needles that came with the kit? If so, mess with the needle height. That's about the rpm where you are in the needles coming off the low onto the high.

[Zanderk]

Did you install the adhudtable tapers needles that came with the kit? If so, mess with the needle height. That's about the rpm where you are in the needles coming off the low onto the high.

Yes I installed the adjustable needles. I’ve dropped the needle one clip position to run it a bit leaner and cleaned the plugs. I’ll take it out for a blat at the weekend and see how she goes .Got some BT016’s due to be fitted and looking forward to losing the 120/70 that’s on the front

[6]

Id probable raise the needle. I've had a bunch that will stutter coming off the low onto the needle, they are almost always lean. You have a setup in there now, worth testing first tho.

i love b-stones.

[Zanderk]

Thanks for the tip, I’ll try the change I’ve made and then If still the same, raise the needle 2 positions as I’ve already tried the +1 setting. Thanks for the help, appreciated