6-7K flat spot

[britaxcooper]

When I bought my VFR it had a flat spot between 6-7Krpm.... nothing major but a very obvious dip in power.
Anyway, I decided to try sort it out last night. So I've raised the needles by 0.5mm and drilled the slide as per HRC mods to have a 2.5mm air hole in. I also wound out the pilots 1/4 turn to be at 2.25 turns out.

The bike is standard with a slip on can.

Now when riding the bike it's perfect apart from between 6-7K I now get a stutter as if its missing. I thought this it being too rich.... however I've now removed the needles so just have the standard washers under the needle and I've just tried the bike and its still similar. However if I drive the bike through 6-7K on part throttle its fine.... big handfuls of throttle and it stutters...... which to me means its actually lean?

Anyone got any helpful advise?

[scubasteve]

I had this exact same problem when I bought my bike. It stuttered at 6-7k but if you were powering past this point on the throttle it would be fine. I investigated and found that the jets were wrong so changed them to a 118/120 setup. Also I found I had tapered adjustable aftermarket needles is the carbs which were set quite high. I dropped them 0.5mm and this took away the stutter and no flat spot either. I've heard that drilling the carb slides can affect some NC's for the better and some for the worst so didn't bother with this.

[Neosophist]

When I bought my VFR it had a flat spot between 6-7Krpm.... nothing major but a very obvious dip in power.
Anyway, I decided to try sort it out last night. So I've raised the needles by 0.5mm and drilled the slide as per HRC mods to have a 2.5mm air hole in. I also wound out the pilots 1/4 turn to be at 2.25 turns out.

The bike is standard with a slip on can.

Now when riding the bike it's perfect apart from between 6-7K I now get a stutter as if its missing. I thought this it being too rich.... however I've now removed the needles so just have the standard washers under the needle and I've just tried the bike and its still similar. However if I drive the bike through 6-7K on part throttle its fine.... big handfuls of throttle and it stutters...... which to me means its actually lean?

Anyone got any helpful advise?

Its common for it to be lean in the transition phase, Rick Oliver sells a set of extra shim washers that will raise the needles a little and richen up the area and mostly cure this

[britaxcooper]

I did the Cadwell trackday yesterday and the bike went well although it was frustrating because of the carburation being out.

It had a stutter when flat out on main jet, a problem that's never shown itself on the road.... but the 6-7K stutter is still there. I brought home a load of washers from work so I lifted the needle 1mm over standard (2 washers, plus the standard one fitted) which still didn't cure it. I'd be surprised it if needs raising further?

I think my options are to drop the needles totally so no washers at all, just to prove a point? Try a set of standard slides? Or try changing the main jets, but didn't think this would cause the 6-7K stutter....

[CRM]

check float heights. i cant stress enough how important it is for heights to be correct.
obviously they shouldn't move - however when carbs have been opened over and over there is always chance someone has tinkered.
are you sure it wasnt the speed limiter kicking in ? and was indeed fueling ?

for a typical 30 with perfect valve clearances, and a can / pipe and stock air intake setup the 118/120 is plenty and works well. drilling and extra needles may compensate for other items of wear or differences between bikes.

I have a UK 30 which was terrible for years, loads of trial and error and i ended up running a 130/135 setup and it was OK, however the cam timing was all wrong, heads were cast poorly and off centre, once all that was corrected (new heads and RVF cams) i was back on the 115/118 setting and its quicker than my RVF by quite a margin.

[scubasteve]

I did the Cadwell trackday yesterday and the bike went well although it was frustrating because of the carburation being out.

It had a stutter when flat out on main jet, a problem that's never shown itself on the road.... but the 6-7K stutter is still there. I brought home a load of washers from work so I lifted the needle 1mm over standard (2 washers, plus the standard one fitted) which still didn't cure it. I'd be surprised it if needs raising further?

I think my options are to drop the needles totally so no washers at all, just to prove a point? Try a set of standard slides? Or try changing the main jets, but didn't think this would cause the 6-7K stutter....

I think you should be dropping your needles rather than raising them to cure the 6-7k stutter. Like I said in my original post, I had the same issue as you and I cured it by dropping them 0.5mm. If it was flat spotting then you would say it needs more fuel and to raise the needles but it's stuttering like mine was which was due to too much fuel and it running rich hence dropping the needles. The top end issue when flat out could well be the main jet like you say tho. Let us know how you get on.

[britaxcooper]

Thanks guys, I'll keep you posted!

[Neosophist]

I did the Cadwell trackday yesterday and the bike went well although it was frustrating because of the carburation being out.

It had a stutter when flat out on main jet, a problem that's never shown itself on the road.... but the 6-7K stutter is still there. I brought home a load of washers from work so I lifted the needle 1mm over standard (2 washers, plus the standard one fitted) which still didn't cure it. I'd be surprised it if needs raising further?

I think my options are to drop the needles totally so no washers at all, just to prove a point? Try a set of standard slides? Or try changing the main jets, but didn't think this would cause the 6-7K stutter....

I think you should be dropping your needles rather than raising them to cure the 6-7k stutter. Like I said in my original post, I had the same issue as you and I cured it by dropping them 0.5mm. If it was flat spotting then you would say it needs more fuel and to raise the needles but it's stuttering like mine was which was due to too much fuel and it running rich hence dropping the needles. The top end issue when flat out could well be the main jet like you say tho. Let us know how you get on.

This might cure your specifc issue but it is just fixing a symptom not the problem.

As CRM has mentioned, a properly setup NC30 with stock jetting will have a slightly lean transition phase which willl show up when racing.

If its stumbling at high speed id check you fuel vaccum hose isnt collapsing and cutting off the fuel.

I assumed carbs are setup but there is not point at all in doing anythign utnil you make sure

1. valve clerances are in spec, cams are aligned and cydliner compression is ok.
2. carbs are clean (including all passageways) and set to stock, not forgetting float heigts.. andall o-rings are present and correct (including pilot screws)
3. carbs are seated propeprly and inlet rubbers are in good conditino and sealing.

Any vaccum leaks or somethign being off will mess up everything else.

[Viffer]

I know this is an old thread, please let me know about your experience.

I bought a nice NC30 with an aftermarket end can (LeoVinci). The bike runs excellent except from the flat spot/stutter between 5-7k. A Dyno test showed me the air/fuel mix - it's WAY too rich between 5-7k. Any ideas? Thanks

[Shinymcprice]

They're really fussy, fitted a baffle to my tyga exhaust to get it under trackday db, that cured the flat spot! Also lifted the needles by 0.5mm. It came back the other day, turned out the tank rubber had dropped down in front of the air intake.....