fuel issue?

[randallbass]

hey guys i recently had a problem with my bike only running on 3 cylinders, i check floats and reset some made shore jets were clear and tried it, no go,,,, it ended up being it just needed the pilot screws 1/2 a turn richer (now at 3 full turns from their seat)
idles beaut, revs spot on till about 8,000 then hesitated a bit till 9,000 then cruizes to red line no worries
the bike wasnt at full operating temp (just above blue and below 70) as it was in my shed and being loud as i coulndt have it running for to long i need to get the wheels back on and get her out on the street
what would cause this?
i am thinking its running a bit rich, which may get better when shes fully warm?
maby the needles need to be shimmed different
or maby the pilot screws are to far out and i should go in 1/4 turn and see it that helps :)
what u guys reckon?
cheers Tim

[viper_biker]

Hi Tim, 3 full turns out or 3 half turns out on the a/f screw? If memory serves me right, it 2 and a quarter turns out is standard VFR (and 1 and five eigths on RVF), now I only know of a few folks that are running 3 full turns out and they are running no-air-box HRC carbs. What is the rest of your set-up, jet size, needle height, exhaust, air-filter age etc? The only real way to sort out finer fueling issues is on a dyno. You can only guage so much by the seat of your pants. Also, what bike do you have?

[royster81]

gareth ,he's running an nc30 with an open HRC carbs

[viper_biker]

Sounds like a similar issue you have had then Rhory with a plug fouling. Have you diagnosed the problem yet, or go back to standard carbs?

SorryTim, but I don't know enough about HRC carb set-up so just ignore me.

[randallbass]

yer sorry its an nc30 open airbox with a race exhaust fitted
the main jets are 160
idle jets stock
float heights stock
needles, not to shore
so 3 turns is not to much for an open airbox with a race exhaust? even with the big 160 mains?
cheers Tim

[viper_biker]

I think Rhory is running something in the mid 150's but no idea what needles or height he's using. There were 2 flat-spots on his track-bike but from 9,000rpm onwards it was starting to pull well, but we ran out of 'test track'

His pilots are up to #38 (as is my own track-bike on a different set-up (I use and air-box on mine)) but that should only count down low. Contact Rhory or Rick and see what they say but I think a dyno will sort it out best.

[randallbass]

yer the flat spot is just from around 7500,8000 to bout 9000 then goes off like a frog in a sock :)
i guess i will let her warm fully up tomorro, hopefully its running rich not lean so when shes warmer it wont mind the extra fuel :)
cheers guys Tim

[royster81]

I think Rhory is running something in the mid 150's but no idea what needles or height he's using. There were 2 flat-spots on his track-bike but from 9,000rpm onwards it was starting to pull well, but we ran out of 'test track'
His pilots are up to #38 (as is my own track-bike on a different set-up (I use and air-box on mine)) but that should only count down low. Contact Rhory or Rick and see what they say but I think a dyno will sort it out best.

close...
jets are 150/152
pilots 40
needles 8ZF
shims 0.5 front /1.0 at rear

i think mine is rich but don't have the knowledge or test track to run it all day to make small adjustments so dyno seems to be the best option

rick might not be the best person to ask as he has little experience of setting up the HRC open carbs,best bet would be contact stickshift as he's in OZ and races (or will race) a '30 with the same set up,maybe he could at leats point you in the right direct as to where you'rs going wrong

[Cammo]

idles beaut, revs spot on till about 8,000 then hesitated a bit till 9,000 then cruizes to red line no worries
the bike wasnt at full operating temp (just above blue and below 70) as it was in my shed

Have you ridden it?

I would guess that this flat spot will be major on the road/track if it shows up at no load (neutral).


The flat spot is common on carbs that have just had the airbox removed and big jets installed. The gap in fuelling between the needle to main jet transition will be too big. A way around this is to restrict the air jets, then you can run smaller mains and it will get rid of the flat spot.

The hrc nc30 velocity stacks essesntially shroud the air jet, restricting the flow. If you don't have the stacks you can tap the air jets and use an insert (old pilot jets are useful for this), then drill the pilots out to the required (restricted) size (2-3mm?).


Many track bikes have this problem, I've seen plenty passed on cheap (some to me!) because a lot of owners can't get around this problem.

Rvf carbs don't suffer the problem to the same extent as nc30 carbs due to the very different air circuit.

[randallbass]

hey i was tinkering today and noticed i had a little leak in my exhaust joint, so i fixed that started her and warmed it up properly and the flat spot seems to be gone hehe or very minimal i will take it for a spin tommorro and let you know how its pulls up but yer in the shed it seemed to be alot better then yesturday ay.
i doubt the exhaust leak had anything to do with it as it was only minor but ay
it just has a little delay when sitting at any revs when you blip the throttle delays for a poofteenth then gets up and goes.
i think i was blippin the throttle at the same point or something yesturday lolz
anyways i will let ya knoe how she goes on the road tomorro
cheers Tim
oh btw would changing clip position help this in any way if its still there as i dont really wanna go drilling things