carb needles...........and the common prob of weak mixtures

[sean brady scooters]

carb needles and the common prob of weakness at approx 5000 revs or so..........
it can be seen on a lot of dyno graphs etc..........on many two strokes.........
and occurs roughly at the point where the torque curve should cross the bhp curve.........
sometimes a bit earlier..................
much of this is explained away often by saying that its just natural........and is just what happens as a pipe comes "on song"........
to a certain degree this may be inevitable..........but then again there are things that could be done to at least lessen this,if not actually totally cure it..........

And............just to back track slightly.........in order to explain this a bit more.......

this common dip/flat spot/lean area.......isnt often much of a prob when accelerating and putting the power on..........because its in and out of it so quickly........and back into the safe range.............
where it is a prob.......is when someone is just cruising along..........and is in this range for a while...........
which goes some way to explain why most seizes/faults can occur ,usually at about only 50mph............

and so......back to needles......
most of these that are massed produced for most common carbs are simply a matter of tapers and straight bits.......
but what would be usefull maybe in order to correct the mid range lean spot would be a slightly concave shape on the needle............
this was suggested to me years ago.........by a well known racer..........
that in order to get a smooth mixture it was nessesary to hand finish and modify needles......................in this way..........
i have tried it a couple of times before............with quite good "seat in the pants "results.............
but it is very labour intensive............

i,d be very interested to see what anyone else thinks about this...........

also i appreciate the fact that Mikunis for eg are not as adaptable as delortos in regards mid range......tuning..........
and also that different pipes demand vastly different mixtures at certain revs........

[Diablo]

Ah ha Sean you could have been in our dyno room on Sunday with Eden and co.
This leaness in the fueling showed up on every bike we dynoed and was very difficult to cure with the 35mm mikuni. In fact we ended up going full circle with Scottys bike with jets and needles and ended up with a compromise which was at least safe but by no means as good as I would have liked.
As you rightly say its very difficult to pin down on the road but could quite easily explain those bikes which run for ages with no problems then seemingly blow up for now reason at relatively low speeds.
I think you may also be right about tayloring needles to suit because the fueling can be nigh on perfect either side of the blip at 5500rpm. Its also perhaps significant that many of the top tuners favour dellortos because of the more finite options available.
I think differing exhausts does come into play here as well though as some bikes seem to produce this effect more than others although there was some discussion as to whether the inlet was a factor as well.

[carlos fandango]

This is the flat/lean spot i was on about in my "rb20 Dyno" post.
Are dellortos less prone to this than mikunis? or is a mixture of carbs, exhausts,porting,compression etc.?
As i said its the first dyno i have had done so just trying to get my head around the info

[sean brady scooters]

i,m sure theres a very good reason why charlie edmonds uses a dellorto.................

[drunkmunkey6969]

Lecton needles are both tapered and curved.....sadly i dont have a ready supply of them on in my tool box!

Image: http://images.google.co.uk/imgres?imgur ... 8%26um%3D1

[RinB]

Ill put my head on block here

And say this is one of the reasons I stay with Delly
For one ive been playing with them for years and know them well and have massive spares.

But I do believe that a Delly can be dialed in better then a mikuni
Even some of the top racers say this and Im certain this is the case.

Saying this there are some charateristic benifits for both carbs

Discuss.....

Smiffy

[ralph95]

dellorto every time
hi smiffy

[Dryballs]

dellorto every time
hi smiffy

But dellorto what? VHSB34? surely not PHBH?!?!?

[rbgaz]

hi all , if i was to put a head gasket on my rb and lower the comprression would this also make it run cooler ?

[RinB]

dellorto every time
hi smiffy

But dellorto what? VHSB34? surely not PHBH?!?!?

I predominately use VHSx 28,30 32 34 36 mm

YO Ralph hope your well m8

I forgot you 2 are a delly man