fine tuning needles

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sean brady scooters
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Thu Aug 27, 2009 9:11 pm

in the motorcycle world.........the dynojet kits are well known........basically these comprise of amongst other things tailored needles...........
in the lambretta/vespa scooter world........these are not of course available........but in some circumstances are needed.......
with some set ups/using certain pipes and carbs.........its quite impossible to obtain a perfect
air/fuel delivery ratio......despite the array of std carb parts available..........
typically this will show up on the dyno as a weak area at approx 5000 to 6500 revs......or thereabouts........usually just where the pipe starts to come on song...........
the more efficient the pipe is ,the more pronounced this appears...............jl4,s...for eg
with dellortos.....this can be worked around in most cases.......
but with say mikunis.........it can be a pain............

and so......when all else has been tried..........modifying /tailoring the needle can only be a good thing.........
i know for a fact that this has been done by many a scooter racer .............
with great results...........
and so again..........if this is what you think you need to do..........this is how we would now do it
firstley of course ......you need to identify exactly at which point in your rev range needs sorting.........
easily done using a dyno............
just hold it at the revs in the weakest air flow/ bar graph indicator.........red zone............
then quickly mark throttle opening on bars using tippex etc............
then turn off.........and open throttle to marked setting..............and then mark on needle.......................just above diffuser/etc........
its then the area below this mark on needle that needs relieving/reducing in diameter..........
in order to richen up...........
the best way probably to do this is to put needle in a drill chuck...........and hold on at the desired point some sand paper............removing only say .2mm at a time...........
then try again...........repeat process until lean spot is evened out..............
i know it all sounds a bit "heath robinson"...........but it can work out very well............. :D
Sean Brady Scooters - 01765 690 698

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celt
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Thu Aug 27, 2009 11:18 pm

I was present when Jerome of Reedspeed altered the taper on a Dellorto needle of my mates carb to clean up a weak area when dynoed.

Also he used a small and thin washer as well as circlip to drop/raise needle setting when circlip position was inbetween desired clip setting

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rog60
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Thu Aug 27, 2009 11:35 pm

Camlam, for one, sell the spacer for a half clip position :idea: :idea:
NO IT'S NOT A F***IN* MOPED!!!!!!

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Diablo
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Fri Aug 28, 2009 1:10 am

Sean I'm not sure altering the needle in this way would alieviete the problem you describe though. When I've seen Jerome do it its been with the edge of the needle taper especially X2 dellorto needles.
The reason I think this is because the weak spot you mention usually happens at a specific point in the rev range not neccessarily coresponding to a throttle opening.
For instance if you do a full power run on the dyno and pull the throttle wide open this spot often happens(but not always) at around 5500rpm. The throttle has been wide open for some time before its reached this point so for me is not neccesarily anything to do with the needle.
Often time if you do a half throttle run the effect is less but often still there with a rich spot either side. It will still also happen at 1/4 and 3/4 throttle power runs so wwhere do you modify? If you modify the needle to compensate you end up making it still richer either side of the blip.
Modifying the needle doesn't neccesarily get rid of the weak spot because say a bike is labouring into a head wind two up at full throttle it could concievably drop into that weak part of the rev range. My advice to dyno customers has been thats it better and safer to cane it in third than chug along in this weak area in fourth should it happen.
I must admit that its a problem I've been looking for a solution to (and greater understanding of why it happens)for some time but I'm reasonably satisfied now that its something to do with both the relationship between the transfer and exhaust timing and also the tuned length of the exhaust and possibly even the inlet.
When we have dynoed bike engined scoots we have managed to change the fuel air ratio by correctly mapping the exhaust valve which would tend to back up the above.
Also this effect seems to happen more with race type pipes .
Sticky has a theory we hope to test soon that this is why the Japs use boost bottles on singles and balance pipes on twins. After all if the Japs felt the need to fit a boost bottle to normal production bikes like a DT125R then there must be something in the way a 2 stroke behaves that can't be ironed out just be adjusting the carburation.

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tony
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Fri Aug 28, 2009 1:50 am

I agree with Alan.
revs and throttle opening depend on load and the carburation varies with load.
The exhaust is the 'suction' on the motor so by altering the amount of suction (eg the valve, cones etc) the mix will be different too.
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Rude De
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Fri Aug 28, 2009 3:53 am

I have been intrigued by this guys custom needles
http://www.jdjetting.com/
These folks have some interesting power jets
http://www.jdjetting.com/

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drunkmunkey6969
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Fri Aug 28, 2009 10:29 am

Al....do you not find the problem worse on TMX35mm where there is no adjustable atomiser, unlike say an equivalent sized Delly? I also agree with what you are saying regarding throttle position, and the spike being there regardles....the only caveat being it is worse when the throttle is 'whacked open' as opposed to being fed on according to revs/engine demand.....this lessens the effect slightly. But TMX35mm carbs definitely seem to be the worse.

On the dyno, i seem to find that 35mm TMX running with a revvy pipe is a bitch......the weak spot in the mid-range is cavernous, and the only way i can make the scooter safe is to lift the whole spike up by running the lowest clip on the needle that the carb can manage, before the rest of the rev range becomes too rich.

However, the same set-up, but with a 34mm Delly has a similar spike, but less exagerated, and can be somewhat smoothed with the right jetting/needle/atomiser combos. Do you find the same?

I'm struggling with the jetting of my race scooter, as the port timings are very 'revvy' now, along with a JL4 and because i run the 38mm TMX.....the spike is even worse on that! I picked up a cheap 38MM VHSB for £20 yesterday and so am going to experiment with that, and see if the superior adjustability of the Delly pays off.

I also have a contact that is currently having a lot of success with these: http://www.thunderproducts.com/dial_a_jet.htm It is supposed to be different from a 'standard' power jet in the sense that the fuel it delivers is atomised and can therefore fuel weak spots on demand, although i myself havent tried them yet. He has some in stock, and they seem to be working for him, so if i get no joy on the race scooter from the 38mm Delly, i'll try one of the dial-a-jets on the Mikuni and see how it goes.

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Diablo
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Fri Aug 28, 2009 12:03 pm

Dan yes I do find TMX more difficult to get rid of this problem but also Sticky has chased his tail with his wifes TS1 with both OKO and PHBH carbs. I do agree its more difficult to compensate for with the TMX though.
I'm lucky at the moment in that there is a guy called Chris Sturgis who brings scoots over virtually every week for me to dyno so I'm building lots of data on all sorts of engines.
We are trying to see patterns with these scoots because some have this blip much more pronounced than others.
I do the same as you in that I try and compensate for it but inevitably this leads to richness elsewhere which isn't always favourable.
Interestingly I have almost cured the effect a couple of times by fitting either a bellmouth or in one case a remote filter(but other times its made no difference). Personally from what I've read on bellmouths I think its because in those instances a boost bottle effect has been achieved in that the excess fuel is sprayed into the bellmouth and stays there untill its drawn back in during the lean spot.
One thing your ahead of me here is that I see very few big dellortos on scoots for some reason so don't have enough data to compare to a TMX but I do think you are right there.
One thing I'm certain of is that the problem has always been there as I have always had problems jetting mid range on road going lammys with JL4s. I think people need to be aware of it if they choose a race pipe or have race type port timings with there scoot and then intend to "cruise" in the mid range.
I've got several bikes out there at the moment where this spike is very pronounced but the bikes have been very reliable because they are thrashed everywhere.
Beware!!!! A limp wrist is bad for your scooter :lol:

Off to IOW now so will look in and see what other minds think wwhen I get back :lol:

J1MS
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Fri Aug 28, 2009 4:02 pm

Not sure of the exact science around the boost bottle, as I have only ever worked onthe basic principles of 1100ft/sec on air intake doing the normal sums timings, areas, and inlet tract length sort of stuff. But I have noticed the air/fuel dosnt lean off as much when some form of air intake system is attached such as ar remote filter... On one scooter I used a plastic cup with a hole in the end to keep a supply of enrichened mixture in the inlet tract when the scooter started running hot. It does seem to equate a little towards the exhaust system used. I always refer to them as hungry pipes as they Sometimes try to steal the mixture at certain revs. The boost bottle I am sure has a part in offsetting this but I would think ther is a little more to Yamahas thinkig than just any old size boost bottle will do. I would think it would be balanced to the pressures in the intake with originally some sort of formula to work it out. But it's obviously there to enrich the mixture when air pressure drops in the inlet tract...as it would hold a small supply of pre atomised fuel in the boost bottle...

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sean brady scooters
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Fri Aug 28, 2009 9:50 pm

as far as i know and remember..........boost bottles worked on the principal that when the throttle was shut off.....the boost bottle temporarially stored mixture that was released again once the throttle was opened again..............
it was designed at first on crossers i think.............so that after shutting off,and opening quickley again to accellerate hard.........it produced a resovoir of instantly available rich mixture............
this principal of boost bottles really would not help a scoot toodling around in its weak area.................that is usually at about 5,500 revs /60mph..............
and especially the ones fitted with JL4,s,mikunis..........
Sean Brady Scooters - 01765 690 698

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