fine tuning needles

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tavspeed
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Sat Aug 29, 2009 8:46 am

what about the egt linked up with a solenoid operate power jet,when the engine is in the the weak area the exhaust gas temp will rise,this then tells the power jet to operate,dont cam lam sell this set up,cheers all.

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sean brady scooters
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Sun Aug 30, 2009 9:55 pm

ok chaps..........yet again i,m back tracking a little and trying to approach this prob from a different angle............
could it be that mikunis are just very efficient at air flow.........
what i mean is..........could a 35mm mikuni be far better in regards of air flow than say a 34 delly
...........
i think yes........
and so ,i think the whole prob lies not so much with JL4,s.........but with the fact that with the combination of a 35mm mikuni ........that flows better than say a 36mm delly........(which is far more adaptable jetting wise).......its causing probs............basically cos it is still too big
regarding its flow rating.............
many folk have proved....that a 30mm delly on a TS1 will produce the same power.......torque and BMEP .........as those fitted with bigger carbs
and so ,as in the case of dans racer..........with a 38mm mikuni fitted.......it did provide a boost in peak BHP at high revs..........but not in torque or BMEP..........and also created a bigger lean spot than ever at that 5,500 revs range...........
i could be wrong.......but i just think this carb is TOO big..........
when using the 35mm..........peak BHP was less BUT torque/BMEP was the same...........also the dip into leaness was less at the point in question.........
although peak BHP was raised with the 38mm mikuni............i dont actually think it made the scoot any faster...........around the tracks

dan may disagree of course...........but i think his better pace was more down to his on comming skills as a rider than anything else..........

page 96 in the bell book sums up pretty much my point here,see second paragraph........ :D

"
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drunkmunkey6969
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Sun Aug 30, 2009 10:09 pm

tony wrote:Personally I wouldnt be looking to 'cure' the mikuni when the solution is already out there and its so simple.
I'm guessing you either mean fit a Dellorto, or you mean fit a powerjet? Yes/No?

If not....can you expand?

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sean brady scooters
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Sun Aug 30, 2009 10:26 pm

dont get your hopes up dan by asking tony......... :D
he,s great at telling us what does NOT work and why.......but not so great at actually telling us exactly what DOES work and why.............. :lol:
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jonny snatchsniffer
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Mon Aug 31, 2009 2:53 pm

is this lean spot around 5/6000 rpm just consigned to the ts1/reed inducted or are piston ported barrels the same ?
have you not tried a kehin carb ?

J1MS
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Mon Aug 31, 2009 4:31 pm

I have been doing a little reading on the Internet about boost bottle theory... Not really much wiser there are arguments for and against and some contradiction between articles... Here's three of the few I have looked at... The good thing about these pieces is that it shows the principles and possible theories behind the boost bottle... http://www.totallyamaha.com/snowmobiles ... theory.htm ... If you only click on one of these links Then Click on the second one.... as it explains fairly well the principles in theory & very clearly> http://wikiscootia.wikidot.com/boost-bottle ... And finally someone who wasn't too impressed ... http://forums.off-road.com/atv-lounge/2 ... ottle.html ...

From what little I have read (and as I stated before I have no real previous knowledge of the theory or formula).... But it seems its there to prevent the premixed fuel that has entered the manifold passing back over the needle jet aperture, picking up more fuel as the carburation pulses vary in length, reverberation also at varying engine speeds and throttle openings start pushing this or some of this charge into the boost bottle (caused by the charge reversing stopping or even just slowing up) the ramming effect of the venturi compresses the charge to a point, but then if the ramming effect is overcome, then the inlet charge without a boost bottle fitted reverses further back passing over the needle jet aperture, again picking up more fuel... this then richens up the incoming charge as it then passes the needle jet aperture a third time before entering the cylinder....

So it looks like its there to try to even out the fuel air mix to prevent a rich spot that then irons out as the engine comes on song as its been jetted down at lower air speeds this weakness then shows up as air speed increases, but throttle position is still slight... The problem from the little I have read, is an over large carb for the intake exaggerates this...

I don't fully agree with what I have read up to now, and I still do not know enough to give my own opinion, this is really only a little of what I have been browsing through ...Just thought I would post here for others to read and give their thoughts... Anyone..??

But it proves one thing its not just happening to scooters... and a Malossi boost bottle flexible to absorbe the pulses that compress the charge and in theory fill the boost bottle between pulses...http://www.adrenalin-pedstop.co.uk/p/MA ... M-549.aspx
Last edited by J1MS on Mon Aug 31, 2009 5:24 pm, edited 2 times in total.

mickdale
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Mon Aug 31, 2009 5:08 pm

anyone tried to speak to mikuni themselves about this issue?

mick

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soulsurfer
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Sun Sep 06, 2009 12:14 pm

tavspeed wrote:what about the egt linked up with a solenoid operate power jet,when the engine is in the the weak area the exhaust gas temp will rise,this then tells the power jet to operate,dont cam lam sell this set up,cheers all.
I was talking to Ben at AF the other day and they use a Mychron with EGT and set a max temp alarm with a thumb operated choke. When the alarm sounds, put the choke on. Sound crude, but effective.
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tony
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Mon Sep 07, 2009 12:36 am

Sean you cheeky monkey. :D
Dellortos work! Mikunis dont work when you are after this level of adjustment. The mikuni is generally designed for a specific motor and thats why it doesnt have the range we need for ours. The dellorto carb is approached differently and is more versatile as that company make the carb adjustable to suit the different applications.
Your lean spot is there because you cant adjust it out. Most of our road motors spend most of their life in the mid range. So this is where we should start. Now if we cant get that right and start trying to over compensate with big mains,pilots or hand sanded needles then surely this is crazy? As I said .. the solution is there already... its so simple.. buy a dellorto.
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tony
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Mon Sep 07, 2009 12:43 am

oh and re the powerjet.. well the camlam one is excellent for road an race use. This will allow even finer set up of the carb. If you use an egt and you see it getting silly when on a rally for example just stop, tweak the adjustable screw on the carb and carry on your way. There is no need to strip the carb down to change jets.-of course there is a limit to its 'bleed over'. so You'd want the carb set up to begin with. But if the motor gets hot, fuel is a little dodgy,conditions change then this is ideal.

there... two posts on what works.
;)
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