fine tuning needles

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sean brady scooters
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Mon Sep 07, 2009 12:46 am

thanks tony for verifying what i have been trying to tell dan for ages................. :D
i always knew that eventually .........you could help tony..................... :lol:
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tony
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Mon Sep 07, 2009 1:27 am

:lol:
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J1MS
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Mon Sep 07, 2009 11:40 am

Tony, Your right... totally right in that a dellorto is sold as an after market carburetor, with many variants of single & dual taper needles with the ability to fine tune further with differing sizes and lengths of needle jets readily available.... Where as the Mikuni fitted to many Japanese engines are model specific... and not relly intended to be altered when first fitted... The many needles listed for theses Mikuni carbs are for individual engine setups... Engines are modified year on year at the factory... then a new needle introduced into the range... each needle made for a certain application... Which is why many have have multi taper needles as they are model specific... Picking a multi taper needle and getting it to work right on a lambretta with no fine tuning, only needle position is difficult even with a dyno...

But the data base on these Mikuni carbs is growing and some now are getting very good results....

Having re read alot of old formula & re educating myself (and with no small amount of help from Smiffy.. Thanks) I can understand why some running the 24mm Mikuni swear by them... But I can also see why some for road use hole pistons on much larger sized carbs... The more difficult it is to control the mid range jetting for all situations... Then there is a greater chance of a damaged engine, especially when the person setting the carb is after power and leaning the jetting to suit...

A little that I have read, and there is alot more readily available, might not stop me holing a piston, but I might have a greater understand of why it happened, then better be able to alter the dynamics of the induction to suit...

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Tue Sep 08, 2009 3:48 am

Interesting i have brought a TMX35 for my new bike i will be running mychron 4 with cht & egt also 2 power jets one of which wil be manually
turned on & off via a switch in ser11 light housing i have spoke to allens who have told me there is leaner neddles but are not tmx ones i was only told after i had bought carb about the neddle problem but also how fine do we need to tune a road bike any way as long as its safe thats ok so when bike is built and set up i will post our findings
thanks

J1MS
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Tue Sep 08, 2009 3:53 am

But the data base on these Mikuni carbs is growing and some now are getting very good results....


And a power jet will increase fuel as air speed demands depending on how far into the air stream its placed it will enter sooner or later... But I think you have a couple of friends with more than a fair idea of what they are doing, that can help.... ;)

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soulsurfer
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Tue Sep 08, 2009 7:55 am

I was told yesterday by a well known tuner/dyno operator that this problem with Mikuni is down to compression ratios and running far too lean needles.
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Tue Sep 08, 2009 8:06 am

J1MS wrote:But the data base on these Mikuni carbs is growing and some now are getting very good results....


And a power jet will increase fuel as air speed demands depending on how far into the air stream its placed it will enter sooner or later... But I think you have a couple of friends with more than a fair idea of what they are doing, that can help.... ;)
So i thought i was being so clever, but yes you are right i do have a lot of friends lets say the more older type 2of which race now and then
and win most of the time plus of coarse 7 times road race champ in a row is a close mate(i think they called me his rent boy).

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RinB
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Tue Sep 08, 2009 8:07 pm

J1MS wrote:Tony, Your right... totally right in that a dellorto is sold as an after market carburetor, with many variants of single & dual taper needles with the ability to fine tune further with differing sizes and lengths of needle jets readily available.... Where as the Mikuni fitted to many Japanese engines are model specific... and not relly intended to be altered when first fitted... The many needles listed for theses Mikuni carbs are for individual engine setups... Engines are modified year on year at the factory... then a new needle introduced into the range... each needle made for a certain application... Which is why many have have multi taper needles as they are model specific... Picking a multi taper needle and getting it to work right on a lambretta with no fine tuning, only needle position is difficult even with a dyno...

But the data base on these Mikuni carbs is growing and some now are getting very good results....

Having re read alot of old formula & re educating myself (and with no small amount of help from Smiffy.. Thanks) I can understand why some running the 24mm Mikuni swear by them... But I can also see why some for road use hole pistons on much larger sized carbs... The more difficult it is to control the mid range jetting for all situations... Then there is a greater chance of a damaged engine, especially when the person setting the carb is after power and leaning the jetting to suit...

A little that I have read, and there is alot more readily available, might not stop me holing a piston, but I might have a greater understand of why it happened, then better be able to alter the dynamics of the induction to suit...

J1MS

Im glad to be of help m8 the theory on this sort of subject needs to be understood I believe, hence how I was able to give the info. I have read it over & over again, then wrote formula's into spreadsheets and then started entering numbers,to see results one way or other, then started to fettle & adjust.

I for one have always preached the benifits of Dellies over Mikunis which has been described here in-depth .

But I do think there could be a fix for the mikuni and the lean spot in 5K rev range & this could be the boost bottle
If calculated right & set up right it may over come this problem.

Remember boost bottle design is that the bottle be at least as big as the displacement of the cylinder
and the hose be at least as big as the throttle opening at the intended RPM.

ie for example for 223cc a starting point
Using the formular from Yamaha's SAE paper

Speed of Sound (m/s) = 340.00
Hose Diameter (mm) = 8.00
Hose Length (cm) = 9.00
Calculated RPM = 4967.81

Smiffy
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tony
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Wed Sep 09, 2009 12:00 am

soulsurfer wrote:I was told yesterday by a well known tuner/dyno operator that this problem with Mikuni is down to compression ratios and running far too lean needles.
Interesting ;) Compression ratios are affecting the lack of adjustment on the carb? Did you ask the 'tuner' to explain mate? Thats weird to me.
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soulsurfer
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Wed Sep 09, 2009 7:20 am

tony wrote:
soulsurfer wrote:I was told yesterday by a well known tuner/dyno operator that this problem with Mikuni is down to compression ratios and running far too lean needles.
Interesting ;) Compression ratios are affecting the lack of adjustment on the carb? Did you ask the 'tuner' to explain mate? Thats weird to me.
He said that using a low comp head (large bowl) meant that you'd need to use leaner needles (57/58) to run properly, whereas with smaller bowls you would run richer needles (55/56) for the engine to run properly and this was the problem people had with setting up Mikunis. But then again another told me my smaller bowl head should be changed to a larger as that was the reason I was running a hot spot, so go figure :?

The other thing that has occurred to me is that a heat spike on a egt guage and a lean spot on a dyno using a/f sensors are very different and that the heat in the exhaust may not necessarlily relate to the heat experienced at the piston crown and certainly doesn't mean that it's lean. Perhaps CHT is more important/relevant?
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