Mikuni VM28's on a 1976 3 1/2 Sport

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Mikuni VM28's on a 1976 3 1/2 Sport

Postby 70sbikes » 16 Nov 2014 06:51

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Re: Mikuni VM28's on a 1976 3 1/2 Sport

Postby chirpy999 » 18 Nov 2014 07:15

Looks like a nice job, is there any improvement over the dellortos?
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Re: Mikuni VM28's on a 1976 3 1/2 Sport

Postby 70sbikes » 18 Nov 2014 10:09

is there any improvement over the dellortos?


There certainly is. Easier starting, a beautiful low idle & instant throttle response throughout the rev range (it also helps that I have the NLM pick-up, Tritsch transducers & a rewound ignition coil)

Cheers

Trevor
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Re: Mikuni VM28's on a 1976 3 1/2 Sport

Postby Chips1953 » 30 Nov 2014 14:49

This is of interest to me. My Camel has asked for a breath of fresh air so I was wondering how you came to choose the settings for the Mikuni carbs. Jet size and needle position if any. etc.
I believe my Camel initially with "pump" carbs.
Trying to make the old girl easier to start with any means possible. That's why I am also fitting electric start. Just waiting for NLM to come up with new shoe friction material on my starter. (Also Gym work to create muscular leg.) :D
I was intrigued by the 1/2 cup on the air filter. Something to do with heat?
Any info would be helpful about your fitting method as well. Many thanks.
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Re: Mikuni VM28's on a 1976 3 1/2 Sport

Postby EVguru » 01 Dec 2014 11:46

Chips1953 wrote:This is of interest to me. My Camel has asked for a breath of fresh air so I was wondering how you came to choose the settings for the Mikuni carbs. Jet size and needle position if any. etc.
I believe my Camel initially with "pump" carbs.
Trying to make the old girl easier to start with any means possible. That's why I am also fitting electric start. Just waiting for NLM to come up with new shoe friction material on my starter. (Also Gym work to create muscular leg.) :D
I was intrigued by the 1/2 cup on the air filter. Something to do with heat?
Any info would be helpful about your fitting method as well. Many thanks.


The 'pumper' carbs were for the American market where the bikes had to be jetted quite lean for emissions. They used a version of the carb that had an acceleration pump (almost a crude injection device) that gave a squirt of fuel when the throttle was opened suddenly. Unfortunately, the smallest version of the carb with the feature was a 30mm version, where a stock 500 ran 26mm and the 501 ran 28mm. They were notorious for being very difficult to set up because they're really too big for the state of tune.

The shield on the filter will be to stop it getting soaked with water!
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Re: Mikuni VM28's on a 1976 3 1/2 Sport

Postby Steve Brown » 01 Dec 2014 18:42

Chips1953 wrote:This is of interest to me. My Camel has asked for a breath of fresh air so I was wondering how you came to choose the settings for the Mikuni carbs. Jet size and needle position if any. etc.
I believe my Camel initially with "pump" carbs.
Trying to make the old girl easier to start with any means possible. That's why I am also fitting electric start. Just waiting for NLM to come up with new shoe friction material on my starter. (Also Gym work to create muscular leg.) :D
I was intrigued by the 1/2 cup on the air filter. Something to do with heat?
Any info would be helpful about your fitting method as well. Many thanks.


George, you're absolutely right, your Camel did come with 30mm pumpers on and it was me that replaced them with the 28 Dellortos that I borrowed from my 350 Sport. I fitted the 350 with-guess what? 28mm Mikunis! I would prefer the 350 to have dellortos again. fancy a chat? :roll:

I wanted rid of the 30mm pumpers for all the reasons given by Paul (EV) above. The Mikunis are now pretty well jetted for the 350 (I forget what is in them but have it recorded somewhere)
All donations to the rest home for old Camels, Leicestershire.
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Re: Mikuni VM28's on a 1976 3 1/2 Sport

Postby Tom » 05 Dec 2014 09:34

how about using the modern round slide dell'orto's 25mm as used on modern scooters?
I was planning to use a set of 26mm Mikunis but for the price of a setting of correct Mikuni jetting (200+ euro's) i can buy a brand new set of 25mm dell'orto's from Stein-Dinse.
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Re: Mikuni VM28's on a 1976 3 1/2 Sport

Postby hendre » 08 Dec 2014 16:45

Tom wrote:how about using the modern round slide dell'orto's 25mm as used on modern scooters?
I was planning to use a set of 26mm Mikunis but for the price of a setting of correct Mikuni jetting (200+ euro's) i can buy a brand new set of 25mm dell'orto's from Stein-Dinse.

PHBL25BS... plug&play and engine runs beautifull on it (with correct jets)
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Re: Mikuni VM28's on a 1976 3 1/2 Sport

Postby Ralph » 09 Dec 2014 12:24

These people have a large selection of Dell Auto stuff I have had
the choke units from them, they are nr Morecambe

http://www.pmtuning.co.uk/brand/dellorto?p=8
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Re: Mikuni VM28's on a 1976 3 1/2 Sport

Postby EVguru » 09 Dec 2014 13:50

Ralph wrote:These people have a large selection of Dell Auto stuff I have had
the choke units from them, they are nr Morecambe

http://www.pmtuning.co.uk/brand/dellorto?p=8


Just remember that they specialise in scooters and their carbs are all 2 stroke versions!
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Re: Mikuni VM28's on a 1976 3 1/2 Sport

Postby 70sbikes » 28 Dec 2014 08:25

I was wondering how you came to choose the settings for the Mikuni carbs. Jet size and needle position if any. etc


I found the Dellorto specifications for converting a 3 1/2 Morini from square slides to round slides on the internet, then I found the nearest Mikuni match. I ended up with the following:

Dellorto to Mikuni conversion (VM28)
(1) Needle jet 268 T = P-6 (series 175)
(2) Jet needle X6 = 5L1
(3) Main jet 120 = 110 (series 4/042)
(4) Pilot jet 45 = 37.5 (VM22/210)
(5) Throttle Valve 40mm = 2.5 (VM28/56)
(6) Air jet = 1.5 (BS30/97)

I was intrigued by the 1/2 cup on the air filter. Something to do with heat?


It's to stop the front carburetor "ram air" effect, and it works.

their carbs are all 2 stroke versions!


Carbies are not "2 stroke" or "4 stroke". The jetting is...

I would prefer the 350 to have dellortos again.


Any particular reason? My Mikuni's were not quite right, then I (eventually) worked out that they had no air jets fitted. 2 Stroke jetting!
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Re: Mikuni VM28's on a 1976 3 1/2 Sport

Postby EVguru » 28 Dec 2014 08:47

70sbikes wrote:Carbies are not "2 stroke" or "4 stroke". The jetting is...


Except where there are differences in internal drillings, which is apparently the case on at least some Dellortos.

In any case by the time you've re-jetted for a 4 stroke, they will no longer be cheap.
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Re: Mikuni VM28's on a 1976 3 1/2 Sport

Postby 70sbikes » 28 Dec 2014 11:30

they will no longer be cheap


You're not wrong there! But the Mikuni's are worth every cent...
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