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Ossa Mar Carby Jetting For Mikuni


paul wells
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Hello all, I am new to this forum. I just purchased a 1971Ossa Mar from an individual who lives in denver colorado at a very high altitude. He has a mikuni on the bike. I live in sacramento California at sea level and occaisionally ride at around 2 thousand feet. My question is do I need to rejet the mikiuni to run richer e.g. needle position, main jet size, etc or can I leave it. What I don't want is for the bike to run too lean. Sorry but I don't know the model of the carby yet as the bike is still on the way. In any case your help is greatly appreciated. Sincerely Paul and Donna

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Hi and welcome to the group. Nice bike. I remember idolizing MA and his Ossa... it got me into trials.

I got my Carb tuning book out and it had limited info compared to Googling the question. Try...

http://justkdx.dirtrider.net/printcarbtuning.html or

http://www.iwt.com.au/mikunicarb.htm

From these charts i'd think you should be running 8-9% larger jets - assuming it was correctly jetted for altitude.... But there is enough info at these pages for tuning it correctly anyway.

Ralph

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Thanks gents,

Guess what? I received the bike yesterday there was no needle in the carby...lol!I was able to take a wild guess and put one in just to make sure the bike runs which it did.

Now what I need to know now is specifically nwhich needle and needle jet combo to use with a 150 main and 35 pilot? There are so many choices and even the documentation I have seen seems to vary considerably. The carby is a 26mm vm with a main jet of 140 which I just changed to a 150. Will change the pilot as recommended today.

Dissapointingly, in addition to the missing needle the bike has a completely stripped kick start knuckle, rusted cables, and flat rear tire.

These are minor items and I wouldn't mention them except for the fact that the seller's advert read "starts first kick / new cables" he also said he rode it three weeks ago. hmmm...I don't think so.

Live and learn as they say...the good news is I now have a mar and it is certainly is a beautiful bike for it's time in terms of it's simplicity and lines.

looking forward to a reply on the needle.

Cheers

Lizzy

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Paul,

I have a 250 MAR with a Mikuni carby but it is an OEM Kawasaki KT250 26mm Mikuni carby, which is quite different to an off-the-shelf 26mm VM Mikuni carby and the main jet in it uses a different sizing code to an off-the-shelf VM26 Mikuni. I've not yet looked at the sizing codes on the needle and needle jet to see if they are recognisable as being the same system as for off-the-shelf Mikunis, but will have a look today and reply again.

26mm Mikuni OEM carbies from DT175, KT250, RD350, TY250 are all quite different designs and also to what you get if you buy a new carby from Mikuni.

My other comment is that a friend has his M198 (238cc) Sherpa T running wonderfully with a DT175 OEM carby and somewhere in my shed I have his jetting specs. From what I have found with swapping carbies between 250cc 2 stroke trials bikes, a carby that works well on one non-reed motor will usually work quite well on another non-reed motor so his DT175 carby specs might also work with your OSSA.

It would help if you could tell us what sort of Mikuni carby you have there and what size it is.

David

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Feetupfun,

the carb is once again a 26mm vm flange mount...origin unknown, with a 150 main and a 35 pilot. I can find no other markings on the carb. I could I suppose find a number on the slide if that would help. Let me know. I could also send some pictures of the unit. In any case I am once again assuming that a needle and needle jet combo from any of the japanese tirals 250s will close.

I did speak with a company I heard of on this message board called B & J racing products in the states who sell bolt on oem vintage trials carbys.

A gentleman at this company told me he could sell me the proper needle and needle jet combo but declined to comment on the spec of said items.

I am holding off on this route until I am comfortable that I know the correct part for the job rather than relying upon someone's word alone.

Regarding the belief that a carby from a bul sherpa t will work with an ossa mar I have heard the same although I did read somewhere..not sure where...that an ossa requires a slightly larger main jet than a sherpa t e.g. 160 vs 150.

In any case thank you so much for your time and assistance.

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Lizzy

The jetting in the 26mm Mikuni KT250 carby on my 250 OSSA is:

Main Jet 85R (be careful here because the type of main jet in this carby is different to what is in an aftermarket pattern Mikuni and probably uses a different numbering/hole size system also) - the hole size is much bigger than an 85 size jet would be in an aftermarket VM Mikuni main jet.

Air jet 2.5mm

Needle 5D1 with clip in mid position

Needle jet O-8

Needle jet type 8 172

Slide cutaway 2.0mm

Pilot jet 30

Air screw 1.5 turns

Checking in my Mikuni catalog, I see that 5D1 needles and 172 pattern needle jets are not not listed as spare parts. This is common with OEM Mikuni carbies and probably means you will need to either buy the items from Kawasaki as spare parts or use the Mikuni catalogue information to choose something dimensionally close to them.

I still haven't found the DT175 carby jetting specs for use on a M198 but I do remember it was very close to standard DT175 jetting.

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Lizzy, you really need to know which carb you have because as Feetupfun has mentioned there are quite a few different types of Mikuni and parts aren't interchangeable between all of them. I can't see how B&J or any dealer can supply you the jets without knowing which type you have.

Even the new VM carbs that you can buy come with two different types of needle and needle jet and they must be paired correctly. They won't work with each other. Mikunis are an absolute minefield to jet because of the many variants. If you could post a picture someone may recognise it if it is an OEM carb from another bike (which is probable)

Does it bolt on to the cylinder or does it push into a rubber hose.

This is a link to the website of the Mikuni importer here in the UK, Allens, and it lists all current carbs that they can supply new. Take a look and you will get an idea of the number of variants for carb bodies, jets, needles and needle jets, just for the current range....

Allens

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all,

here are some pics I took of the carb. Let me know if anyone recognizes it. btw there are no markings on the side of the carb whatsoever except for "mikuni"

in reference to the statement "the parts aren't interchangeable" are you saying that I can't mix and match needles and needle jets (couldn't I just buy both) or that needle / needle jet combos (matched pairs) won't work in different carb bodies?

thanks again

paulwellscarbinnerbody.jpg

paulwellscarbrightside.jpg

paulwellscarbslide.jpg

paulwellscarbfront.jpg

paulwellscarbleftsidel.jpg

paulwellscarbrear.jpg

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Good to see you have put some photos up. What you need to do to know of you can buy a different needle and needle jet is to take the needle jet out and read the stampings on it and on the needle and compare them with what is available from the Mikuni agents and maybe from the manufacturer of the bike that the carby came from originally. I would imagine it came from something like a late '60s early '70s trailbike of about 175cc like a Yamaha CT3 or Suzuki TS185.

The needle jet will have both size and type ID stampings like the stampings on the slide. The square-in-a-square stamping just means the part is genuine Mikuni.

I also suggest you post your next questions about your OSSA in the Twinshock forum as I only noticed your latest posting by accident in the "Introductions" forum.

Edited by feetupfun
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