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OSSA Mar carb issues - Mikuni


seedoubleyou
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I went for a first ride on my restored Ossa MAR today and experienced a few issues I have put down to carb. The AMAl is the original from 1973 so I figure it has probably seen better days. All I know is that despite new jets and refurb kit the bike has no zip making climbing banks almost impossible. I'm thinking therefore of changing carbs to a Mikuni.

I appreciate that this one has been done to death but...The general concensus seems to be a 150 main and 35 pilot on a VM26  Mikuni but this is only part of the story. The Needle and Jet needle are very important also. The standard VM26 comes with a O-0 needle Jet and a 4J13 Jet needle. For those who have fitted a VM26 to an Ossa Mar, did you need to play around or change these jets? 

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I've had a mikuni on my Ossa gripper for years because i was after better pickup from low revs and the old Amal was badly worn. It has served me well but i was finding it would  gas up on steep down hills and also flood the engine if i dropped the bike, so i have invested in a new OKO from scooter assasins which cost me just under £60.00 plus a couple of different jets and new air filter.

It hasnt improoved the perfomance much but it has stopped the flooding... Bargin

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  • 4 weeks later...

I have taken Spens advice and Steve Panter sorted me out with a new carb. The result, the bike is a lot easier to start with a choke and runs a little better. However on checking the plugs after a trial I found it was running way to lean, so raised the needles a little to the last slot and that seems to improve things. A new main is on its way.

However I still struggle with bogging coming off tickover onto the needle. The carb is fitted with an O-2 needle jet and 5C23 needle. I'm not good with jetting in this area so has anyone changed this part if the circuit. In summary when I snap the throttle open it just doesn't pick up. I can't decide if it's too weak, not enough fuel, or too rich, too much fuel. Any help appreciated as I have a trial next weekend I'd like to attend . Thanks 

 

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  • 2 years later...
On 4/1/2017 at 6:42 PM, bisby said:

I've had a mikuni on my Ossa gripper for years because i was after better pickup from low revs and the old Amal was badly worn. It has served me well but i was finding it would  gas up on steep down hills and also flood the engine if i dropped the bike, so i have invested in a new OKO from scooter assasins which cost me just under £60.00 plus a couple of different jets and new air filter.

It hasnt improoved the perfomance much but it has stopped the flooding... Bargin

Bisby, can I be cheeky and ask what size OKO you chose, 26 or 28 and what jets your using. I was going to get one from scooter assassins but not sure if you get jets with the standard carb. My Gripper original Amal is worn out.

Many thanks.

Dave.

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  • 2 weeks later...

Hi Dave just spotted this, my oko is a 28mm it was not far out as it came but i have got it on a 112 main jet and either a 52 or a 55 pilot and i bought a rubber carb pipe from a Sherco to go from the  rear of carb to an angled piece of plastic 60mm? drain pipe tube then a Ram airfilter and i run the air box with no top or snorkel tube but have the rear mudguard well taped up. This sounds like a bit of a bodge but it works fine for me.   Steve  

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On 4/30/2017 at 9:43 AM, seedoubleyou said:

I have taken Spens advice and Steve Panter sorted me out with a new carb. The result, the bike is a lot easier to start with a choke and runs a little better. However on checking the plugs after a trial I found it was running way to lean, so raised the needles a little to the last slot and that seems to improve things. A new main is on its way.

However I still struggle with bogging coming off tickover onto the needle. The carb is fitted with an O-2 needle jet and 5C23 needle. I'm not good with jetting in this area so has anyone changed this part if the circuit. In summary when I snap the throttle open it just doesn't pick up. I can't decide if it's too weak, not enough fuel, or too rich, too much fuel. Any help appreciated as I have a trial next weekend I'd like to attend . Thanks 

 

Can't help with jetting, Mikunis have too many variables for me to sort out and there is no base setting for a MAR to even start with, so I don't use them. However, might be worth checking which head gasket you have fitted as they range in thickness from 3mm down to about 0.5mm.  A MAR isn't the liveliest of bikes anyway, if you snap the throttle it's never going to leap out of your hands, but if you have the 3mm fitted you could go own to a 1mm which should improve pick up.

I;m assuming your crank seals are good and timing is ok (you can advance to 3mm BTDC)

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