TY250A Carby - Mikuni Upgrade

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Rob_W
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TY250A Carby - Mikuni Upgrade

Postby Rob_W » Mon Feb 29, 2016 9:42 pm

Hi All,

I have recently restored a 1974 TY250A and fitted a Mikuni VM26-8074 round sided carb. From what I have read this seems to be one of the generally accepted replacement carbs.
Unfortunately I'm having a few problems getting the bike to run properly, so I was hoping that someone has had some experience in running this type of carb on the old Ty250's.
I originally started with #35 pilot jet, #150 main, 4L6 jet needle and now down to #22.5 pilot, #120 main and 5L1/5F3 needle. Slide cutaway has stayed at #2.0, but next moved is to increase to a #3.
Bike has no power under load and in the first 50% throttle. Temporarily fitted a 30mm Mikuni and it improved it's performance but still running too rich.
Would like to persist with the 26mm carb before playing with the 30mm but would be good to hear if anyone else has been through this problem.
Any advise would be greatly appreciated.

Thanks
Rob



David Lahey
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Re: TY250A Carby - Mikuni Upgrade

Postby David Lahey » Mon Feb 29, 2016 10:45 pm

If that's the modern VM26 yes I tried for about 15 months to get one to work on a TY250D motor in 2008. I eventually gave up on it and refurbished the original Yamaha-Mikuni and rode happily ever after

Mark Beechey (RIP) also went through that exercise at the same time, with that same VM26 carby and a TY250 motor. For both of us it was a very expensive and frustrating experience. Mark was eventually satisfied with the way his bike ran, but when I tried the setup he had, I couldn't get mine to work as well as I wanted

There were a couple of things we discovered that might help you (or not)

There are two different airboxes on TY250 twinshocks, and they differ in the amount of restriction they create for the incoming airflow. The A and B airbox breathes through two circular holes in the LH side of the airbox. The C and D model airbox breathes through the airbox lid. The amount of restriction to air flow upstream of the carby makes a big difference to the way the carby works

My carby had a problem with the needle retaining plate winding it's way up the slide spring, allowing the needle to slide up and down through the hole in the slide randomly. I fixed it by putting a bend in the wire at the bottom end of the spring

The size of the air jet makes a big difference. I ended up with the air jet removed altogether and I think Mark did the same

That carby is fairly tall compared with the standard carby and because of this the float bowl drain plug is at risk of touching the top of the gearbox on a Ty250. If it does touch it will probably foam up the fuel in the bowl in some conditions

The best needle shape we found was the standard needle fitted to the Beta Rev 3, but I could not source a new needle of that type at the time. I experimented with the needle from my wife's 2003 Rev 3 200. It still wasn't quite right.

The carby I bought was "pre-jetted" by a supplier in the USA, cost a lot of money, was absolutely useless the way it came to me and cost a lot more money and man-hours attempting to get it to work properly. If I was looking for a new carby for a TY250 again I would get something else


relax, nothing is under control

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OKOAustralia
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Re: TY250A Carby - Mikuni Upgrade

Postby OKOAustralia » Tue Mar 01, 2016 2:45 am

Seems odd that more than one person experiences the same tuning issue with same model carb on same model bike especially when thats what the bike had from onset.

If we had a second hand roundslide PE K series carb we'd lend that to you for testing but unfortunately we don't. ..they're not a fussy carb at all when it comes to tuning... could offer you a 2nd hand VM26 off a Rev3 if you'd like to play with that but then I think that's the same as what you've purchased but at least you could rule out or not rule out your new carb as the potential problem. Let me know if there's any way I can be of assistance.

Regards
Denis Lovett
OKO AUSTRALIA
07.3040.0231
http://www.oko-australia.com.au

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David Lahey
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Re: TY250A Carby - Mikuni Upgrade

Postby David Lahey » Tue Mar 01, 2016 10:38 am

OKOAustralia wrote:Seems odd that more than one person experiences the same tuning issue with same model carb on same model bike especially when thats what the bike had from onset.


TY250s didn't come with the VM Mikuni like what came on Rev3
The TY250A originally came with a TK carby that worked beautifully
The TY250B originally came with a Yamaha-Mikuni VM26 that worked beautifully
The TY250C and D originally came with a Yamaha-Mikuni VM26 that worked beautifully and has different jetting to the carby that came on the B model

The Yamaha-Mikunis that came on TY250 twinshocks are more compact than the modern off-the-shelf Mikuni VM26 and have an external air supply for the emulsion tube


relax, nothing is under control

Rob_W
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Re: TY250A Carby - Mikuni Upgrade

Postby Rob_W » Tue Mar 01, 2016 9:47 pm

Thanks David for the advice and thanks Denis for the offer. Makes me feel a little better to know I'm not the only one struggling with this carby.
I'll experiment a little further with the needles/slides and try your suggestion with the breathing and air jet...hadn't thought of it as being a factor to consider. Determined not to let it get the better but I think I should have researched on the Australian websites a little deeper before buying.
Denis, if neither the 26 or 30mm work in the end I'll have to give you a ring to discuss further, though not exactly sure what the difference between the VM26-8074 version from Townsville and your VM26 from the Rev 3 would be.

Thanks again. Will let you know the outcome.
Rob




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