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Tillerman6

New carb anyone?

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So another general question- #2

  I have OKO 26mm on my TY 250 A and still trying to get the best operation out of it.

It starts on about the 5th kick when cold, but when hot you need to open the throttle to maybe 1/3rd to get it to catch.  There is no longer any kick back - but that is mostly due to the new Rex's ignition.

But my concern now is that if you are just running on flat ground, the engine will fire every other revolution.   This is  annoying because of the vibration that is set up while this is going on. 

If you advance the throttle, the engine fires every stroke and smooths out,  or uphill- same thing sounds solid and smooth- , but you only get that operation while you are accelerating, or going uphill or running fast in 5th gear so that the wind resistance puts a load on the engine.   

If you are just idling along - 10 -25 mph,(which is most of the level part of the trail riding I do, there is that "4 stroking" sound which I can't understand.  

 

There is also the usual popcorn sound mixed with this if decelerating, which I think is normal for any 2 stroke.  If you let off the throttle in neutral, the engine returns to idle without hesitation- very little extra firing.

The air filter has just been cleaned and oiled, - the stinger in the exhaust has new material, new plug, Premium gas and 50:1 premix.

I don't know if Trials central can accept videos with sound- maybe that would be the best way for everyone to see and hear what is going on?

This 4 stroking scenario existed prior to the new ignition being installed, so I'm thinking it's the OKO carb.

I went from the 2nd notch on the needle to the first notch plus the spacer - no difference.  The idle air bleed is at 1 1/2 turns out and there is a normal idle speed observed.

Any ideas are much appreciated!

 

 

Edited by Tillerman6
Symptoms have changed

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I tuned many Mikuni over the years but I never had luck with a Mikuni VM 26 on a TY twinshock, I had an old one with all the jetting I needed on the shelve to replace the TK, I couldn't make it work properly. I bought a new one with the same result. I found a Mikuni  that came from a B model, it was an easy job to tune it perfectly for my bike, when you have both ( Yamaha TY Mikuni and regular VM ) there are obvius difference. I'm sure that many here will tell you to go for an OKO, there are many user that can help you on this.

Guy

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guy53,

  That's good info to know.  I would probably have gone for one of the new flat slide Mikuni carbs rather than the old round slide VM type anyway, but there must be some differences with the skirt on the slide itself that make the VM 26 not work as it should in the TY 250.  My other complaint with the TK is that if you can't get the bike started in about a minute or two it tends to flood.  So it probably needs a rebuild kit which is impossible to find???    So you're running an OKO?  how do you like it?  Are you running the stock reeds or something else?

I mostly ride fire roads and single track trails and the 13/53 gearing if that makes any difference.

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I may have not been clear, I run a TY B ( and later model twinshock Yamaha ) Mikuni carb on my bike. I know that many on this and other trials site run OKO with a lot of success.

Guy

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Guy 53,

   From your post, it sounds like the OKO is easier to get running properly.  If you have any more information on them, please send me a link.

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Just now, Tillerman6 said:

 

Just ordered a new 26mm OKO from mid-atlantic.  Fingers crossed it fits and works fine.  The Mikuni carbs don't seem to work too well on a TY and you can't get parts for the TK that was on the bike originally.  

On 5/28/2021 at 2:51 PM, oldtrialer said:

Contact Mid Atlantic Trials.

Index (mid-atlantictrials.com)

He sells them set up for most bikes.

 

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It will probably be a good idea to get new airbox connector from clauss studios for the OKO carb, the bellmouth is bigger so you need to expand the rubber to fit & your original has probably gone hard with age & no amount of hot water will make it pliable enough to stretch to fit without splitting

I have the 28mm for mine but the plan is to remake the airbox with a straight hose & modern air filter in the lid design to get rid of all the restrictions 

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4 hours ago, tony27 said:

It will probably be a good idea to get new airbox connector from clauss studios for the OKO carb, the bellmouth is bigger so you need to expand the rubber to fit & your original has probably gone hard with age & no amount of hot water will make it pliable enough to stretch to fit without splitting

I have the 28mm for mine but the plan is to remake the airbox with a straight hose & modern air filter in the lid design to get rid of all the restrictions 

Tony 27- I was hoping for a simple install, but sounds like that is not to be.  And I do need a new rubber boot for the air box.  The old one is pretty stiff.  Somehow it also seems to be shorter than it really should be for proper fit.  But I did order the 26 mm OKO carb so as not to create a fitment problem.  But from what you are saying- the new 26mm OKO is different on the air box end?  Do you think just the standard hose for the TY 250 A model from Claus Studios will fit the OKO properly?  And thanks again for the help!

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The installation instructions from mid-atlantic mention needing to soften the up the hose to make it fit the OKO carb, only modification needed to fit the new carb.

The mikunis used on the TYs are unique to them, small float bowl & I think the airbox side is smaller than a generic new replacement mikuni measures. There isn't much clearance under the float bowl of the OKO but they fit straight in the cylinder side manifold so it's really only the airbox side that is different

Your original hose will have shrunk as well as hardened with age, the clauss studios hose fits perfectly with the standard carb etc so I think it should be able to fit the OKO with the boiling water treatment

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On 6/1/2021 at 12:57 AM, tony27 said:

The installation instructions from mid-atlantic mention needing to soften the up the hose to make it fit the OKO carb, only modification needed to fit the new carb.

The mikunis used on the TYs are unique to them, small float bowl & I think the airbox side is smaller than a generic new replacement mikuni measures. There isn't much clearance under the float bowl of the OKO but they fit straight in the cylinder side manifold so it's really only the airbox side that is different

Your original hose will have shrunk as well as hardened with age, the clauss studios hose fits perfectly with the standard carb etc so I think it should be able to fit the OKO with the boiling water treatment

It sounds like you are talking from experience on this? Do you have a bike with the OKO installed?  What are your thoughts other than the installation?

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Don't have it fitted yet as the plan is to make a more modern airbox with a short, straight connection to the carb but ran out of time to get it done before the 75th Kaikoura 3 day this weekend so I'm still running the standard carb with the modified airbox

Looking forward to finishing everything up & getting some testing done in the next couple of months

I've run a OKO on my Jarvis Replica for a large number of years, it's a nice carb that works far better than the original Dellorto down low & doesn't seem to overflow as much as Keihin PWKs do. Well worth the money as long as it's a genuine carb & not a chinese knockoff

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On 6/2/2021 at 4:03 PM, tony27 said:

Don't have it fitted yet as the plan is to make a more modern airbox with a short, straight connection to the carb but ran out of time to get it done before the 75th Kaikoura 3 day this weekend so I'm still running the standard carb with the modified airbox

Looking forward to finishing everything up & getting some testing done in the next couple of months

I've run a OKO on my Jarvis Replica for a large number of years, it's a nice carb that works far better than the original Dellorto down low & doesn't seem to overflow as much as Keihin PWKs do. Well worth the money as long as it's a genuine carb & not a chinese knockoff

Well, It's made in Taiwan with all the other OKO's and the original OKO's are knockoffs of the Keihin PWK's but apparently these knockoffs are not directly from China.  Still waiting on the rubber boot that goes to the air box- it might come in today. - I noticed that this one has a metal float bowl as opposed to the clear plastic one that was mentioned in one of the previous posts.

Edited by Tillerman6
updating info

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On 6/1/2021 at 12:57 AM, tony27 said:

The installation instructions from mid-atlantic mention needing to soften the up the hose to make it fit the OKO carb, only modification needed to fit the new carb.

The mikunis used on the TYs are unique to them, small float bowl & I think the airbox side is smaller than a generic new replacement mikuni measures. There isn't much clearance under the float bowl of the OKO but they fit straight in the cylinder side manifold so it's really only the airbox side that is different

Your original hose will have shrunk as well as hardened with age, the clauss studios hose fits perfectly with the standard carb etc so I think it should be able to fit the OKO with the boiling water treatment

Tony 27- 

Had a time with the OKO install today.  First thing was that the fitting on top of the new carb has a fine pitch thread that is different than what was on my TKO throttle cable.  That may not sound like much, but I don't have another cable kit to draw parts from, so I had to mill off the old fitting and cable ferrule and keep the inner stranded part of the cable intact.  That went well, but not everybody has a milling machine and a lathe, so depending on what threads are on the end of your existing throttle cable, you might have to do some scrounging for parts. The OKO fitting has a large Inner Diameter that fits over the carb end of the cable, so that was a plus, but the main thing is not to have to wait on another delivery.

So with that done I started getting the OKO carb body mated up with the new rubber boot from Claus Studios.  This was a bit more of a challenge as the opening on the OKO is larger than the TKO by about 4mm.  I ended up making tapered plugs that were oversized for both ends of the new boot.   These tapered plugs are inserted and left in the boot while the whole assembly is warmed up and then boiled with a lid on it for about 2-3 minutes. I had more trouble with the carb end than the airbox end and I would say that if you clamp the carb end first  and leave the tapered plug in the airbox end till the last second, you stand a better chance of getting everything assembled correctly.  I also had to take all the mounting bolts out of the airbox and let it float around as much as possible.  After the carb is hose clamped to the boot, and the boot is hose clamped to the air box, you can insert the front end of the carb into the rubber manifold and I'm hoping the airbox will still bolt back down to the frame.  The instructions with the kit are OK, but don't say what to do in case you can't get the new boot stretched over the carb intake diameter.  I had to take the hot boiling pan and all out to the bike to keep it hot enough so that the boot didn't shrink down before I could get it in place.  You only have about 5 seconds to get this done.

There are also a couple of considerations that you might want to think about if you are going to install an OKO on your TY 250 A. 

#1 is that with the standard (furnished) metal float bowl- it is large enough that it touches the top of the gearcase.  However, if you ordered the clear plastic float bowl, there will be more room under because there is no access plug for changing the jets.

#2- It might be necessary to have on hand another throttle cable kit.  _ I was lucky to have the machinery to re-work the one I had- there is another work around - you could re- tap the carb cap to accept the threads of your existing throttle cable.  

I should have the bike running tomorrow if all goes well- wish me luck!

 

On 6/2/2021 at 4:03 PM, tony27 said:

Don't have it fitted yet as the plan is to make a more modern airbox with a short, straight connection to the carb but ran out of time to get it done before the 75th Kaikoura 3 day this weekend so I'm still running the standard carb with the modified airbox

Looking forward to finishing everything up & getting some testing done in the next couple of months

I've run a OKO on my Jarvis Replica for a large number of years, it's a nice carb that works far better than the original Dellorto down low & doesn't seem to overflow as much as Keihin PWKs do. Well worth the money as long as it's a genuine carb & not a chinese knockoff

 

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