TT250very VM26 tuning struggles

[pistolclass]

Agreed ARH. Once I get a couple of free hours I will pull it and square it away. only real issue is when I have a couple of free hours I'd rather be straddling the tank then getting my hands all stinky from gas.

I have the pilot jets on my desk. There is a 60% chance I or my 5 year old son will lose them before I get those free hours to work on it.

I wished you lived closer by I drop the bike off with a 6 pack and $100 and tell you to call me when it's tuned.

[pistolclass]

here is my needle. I know it should be middle, I promise I will get to it

[BlackBike]

Zip locks, magic markers and top shelf hiding places

[Ariel Red Hunter]

Originally Posted by pistolclass

Agreed ARH. Once I get a couple of free hours I will pull it and square it away. only real issue is when I have a couple of free hours I'd rather be straddling the tank then getting my hands all stinky from gas.

I have the pilot jets on my desk. There is a 60% chance I or my 5 year old son will lose them before I get those free hours to work on it.

I wished you lived closer by I drop the bike off with a 6 pack and $100 and tell you to call me when it's tuned.

But you don't LEARN anything from buying me a six pack of Guinness Stout, and a C-note....ARH

[2LZ]

Originally Posted by pistolclass

here is my needle. I know it should be middle, I promise I will get to it

I usually go just one more notch up to the fourth....but all my bikes have after market pipes or mufflers.

[Sullybiker]

I have of course ordered more jets to play with and will probably be trying again this weekend.

[pistolclass]

Originally Posted by Ariel Red Hunter

But you don't LEARN anything from buying me a six pack of Guinness Stout, and a C-note....ARH

Guinness, my favorite too. We will get along just fine ARH.

My big problem is time. I'm an investment accountant for my day job therefore Jan-April I'm sort of tied up keeping the SEC happy with a bunch of filings.

[Sullybiker]

I had a brief tinker with the new jets this morning between work and my perpetually sick (at the moment) kids, and I got exactly the same results, hesitant idle, and slow return to idle. I think I may have an air leak; it has to be something outside of the jets as going up two sizes with no change can't be right, but I didn't get any more time to look today.

I attached the VM26 in the same way as the stock carb, just snugging the bolts on the manifold. Perhaps they're not tight enough (I tend to err on the side of gentle with these things). Now, the stock was fine, but perhaps this VM26 needs a more assertive mating to the boot?

Should I use something like silicon to help, or not bother?

[Ariel Red Hunter]

Originally Posted by Sullybiker

I had a brief tinker with the new jets this morning between work and my perpetually sick (at the moment) kids, and I got exactly the same results, hesitant idle, and slow return to idle. I think I may have an air leak; it has to be something outside of the jets as going up two sizes with no change can't be right, but I didn't get any more time to look today.

I attached the VM26 in the same way as the stock carb, just snugging the bolts on the manifold. Perhaps they're not tight enough (I tend to err on the side of gentle with these things). Now, the stock was fine, but perhaps this VM26 needs a more assertive mating to the boot?

Should I use something like silicon to help, or not bother?

H-m-m, I would check for air leaks between the carburetor and the head. The old fashioned way to do this is to squirt oil on all of the joints between the carb and the head while the engine is running. Also check for any cracks in the rubber intake tube between carb and head....ARH

[Sullybiker]

I finally got somewhere. I thought the o-ring on the carb flange might not be seating nicely, or it may have hardened in transit a little so I removed it, replaced it, and put a very light coating of grease to help keep it supple (CSC actually recommends this on the stock carb when refitting it).

I then realised, after starting the bike with full choke from cold that the revs rocketed up to somewhere around 2500...it was the idle set screw. It was set far too high. My fault from previous experiments, and probably why the idle was so hard to get right with the air screw. I probably didn't have a leak, either.

Now the bike runs really nicely apart from one flat spot just after I open throttle, which is greatly reduced once fully warmed up. I think the needle clip needs dropping down one notch (from the middle). I did suspect my idle circuit mixture wasn't right, but the engine runs up very smoothly off idle, it is just beyond that the bog occurs and it's at a very specific throttle position which I suspect is the moment the slide valve and needle start blending. It is the same throttle position the stock carb won't cope with on very cold days (30F) until the engine is fully warmed. The mixture screw is set at 1.75 turns. I moved it around a half turn either side and it didn't make a lot of difference other than a slight hunt (lean I guess) in the idle over 2 turns out.

A quick test ride on the butt dyno was interesting; the torque curve felt like it fattened out. I avoided high revs on the stock carb as the engine didn't seem to pull so well; now the powerband feels a little wider. Wide-open the engine feels a lot less hectic, the metallic engine note has gone and it now feels like it could sit there all day.

Thanks for the help everyone. I'm pleased I got this carb in the ball park, because it opens up the possibility of a new pipe which I will probably do in Summer.

[Ariel Red Hunter]

Originally Posted by Sullybiker

I finally got somewhere. I thought the o-ring on the carb flange might not be seating nicely, or it may have hardened in transit a little so I removed it, replaced it, and put a very light coating of grease to help keep it supple (CSC actually recommends this on the stock carb when refitting it).

I then realised, after starting the bike with full choke from cold that the revs rocketed up to somewhere around 2500...it was the idle set screw. It was set far too high. My fault from previous experiments, and probably why the idle was so hard to get right with the air screw. I probably didn't have a leak, either.

Now the bike runs really nicely apart from one flat spot just after I open throttle, which is greatly reduced once fully warmed up. I think the needle clip needs dropping down one notch (from the middle). I did suspect my idle circuit mixture wasn't right, but the engine runs up very smoothly off idle, it is just beyond that the bog occurs and it's at a very specific throttle position which I suspect is the moment the slide valve and needle start blending. It is the same throttle position the stock carb won't cope with on very cold days (30F) until the engine is fully warmed. The mixture screw is set at 1.75 turns. I moved it around a half turn either side and it didn't make a lot of difference other than a slight hunt (lean I guess) in the idle over 2 turns out.

A quick test ride on the butt dyno was interesting; the torque curve felt like it fattened out. I avoided high revs on the stock carb as the engine didn't seem to pull so well; now the powerband feels a little wider. Wide-open the engine feels a lot less hectic, the metallic engine note has gone and it now feels like it could sit there all day.

Thanks for the help everyone. I'm pleased I got this carb in the ball park, because it opens up the possibility of a new pipe which I will probably do in Summer.

H-m-m, a stumble at the transition between full pilot jet and needle control. What pilot jet are you running? I'd try one step richer pilot jet. But if you change it you will have to re-adjust the idle mixture screw......ARH

[Sullybiker]

ARH,

I was thinking along those lines, but I'm running a 27.5 pilot and thought I must have done something wrong as folks here get along with 22.5 - 25 just fine, so I experimented. The bike was suspiciously easy to start when cold in 45F weather (with no choke...). I went a half turn leaner on the idle mixture and it's mostly eliminated it. No real change in idle, but it is much smoother up to the mid range.

I think another 1/4-1/2 turn (which would leave the fuel screw 1 turn out) and it should be about right. I thought about just leaving it, but I know as it gets hotter here it'd be better to have it sorted out.

[2LZ]

How many miles do you have on it? I wouldn't even bother trying to get it to run perfectly under 3 or 400 miles. These CG motors change dramatically as they break in. They're very tight out of box.

[Sullybiker]

Originally Posted by 2LZ

How many miles do you have on it? I wouldn't even bother trying to get it to run perfectly under 3 or 400 miles. These CG motors change dramatically as they break in. They're very tight out of box.

I'm at 800.

It's real close to smooth fueling from top to bottom at the moment, I know I can get it there....

[2LZ]

Originally Posted by Sullybiker

I'm at 800.

It's real close to smooth fueling from top to bottom at the moment, I know I can get it there....

Right on! You're definitely on the right track then!
Generally, a minor flat spot off idle means either you need to go another 1/4 turn (or so) out on the idle mixture (which you're at 1.75?) or go up one more on the pilot jet.
I prefer mine a slightly rich anyway. They seem to run "less weird" there and you can always back it off, if need be.