TT250very VM26 tuning struggles

[Sullybiker]

Hi all,

I got a 'Japan T/A' Mikuni VM26 for my TT250; it arrived with a 20 pilot and a 100 main.

I fitted a 105 main for starters as I'd read this was a good starting point. However, I couldn't get the bike idle smoothly, leading me to believe the 20 pilot was perhaps a bit too small.

I started with the air screw 1.5 turns out, and the bike idled unevenly and would rev, but was very slow to come back to idle, whereas it's normally very responsive. Even putting the screw all the way in didn't really change this.

I went up to a 22 main, and the situation improved, but again the bike seemed lean to me as it would be slow to rev down to idle after it warmed. Again I started out from 1.5 turns out on the air screw. Tightening the air screw (all the way, eventually!) helped the throttle response, but it would bog - perhaps not surprisingly - on snapping the throttle so it seems to be mostly lean > very rich across the air screw range. A quick test ride around the block and the bike clearly wasn't right at low throttle openings. I figure starting out at 1.5-2 turns should have me in the ballpark if the jetting is right, but so far the bike is running terribly. I can get it to idle but it's rough. Half choke smooths it out so I know it's lean.

Am I on the right track thinking of going to a 25? I haven't even tinkered with the main and needle yet as I haven't had the engine working properly at quarter throttle. I'm at about 1000ft here, and it was about 65F today.

Also any tips at working the air screw? I have an HF right angle screwdriver (perfect for taking the float bowl off the carb) but I've been having to take the bit in my fingers for the air screw adjustment and it gets ouchy when the engine is hot.

[pete]

wined the air screw in/out till you get the highest idle rpms then
back it in 1/8 to 14 turn...

{funny hearing people talk about main jets in the 105/110 range on a 250
my 250 running 157 main / 52 pilot ,, It is a Yamaha but even so it's
still a 250...}


...

[Ariel Red Hunter]

Originally Posted by Sullybiker

Hi all,

I got a 'Japan T/A' Mikuni VM26 for my TT250; it arrived with a 20 pilot and a 100 main.

I fitted a 105 main for starters as I'd read this was a good starting point. However, I couldn't get the bike idle smoothly, leading me to believe the 20 pilot was perhaps a bit too small.

I started with the air screw 1.5 turns out, and the bike idled unevenly and would rev, but was very slow to come back to idle, whereas it's normally very responsive. Even putting the screw all the way in didn't really change this.

I went up to a 22 main, and the situation improved, but again the bike seemed lean to me as it would be slow to rev down to idle after it warmed. Again I started out from 1.5 turns out on the air screw. Tightening the air screw (all the way, eventually!) helped the throttle response, but it would bog - perhaps not surprisingly - on snapping the throttle so it seems to be mostly lean > very rich across the air screw range. A quick test ride around the block and the bike clearly wasn't right at low throttle openings. I figure starting out at 1.5-2 turns should have me in the ballpark if the jetting is right, but so far the bike is running terribly. I can get it to idle but it's rough. Half choke smooths it out so I know it's lean.

Am I on the right track thinking of going to a 25? I haven't even tinkered with the main and needle yet as I haven't had the engine working properly at quarter throttle. I'm at about 1000ft here, and it was about 65F today.

Also any tips at working the air screw? I have an HF right angle screwdriver (perfect for taking the float bowl off the carb) but I've been having to take the bit in my fingers for the air screw adjustment and it gets ouchy when the engine is hot.

Yeah, I'd try a 25 pilot. You are sure the carburetor slide is actually going all the way down to the idle speed adjustment screw? When I set up bikes way back when, I always made sure the throttle would snap shut, with the engine shut off. I don't know about a TT250, but on a Hawk I'd get the air cleaner hose off of the carburetor mouth so that I could make sure, visually, that the throttle is snapping shut. The cables are non-too long on Chinese bikes, so check that as well.

[Adjuster]

I would also be concerned about cable and slide operation since you switched out carbs.


/

[Ariel Red Hunter]

Originally Posted by Ariel Red Hunter

Yeah, I'd try a 25 pilot. You are sure the carburetor slide is actually going all the way down to the idle speed adjustment screw? When I set up bikes way back when, I always made sure the throttle would snap shut, with the engine shut off. I don't know about a TT250, but on a Hawk I'd get the air cleaner hose off of the carburetor mouth so that I could make sure, visually, that the throttle is snapping shut. The cables are non-too long on Chinese bikes, so check that as well.

What I should have said in this post is that it is POSSIBLE that you have two issues with your carb woes. Too lean pilot jet AND slide not dropping instantly when you release the twist grip. It is very easy to have too much drag in the throttle cable, because the stock cable is so short. It has to be routed perfectly in order to let the spring in the top of the carb to over come friction in the cable. Avoid tight turns in that cable as much as possible. ........ARH

[pistolclass]

105 and 20 sounds too lean to me. I'm running a 110 with a 20 ( have a 22.5 and a 25 but not time to tinker) With this set up I had to run the needle in the highest slot. I'm 2 turns out. It runs great with this set up so I'm a little reluctant to tinker when I do find time.

[Ariel Red Hunter]

Originally Posted by pistolclass

105 and 20 sounds too lean to me. I'm running a 110 with a 20 ( have a 22.5 and a 25 but not time to tinker) With this set up I had to run the needle in the highest slot. I'm 2 turns out. It runs great with this set up so I'm a little reluctant to tinker when I do find time.

You have to run the needle in the highest slot? Maximum lean on the needle. Something's out of whack. ......ARH

[Sullybiker]

Originally Posted by Ariel Red Hunter

What I should have said in this post is that it is POSSIBLE that you have two issues with your carb woes. Too lean pilot jet AND slide not dropping instantly when you release the twist grip. It is very easy to have too much drag in the throttle cable, because the stock cable is so short. It has to be routed perfectly in order to let the spring in the top of the carb to over come friction in the cable. Avoid tight turns in that cable as much as possible. ........ARH

Thanks, I think this is okay on the bike but I will double check as it would explain what I'd been hearing, I'm going to go out and tinker some more this morning.

I do think a 22 pilot ought to be just enough given what some people have been saying, so I'll try the full range of mixture screw adjustments to be sure.

[2LZ]

Is the slide in backwards? It happens....... You may want to double check that and your needle clip position. The 4th slot down (clip second from the bottom, lifting the needle) gives me best performance.

If you have the stock exhaust, a 110/22.5 or 25 should be fine and run great, depending on your elevation. Start at two turns out on the mixture screw.

These things should do the trick, unless you have a major air leak somewhere. The slow to rev and slow to come back to idle is very suspect of a bigger issue. Again, check to make sure the slide is installed with correct orientation (wedge-side toward the air filter).

[pistolclass]

Originally Posted by Ariel Red Hunter

You have to run the needle in the highest slot? Maximum lean on the needle. Something's out of whack. ......ARH

Top meaning closest to the point. I'm sure I'm saying it backwards. Whatever slot lifts the pointy thing.

[Sullybiker]

I got something approximating normal throttle response this morning out to about 1/4 throttle, but it was still running unevenly & rough. I do have a queston about the mixture screw: It's metering fuel on that side of the carb, right? Screwing all the way in is lean? I went by this schematic, I know it's a vacuum carb:




Either way, it didn't seem to make much difference. I started at 1.5-2 turns out (I have blistered knuckles from trying to adjust that damn thing) and attempted 1/4 turns either direction. I never felt I was getting anywhere near the right fuel/air ratio.

I'm ordering a 25 pilot and 110 main as a sane basepoint (which I probabl should have done in the first place) but in order to get some riding in this nice weather I'm back on the stock for now (I realised just how off my aftermarket carb was when I listened to the engine idle when I put the stock one back) and will probably play around again later in the year, as the warmer temperatures seem to favour the current setup, but I want to have a solution when it gets much colder again, or if I get the fancy exhaust.

[Sullybiker]

Originally Posted by 2LZ

If you have the stock exhaust, a 110/22.5 or 25 should be fine and run great, depending on your elevation. Start at two turns out on the mixture screw.

These things should do the trick, unless you have a major air leak somewhere. The slow to rev and slow to come back to idle is very suspect of a bigger issue. Again, check to make sure the slide is installed with correct orientation (wedge-side toward the air filter).

I was trying a 105 main and a 22 pilot, clip in the middle position.

I thought the slide was keyed as such that you couldn't put it in the barrel the wrong way? You did make me wonder if I'd managed it though...

[Ariel Red Hunter]

Originally Posted by pistolclass

Top meaning closest to the point. I'm sure I'm saying it backwards. Whatever slot lifts the pointy thing.

Oh, thanks for clearing that up. You are running on maximum rich on the needle. This is why you are not getting a smooth transition between idle to 1/4 throttle, and beyond to 3/4 throttle. You will have a very hard time figuring out which pilot jet is right when you have the needle set in the richest possible setting. Try setting the needle in the middle slot, AND LEAVE IT THERE while you decide which main jet and which pilot jet works best for you. After you have the best (for your conditions) main and pilot jets in the carburetor, THEN you can fine tune by going up or down one notch on the needle.....ARH

[Ariel Red Hunter]

Originally Posted by Sullybiker

I was trying a 105 main and a 22 pilot, clip in the middle position.

I thought the slide was keyed as such that you couldn't put it in the barrel the wrong way? You did make me wonder if I'd managed it though...

Yes, it is keyed to only go in one way. But Bubba can put enough pressure on it when re-assembling the carb, he can break off the Key.

[Ariel Red Hunter]

Originally Posted by pistolclass

Top meaning closest to the point. I'm sure I'm saying it backwards. Whatever slot lifts the pointy thing.

OK, that is the richest position. It is going to be difficult getting your pilot jet figured out with an over-rich needle setting. If it was me, I would move the needle clip to the middle position, then change pilot jets until I had dead smooth carburation from the first "thump" at 1350-1400 rpm all the way to 1/4 throttle. And a little beyond. This is the transition zone to where the pilot jet is flowing all the fuel it can, and the main jet is progressivly entering more and more into the mixture, until at 3/4 throttle and above the main jet controls the mixture almost entirely.....ARH