TT250very VM26 tuning struggles - Page 2

Sullybiker · 02-18-2017, 04:06 PM

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.

02-18-2017, 04:06 PM	#1
Sullybiker Join Date: Dec 2016 Location: Western PA Posts: 363	TT250very VM26 tuning struggles 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.
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