Technical Still confused with carburettor

[JonPiz]

I have jetted my 28imb correctly for my 650cc engine and it starts fine, however I unscrewed the idle adjustment screw 1.5 turns and it runs a bit rough. It runs brilliantly if I screw this all the way in

 

[smart51]

If you screw the idle screw in, you reduce the fuel flow to the idle jet. If it runs better like this, it suggests that the idle jet is too big. What size jets are you using?

 

[JonPiz]

If you screw the idle screw in, you reduce the fuel flow to the idle jet. If it runs better like this, it suggests that the idle jet is too big. What size jets are you using?

• 23mm venturi
• 115 Main jet
• 190 Air corrector
• F8 Emulsion tube
• 50 Idle jet
• Starter Jet F5/90
• 125 Needle valve

 

[smart51]

That jetting looks right for an 28IMB4/250, which is right for a 650 engine. Mind you, I tried the right jetting for my 594 and that wasn't right either. It's worth trying a 45 idle jet to see if that's better.

 

[the hobbler]

That jetting looks right for an 28IMB4/250, which is right for a 650 engine. Mind you, I tried the right jetting for my 594 and that wasn't right either. It's worth trying a 45 idle jet to see if that's better.

It is quite common for the larger engine to require smaller jets than the smaller engine when they are using the same carburettor---in this case a Weber 28IMB. For the '594'engine either the "3" or "1" variants, and for the '652'engine either the "5/250" or the "12/250" variants.Quite what the differences between the variants are, I honestly don't know. The reason that the bigger engine can get away with a SMALLER jet is that the bigger engine sucks in more air, and at the same time, more fuel. Whilst the '652' engines have a 1 size BIGGER "idle" jet, they go DOWN a couple of sizes on the Main jet! You say that the engine runs rough when the mixture screw is out 1-1/2 turns, but smoothly when fully in---have you tried turning the idle-mixture screw OUT a bit more---does it then run smoothly?

 

[JonPiz]

It is quite common for the larger engine to require smaller jets than the smaller engine when they are using the same carburettor---in this case a Weber 28IMB. For the '594'engine either the "3" or "1" variants, and for the '652'engine either the "5/250" or the "12/250" variants.Quite what the differences between the variants are, I honestly don't know. The reason that the bigger engine can get away with a SMALLER jet is that the bigger engine sucks in more air, and at the same time, more fuel. Whilst the '652' engines have a 1 size BIGGER "idle" jet, they go DOWN a couple of sizes on the Main jet! You say that the engine runs rough when the mixture screw is out 1-1/2 turns, but smoothly when fully in---have you tried turning the idle-mixture screw OUT a bit more---does it then run smoothly?

It runs when the idle adjustment screwed out 1.5 turns but the revs speed up as it’s screwed in.