SU tuning

[jackkelleher]

Ok, I know this is probably the simplest carb in the world to tune, but I need good results! I've done it once or twice before, after carb overhauls, following the Haynes, but somehow the dense prose of the HBOL has never left me feeling I've done it correctly...
So can someone give a comprehensive way to tune the engine satisfactorily? Preferably without special tools...

[MoggyTech]

Assuming you have the standard SU carb, there are only two major adjustments. Idle speed, and mixture.

1: Ensure timing is OK and ignition circuit in good order, plugs,points,condensor,HT leads, rotr arm, dizzy cap.

2: Remove air cleaner, and using a small screwdriver or the carb lifting pin, lift the carb piston 0.8mm If Revs decrease mixture is too lean, If Revs increase mixture is too rich, if Revs stay the same or increase just slightly mixture is spot on. Mixture adjustment is done with the bottom nut on the jet assembly.

3: Once mixture set, adjust idle speed for 600 RPM

Done.

[jackkelleher]

Splendid! One more question: how does one tell when the engine is at 600 rpm? Will I need a rev meter?

[Alec]

Hello Jack,

just to add to the above, the engine needs to be at working temperature. Yes you need a rev meter to set it to 600, however you can just set it by ear to get a steady idle. If you find it tends to stall when driving and coming to a stop, increase it slightly.

One other tip, if you get totally lost in setting the mixture, remove the piston and dashpot assembly. Adjust the jet until it is flush with the jet tube top, then adjust the jet down again two full turns (12 flats of the adjusting nut) This is the standard reference setting for the mixture and should be pretty close.

Alec

[jackkelleher]

if you get totally lost in setting the mixture

Sounds familiar
That's perfect. I can tune it properly now!

[bmcecosse]

No need for 'rev counter' - just adjust both carb and dizzy timing to get the smoothest idle that doesn't die on you when you press the clutch. Timing first - then the carb - then back to the timing etc until it's right.

[jackkelleher]

All right... Yes, I think timing needs a bit of sorting too, so that's a good strategy!

[jothina]

hi

i'm trying to tune my carb and timing, can anyone tell me roughtly how many degrees advance the timing should be about right at . setting dynamicly using a strobe?

many thanks
john

[bmcecosse]

Depends on the engine - and of course it's now unleaded fuel so original settings are not going to be dead right. Just set it for the best/smoothest idle. Twist the dizzy slowly by hand back and forth while the engine is running - you will soon notice the 'sweet' spot. But of course that's just the idling setting - you need to be sure it's not pinking when slogging up a hill. In fact for very best performance you should advance it until it does pink - then back it off slightly until no pinking. This of course may not give the best idle ! So it may be a matter for a little compromise, although the real answer is to have the dizzy advance curve properly tuned to the engine characteristics. A ' 123 ' electronic dizzy has 16 different curves built in - but they are not cheap!!

[RogerRust]

hi

I'm trying to tune my carb and timing, can anyone tell me roughly how many degrees advance the timing should be about right at . setting dynamically using a strobe?

many thanks
john

I always set it up static timing, with a 12v lamp across the points and set it to 7 degrees BTDC. The timing marks are at the bottom of the pulley the big one is TDC and the others 5 and 10 degrees BTDC.

Then as BMC says take it from there.

Roger.

[bmcecosse]

Yes -good starting point if it's totally lost!

[Bazzalucas]

Hey- silly question, but turning the bottom nut to adjust the jet, which direction is which? I.e., anti-clockwse to richen, clockwise to lean?

[moggydriver62]

Turn the nut down = rich
turn the nut up =lean.

[Bazzalucas]

Thanks, moggydriver!