3 replies to this topic

[crock]

Three questions about the best way to dial in the optimum carb needle and dizzy curve for my 1380:

1. With my HIF 44 carb, how do I determine the best damper spring and oil?
2. Using an oxygen sensor for A/F ratio read out while driving, how do I know where the needle is positioned in the carb? I can't figure out where I need to file the needle.
3. It is straight forward to set up maximum ignition advance on the dyno. The vacuum advance is much more difficult for me to figure out. If this were an OEM job, we would dial in the vacuum advance for optimum fuel consumption, however that is pretty much impossible to determine with the tools I can afford. Is there a reliable way to set up the optimum vacuum advance on the cheap?

Thank you for your assistance.

[KernowCooper]

Also a needle point gauge which you can get by google SU Needle Positions

Positions 1/2 =Idle and datum position
Positions 3/4/5/6 =these govern pick up in top gear from about 20-50mph and part of the needle part throttle and crusing
Positions 7/8/9 and upwards = affect the full throttle accleration
Positions 9 upwards these do not take part in the metering on a 1.25 carb

Edited by KernowCooper, 11 February 2013 - 12:12 PM.

[KernowCooper]

Needle profile, SU carb, guide to throttle opening
Here is a rough guide of where to file the carb needle (assuming the carb throat diameter is suited to the engine cc):
Top (thick) 10% idle < 2000rpm
Middle 30% mid 2000 - 4000rpm
End (thin) 60% top end > 4000rpm
Take care when filing the idle area. It is very sensitive. To leanen an area, file the rest and wind the mixture screw out slightly.

Get a copy of the SU Booklet SU Carburetter Needle Profile \Charts is my advice.
What needle do you have now? there may be a lot better needle to start with in the first place.

Also worth noting on your CO meter on road setup (or Dyno) the A Series produces the max power between 4-5.25% CO Reading, any more its to rich and any less than 3% under full throttle its obviously to weak and you need to watch for increased engine temp with the risk of holing a piston and severe detonation (pinking)

AC here on the forum is the man to speak to about custom needles

Edited by KernowCooper, 11 February 2013 - 12:20 PM.

[ACDodd]

Vacuum Advance is set using a vacuum gauge basically tune the engine such that you get the maximum vacuum while cruising. Too little advance the vacuum drops while cruising, too much, the same thing happens. You can assist this by using a vacuum pump and gauge and manually adjust the vac while driving. This will assist in finding the sweet spot. Follow the vac gauge for maximum efficiency.

Part throttle on the HIF44 is station 3 through 5 1500rpm to 4000rpm. aim for 1.5 to 2.25%CO on part throttle, 5 to 6% at full power. Full power point is number 12 on the '44.

AC

Edited by ACDodd, 11 February 2013 - 01:24 PM.