3 screws round top SU CARB tuning correct or not?

[jalexquijano 93]

while tuning my 3 screws round top su carbs this weekend after setting the timing to 14 btdc, i found out that even if i turn the fuel mixture knob 5 turns clockwise when i raise the piston the engine shuts down. Could the float level in this rear carb be wrong? It does not happen on the front carb.

My tuning procedure starts at 2.5 turns clockwise (lowering the jet 2.5 turns). After that i set the idle rpm at 1000 RPM and doublé check the air with the synchometer. Then i turn the fast idle screw until it hits 3000 RPM on the tachometer and verify the balance amongst both carbs with the synchometer. i tend to increase the fuel mixture knob to 3 turns each depending on the gunson colortune. Is there something i am not doing correctly? Valves have been perfectly adjusted.   

[240260280 2,151 1 Vehicles]

Try the piston lift experiment but with the engine at 2000rpm (use the fast idle screw in the middle of the carb linkage to get the 2000 rpm).

If both carbs are getting the same air and same fuel, the rpm drop should be the same when you lift a piston.

 

Note:

1. SU's are air balanced at idle using the stop screws on each side of the carb and a flow meter. 

2. SU's are air balanced at 2,000 to 3000rm using the screw in the linkage near the back carb (the back carb's air is matched to the front carb).

3. The fuel is matched by jet height and needle shape.

[Hardway 280]

240260280 is on point above.  One thing I would recommend is using an aftermarket tachometer or dwell/tach to ensure you know your true engine speed when doing any tuning.  These are easy to hook up and allow you to see the engine RPM's while working under the hood.  The original tachs in our cars seem to suffer from a little to sometimes a lot of inaccuracy. 

[jalexquijano 93]

thanks! ill set my rpm to 2000 with the fast idle screw and check to see if when i lift the piston the engines shuts down.

[240260280 2,151 1 Vehicles]

2000 is just a ball park to give the engine a bit more air and speed.  Go as high as you feel comfortable with.

The key is to make sure they have the same air flow then adjust the fuel jet height to make sure they each drop to the same  RPM.  It is iterative so you have to repeat a few times to get it dialed in.

Edited June 6, 2017 by 240260280

[240260280 2,151 1 Vehicles]

4 hours ago, Hardway said:

240260280 is on point above.  One thing I would recommend is using an aftermarket tachometer or dwell/tach to ensure you know your true engine speed when doing any tuning.  These are easy to hook up and allow you to see the engine RPM's while working under the hood.  The original tachs in our cars seem to suffer from a little to sometimes a lot of inaccuracy. 

I agree.  A great tool is a timing light with built-in tach. and advance.  I use mine all the time.

 

 

Edited June 6, 2017 by 240260280

[jalexquijano 93]

Is there any more effective way to adjust the right amount of turns of the fuel mixture knobs other than using the gunson colortune? I know you start at 2.5 turns clockwise and then keep lowering the jets.

[240260280 2,151 1 Vehicles]

Air/Fuel O2 measurement is best method.

[jalexquijano 93]

Which is this method? Colortune method?