Tuning SU carbs

[Timothy Keith-Lucas]

I've got a beautiful rebuilt set of 1 1/4" SU carbs on my Austin 948 engine. I've been following the step-by-step in the Street Sport "Tuning SU Carburetters" guide that MV8 recommended. I'm on page 26, step #19, with the choke and throttle disconnected and the jet adjusting nuts two turns out. 

 

The problem is that the engine is cold and even with starter fluid and holding one choke in place I can't get the engine to run long enough to warm up so I proceed to actually tuning the carbs. Any advice on the next step?

[Timothy Keith-Lucas]

Two turns was from the point of the jet adjusting nuts fully compressing the springs. It's acting like it really isn't getting any fuel from the carbs, just starter fluid. Perhaps open the jet adjusting nuts some more?

[MV8]

Step 17 in this process was to have the engine warmed up. You can snug the interconnections and start over using choke and/or throttle to get warmed up or instead of starting over, turn both throttle screws in a half turn or so, start, then back them off evenly as it warms up to the desired warm idle speed, chokes off.

[fastg]

It make me laugh when people say FI is to complicated, we are already at step 17:)

 

Graham 

[ashyers]

Tim,

Set the jets flush with the bridge and wind them out 2 turns from there. While you’re doing that check the fuel level. It should be very close to the top of the jet, but not spilling over. Adjust the float height as needed. Toss the pistons/needles/dampers back on and see if you have better results. You can always lower the jets to get it running and warm, then wind them in once up to temp.

Andy

[Timothy Keith-Lucas]

Thank you, all. I'll get back to work on it in the morning.  The central problem is that this is a new installation, so a cold engine, rather than tuning existing carbs on a running engine.

[Timothy Keith-Lucas]

I Got it!  Typical "I'm new to this piece of machinery" mistake. I followed your instructions (all of them, THANK YOU!).  No joy, so something had to be wrong beyond adjustment. I took the anti-bird nesting screens off the carbs to watch the throttles rise and fall. Both throttles were stuck in the full-up position.....    I had not noticed the tongue and grove on the sides of the piston and the dashpot case. Locked them full open.  Runs one hell of a lot better with working throttles.  Still needs some tuning, but that I can do.

 

So thanks to all, and to all a happy holiday season.

 

--Tim.