R90S - balancing carburetors

[tiktok]

My 1976 R90S is now running, but not quite as well as I would like after I took it for a test run for 5 miles today.

Issue 1: When I pull the right side spark plug cap - the bike dies immediately. When I pull the left hand cap - it makes no change to the idle - at any rpm.
Issue 2: The throttle stop screw on the left carb seems to make no impact on idle speed - I have swapped the screw and spring assembly between the carbs, no change.

• Idles at ~850rpm, idle mix at 1 turn out and throttle grip set to 4mm slack. throttle stop screws almost all the way in, to enable idle - presumably linked to the issues below.
• Carbs: both ultrasonically cleaned, accelerator pumps are squirting well, no blocked airways detected. Float height set to 18mm
• Timing: I static timed the engine and set the points gap to 0.4mm. I don't have a strobe - should I buy one to check timing whilst engine is running?

I'm thinking there is an issue with the left hand carb that is causing the running issues, but short of replacing the carb body and float I'm unsure what's a sensible course of action to take next.

 

[Wierpig]

Get some vacuum guages or a harmoniser on it and check the carb balace.

 

[tiktok]

Get some vacuum guages or a harmoniser on it and check the carb balace.

I’ll be borrowing a carb tuner, but I need to fix the above problems first, I think.

 

[Stirlingmoz]

I don’t have much experience of Dellorto carbs but the symptoms you describe are consistent with a blocked idle jet on the left side.

Is there slack in the enrichener cables and are you confident both plungers are fully closed when the engine is warm ?

A strobe would be useful to check ignition timing at idle and at full advance.

 

[tiktok]

I don’t have much experience of Dellorto carbs but the symptoms you describe are consistent with a blocked idle jet on the left side.

Is there slack in the enrichener cables and are you confident both plungers are fully closed when the engine is warm ?

A strobe would be useful to check ignition timing at idle and at full advance.

Thanks for this - sanity checks are appreciated.

The enricher cable adjusters are wound in, but there is relatively little slack. I took it apart and reassembled, so I’ll see if I can adjust the toothed sliders that the lever meshes with.

I’ll take the carb apart tomorrow and see if I can find a blockage in the idle jet/circuit. Once I’ve done this, I’ll check the plungers.

 

[tiktok]

Now running on both carbs - airways were clear, but I had (stupidly) not tightenting the float needle housing, so the float height was way too high.

Now idling correctly on each cylinder and cables are synced (I used wooden kebab skewers as I've eaten all the lollipops in the freezer).