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Hi folks, our Strada is still misfiring under load. Substituted ignition rotor , pickups and transducer, fitted new plugs and HT leads, checked valve clearance and adjusted, ultrasonically cleaned carbs, checked cam timing, wiring connections etc etc. I'm slowly going mad. Has anyone swapped carbs for something more modern? I don't care what they look like, I just want the bl---y thing to run! It starts great and revs up nicely in the workshop, just won't pick up over 10-15 mph in second, misses like mad up hills, and we're surrounded the things . HELP

Float levels?

Fuel taps blocked?

Checked compression?

What size main jets and idle jets are you using?

I have Mikuni VM28's on my 3½ Sport. Turned out to be an expensive conversion, however, the Indian Mikuni VM28 copy (Mikcarb) is now readily available and a lot cheaper. I experimented with different tapers for the needles, but my original setup worked the best.

I started by finding a website that gave the jetting required for fitment of Morini 500 roundslide Dellorto’s to a Morini 3½ Sport.

I then found websites that gave physical dimensions for Dellorto and Mikuni needles and came up with the closest possible match.

I came up with:

Dellorto to Mikuni conversion (VM28)
(1) Needle jet 268 T = P-6 (series 175)
(2) Jet needle X6 = 5L1
(3) Main jet 120 = 110 (series 4/042)
(4) Pilot jet 45 = 37.5 (VM22/210)
(5) Throttle Valve 40mm = 2.5 (VM28/56)
(6) Air jet = 1.5 (BS30/97)

I no longer have the website links. The above should be a good starting point. The air jet is critical! My VM28's did not have any fitted, and this really messed with the whole setup.

With the Mikuni's my bike was much easier to start, idled at 1200 rpm, and reacted very quickly to throttle inputs (or it did until the left hand main bearing race came apart - the rebuilt motor is ready to go back in the frame).

Cheers

Trevor

Breaking down under load most likely points to an ignition problem. Spark plugs can show a nice spark outside the combustion chamber, but not be sparking under load. Are the spark plugs non-resistor? Are the HT leads non-resistor? I run non-resistor plugs and non-resistor Dyna leads. Where did the transducers come from? I tried different providers, but the only decent transducers came from Tritsch (no longer in business). Are you earthing the transducers to the frame? I run the transducer earth wires to the motor, behind the rear cylinder. How many ohms is the ignition coil on the stator showing? Mine was about 160, so I had it rewound. How is the magnetism on the flywheel? Mine was fine, which was about the only thing I didn't have to change!

Also, what year is the Strada? On an early model Morini you set the ignition timing on a mark at the 9 o'clock position, not at the 12 o'clock position...

70sbikes wrote: ↑15 May 2022 10:41 How many ohms is the ignition coil on the stator showing? Mine was about 160, so I had it rewound.

This. Unless the ignition winding on the alternator is doing its stuff properly, changing anything else is unlikely to sort spark problems out.