I have one of these carburettor problems that could be an ignition problem but just to be sure, would drawing air get worse with load/revs? What’s a good test for it? I was thinking of scooshing WD40 round the inlet rubbers and if it’s drawn in I would get a stutter and smoke? Does that seem reasonable? FCR41’s and JC HC pistons. It sort of whistles around 5/6K on a hard throttle then makes a horrible noise and loses power. My immediate thought was pinking (ignition is already retarded though) but it feels like rear cylinder only and plugs show rear is markedly leaner than front. Idles and revs freely with no load, and pulls cleanly enough up to 5K so I think valve clearances are fine, but easy checked. Any ideas?
Take the manifold rubbers off and inspect them for cracks, opening them out as much as possible. It's the only way to be sure. I doubt it's a timing issue if it's only affecting one cylinder. Does it idle OK?
Yes idles OK. I'm not 100% that it's just one cylinder but that's what it feels like. Only happens pulling hard at 5-6K RPM.
The rear cylinder head is always going to run hotter as the exhaust port is facing the wrong way. It's a carry over from the 750 sport when Ducati inexplicably fitted Weber carbs and had to reverse the cylinder head. It was avoidable with Mikunis as standard but not with fuel injection, although Ducati have tried to resolve this issue with the more recent air cooled engines. You're exacerbating this issue by running high compression pistons. Aside from all the standard checks: Plug gaps and condition Air filter Manifold rubbers Valve clearances Ignition timing Carb balance (assuming all settings are equal) I'd consider running a one grade colder plug in the rear cylinder. It's definitely worth a try and is an inexpensive option. In terms of fuelling; a relatively easy check of main jet size (if you can do it safely) is to turn the choke on as you're accelerating at over 3/4 to wide open throttle. If the pickup improves your main jet is too small and if it bogs down or there's no change, it's the right size. I don't know Keihin carbs at all and you may not have manual chokes on each carb though. This also assumes they're fitted with accelerator pumps. It's also worth checking float heights as this will affect fuelling across the whole rpm range. If there's a difference between the two you may be getting slight fuel starvation on one cylinder.
Unless i'm losing the plot, Pict's bike has separate ignition pickups for each cylinder? also it's perfectly possible for one ignition pickup to partially breakdown in a way that is similar to symptoms described, something i've experienced first-hand more times than I can remember:- the hot oil slowly leeches into the pickup body over the years with the inevitable result. The giveaway confirmation is usually the visible cracking of the pickup body on inspection during strip-down.
Great news that you still have it, stunning bike and I hope you get to the root cause. For something like you describe, I'm thinking it may be ignition too. The coils do breakdown and give intermittent problems, can be fine at low revs then problems higher up. You are running an Ignitech aren't you? From memory most of your bike was gone over, renewed and replaced so its an interesting one...
might be worth checking the ohms resistance of the ht leads and caps and maybe check the earth connections for the ignition coils spark plugs may show a good spark but could be braking down under load maybe swap for a known good one all silly little things but worth a check most other things already mentioned the only other thing i had that caused running issues turned out to be a faulty ignition switch just a few long shots but might help
If you have a propane or butane torch (and no sparks from the ignition), point the unlit torch around the area suspected of leaking . Revs will raise if there’s a leak as the gas is drawn in and ignited.
yip, with air leaks wouldn't you expect popping on the over run. not a ducati expert by any means tho in my experience when air leaks are present there is a distinctive smell from the exhausts. i'm not familiar with the carbs on your bike, do they have diaphragms on them? as the symptoms, bar the pinking are similar to the old VV carbs back in the day.
