My Cento has recently developed a mystery fuel feed problem. It was all working fine until a couple of weeks ago. The engine starts easily, 1st or 2nd kick, but will now only run for about a minute before cutting out.
I first suspected an ignition fault, decided to check everything methodically, replaced some ignition components one at a time, testing after each change, , so it has new Condensor, (genuine Ducati), new points, I tried a known good HT Coil, (borrowed off a running J125), new HT lead, new Champion spark plug, rechecked ignition timing and points gap, none of these changes had any effect on the problem, It was producing healthy sparks before and still does after each of these changes, and I'm sure it is firing at the right time, (because it starts easily, and runs sweet for that one minute every time).
Next step. checking fuel system. I noticed shortly after the engine cut out, if I tap the fuel pipe, (clear plastic), I see a stream of bubbles going up from the carb to the fuel tap. After this, the engine will start again, and run for another minute. So I'm thinking Sticking float or needle. The carb is the standard old Dell'Orto SHB 18/16 SHB, correctly jetted for the Cento engine. I removed it stripped it down to check properly. Other than a worn looking Float needle, and squashed float bowl gasket, I saw nothing unusual, replaced those parts, (new needle is Viton tipped), with genuine spares from EuroCarb. the float moves freely inside the bowl, when I tip it over.
Refitted the carb, but same problem. getting a little annoyed now.
I have a NOS Dell'Orto SHB 18/16, I was keeping as a spare, I had tested it when I got it, and it ran fine on the engine at that time, and it got drained & cleaned before storage, so I fitted that, but the same problem was still there. Really annoyed now.
I stripped the NOS carb, checking for blockages etc., but all the drillings and jets are clear & easy to blow through.
There is no evidence of the float sticking in either the original or the NOS Carb. (would be massive coincidence if 2 carbs, which were working fine before, both developed sticking float at the same time), and, I can hear the float moving when I tip the carbs over, but, of course, you can't see what it is doing when the carb is assembled and fitted on the engine.
Checks to the rest of the fuel system revealed nothing obviously wrong either.
I know the fuel tank cap vent is clear, (even tried running it with the cap off), and if I disconnect the fuel line, the fuel runs freely into a bottle, from both the main and reserve tap positions, so the tank filter and tap seem fine. The filter disc under the fuel inlet union is clean, I can blow through the union easily.
The stream of bubbles after tapping the fuel line seems strange. I would expect, if the bowl had run dry, that any air in the float bowl would escape via the float bowl vents, as fresh fuel runs in. the vents are clear on both carbs.
Fuel just doesn't seem to get past the inlet union, unless I tap the fuel pipe, which appears to release an air lock, possibly inside the inlet union, (I can keep the engine running, as long as I continuously tap the fuel pipe, and I can see a constant stream of bubbles when I do that.
Sorry for the long tale, just trying to cover everything.
Has anyone experienced anything like this, and can advise me ? (before I damage something out of sheer frustration) .
Any advice much appreciated.