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Fuel feed problem

Technical help for Lui, Vega, Cometa, J50, Cento, J125, Starstream.

Fuel feed problem

Postby 57Speedster » Thu Mar 30, 2017 8:57 pm

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.
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Re: Fuel feed problem

Postby coaster » Thu Mar 30, 2017 9:34 pm

It is possible that the float has swelled and is catching on the side of the float bowl and frees off a bit when you tap it. Either try a new float or you could try gently sanding the high spot on the float.
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Re: Fuel feed problem

Postby 57Speedster » Fri Mar 31, 2017 9:16 am

coaster wrote:It is possible that the float has swelled and is catching on the side of the float bowl and frees off a bit when you tap it. Either try a new float or you could try gently sanding the high spot on the float.


Thanks Coaster, I did notice a slightly rough edge where the top and bottom halves of the NOS float are fused together, I sanded it smooth with Wet & Dry paper. The old float is smooth and shows no signs of a high spot or rubbing .
It may well come down to Replacing components until the fault clears, and your suggestion of replacing the float is a logical starting point.
Cheers
57Speedster
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Re: Fuel feed problem

Postby Wack » Fri Mar 31, 2017 8:41 pm

From what I've seen most pipes seem to have bubbles in but they tend to be worse when the fuel tap is playing up,such as when the small feed tube inside the filter falls out or becomes loose or the tap rubber seal breaks up.To rule the tap out rig up an auxillary tank or even a washing up liquid bottle if the pipe will fit the cap and see if you still have a problem.Have you tried just running on the reserve position on the tap?
Last edited by Wack on Sat Apr 01, 2017 9:18 am, edited 1 time in total.
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Re: Fuel feed problem

Postby Fast n Furious » Sat Apr 01, 2017 2:13 am

I've recently seen this problem with a Mobecane moped I've just revived. It had been dry stored since 1973 and so the remaining contents of the fuel tank were totally rank. The tank is an integral part of the frame so, I flushed it with Acetone to get most of the shellac out.Then it was steam cleaned.
When new fuel was added I was getting the occasional fuel starvation problem. Turned out, that the tank wasn't as clean as I'd hoped and after thrashing it around the paddock like a giddy 16 year old it would clog the tap filter. It was a right pain in the arse to get it flowing clear.

Try flushing your tank through and observe the flow rate as it discharges from the fuel pipe into a container.
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Re: Fuel feed problem

Postby 57Speedster » Tue Apr 04, 2017 4:48 pm

Thanks Wack and Fast n Furious, I'm going to try an auxiliary tank / washing-up liquid bottle.
My standard tank has been thoroughly cleaned, first by part filling it with a smashed car window, and running it through a cycle in the tumble drier, followed by filling it with Phosphoric acid, and leaving it to fizz overnight. Flushed a few times, and the part of the inside that I can see, looks good, just pitted grey metal surface. The Fuel tap is the right one for the Cento, it is a NOS "OMG" brand tap, but I don't know how old it is.
Although the fuel flows steadily into a bottle when I disconnect the pipe from the carb, with the Tap in either the Normal or Reserve positions, both Tank and tap are now coming under suspicion, so I'll take them out and check thoroughly.
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