Suspected fuelling problem

Hi Mark,

I had an almost identical fault on a BSA A10 and that was traced to an air leak on the top of the carb. Some vandal had mashed the threads and it was letting air in weakening the mixture to the extent that if you let go the throttle or moved the choke it died.

When a new carb was fitted it started second kick and ran perfect.

Cheers

Hello Mark,

Given that the logical conclusion would be that this is a fuel issue, Murphyâs Law says:

âDryâ or otherwise loose joint in the ignition system?

Low battery voltage?

Do you have a spare set of coils to try?

Jamesâ point is relevant as well. I donât know whether the Mk11 concentric has a bolt down top (my development stopped with monoblocs); but a leaking manifold flange, damaged tufnol washer, or warped manifold face would develop the fault as the engine started to warm.

If your Mk11 is rubber mounted, try a blast of WD40 around the joints, when the engine is faltering or Phil Hannamâs wet cloth around the manifold joints, when the fault reappears.

While Iâm whistling in the dark, perhaps one or more of your valve clearances are tight....

Hereâs another:

You have a few joints in your breather pipe, donât you? Perhaps they could do with a tweak. Especially if you noticed a rise in idle speed, when you originally fitted the one-way valve...

When you next try a ride down the road, take a couple of (known good) plugs in your pocket. A couple of large crocodile-clips come in handy then, for clamping plugs to a hot engine for spark checking!

âTis worse than having children, ainât it!

Paul

Hello Mark,

A bit more about your 920 cutting out.

You said you put fresh fuel in, after it started cutting out. Would that have been from a can in your garage? So maybe just less than a gallon?

I donât want to offend; but are those fuel taps set to âONâ or âRESâ.......?

I think you have an RGM aluminium alloy tank, donât you? I wonder how much fuel has to be in that tank to run the taps at âONâ...

My last message was way after Beer-OâClock and I hadnât had one; so ignore what I suggested about the crankcase breather.

Next thought:

Do you have the carb mounted on some kind of rubber manifold pipe? If so, it may be worth checking that it hasnât split. If there are clips fastening it together, over tightening them can split the pipe. However, I would expect the engine to run very badly all the time like that, not just after 100yds.

If you can get the engine to start, then run with the choke off. When it starts to miss-fire & play up, press the tickler and see if the float chamber actually floods. If it does and you can confirm that the float height (with the engine running) is correct, then the sparks department are the next suspects.

Paul

Have you got a Boyer fitted, and has it been on there for a while? If yes then renew the pick up plate. I had a year's worth of 'strange carburation problems' with it running rich when warm and misfiring on certain throttle openings. It was all due to the 'classic' (that's a euphemism!) Boyer fractured pick-up wire.

Previously wrote:

Previously wrote:

.................The strange thing is that I'm sure I can hear a faint 'ticking' noise coming from the ignition switch when 'off'. All other electrics (lights etc.) seem OK. The battery is at 12.6v. I'll have to re-check all of the ignition wiring, switch contacts etc......

Cheers

Hello Mark,

Too bad the fault hasnât shown up yet.

How old is that switch?

In the early 80âs I was given a Ducati 900SS to tidy up and sell for a friend. It had been dropped then stood in a barn for a year or so. After about a month of getting the tank restored and replacing what else was damaged, I came to fire it up.

It wouldnât go apart from a few pops & bangs. When I switched it off I noticed there was a slight âbackfireâ. I stripped the switch and found it quite corroded inside. After cleaning up & reassembling all was well and I felt Iâd been really lucky to fix it so easily.

Worth a shot?

Paul