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AllThumbs
25-01-2017, 06:07 PM
Hi All

I’m struggling to get my mind around this one. Any ideas welcome.

So…

As my bike (a 1995 M900) hadn’t been run for 3 years (I know, I know) I had it serviced at the local Ducati dealer before taking it away for long weekend.

On the drive from the dealers to home, it misfired a couple of times. I put it down to it “settling back in”. As I neared home however, one cylinder gave up completely. I managed to nurse it to a supermarket car park where it conked out completely and wouldn’t restart.

I called the dealer who promptly sent a man in a van to recover the bike.

He cranked the bike over (still wouldn't start) and suggested I put some more fuel in it (there was approximately half a tank of fuel in the bike at the point of breakdown).

I looked at him incredulously. Given there was half a tank in there what good would that do?

However, I pushed the bike to the nearby station and filled the tank.

Voila! Bike started. Took it for a run around the block – no problems. The engineer suggested the “weight of fuel” may have been an issue. Again, I looked at him incredulously. The bike has never exhibited this problem before and, frankly, I didn’t buy it.

Confused, I got the bike home without issue. That weekend I set off for my trip. First day went fine – in fair weather did about 200km from a full tank – no issues. Happy days.

The following day, in pouring rain, I had the same problems as before – this time with even more fuel in the tank – bike went down to one cylinder and a few moments later, none. Bike wouldn’t restart.

I was recovered to the local town where I simply filled up the tank and Voila! The bike started fine and showed no issues.

Here are my assumptions:

The problem is not electrical, given the bike works fine after refuelling.
The problem is not dirt in the fuel, given the bike works fine after refuelling?
The problem is not water ingress into the fuel, given the bike works fine after refuelling (as water is heavier than the fuel it would sit at the bottom of the tank and would be drawn into the carburettor before the fuel - regardless of the level of fuel above it in the tank).
The problem is not a vacuum in the petrol tank, given I opened the filler cap to check fuel levels on each occasion the bike has broken down.

So, what fuelling problem could be overcome by simply adding more fuel?

I’m at a loss here. Bottom line is I don’t trust the bike to get me back home each time I take it out which means it’s sitting in the garage.

All ideas welcome.

Cheers!

Col

Dirty
25-01-2017, 06:21 PM
Paco reckons it might be a vacuum fuel pump issue.

Kato
25-01-2017, 06:33 PM
Perished diaphram in the vacc pump on its way out

Darren69
25-01-2017, 06:36 PM
Tell Paco he could be right, but it could also be a blocked fuel filter issue if there is an in tank filter on the carby bikes, I don't know off hand but if there is putting more fuel in might help overcome the blockage. Or maybe a venting issue, blocked air vent in the tank, causing a vacuum lock? Weird one is that.

Kato
25-01-2017, 07:15 PM
But the OP says he had the bike serviced at a dealer therefore you would imagine the fuel filter would be new?

utopia
25-01-2017, 08:09 PM
Yep, the diaphragms in the vac pump apparently don't like drying out, as yours almost certainly will have done during its longish lay-up.
Allen's Performance will sell you a rebuild kit, I believe.

The pressure from the "weight of fuel" is called the "static head" and is determined by the height (not volume) of fuel in the tank.
The pressure required to fill the carbs is very small, and a full tank will have more than enough static head to do that without a pump .. indeed that's how just about all bikes used to be.
But the carbs on a monster are relatively high, so they aren't far below the fuel level in the tank when its getting low .. which is presumably why Ducati fitted a pump in the first place.
The vacuum pump fitted to carby bikes is a very low pressure device, vastly different from that on an injection bike.

Of course there are other possibilities as suggested, but the diaphragm in the pump is a known issue on bikes that have stood for a while so its the most likely, particularly if you're on a new filter.
And even if it doesn't cure the problem, your old one is likely to be on the way out anyway, so its a reliability benefit to replace it .. and they're fairly cheap.
Do you have a manual fuel tap btw ? ..later bikes had a vacuum tap, where there may be a similar problem.

It could also be a restriction in the delivery tube outlet from the tank .. corrosion from condensation in the tank while it was standing maybe.
Might be worth checking that fuel flows freely when you disconnect at the tank.
... when removing the pump to inspect the diaphragm. ;)

Flip
25-01-2017, 08:25 PM
I agree the dealer that did the service should have changed the fuel filter and I think it would also be prudent to physically empty the tank as well just to be certain there is no accumulation of water and or rust in there.

I also think the symptoms are pointing towards something vacuum pump related- I'm not sure if the '95 bikes are the same but my '97 has two. The Mikuni (Snow Mobile) one mounted to the frame and also a smaller one on the underside of the tank.

A refurb kit for the Mikuni one can be bought here:

https://www.catalyst-findit.co.uk/index.php?c=2f74fd408728adaf828b5b9373baedbb&w=900&nav=r&h=n&rpp=100&dest=prod&p=164&pid=MK-DF62

But the little one is not rebuildable and so has to be replaced (around £35.00 from Moto Rapido if I remember correctly- don't forget to ask for club discount when calling).

While on the phone to Luke or Craig it might be worth also order the vacuum hoses (one to each of the intakes) that make each of the pumps do their thing as they too could be causing the issue.

Moto Rapido: 01962 873981

Obviously doing everything at once won't really identify the problem but for the sake of around £60.00 how many times do you want it to konk out only for you to have to pull it all apart to change something else?

I replaced all the other hoses as well on my bike last year- a few photos and details are here:

http://www.ukmonster.co.uk/monster/showthread.php?t=54087&highlight=monster+refresh&page=3

Zimbo
26-01-2017, 08:27 AM
Could it be a blocked tank breather? As fuel is used a partial vacuum builds up in the tank, making fuel reluctant to flow out of it. Releasing the fuel cap results in an audible rush of air into the tank and instant fix. Have seen this happen on an M600 in the past ...

slob
26-01-2017, 09:01 AM
...The problem is not a vacuum in the petrol tank, given I opened the filler cap to check fuel levels on each occasion the bike has broken down...

he covered that

Zimbo
26-01-2017, 04:08 PM
Ah, I should read the post more carefully then!

AllThumbs
31-01-2017, 11:27 AM
Thanks for the comments = definitely food for thought.
Will investigate and hopefully report solution soon!

Cheers!