rich running

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Duccout
Posts: 261
Joined: Wed Feb 05, 2020 1:20 pm
Location: Essex UK

Re: rich running

Postby Duccout » Sat May 30, 2020 11:33 am

Hi Zed, great to see you still riding Ducatis, and to still have the 900SS (I presume it is the one that you bought in the early days?). I'm an old timer who remembers you from 'Bike' magazine, all those years ago.

In Two
Posts: 8
Joined: Fri May 29, 2020 5:50 pm
Location: Warszawa/Roma/London

Re: rich running

Postby In Two » Sat May 30, 2020 2:41 pm

Hi Duccout,
Yes it's me, and the Ducati madness is strong still, I'll post an introduction on the photos page so as not to hijack this thread, which has been helpful and has yet to come to a successful conclusion for scottish 888, or me!
Zed
1974 250D yellow
1974 750ss
1975 900ss

ducwiz
Posts: 429
Joined: Tue May 22, 2012 12:52 pm
Location: near Frankfurt, Germany

Re: rich running

Postby ducwiz » Sat May 30, 2020 3:07 pm

Gentlemen,

imho you cannot run these machines with this 0688.91.110 convoluted rubber bellows thing. This was the first part I omitted from my 350M3D after her rebuild. The bike simply couldn't be run with it, tuning of the carb was nearly impossible, the engine's performance totally down. The jetting given in the user manual didn't work at all. Full power and max. rev speed came, after I attached a short bellmouth and a push-on K&N air filter. The best jetting then was found close to the factory values.

Hans

In Two
Posts: 8
Joined: Fri May 29, 2020 5:50 pm
Location: Warszawa/Roma/London

Re: rich running

Postby In Two » Sat May 30, 2020 5:33 pm

Ducwiz, firstly thank you for your clear and informed comments.
I always used to run my singles on open bellmouths and exhausts when a teenager and do the same with both of the twins, often explaining to the police forces of various countries that "It was designed and delivered from the factory like that, I couldn't possibly change it..."
I actually bought a new 'bellows thing" -great description,for the 250D, and felt very responsible and grown up fitting it! I will now feel even better tearing it off! I'll report back tomorrow after testing, it's raining and dark now, and I am no longer 18 years old...
1974 250D yellow
1974 750ss
1975 900ss

blethermaskite
Posts: 133
Joined: Thu Mar 01, 2018 1:06 am
Location: northern ireland

Re: rich running

Postby blethermaskite » Sat May 30, 2020 7:17 pm

When I bought my 350mk3 new in 1974 it was running so bad I couldn't even run it in in any sensible fashion, with about 40 miles on the clock I temporarily fitted a 32mm Mk 1 concentric with velocity stack I has scrounged from a mates dead Norton commando race bike, the awful VHB carb complete with its "rubber bellow thingy" was chucked under the bench (and is long gone), the bike is still wearing that same "temporary" carb to this day 46 years later, I am sure I have said before on this forum .......but those Vhb carbs are fit for nothing....other opinions are of course available :lol:
Stay well,
Cheers,
George

Jordan
Posts: 983
Joined: Fri Dec 24, 2010 11:29 am

Re: rich running

Postby Jordan » Sun May 31, 2020 4:45 am

I'm also an enthusiast of Amal Concentric carbs on Ducatis.
I had a 930 on my 250 Mk3, and it worked perfectly with a short bellmouth.
My current 350 with VHB is very good too, and I like the "slack wire off" action of the enrichener, and how I don't touch the twistgrip when starting from cold - not generally possible with Concentric Mk1 carbs.
It's great to find an easy fix for bad carburetion, but you missed an opportunity to be scientific about it when you removed both your VHB and the troublesome rubber bellows at the same time. Could be a case of guilt by association?

Duccout
Posts: 261
Joined: Wed Feb 05, 2020 1:20 pm
Location: Essex UK

Re: rich running

Postby Duccout » Sun May 31, 2020 11:45 am

I remember Mick Walker advising, way back...possibly 1973... to fit a coil of copper wire inside the bellows to stop them collapsing under induction.

blethermaskite
Posts: 133
Joined: Thu Mar 01, 2018 1:06 am
Location: northern ireland

Re: rich running

Postby blethermaskite » Sun May 31, 2020 12:39 pm

That's quite interesting, I built a 450 Mk3 for a guy about twenty years ago with an Amal concentric carb but he insisted on having the air filter and that daft rubber bellow thingy in place.....on a test ride I noticed the bellows contracting on large throttle openings and I also (independently) came up with the idea of a coil of stiff wire inside to keep the bellow at a constant diameter.....it worked fine.
Cheers,
George

In Two
Posts: 8
Joined: Fri May 29, 2020 5:50 pm
Location: Warszawa/Roma/London

Re: rich running

Postby In Two » Mon Jun 08, 2020 4:11 pm

So, it took a week rather than a day to get around to removing the 'bellows thingy' and going for a ride, just pulled it off the bellmouth and tucked it out of the way.
It hasn't cured the mid-range misfire, but it has changed it considerably, in that it now occurs at a more defined and narrower rev range, 4.5 to 5.5 k and clears at full throttle/higher revs. It's difficult to pin down exactly what is happening on a public road with my tiny, wobbly instruments.
I'm wondering if I have both issues, poor/rich carburation courtesy of of Mr Bellows Thingy and a classic ignition misfire. In any event, a shiny new Sachse ignition and Dyna coil has just turned up so I'll throw that on and see if that exorcises the gremlins.
On a slightly different subject, but connected, I decided not to go for a 12v conversion on the 250D, rather fitted LED lighting, thinking that the real reason for 12v was better(some...) lights. The LEDs are very bright and consume about a 10th of the power . This has worked well in practice although the headlamp needs a tiny electronic 6-12 v converter to spark up, and Ba 6 LED headlamp bulbs have famously dodgy dip/main patterns. If anybody is interested I'll write up what I did, suppliers etc. in a separate thread .
Cheers
Zed
1974 250D yellow
1974 750ss
1975 900ss

Ventodue
Posts: 627
Joined: Mon Mar 08, 2010 3:23 pm
Location: Montpellier, France

Re: rich running

Postby Ventodue » Tue Jun 09, 2020 9:38 am

In Two wrote:<snip> If anybody is interested I'll write up what I did, suppliers etc. in a separate thread.

Yes, please, Zed! Glutton for good info, me!


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