1981 Darmah - Carb Mods - FWIW

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GT7307
Cucciolo - the Lil Pup
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1981 Darmah - Carb Mods - FWIW

Post by GT7307 » Sat Dec 01, 2012 6:17 pm

Just thought I'd share my experience with my Darmah carbs.
This bike is comparatively low mileage - 20,000 - and when I got it, it ran pretty well when warmed up and up to road speed. However, the idle was always ragged and it didn't like off-idle transition to road speed. It always felt lean and would buck, lurch, and occasionally spit back when doing lower speed riding in traffic.
I tried all the usual idle adjustments, searched for manifold air leaks, move the needle a notch or two higher, etc.
Nothing made much difference. I finally bought rebuild kits and went completely through the carbs. I didn't find anything particularly wrong other than old rubber parts, but thought it was best to start fresh.
Long story short, that didn't make any difference either.
So - I looked through my old Delorto manual at options. There are many slides, needles, and needle jets (atomizers) for the PHF's, and I saw that there were richer (lower cutaway) slides than the 503 slides that were in the bike.
As far as I can find, 503's were the stock slides.
I did eventually find some 402 slides and installed them. Presto - it cured about 95% of the low speed transition problem.
Second, I raised the needle one notch to the middle (3rd from top)notch and that cured it 99.5%.
Now it responds to the idle mixture screw and slide stop properly.
It starts on the first hit of the starter with the chokes on, idles fast and a bit rich till it gets the heads a little warm, (maybe a minute) and settles down to its normal lovely cammy idle. Motoring off from idle it immediately picks up both cylinders evenly and cleanly motors away with very little clutch slip or extra revs.
For the record, the carb specs are as follows:
Stock PHF 32 AD & AS
Slides 402
Needle K16 in 3rd notch
AB1-262 Needle jet (atomizer)
#122 Main jet
#60 Idle jet
Idle mix screw 2.75 turns out
Idle speed about 1200 - 1300 warm
(the 402 slides have a later but quicker accelerator pump ramp than the 503, but it seems fine. Now at steady road speed of 60 mph or so, the slide must be sitting just below the ramp as hitting the throttle gives a response like the secondaries on a 4 bbl carb kicking in - A really nice hit -great for passing. Just in case you wonder about what this does for mileage, it gets low to mid 50's mpg at steady 60 - 65 mph.)
The bike is now an absolute pleasure to ride without having to contantly baby it in slower traffic. I live way out past the suburbs and and can enjoy low traffic secondary roads most of the time, but I used to hate taking this bike to town.

I assume Ducati set these a bit lean for the then emerging emissions specs but it didn't do the carburation smoothness any favors.
Has anyone else gone down this road ?
GT7307
70 350 Desmo
73 750 GT
81 Darmah SD
BMW R's and K's
Spanish bikes
Rickmans
Assorted Japanese

idontknowher

Re: 1981 Darmah - Carb Mods - FWIW

Post by idontknowher » Mon Dec 03, 2012 12:57 pm

We're right in the middle of this road now. :) Although the carbs on my '81 Darmah are 38's.

Been having a heck of a time with it, was spitting & sputtering, running rich, not idling, backfiring & even shot flames a couple times (that was exciting, & loud).
We did a carb rebuild & made adjustments & it seems to be better, but still needs fine tuning, or new carbs.
I'll forward this info to the Mr.
Last edited by idontknowher on Wed Dec 05, 2012 8:31 am, edited 1 time in total.

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Craig in France
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Re: 1981 Darmah - Carb Mods - FWIW

Post by Craig in France » Wed Dec 05, 2012 2:17 am

Interesting - thanks. It's true that Ducati themselves played around a bit with the settings during the life of the model. For example, here are the stock settings for a Darmah with PHF 32s (AD/AS) and Lafranconis, as given in the undated 'Whaletail' parts book:

Slide: 70/2
Atomiser: 265AB
Needle: K6, middle notch
Idle: 60
Mains: 122 (note: the second edition of the owners handbook, dated July 1977, spec'd a 118 in the front).

This is what I use on my pre-April '79 SSD, btw. They seem to suit it well :)

The 1980, non "Whaletail", parts book lists the CD/CS carbs (and Silentium silencers), with the following differences:
Slide: 60/3
Needle: K16, middle notch
Idle: 62

However, the third edition of the owners handbook from June 1979; and the SSD owners handbook from July 1979 list a 55 idle jet.

Ciao

Craig

427man
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Re: 1981 Darmah - Carb Mods - FWIW

Post by 427man » Wed Nov 12, 2014 10:10 pm

Hello GT3707 -
I know it's been 2 years since this post. I just finished my first bevelhead - also a 1981 900 Darmah SD. I've only ridden it once, and it is exhibiting all the same symptoms yours did. I just tore the carbs back apart to see what was in them. I assumed they were right, obviously a mistake. I am running K&N cone filters and stock exhaust with Conti replicas. in both carbs (PHF 32CD) I found:

Main = 122
Idle = 55
Slide = 50/3
Needle = K27 2nd position from bottom
Atomizer = 262AB

The bike seems to be running very lean at lower RPM. I'm guessing I need to go up on the idle jets. I don't know what the difference in needles will cause. I assume a K27 is larger and more restrictive than a K16. Same for the atomizer, do I go up or down or leave it alone.

The slide is a big deal. Where did you find a 40/2? Slides are so expensive, I hate to spend $ on the wrong unit.

Any and all help is greatly appreciated. Thanks, Darrel
1981 Ducati 900SD Darmah
2007 Ducati 1100S Multistrada
1973 Honda CL350
1979 Honda CBX
2001 Kawsaki ZRX1200R
1968 Norton Atlas
1973 Norton Commando 750
1974 Norton Commando 850

GT7307
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Re: 1981 Darmah - Carb Mods - FWIW

Post by GT7307 » Thu Nov 13, 2014 7:05 am

Hi 427man,
I found my 40-2 slides at Vintage European Mototrcycles and Parts.
Bill Herring - cell phone 419-466-7444
Lives in Perrysburg, Ohio, near Toledo
Has a huge stock of European stuff as he buys out dealers stock when he can.
In this case, he bought a big pile of parts from Cosmo and it included boxes of Dellorto bits.
When I bought my slides, he had a whole bag full of them at about $40 each.
After all the unsuccesful messing with the carbs, $40 each for the "fix" was a relief.
Give us a follow up post if this helps.
GT7307
70 350 Desmo
73 750 GT
81 Darmah SD
BMW R's and K's
Spanish bikes
Rickmans
Assorted Japanese

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Craig in France
Cagiva Gran Canyon
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Re: 1981 Darmah - Carb Mods - FWIW

Post by Craig in France » Thu Nov 13, 2014 10:55 am

427man wrote: The bike seems to be running very lean at lower RPM. I'm guessing I need to go up on the idle jets. I don't know what the difference in needles will cause. I assume a K27 is larger and more restrictive than a K16.
Hi Darrel,

Some thoughts:

1. The K27 needle measures 1.80 at the sharp end against 1.75 of both the K6 and K16. It's taper is also longer: 44mmm vs. 39mm. So it will be more restrictive. Me, I'd change it/them. (The K6 is slightly smaller overall diameter than the K16, btw: 2.45 vs. 2.5).

2. Changing to a 40 slide will cause more vaccum - and therefore a richer mix - when moving off progression = up around ¼ throttle opening. Note: 'throttle opening' - forget the engine revs.

3. If the bike is idling ok, then I suggest you leave the 55 in there for now.

4. Ditto for the atomizer. Yes, you could go up to a 265; but it probably wouldn't be the first thing I'd look at.

Just thoughts ...

Craig

427man
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Re: 1981 Darmah - Carb Mods - FWIW

Post by 427man » Fri Nov 14, 2014 3:00 pm

Thanks Craig. After more discussion and thought I am going to try the following:
Idle jets = 62
Needle = K6
Atomizer = 265

I am going to stick with the 50-3 slides that it came with. Hopefully this combination will reduce this drastic lean condition down low. I'll let you know what happens. Thanks
1981 Ducati 900SD Darmah
2007 Ducati 1100S Multistrada
1973 Honda CL350
1979 Honda CBX
2001 Kawsaki ZRX1200R
1968 Norton Atlas
1973 Norton Commando 750
1974 Norton Commando 850

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Craig in France
Cagiva Gran Canyon
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Joined: Tue Mar 23, 2004 3:58 pm
Location: Montpellier, France

Re: 1981 Darmah - Carb Mods - FWIW

Post by Craig in France » Sat Nov 15, 2014 5:52 am

427man wrote:I'll let you know what happens.
Yes, please do! Good luck!

427man
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Re: 1981 Darmah - Carb Mods - FWIW

Post by 427man » Thu Dec 18, 2014 7:49 am

Well - the parts are installed. Between a new job, the holidays and generally lousy weather, I have not had a chance to ride it yet. But, if the way it starts and runs initially is any indication, I think a big improvement has been made. It runs MUCH better so far. :-D If I get a dry weekend above 40 degrees (and a get out of jail card from the warden), I'll put some miles on it and report back. Thanks for everyone's help.
1981 Ducati 900SD Darmah
2007 Ducati 1100S Multistrada
1973 Honda CL350
1979 Honda CBX
2001 Kawsaki ZRX1200R
1968 Norton Atlas
1973 Norton Commando 750
1974 Norton Commando 850

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dessmo
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Re: 1981 Darmah - Carb Mods - FWIW

Post by dessmo » Thu Jun 06, 2019 2:35 am

Bumping this old thread, any more suggestions regarding what's the best set up for a 83 900SD running PHF 32. I have recently installed Silent Hektik ignition, but still it runs a bit rough. Especially the front cylinder seems to struggle. I have refurbished the carbs, looked for air leaks and adjusted the carbs with vacuum clocks. It seems like the exhaust from the front cylinder is a bit smokey, possible an oil leak or just running rich? I recently had the engine apart to include overhauling the valves, valve seats, and new shims. I also replaced the little rubber gasket over the valve steam. I have had the bikes for 18 years, and the rough has always been running a bit rough from time to time. I think it now is time to have a closer look at the carbs, and many change the set up. I don't have the detailed set up as of today, but will post later. All good ideas and recommendations are highly appreciated:)
Magnus
Norway
83 900 SD Darmah

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