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900 Darmah sputtering around 5000 rpm

Posted: Sun Aug 26, 2018 10:50 am
by dessmo
My 900 Darmah runs nice at lower RPM, but around 5000 I am loosing one cylinder. If I accelerate hard it starts around 4500, and if I go slowly I can push it to 6000. So my conclusion is that it is fuel related. I adjusted the needle to slot 3, which made the engine run better but did not resolve the issue. Running 32mm carbs. Any tips regarding where to start is highly appreciated.

Re: 900 Darmah sputtering around 5000 rpm

Posted: Sun Aug 26, 2018 5:01 pm
by Lumpy
Someone told me once the best way to solve carby issues was to start with the ignition. If your Darmah is Bosch ignition the first place I would start is the lead that goes to your left hand engine side cover. This lead runs from the ignition pick ups inside the side cover to the Bosch ignition boxes under the tank. The insulation on these wires turns to plop and the leads short out against one another. This should be done, if it hasn't been done already, even if its not your main problem cause sooner or later it will be.
Another remote possibility is if your tank is rusty the rusty flakes and other crud gather around the strainers on top of your fuel cocks. My GTS did something similar however it would pretty much just shut down 1 pot. I would then have to pull the fuel line off and blow into it to get me a little further down the road. Love the taste of unleaded in the morning....................

Re: 900 Darmah sputtering around 5000 rpm

Posted: Sun Aug 26, 2018 5:33 pm
by wdietz186
What Lumpy said, but also check that the float levels are correct. You could be running out of fuel under load if the float level is too low. In addition to a possible clogging petcock have a look at the filter screen under the carb banjo as they can fill with rust and become blocked.

Re: 900 Darmah sputtering around 5000 rpm

Posted: Sun Aug 26, 2018 9:36 pm
by dessmo
Thanks for the tips. I forgot to mention that I recently replaced the Bosch ignition with Silent-Hektik, so I quite sure it is not the ignition. How to I check the float levels?

Re: 900 Darmah sputtering around 5000 rpm

Posted: Mon Aug 27, 2018 6:33 pm
by Lumpy
This is a blurb off Ducati meccanica that lays it out quite simply

https://www.ducatimeccanica.com/float_levels.html

And here is a handy little booklet on all round tuning and construction of Dellorto carbs

https://www.ducatimeccanica.com/dellort ... lorto.html

If your going to do some carb surgery, before you remove the carbs, turn your fuel taps off. Remove the float bowls one at a time. Hold the float up and turn the fuel on. Gently lower the float till you get a good flow of fuel and then raise it. The fuel flow should cut off cleanly. Repeat for the other carb. This will ensure your needle and seat is free to move and is also sealing. I find it easier to check these while on the bike. If alls well there carry on to float level checks.
You don't mention if there has been any work done on the carbs prior to this issue. Did the bike run well one day and not the next?? Was It a slow build up or was the problem there suddenly??

Re: 900 Darmah sputtering around 5000 rpm

Posted: Wed Aug 29, 2018 10:27 am
by dessmo
Thanks Lumpy, I will start with having a look at the floats.

Re: 900 Darmah sputtering around 5000 rpm

Posted: Sun Sep 02, 2018 4:58 am
by dessmo
Thanks again for the tips. I had a brief look at the carbs today still mounted on the bike. Measured the floats which are 24mm below the edge on the front cylinder, and 34 mm for the rear. I guess here is something wrong, they should be the same or? When I put the carbs on the table with the air filter side facing down the floats can actually stay in two positions, all the way down/open and up which i closed or close to closed. So what distance should be measured? And what is the safest way to bend the arm into the right distance?

Re: 900 Darmah sputtering around 5000 rpm

Posted: Sun Sep 02, 2018 8:55 am
by wdietz186
You need to check them with the float needle just closed and not compressing the little springy thing on the float needle. The float itself should move freely on the spindle. Scum can form on the spindle if the carbs were stored with fuel in the float bowls and it evaporated over time.

Re: 900 Darmah sputtering around 5000 rpm

Posted: Sun Sep 02, 2018 9:59 am
by dessmo
wdietz186 wrote:
Sun Sep 02, 2018 8:55 am
You need to check them with the float needle just closed and not compressing the little springy thing on the float needle. The float itself should move freely on the spindle. Scum can form on the spindle if the carbs were stored with fuel in the float bowls and it evaporated over time.
Thanks wdietz186, iow when it is closed. What is the best way to bend the float arm, do I dismount it from the carb?

Re: 900 Darmah sputtering around 5000 rpm

Posted: Tue Sep 04, 2018 5:28 pm
by wdietz186
It would be prudent to remove it before tweaking it.