Weber DCNF fuel pressure issues

discussions specific to the 906 Paso
Fulvia-hf
Posts: 18
Joined: Sun Jun 06, 2021 1:19 pm
model: 906 Paso
year: 1989
Location: Netherlands

Weber DCNF fuel pressure issues

Post by Fulvia-hf »

The Weber DCNF was always considered troublesome for our L-twin engines. Some problems were related to fuel pressure However as I see it it is a very simple and effective system as used on many cars the DCNF was fitted to. Fuel tap, filter, pump, T-piece splitting the supply to the carburator and the return line. Overpressure is prevented due to the installation of the return line.

Can somebody explain what is the problem with this system and what can be done to improve it?

Also I wonder if the pump needs to be running al the time. It seems to be the case due to the return line so the pump never gets enough back pressure to make it stop. But I cannot remember the pump always running on the 750paso I had 25 years back.

Cheers,

William
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paso750
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Re: Weber DCNF fuel pressure issues

Post by paso750 »

The problem is that the fuel pump is too strong. You can try to reduce pressure by removing the jet that is installed in the Y-piece. It sits in the exit for the return hose. Next option would be to install a fuel pressure regulator. I personally don't see the sense in adding another item. Better get rid of the electric fuel pump and install a vacuum pump from a later Ducati. Those work fine and also only when the engine is turning.
Fulvia-hf
Posts: 18
Joined: Sun Jun 06, 2021 1:19 pm
model: 906 Paso
year: 1989
Location: Netherlands

Re: Weber DCNF fuel pressure issues

Post by Fulvia-hf »

Thank you for your reply. is there a calibrated orifice in the Y piece? I bought a new Y piece from Stein Dinse and did not notice one 'branch' having a smaller hole. Or is the restriction in the fuel line or in the fuel tank connection?
In any case I have ordered a Yamaha TXR mechanical fuel pump so all this does not really matter anymore.
I think when installing the yamaha pump I need to plug the return line?

Cheers,

William
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paso750
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Re: Weber DCNF fuel pressure issues

Post by paso750 »

I kept the return line but you probably don't need it. The Monster doesn't have one.

You obviously didn't have a look at the parts catalog. :D There's a Dell'Orto main jet screwed into one exit of the Y-piece. It's size 120. Sometimes if the fuel lines incl. Y piece are replaced it disappears. Other times when the old fuel hoses have become hard and are pulled off the Y-piece the jet is pulled out, too and is thrown away with the hoses.

#24 in the pic below
Image
Fulvia-hf
Posts: 18
Joined: Sun Jun 06, 2021 1:19 pm
model: 906 Paso
year: 1989
Location: Netherlands

Re: Weber DCNF fuel pressure issues

Post by Fulvia-hf »

I did have a look at the parts catalogue and it strikes me that I missed it;-) Must start to use my reading glasses.
Thank you very much for pointing it out though. This explains the system for me. There had to be a restriction in the return line. And here it is! A rather crude solution it has to be said.
I replaced the old OEM hoses including the Y piece and threw them away. As I will fit the TRX pump needing no return I will plug the return line completely using a bolt shank or the like.

Cheers,

William
Fulvia-hf
Posts: 18
Joined: Sun Jun 06, 2021 1:19 pm
model: 906 Paso
year: 1989
Location: Netherlands

Re: Weber DCNF fuel pressure issues

Post by Fulvia-hf »

Another question, is a vacuum port in one manifold leg enough to power the Yamaha pump? I have replaced one of the two Alen bolts which fixes the bracket to the manifold, which in turn holds the throttle cable, with an M6 Phillips head screw in which I drilled a 2.5mm through hole. The 6mm thread in the manifold already goes through all the way. If this works it is a simple solution.
Both existing vacuum ports are already in use by the CDI.
Last edited by Fulvia-hf on Wed Sep 08, 2021 3:14 pm, edited 1 time in total.
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paso750
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Re: Weber DCNF fuel pressure issues

Post by paso750 »

The answer to that question is in the parts catalog :cool: (of the Monster or TRX/TDM). Yes, connecting to one intake manifold is enough.
Fulvia-hf
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Joined: Sun Jun 06, 2021 1:19 pm
model: 906 Paso
year: 1989
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Re: Weber DCNF fuel pressure issues

Post by Fulvia-hf »

Just came back from a test ride. The TRX pump works flawlessly. A few issues need addressing. The temp gauge indicates between 90 and 100 deg. is that normal?
When the engine is very hot (fan comes on occasionally) the rev drop slower to idle when releasing the throttle than when the engine is less hot. What can this be?
Tyre pressure seems not Ok.
What is the recommended tyre pressure for the Shinko's?
Brakes are not very sharp. It still has the OEM rubber lines. What pads are recommended?
Slight vibration through the bars. My hands start to tingle after 20km's or so. Can this be a little bit ironed out by synchronizing the Weber?

the rest is fine and the bike gives me the pleasure my old 750 gave me 25 years ago. So money well spend!
Fulvia-hf
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Joined: Sun Jun 06, 2021 1:19 pm
model: 906 Paso
year: 1989
Location: Netherlands

Re: Weber DCNF fuel pressure issues

Post by Fulvia-hf »

Restored the water temp by topping up. It seems it takes a lot of time to get all the air out.
I have set the tyres at 2.4 front and 2.7 rear (nitrogen). Also the Marzocchi duo-shock reservoir was empty so I had that filled up with nitrogen also to about 70psi. Makes quite a difference!
Synching the Weber obviously has no impact on vibration off idle. I will try to set the mixture screws when I have the fuel tank off someday. But first a nice multi day trio through the german Sauerland region next week. looking forward to it!
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paso750
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Re: Weber DCNF fuel pressure issues

Post by paso750 »

Brake hoses are recommended to be changed every 6-8 years (which noone does). The ones from the Paso are so old they can start to dissolve internally. In worst case and under hard braking the brakes then can lock up. Ideally noone is still using them. There's a date code on the brake hoses btw. You get an improvement by replacing them with modern steel braided ones. Actually, if you're interested I may still have a set from Brembo that I once installed but then never used as I upgraded the calipers and then had to replace the hoses with longer ones.

Sintered pads are generally not recommended in combination with cast iron rotors as they generate more heat and can wear the rotors quicker but they do have more bite. And regarding a potential stronger wear. Most Pasos aren't ridden that much anyway, so this may not even get noticed. However, there's a huge amount of organic brake pads on the market as well.

Try to ride until you feel the vibration the strongest, then pull the clutch (obviously while closing throttle), let the bike roll and see if the vibration is gone or not. If it's still there it's from the suspension, if it's gone it's from the engine.
Fulvia-hf
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Joined: Sun Jun 06, 2021 1:19 pm
model: 906 Paso
year: 1989
Location: Netherlands

Re: Weber DCNF fuel pressure issues

Post by Fulvia-hf »

Brake lines are on the to-do list. I will search for black lines and preferably made by a shop who wants to transfer the plastic cone shaped covers from my old lines to the new.
I will go for a set of organic (disk friendly) pads as I have the rare full floater disks.
The vibration is most present around 5000rpm and disappears when I pull the clutch.
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paso750
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Re: Weber DCNF fuel pressure issues

Post by paso750 »

Keep an eye on that vibration. I had vibrations that became stronger over time. On the Autobahn also the feet would start to get numb as at a certain rpm the footpegs started to vibrate. It was a crankshaft bearing that was on its way out.
Fulvia-hf
Posts: 18
Joined: Sun Jun 06, 2021 1:19 pm
model: 906 Paso
year: 1989
Location: Netherlands

Re: Weber DCNF fuel pressure issues

Post by Fulvia-hf »

Today I rode the bike for 210Km and it was good. Brakes can be better and I think the vacuum TRX pump needs a second vacuum port. When I ride the bike with low rpms and a small throttle opening it can occur that the bike starves. Opening and closing the throttle a couple of times restores everything and if I'm fast with this the engines does not stall. Vibrations are acceptable. What is new to me that there seems to be a false neutral between all the gears. Is this normal for the 906?
Fulvia-hf
Posts: 18
Joined: Sun Jun 06, 2021 1:19 pm
model: 906 Paso
year: 1989
Location: Netherlands

Re: Weber DCNF fuel pressure issues

Post by Fulvia-hf »

I just came back from a 1200Km tour through the Deutsch Sauerland region. I had two problems needing mending at the hotel and a day later at the roadside. First the clutch was slipping when accelerating up hill. As holland has no hills to speak of I had not noticed this before in the few months a had this bike. Turned out one disk was complete worn and the adjacent steel plate had started to dissolve into metal dust as well. Luckily Witten und Weber in Bad Berleburg came to the rescue. From their extensive used clutch plate collection I could pick just what I needed to repack the clutch and continue our journey. No charge. Top shop and their collection of bikes is stunning.
The next day I noticed that I lost power, again going uphill. One cilinder was not fully participating. We stripped the 906 from all her clothes by the roadside and found a blocked main jet. Never had that before. Especially with a main as big as this one. But having removed the dirt and after replacing the (new) fuel filter the bike ran flawlessly for the next 600km's. A problem to have fixed later is a slight wobble in the front rim. Very annoying around 50Kph and again somewhere between 100 and 140kph. The brakes can be more powerful and the front fork compression damping is harsh. Oil weight is 8 so maybe a change of the oil in the right leg to 5 will soften everything up a little.
Cheers,
william
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Re: Weber DCNF fuel pressure issues

Post by higgy »

Sounds like charging system issues. Very common on our bikes. Start by checking all the grounds for good uncorroded connections. Then check all connections between the regulator and stator.
If you are very lucky that should do it. If not, welcome to the lad of the Paso.
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