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 Post subject: Weber Carb info, very detailed
PostPosted: Sat Nov 12, 2005 2:39 am 
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paso grand pooh-bah
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Joined: Mon Jul 19, 2004 12:00 am
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Location: Hudson Valley, NY
model: 750 Paso
year: 1988
This was posted on the Yahoo Paso site by Mr. Goose:

Earlier in the year I advised the group that I was going to make the Weber work right, or else! Having purchased the bike new in the year 2000 (that's another story...) I wanted to get more miles on it before modifying the carburetor. Here's how I made my 1988 Paso 750 run right:

Having determined to my satisfaction that the bike was way over-carbed with the 44mm DCNF, I decided to put on a 40mm DCNF instead. The one I used is a "40 DCNF 102", which is nearly identical but not entirely. (Just like with other Weber carburetors, the castings have been changed a bit during the production life to suit changing needs and applications. All of these carbs are/were manufactured primarily for automobile use.) If you select a "40 DCNF 102" you will see the top cover is slightly different, and so are the idle jet screws. That's about it - - oh, the choke cable pulls in the opposite direction.

I decided to re-use the plastic float and the 3.00 float chamber needle valve from Ducati's 44DCNF because I figured there was some good reason for them to switch from the differently-shaped brass float, and for using the larger-than-normal 3.00 needle. Maybe it has something to do with leaning the bike over in turns. I fitted the 40 DCNF with 32mm venturis, 4.5 long nose auxiliary venturis, 135 main jets, re-used the F44 emulsion tube, and used 190 air correctors. I continued with the 55 idle jet, the F7/110 starting jet, and the .35 accelerator pump jet. These decisons were made after months of studying several Weber books, and represent my final jetting choices.

It should be noted that my U.S. version 1988 Paso came with 34mm venturis, not 36mm as stated in the manual. All other carb jets, etc., were the same as stated in the Ducati specs. I suffered, like everyone else, from the mid-throttle "hunting" when cruising, and the gasp and hesitation when opening the throttle further. These are normal symptoms of a momentary leanness caused by too much pressure drop from a too-large carburetor barrel. Otherwise my bike as stock ran OK ( I had installed a fuel pressure regulator when new, set at 2# pressure).

For the backwards choke mechanism, I cut off the cable mount end of the bracket and rewelded it onto the other end. Likewise I removed the 44 DCNF throttle "winder" on the end of the throttle shaft and re-used it on my new 40 DCNF. Everything now fit correctly on the new carb, so it was bolted into place and it started right up on the first try.

In actuality I first used 130 mains and 180 air correctors, but the jets listed above were soon substituted. After balancing both barrels per the factory instructions, I found that the bike starts very easily and needs little choke before settling down to a nice idle. I tested the running characteristics and acceleration in hot 90F August weather here in the south of the USA, and recently tested it again in 50F October weather. The bike runs perfectly smoothly, with no mid-throttle hesitation once it is warmed up. It accelerates like a rocket right up to the redline with great power. I tested this beginning in 2nd gear, cranking on full throttle from about 3000rpm, and again in the upper gears. A friend who followed me in a very fast car said there is a wisp of richness smoke in the higher RPMs, so the carb. might need slightly larger air correctors. However I am tired of removing all the bodywork, tank, and air box at the moment, so I'll leave it as it is. I think the air/fuel ratio is real close for best power.

Before I decided to change carbs I ran a "before & after" on an engine dyno computer program, and it looks like with the 40mm Weber and 32mm venturis, one loses 1 or 2 hp at 8000rpm, but you gain 1 lb.ft. of torque. Good enough for me, since I don't ride at 8000-9000rpm.

I realize changing to a new carburetor is not inexpensive (but you can buy them cheap on Ebay), however it does solve the Paso problem. It's possible you can rejet the 44DCNF very close to my 40DCNF and get nearly as good results, however you'll probably still have some hesitation since the carb throttle plates are still tooo large. If one were to try this on a 44, you might also try the trick of drilling the small hole in the throttle plates opposite the idle running holes, which has been described by other people, and is a recommended Weber modification.

Mr. Goose

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Jon

'88 750 Paso * '87 750F1
'04 Multi 1080s * '88 650 Indy
'65 250 Monza - Moto Giro Project

Click HERE--> 750 Paso Tech FAQ
www.desmoducati.org
www.ducatisuite.com


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 Post subject: Weber Carb Set-up
PostPosted: Sat Nov 12, 2005 10:23 pm 
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Joined: Wed Jul 21, 2004 12:00 am
Posts: 66
Location: Windsor, CT USA
Mr. Goose's info seems very good, and is spot on with what I have observed in my trials on my Weber set-up. I tried to get smaller venturis for the 44 DCNF, but the smallest available for the 44 mm body seem to be the standard 36 mm ones. Going to a smaller carb body seems like a reasonable way to reduce venturi size. The only possible drawback is that the matching of the carb body diameter to the intake manifold diameter may not work as well, unless you perform a little Dremel surgery to the smaller carb body where it mates to the manifold.

Good info!


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PostPosted: Sun Nov 13, 2005 9:20 pm 
This certainly sounds like a viable alternative to hacking the bottom out of the airbox to fit a pair mikunis. I think i will look into this solution before looking at the other alternatives... Otherwise it's back to the 750ss carbs again.


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PostPosted: Fri Nov 18, 2005 2:35 pm 
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paso grand pooh-bah
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Joined: Sun Aug 01, 2004 12:00 am
Posts: 1092
Location: siena, italy
model: 750 Paso
year: 1988
Best trouble with weber carbureteurs is balancing good the mai valves.
It sould not be done at idle speed but about 1800~2000 rpm by pulling the forward handle. It happens that the valves change their balance when you pull the handle, and the best idle is not good for running.
This happens because the forward lever on the carbureteur moves the valves not at the same time.
You sould need a 2 columns balancer, instead of the usual 4 columns, because it's very VERY sharper!
Have a nice ride, Nicola.


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PostPosted: Mon Nov 21, 2005 12:54 am 
I had a mechanic ( BMW) who changed the pilot ( i believe ) and just put on a fuel regulator ( car type ) set the fule regulator to 4 lbs and it has worked fantastic always. I can more details if you need but it was cheap and easy fix plus the Weber really works.


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PostPosted: Tue Nov 22, 2005 11:27 am 
DucatiDan wrote:
I had a mechanic ( BMW) who changed the pilot ( i believe ) and just put on a fuel regulator ( car type ) set the fule regulator to 4 lbs and it has worked fantastic always. I can more details if you need but it was cheap and easy fix plus the Weber really works.
If you have any more details please provide them, as cheap alternatives arealways welcome.

Incidently does anyone know a distributor of weber parts?


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PostPosted: Tue Nov 22, 2005 5:27 pm 
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paso grand pooh-bah
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Joined: Wed Jan 19, 2005 12:00 am
Posts: 691
Location: CNY, US
model: 750 Paso
year: 1988
750pheonix wrote:
Incidently does anyone know a distributor of weber parts?


A quick Google search brings up Weber Carb Direct, WeberCarbs.com, and Pierce Manifolds - of which I have tried none ...

... but I will recommend Greenfield Imported Car Parts. Very knowledgable & helpful.

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1988 Paso 750 #753965
1997 Monster 750


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 Post subject: increasing fuel pressure
PostPosted: Wed Nov 23, 2005 12:03 am 
Hi all.
I have a beautiful 89 paso, and have fitted carbon exhausts on it. I plan to modify the webers , and have ordered a set of larger fuel jets for the carbs

The fuel pressure solution does however sound very interesting !
but:
Where is the logic in the increased fuel pressure solution ?

I thought the floater would shut off the fuel supply at any fuelpressure as soon as the chamber is full. Could it be that the higher pressure more quickly fills up the chamber? Is there any indication that the chamber itself, is starved from gas ? Could it be another solution to just increase the float shut-off level a few millimetres, maintaining the 2 psi fuel-pressure ?


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PostPosted: Wed Nov 23, 2005 1:57 pm 
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paso grand pooh-bah
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Joined: Mon Jul 19, 2004 12:00 am
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Location: Hudson Valley, NY
model: 750 Paso
year: 1988
The higher pressure bends the tab on the float, over time increasing the float height. Getting a pressure regulator and setting it to 3PSI seems to be part of the answer to some of the Weber problems.

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Jon

'88 750 Paso * '87 750F1
'04 Multi 1080s * '88 650 Indy
'65 250 Monza - Moto Giro Project

Click HERE--> 750 Paso Tech FAQ
www.desmoducati.org
www.ducatisuite.com


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 Post subject: Re: Weber Carb Set-up
PostPosted: Tue Dec 06, 2005 9:48 pm 
jfiore wrote:
The only possible drawback is that the matching of the carb body diameter to the intake manifold diameter may not work as well, unless you perform a little Dremel surgery to the smaller carb body where it mates to the manifold.


I have got hold of the weber 40 dcnf and i will post pictures somewhere once i sort out my ftp. the manifolds are a straight match. the only real pain is that the standard bolts to attach the airbox and intake are half as long as the standard weber 44's.

The other trouble is removing the floats from the carbs, seems you need a specific setup to make sure you don't break any of the aluminium (aluminum) carb body.


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PostPosted: Sun Dec 25, 2005 4:00 pm 
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paso grand pooh-bah
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Joined: Sun Aug 01, 2004 12:00 am
Posts: 1092
Location: siena, italy
model: 750 Paso
year: 1988
best help I had from fuel pressure regulator is to cut the vapour lock in fuel pipes after a long time of garage standing by (one month or more) I put my regulator in the bypass pipe, in order to let the vapour flow through the carbureteur I'm trying to make a 3way valve to tell vapour to fuel... I'll make you know how it works!
have a nice ride, Nicola


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PostPosted: Mon Feb 06, 2006 10:35 pm 
has Mr Goose been riding in these colder months?... I am interested in his long term review of his modifications to the carb setup.


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PostPosted: Tue Feb 07, 2006 12:45 am 
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paso grand pooh-bah
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Joined: Mon Jul 19, 2004 12:00 am
Posts: 1714
Location: Hudson Valley, NY
model: 750 Paso
year: 1988
I'll ask and we'll see if he answers.

Here's his response:

don't know if you saw it, but a couple months ago I reported to the Forum that the conversion to a 40mm (vs the stock 44mm) was entirely successful (and I gave all the carb settings and the installation procedure), but due to the Fall weather I wasn't able to test it in year round conditions. I have ridden it some since, but not much (it snowed here today). In the coldest I've ridden (about 45F.) I can report no hesitation once the temperature guage is above about 140F. If the needle hasn't moved off the peg (120F. or less) there is still a niggling of hesitation, but I'm more than happy as it's much less than before. (In order to get it to 140 I had
to ride it some then let it set and idle for awhile 'till the needle started to climb, and then when I rode it again the needle would soon drop back down to the peg. Summer will be a different story.)

Whether or not I can eliminate that last little bit of cold-naturedness is something I've thought about. It would probably mean either the talked-about throttle plate modifications, or some small adjustment to the air correctors or the emulsion tube. I may or may not work at it, because as you know there's quite a bit of disassembly of the bike required to get to the top of the carb, then you have to hang the tank somewhere and route some new fuel lines in order to run it on the stand.. And you have to reassemble everything again in order to take a test ride. So each bit of
testing requires all that ass and deass! I went through a lot of that getting THIS far!

"Highly recommended" is how I would describe the conversion. Hope this answers your question.

Tom Sanor

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Jon

'88 750 Paso * '87 750F1
'04 Multi 1080s * '88 650 Indy
'65 250 Monza - Moto Giro Project

Click HERE--> 750 Paso Tech FAQ
www.desmoducati.org
www.ducatisuite.com


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PostPosted: Thu Mar 02, 2006 11:16 pm 
I'm hoping this pays off... here's the kit. as you can see it's a straight fit onto the manifold. The airbox needs a little skimming off the join as there is overlap at a certain point that stops it sitting flush...

Image


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 Post subject: Weber Part Number
PostPosted: Fri Apr 07, 2006 12:22 am 
Hi All,

I have just started to learn about this Weber carb. I contacted http://www.webercarbs.com and requested info on a "40 DCNF 102" as mentioned in an earlier post. The response that I got back was "You mean a 40 DCNF 12 (There is no 102) We have them in stock. $469 for a New 40 DCNF 12. So besides the fact that my wallet is already begging me to look elsewhere am I chasing the wrong part?

Merlin


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