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 Post subject: Weber with 34mm chokes - experiences? whodunnit?
PostPosted: Mon Dec 31, 2018 2:06 pm 
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Joined: Fri Jul 24, 2015 11:21 am
Posts: 181
Location: Belgium
model: 750 Paso
year: 1989
Hi,

I have a question: “Who has (first hand) experience with a 34mm choke/venturi in his Paso 750 Weber?”.
After having read the Weber article on the 906 Weber from Higgy and Romus, I decided to switch the standard 36mm chokes/venturis for the 34mm versions after the complete rebuild/overhaul of my Weber.

At first I decided to use the standard jet settings for my baseline (57/150/170 idle-main-air) but after having read the various articles/manuals on the Weber I doubt this is the best way to proceed.
From what I’ve read the main jet should be roughly a 0.10mm smaller if the venturi is 2mm smaller, so instead of 150 size I should use a 140 size etc. (again to start with).
The air corrector on the other hand is much more of a mystery to me: I’ve read somewhere that normally the difference between the 2 is about 30 (but to be honest in all application tables for the DCNF I’ve seen so many derogations going from 125/220 to 135/170).

For the idle jet I think I will leave it standard (size 57) since I will buy sizes from 50 to 60 anyway since they are responsible for the sub 3000 rpm region.
Also I assume the change of choke/venture size has not a huge impact on the idle jet size.

So again, all considered I would love to know if someone has actually done this exercise and has any experiences with the jetting.

I struggle to decide if I should change the auxiliary venturis for the 4.5 versions (obtained) but due to lack of experience with the 34mm chokes/venturis, I think it’s better to first wait and see how it behaves with the 3.5 auxiliary venturis and try to optimize it without the 4.5 auxiliary venturis.

Playing with the pump jet cam is an exercise which will have to wait for later (I've bought a spare cam after reading Brutus's work on the Weber's cam profile), since otherwise I will loose track as result of all changes compared to the Original configuration.

Hope for some reactions/advise from you!

Frank


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Bimota SB4-S - 1984
Ducati Paso 750 - 1989
Suzuki SV1000S - 2008
Ducati Monster 796 abs - 2010
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 Post subject: Re: Weber with 34mm chokes - experiences? whodunnit?
PostPosted: Tue Jan 08, 2019 2:26 pm 
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Joined: Fri Jul 24, 2015 11:21 am
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Location: Belgium
model: 750 Paso
year: 1989
No one? :(

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 Post subject: Re: Weber with 34mm chokes - experiences? whodunnit?
PostPosted: Wed Jan 09, 2019 11:45 pm 
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paso grand pooh-bah
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Joined: Wed Dec 05, 2007 12:35 am
Posts: 1636
Location: Newzealand
model: 906 Paso
year: 1990
:D looks like your a pioneer Frank . What is it that your hoping to achieve ?..... I'm guessing a bit better behaviour at lower revs / bit more low end torque ? Personally if I were doing this I would fit the 34mm chokes and take it for a ride on a twisty hill and see how it goes. Im thinkin that if low end improvements are the main objective changing mains at this point aint going to tell you much. When doing this stuff I find it helps to try to separate things you notice whilst on a steady throttle and things you notice whilst changing throttle position ..... and even things that happen while waiting for the motor to catch up with any changes to throttle ..... just helps (me ) theorise later when trying to make sense of what you find :)

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 Post subject: Re: Weber with 34mm chokes - experiences? whodunnit?
PostPosted: Thu Jan 10, 2019 5:32 pm 
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Joined: Fri Jul 24, 2015 11:21 am
Posts: 181
Location: Belgium
model: 750 Paso
year: 1989
What I'm trying to achieve?
Well that's a bit complicated to say in a single sentence: I've actually never ridden a Paso so far.
I just wanted to have one, so I bought it and immediately I have started to take it apart and started to restore it with the idea of bringing it back in an almost as new state.
BUT with some obvious improvements: the well known weak points as were described in lots of posts in the forum.
One of the more known one is the Weber (I did really enjoy reading the “906 improvements” from Higgy and Romus).

Here (also from other sources – books on the Webers) I got the idea that the 44 DCNF is a bit oversized for the (2 valve!) 750 engine.
Maybe not if ridden full out but I intend to use it on the public road somewhere between 2500 and 4500 rpm.
As such the estimated loss of about 2 hp at the top end is fully acceptable for me (didn’t buy the Paso for the sheer power anyway).
After all: they use the 36mm venturis also for the 900 cc engine (is 20% bigger!).
I just want it to behave as good (read: smooth) as possible in the critical transition phase.
Here the all important vacuum signal is a little stronger if the venturi is smaller (11%) and as a result I hope/expect it will function a bit better.

My “problem” lies in the fact that I need a base set up which is roughly in the ballpark.
I will have changed the electrics, ignition, carburetion etc. so I need to know it is about right.
The fine tuning I will start later, I’m aware of that (did that exercise a few times before, although with “normal” carburetors not with Webers).
I want to order a sufficient amount of jets so I will be able to change them without having to wait for the next sizes to arrive.
But I have no idea which sizes I roughly I should order (alfa1750 in Italy).
But I assumed that some of us have made the transition from 36mm venturis/chokes to 34mm, and hoped that from these experiences people could tell me which jets I would need.
But so far no luck here.
I intend to get me in stock:
Idle jets: 55, 56, 57, 58, 59 & 60
Main jets.: 135, 140, 145 & 150
Main air: 170 ???
Pump jet: 35 & 40
I will start with the F36 emulsion tube (standard 750 Europe version) and later on I will try to find me a set of F21 or F22 versions (slightly different OD between the 2 of them).

But like I said: I have no idea what the main air jets will do (standard with 36mm venturis is the 170).
So if some of you have good advise ….. just let me know.

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Bimota SB4-S - 1984
Ducati Paso 750 - 1989
Suzuki SV1000S - 2008
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 Post subject: Re: Weber with 34mm chokes - experiences? whodunnit?
PostPosted: Sun Jan 13, 2019 4:40 pm 
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paso grand pooh-bah
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Joined: Sun Jul 18, 2004 12:00 am
Posts: 5298
Location: southern Germany
model: 750 Paso
year: 1987
Frank, you're thinking way too far ahead. Imho make sure the electics is in best possible condition, add some relays and replace the coils with aftermarket ones. Replace the old HT leads and swap the electric fuel pump with a underpressure version. You probably have done all this already. That's it. Then go out and ride. Use the original jetting as starting point. Who knows you may be happy as it is. As you've said, you've never ridden it before...


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 Post subject: Re: Weber with 34mm chokes - experiences? whodunnit?
PostPosted: Mon Jan 14, 2019 9:52 am 
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Joined: Fri Jul 24, 2015 11:21 am
Posts: 181
Location: Belgium
model: 750 Paso
year: 1989
Gerhard,

I need to order some Weber parts (for my spare dcnf) including jets and would like to know which jets to order to have enough variations once the riding/tuning starts.

I assumed some people had followed up higgy's advise to use the 34mm venturis/chokes for their 750's and could advise me on the jets to be used (in particular the main air corrector jet).

Frank

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Bimota SB4-S - 1984
Ducati Paso 750 - 1989
Suzuki SV1000S - 2008
Ducati Monster 796 abs - 2010


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