1988 Paso 750 airbox gasket placement and accelerator pump lever pivot point

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murphus
Posts: 73
Joined: Wed Jun 24, 2020 6:27 pm
model: 750 Paso
year: 1988
Location: USA

1988 Paso 750 airbox gasket placement and accelerator pump lever pivot point

Post by murphus »

I'm back to trying to fit the Weber on my Paso, and a tear-down has uncovered a few possible issues including incomplete cleaning, and the throttle shaft is bent. Looking at how the airbox mounts to the carburetor, it's unclear to me where the gasket for the airbox is supposed to sit. Is it supposed to be between the airbox and the top of the carburetor, or inside the airbox between the top of the airbox and the air horn? Neither spot actually makes much sense, as the gasket only seals the periphery of whichever two pieces it's between. I hadn't mounted the airbox when I initially tried running the Weber, and realize now that the airbox is specifically drilled/molded to segregate the chokes from the air correctors jets and starter jets. I hadn't realized the importance of this until reading a post by jayh (http://www.forums.ducatipaso.org/viewto ... 5&start=15). Without the airbox mounted the air corrector jets are potentially under a vacuum from the choke (throttle bore) air intake. Based on that, the gasket seems irrelevant and even unwanted in either position, and it seems clear the airbox needs to be mounted for proper running and any running adjustments.
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Further, I ended up with two 44 DCMF carbs. The original has damage from the fuel/air screws being turned in so tight they actually deformed/blew out the interior wall of the throttle bores. The used unit I picked up appears to be identical, save for sealed air-bypass screws (I'm assuming it was fitted to either a later or California emissions bike? It was easy to remove the adjustment locks), and the pivot point for the accelerator pump lever is different. The carb that came with the bike has the pivot in location 1, or closer to the accelerator pump.. The pivot in the used carb is in position 2, farther away from the accelerator pump. Does the point of pivot matter in operation? For a given turn of the throttle shaft position 2 would give a longer stroke/more gas.
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1973 BMW R75/5
1976 Suzuki GT185
1983 Lavarda RGS 1000
1988 Ducati Paso 750
1995 BMW K75
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paso750
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Joined: Sun Jul 18, 2004 12:00 am
model: 750 Paso
year: 1987
Location: southern Germany

Re: 1988 Paso 750 airbox gasket placement and accelerator pump lever pivot point

Post by paso750 »

The gasket is positioned between the Weber and airbox as can be seen in the parts catalog.
The screws of the throttle plates are normally peened so they won't come loose. I'm guessing that this was done w/o placing something underneath as a support. Hence the bent throttle shaft.
Do both Webers have the identical numbers stamped on the float chamber? If not differences are i.e. the number and size of the progression holes.
The different pivot position changes the lever length. It is as you have explained. Longer lever, more stroke, more fuel.
murphus
Posts: 73
Joined: Wed Jun 24, 2020 6:27 pm
model: 750 Paso
year: 1988
Location: USA

Re: 1988 Paso 750 airbox gasket placement and accelerator pump lever pivot point

Post by murphus »

Yes, the parts catalog does indeed show that, which I somehow missed, thanks. And yes, both Webers have the same numbers stamped on the float: 44 DCNF 110 7H. As to the accelerator pump lever, I'm just curious if one position is better than the other. I would think more stroke would be better, but ...
Thanks,
Richard
1973 BMW R75/5
1976 Suzuki GT185
1983 Lavarda RGS 1000
1988 Ducati Paso 750
1995 BMW K75
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paso750
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Joined: Sun Jul 18, 2004 12:00 am
model: 750 Paso
year: 1987
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Re: 1988 Paso 750 airbox gasket placement and accelerator pump lever pivot point

Post by paso750 »

If the pivot point of the accelerator pump arm is in a different position there might be other differences. Did you compare all the jets? 110 is the number of both the US and California version Weber. However the jetting is not the same and the Cali version has a 36mm choke venturi while it's a 34 in the US version. It might be that the accelerator pump with the longer stroke is from a Weber with a leaner jetting. More stroke isn't better. On hard acceleration too much fuel will cause a hesitation (as will too little).
Mc tool
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model: 906 Paso
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Re: 1988 Paso 750 airbox gasket placement and accelerator pump lever pivot point

Post by Mc tool »

The accelerator pump timing , duration and fuel volume has a major influence on throttle opening behavior , which is one of the main bugs with the weber carb in this application ..... flat spots and bogging when applying the throttle . The main reason for fitting Mikunis or Del Ortos
Some forum members were putting a lot of thought and effort fiddling with this and were modifying pump cam profiles and jets and bleed orifices . Some brave souls even claimed to have cured the infamous " weber stumble " . this was a few years ago so you might want to search a bit , but its a real can of worms
I wish I was young again............Id be heaps smarter than last time
murphus
Posts: 73
Joined: Wed Jun 24, 2020 6:27 pm
model: 750 Paso
year: 1988
Location: USA

Re: 1988 Paso 750 airbox gasket placement and accelerator pump lever pivot point

Post by murphus »

I think you're right, Mc tool, it does seem to be a can of worms, but for some reason I'm compelled to see if I can make it work, in large measure because I actually like the Weber installation better. The dual Mikuni TM flat slides it's currently wearing work great, but the only air filters I can fit are oiled foam filters, which I kinda hate. On the upside, it runs great, an obvious advantage ...

paso750, if you're interested, here's how the carbs were jetted (never wrote down the starter jet numbers but I believe they were the same).

Weber #1 is what came with the bike and was claimed original, 1988 Federal model, sold new in New York. It had F36 emulsion tubes, 170 air jets, 145 main jets, 50 idle jets, 3.5 auxiliary venturis, 34mm chokes.
Weber #2 is from an unknown bike. It had F44 emulsion tubes but had the same air jets, main jets, idle jets, auxiliary venturis and 34mm chokes as #1. Other differences were the accelerator pump arm pin in position 2, giving a shorter stroke, and air bypass screws with anti-tamper caps.

It's going back together following Higgy & Romus' suggestions with F27 emulsion tubes, 150 main jets, 60 idle jets, and 4.5 auxiliary venturis.

I'm swapping the throttle shaft from #1 in place of the noticably bent shaft in #2. I had previously replaced the throttle shaft bearings in #2 and I'm certain I did not bend the shaft because I did not stake the throttle plate screws. I loc-tited them and squeezed them with vise-grips. No pounding involved. Either the shaft was bent at the factory or someone's been in there before. I should have noticed it the first time around, but somehow didn't. Curiously, the throttle shaft in #1 is very slightly bent. #2 will install, but takes a minor positioning effort to get it to pass through all 3 support point,s and you can feel it binding when fully rotated. The shaft from #1 installs cleanly, and turns freely and easily once in place.
1973 BMW R75/5
1976 Suzuki GT185
1983 Lavarda RGS 1000
1988 Ducati Paso 750
1995 BMW K75
Mc tool
paso grand pooh-bah
Posts: 1811
Joined: Wed Dec 05, 2007 12:35 am
model: 906 Paso
year: 1990
Location: Newzealand

Re: 1988 Paso 750 airbox gasket placement and accelerator pump lever pivot point

Post by Mc tool »

Oh I do understand the compulsion to try and fix the weber , mostly coz at full noise they are great and also because , to my mind , its just a tuning issue and thusly shouldnt be un-fixable , chuck in a bit of OCD and you'll be up all night :D :D
There are a couple of books by John Passini on weber carbs that are worth a read . ISBN #'s
0-903-192-40-3 Weber carburetors 1 theory
0-85113-0607 Weber carburetors 2 tuning and maintainance
older books but still a good read.
I wish I was young again............Id be heaps smarter than last time
murphus
Posts: 73
Joined: Wed Jun 24, 2020 6:27 pm
model: 750 Paso
year: 1988
Location: USA

Re: 1988 Paso 750 airbox gasket placement and accelerator pump lever pivot point

Post by murphus »

Yeah, there's the rub; OCD and up all night. But hey, keeps me out of the bars. Well, that and COVID. Anyways, thanks for the Passini push, I don't have either of those. Used copies appear to be readily available and cheap at around $15 (Amazon lists a new hardcover copy for $482.59!!). More good material for the moto library, thanks again.
1973 BMW R75/5
1976 Suzuki GT185
1983 Lavarda RGS 1000
1988 Ducati Paso 750
1995 BMW K75
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