US Spec P750 Weber Recalibration

discussions specific to the 750 Paso

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Mike's bikes
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model: 750 Paso
year: 1988
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US Spec P750 Weber Recalibration

Post by Mike's bikes »

I know that this is a well worn subject, but I have not seen a report of successfully recalibrating the originally equipped Weber carburetor on a US spec P750 model. I have gotten mine to run very well using only currently available Weber jets and parts, with no soldering or drilling of emulsion tube holes. I thought this might be useful information or at least another data point for others.

Mine is a standard 1988 US specification P750 with the originally equipped mufflers, airbox and air filter element. Here are the changes from the original calibration:

main jet changed from 145 to 150
idle jet changed from 55 to 60. (edit:. Changed to size 57 due to hot weather richness)
auxiliary venturi changed from 3.5 to 4.5 (Weber part number 71102.450 w/tube height identical to original, not the extra tall version)
plastic float set to 43mm height
fuel pressure set to 2.5psi
All other jets and settings are per original US spec.

Before making the changes, I suffered the same complaints as most other Paso owners; hesitation and flat power on acceleration at 2500 RPM and while cruising from 4000 to 6500 RPM. Now, once warmed up, the bike runs as well as any carbureted motorcycle I have had. Gerhard has reminded me that the US spec P750s have more restrictive mufflers than Euro models and that the US P750 carburetor has a 34mm venturi vs. 36mm for the Euro model. Therefore, my calibration may be useful only for US spec P750s.
Last edited by Mike's bikes on Sat Aug 21, 2021 4:41 pm, edited 1 time in total.
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paso750
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Re: US Spec P750 Weber Recalibration

Post by paso750 »

Thanks for the info :thumbup:
and that the US P750 carburetor has a 34mm venturi vs. 36mm for the Euro model
That I didn't say to you. ;) However I posted that info a while back as that is what the parts manual states. Not entirely sure that this info is correct as mine is a US bike with the 44 DCNF 110 and it has a 36 venturi. But who knows if one of the past owners changed it.
Mike's bikes
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Joined: Tue May 04, 2021 1:25 pm
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Re: US Spec P750 Weber Recalibration

Post by Mike's bikes »

Thank you, Gerhard, , I thought the differences between US and Euro spec carbs were the jet and venturi sizes. My venturi definitely measures 34mm ID, though I don't remember what number is stamped into the carb body.
The unreliable nature of printed specifications is why I refer to such information as merely "data points".
Last edited by Mike's bikes on Sat May 08, 2021 6:44 pm, edited 1 time in total.
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Frank.61
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Re: US Spec P750 Weber Recalibration

Post by Frank.61 »

Hi Mike,

1 question: which emulsion tubes does yours have?

Frank
Bimota SB4-S - 1984
Ducati Paso 750 - 1989
Suzuki SV1000S - 2008
Ducati Monster 796 abs - 2010
murphus
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Re: US Spec P750 Weber Recalibration

Post by murphus »

Thanks for posting the info. I read Romus and Higgy's writeup and using the tall 4.5 venturi, interesting that you've apparently had good success using the "standard" 4.5, which is readily available, whereas the tall 4.5 is not, at least near as I can find. I'd really like to see if I can get my '88 Paso working on the original Weber. It's set up with 38mm Mikuni TM-style flat slides right now, and while they seem to be OK, I'd like to return the bike to stock. From everything I'm reading, it appears more than possible to get the Weber to work properly.
1973 BMW R75/5
1974 Laverda SF2
1976 Suzuki GT185
1983 Lavarda RGS 1000
1988 Ducati Paso 750
1995 BMW K75
Mike's bikes
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Joined: Tue May 04, 2021 1:25 pm
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Re: US Spec P750 Weber Recalibration

Post by Mike's bikes »

Frank.61 wrote:Hi Mike,

1 question: which emulsion tubes does yours have?

Frank
F44 emulsion tubes
Mike's bikes
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Re: US Spec P750 Weber Recalibration

Post by Mike's bikes »

murphus wrote:Thanks for posting the info. I read Romus and Higgy's writeup and using the tall 4.5 venturi, interesting that you've apparently had good success using the "standard" 4.5, which is readily available, whereas the tall 4.5 is not, at least near as I can find. I'd really like to see if I can get my '88 Paso working on the original Weber. It's set up with 38mm Mikuni TM-style flat slides right now, and while they seem to be OK, I'd like to return the bike to stock. From everything I'm reading, it appears more than possible to get the Weber to work properly.
I was not able to find the extra tall 4.5 auxiliary venturi but the "tall" version, matching the original tube height, was available through Redline (edit: it was Pierce Manifolds, not Redline) and seems to have done the job. I did not try changing only the jets with the original 3.5 venturi as I believe that path had been tried without satisfaction by others. Having said that, now flushed with success I might try just the jet changes first.
Last edited by Mike's bikes on Wed May 12, 2021 9:16 pm, edited 1 time in total.
murphus
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Re: US Spec P750 Weber Recalibration

Post by murphus »

Did you have any trouble removing the auxiliary venturi? The Weber manual says to carefully file away any staking marks before removal, but I don't quite see how that's done as they're small and hard to reach. It seems a heat gun might help?
1973 BMW R75/5
1974 Laverda SF2
1976 Suzuki GT185
1983 Lavarda RGS 1000
1988 Ducati Paso 750
1995 BMW K75
Mike's bikes
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Joined: Tue May 04, 2021 1:25 pm
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year: 1988
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Re: US Spec P750 Weber Recalibration

Post by Mike's bikes »

I used a 3-corner scraper and then an exacto knife to remove the staked material. The auxiliary venturi came out fairly easily using a small aluminium drift against the support arms, working side to side.
murphus
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Re: US Spec P750 Weber Recalibration

Post by murphus »

Thanks for the tip. Stripping the Weber down shows - no surprise - that someone's been in it before. A small section is broken out of one of the auxiliary venturis at the top. Further, one of the emulsion tubes is broken at the top where the air jet fits over it, and both main jets show marks from, I assume, being pulled from the emulsion tubes. Which brings me to another question: The Weber manual says to "reform" the main jet seating at the bottom of the emulsion tube using the special Weber tool. I'm not clear what's being done, and is it necessary when installing new emulsion tubes and jets?
1973 BMW R75/5
1974 Laverda SF2
1976 Suzuki GT185
1983 Lavarda RGS 1000
1988 Ducati Paso 750
1995 BMW K75
Mike's bikes
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Joined: Tue May 04, 2021 1:25 pm
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Re: US Spec P750 Weber Recalibration

Post by Mike's bikes »

I do not know what the Weber manual means by "reform". I had no trouble separating the main jet from the emulsion tube ... it just pulled off and the emulsion tube was undamaged. I didn't need anything but a couple of pair of pliers to lightly grab the e-tube and jet. My bike has low mileage, I could guess that less well preserved bikes have corrosion locking the jet to the emulsion tube.

If the parts don't separate easily, you might want to soak the e-tube and jet assembly in penetrating oil.
Mike's bikes
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Joined: Tue May 04, 2021 1:25 pm
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Re: US Spec P750 Weber Recalibration

Post by Mike's bikes »

murphus wrote: The Weber manual says to "reform" the main jet seating at the bottom of the emulsion tube using the special Weber tool. I'm not clear what's being done, and is it necessary when installing new emulsion tubes and jets?
Another thought, the manual may be referring to reforming the cone shaped sealing face at the bottom of the emulsion tube bore in the carburetor casting, in the event that it is pitted or otherwise damaged.
murphus
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Re: US Spec P750 Weber Recalibration

Post by murphus »

I think you're exactly correct. It sounds as if there was/is a forming tool matching the cone shape of the jet used to assure proper seating. Interesting. I wonder how one confirms proper seating without it? Maybe a little Prussian blue dye and push the tube with jet into place and see if there's full contact? Anyway, thanks. I'm getting more and more intrigued by the idea of getting the Weber into operational shape and seeing if I can get the Paso to run well with it. The previous owner couldn't, but looking at the condition of things it's almost no wonder. And I don't think the original Weber has more than a few thousand miles on it.
1973 BMW R75/5
1974 Laverda SF2
1976 Suzuki GT185
1983 Lavarda RGS 1000
1988 Ducati Paso 750
1995 BMW K75
Mike's bikes
Posts: 11
Joined: Tue May 04, 2021 1:25 pm
model: 750 Paso
year: 1988
Location: USA

Re: US Spec P750 Weber Recalibration

Post by Mike's bikes »

murphus, have you measured your main venturi ID? Whether it is 34 or 36mm may make a difference in the jetting.
murphus
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year: 1988
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Re: US Spec P750 Weber Recalibration

Post by murphus »

Haven't yet as I've yet to remove the auxiliary venturis, but will report back when I do. Will be curious to see what size it actually is.
1973 BMW R75/5
1974 Laverda SF2
1976 Suzuki GT185
1983 Lavarda RGS 1000
1988 Ducati Paso 750
1995 BMW K75
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