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 Post subject: Re: 750 Weber woes - same same but different?
PostPosted: Mon Apr 17, 2017 3:28 pm 
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Joined: Sun Jul 30, 2006 12:00 am
Posts: 209
Location: Chicago-NW Suburb
model: 750 Paso
year: 1988
Check for voltage drop throughout the igniton circuit. I had stumbling, misses, fouling of plugs due to fouled contacts in the off/run/off switch. Biggest voltage loss was across it, and from there it feeds the coils. Coils were not getting sufficient voltage. I first tested before bringing out the vom, by cranking while in off position, and while spinning over, I flipped switch to run. It would start much better when I did that, so I opened the switch to find it grungy, and a clean up of it solved the fouling of forward cylinder plug and the stumbling and misses.


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 Post subject: Re: 750 Weber woes - same same but different?
PostPosted: Mon Apr 24, 2017 4:51 am 
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Joined: Wed Mar 02, 2016 2:52 am
Posts: 9
Location: USA
model: 750 Paso
year: 1986
Gotcha, there is quite a drop at the coil - 7.5 at both, so that must be something upstream. Will have a look at the switch.

I can confirm that it's running on the front cylinder only. The weird bit is that I get spark at both. Tried a new plug and get some intermittent back fires and an occasional stumble when I pull the known-running plug cap and try to run on just the rear. With the front plug cap pulled, wouldn't I have enough juice to run just the rear? Should probably avoid too much logic. Will do some more poking tomorrow.


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 Post subject: Re: 750 Weber woes - same same but different?
PostPosted: Mon Apr 24, 2017 7:35 am 
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paso grand pooh-bah
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Joined: Sun Jul 18, 2004 12:00 am
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Location: southern Germany
model: 750 Paso
year: 1987
It's a wonder that it runs at all with 7.5V. Also have a look at the wiring upgrade. You`ll find the info in the downloads thread on top of the P750 forum.


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 Post subject: Re: 750 Weber woes - same same but different?
PostPosted: Mon Apr 24, 2017 10:43 am 
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Joined: Wed Jul 21, 2004 12:00 am
Posts: 66
Location: Windsor, CT USA
Try cleaning the idle jets first, and run the bike for a while after the cleaning. The idle jets, being so small, tend to gum up when the bike has been sitting for a while. If that doesn't seem to help then adjust the idle mixture on both cylinders. I know that my bike can be sensitive to idle adjustment, with similar symptoms to what you describe, if the idle mixture screw is off by even 1/2 turn on one cylinder.

For reference, the idle jets are held by the screw-in carriers circled in yellow in the attached photo and the idle mixture adjustment screws are the ones circled in blue (sorry for the crappy picture quality!).

While you have the idle jets out of the carb, spray some carb cleaner through the open passages in the carb.

Also make sure that the air bypass screws are closed (circled in red), if one is open too far it may also have an effect on idle operation.


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 Post subject: Re: 750 Weber woes - same same but different?
PostPosted: Mon Apr 24, 2017 8:24 pm 
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paso grand pooh-bah
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Joined: Wed Dec 05, 2007 12:35 am
Posts: 1566
Location: Newzealand
model: 906 Paso
year: 1990
One bypass screw should be part open ..... best not to dick about with them if you don't have a set of vacuum gauges ............... and 7.5 volts is way low , get that sorted 1st , mine was showing 9v and it played up

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 Post subject: Re: 750 Weber woes - same same but different?
PostPosted: Wed Apr 26, 2017 4:00 am 
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Joined: Wed Mar 02, 2016 2:52 am
Posts: 9
Location: USA
model: 750 Paso
year: 1986
Woo, thanks guys.
So I got the relays in today and went through the pdf - thanks for that paso750. Now I get 11.5v at the coils AND BOTH CYLINDERS ARE RUNNING. Very fantastic. I haven't ridden it that much with our move this year and I suspect that the little I have done has been on one cylinder the entire time. That would explain a lot.

I also pulled and cleaned the idle jets and passages. Here's a video of it running now, with lots of smoke. https://www.youtube.com/watch?v=0ZoAnwRl3_4&feature=em-upload_owner The smoke eventually dissipated, hopefully it will keep doing so.

The smoke did clear up quickly after I pulled the air filter, wondering if it's over-jetted now. Mixture screws are out 2.5 turns and bypass screws are both closed. I have a set of vacuum gauges somewhere, will try to dig those out. Anyone have a link to a post on tuning w/ gauges?

Are the mixture screws air or fuel regulators? ie, if it's rich, do I screw in or out to meter fuel?


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 Post subject: Re: 750 Weber woes - same same but different?
PostPosted: Wed Apr 26, 2017 1:09 pm 
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Joined: Wed Jul 21, 2004 12:00 am
Posts: 66
Location: Windsor, CT USA
The idle mixture screws are fuel regulators. They screw in to make the mixture leaner. They are very sensitive, so use small increments.

The simplest way to adjust the idle is to set the idle speed to around 1000 rpm using the throttle stop, then pull one spark plug wire, adjust the idle mixture for the highest idle speed on the running cylinder, then do the same for the other cylinder, then re-adjust the idle speed back to normal via the throttle stop screw.

The air bleed screws are to adjust the vacuum balance between the cylinders at idle. If you have a vacuum gauge, all you need to do is compare the two cylinder readings, then open the air bleed screw on the cylinder with the highest vacuum to reduce the vacuum to the same level as the other cylinder. The bleed screw on the cylinder with the lowest vacuum remains closed. If a large adjustment is required (like 1 turn or more), you may need to re-adjust the idle mixture.


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 Post subject: Re: 750 Weber woes - same same but different?
PostPosted: Wed Apr 26, 2017 1:41 pm 
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Joined: Wed Jul 21, 2004 12:00 am
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Location: Windsor, CT USA
Sorry, forgot to point out that the vacuum gauge lines should be connected to the ports on each side of the carburetor, just above the outlet manifold. In the picture from the post above, one of the ports (and the blocking screw) can be seen just below the accelerator pump cam. The other one is in the same relative position on the other side of the carb.


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 Post subject: Re: 750 Weber woes - same same but different?
PostPosted: Wed Apr 26, 2017 4:52 pm 
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paso grand pooh-bah
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Joined: Sun Jul 18, 2004 12:00 am
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Location: southern Germany
model: 750 Paso
year: 1987
Imho the engine still doesn't sound completely right. (But I`m guessing it was cold and maybe running with choke) It sounds ok at idle but when you open throttle or better when you close throttle and open again it chokes a little. But that's hard to say from the sound of a video and some may disagree. It may change with fresh fuel and once you bring the engine up to temperature. Setting the carb right will do the rest.

When you get it sorted buy yourself a set of new tires!


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 Post subject: Re: 750 Weber woes - same same but different?
PostPosted: Wed Apr 26, 2017 10:32 pm 
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paso grand pooh-bah
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Joined: Wed Dec 05, 2007 12:35 am
Posts: 1566
Location: Newzealand
model: 906 Paso
year: 1990
WTH G you selling tires now ...? that's 2 in one day :lol: . I have several new tires available..... original gear ... none of this fake modern stuff :D :lol: :lol:

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 Post subject: Re: 750 Weber woes - same same but different?
PostPosted: Thu Apr 27, 2017 4:10 am 
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Joined: Wed Mar 02, 2016 2:52 am
Posts: 9
Location: USA
model: 750 Paso
year: 1986
paso750 wrote:
Imho the engine still doesn't sound completely right. (But I`m guessing it was cold and maybe running with choke) It sounds ok at idle but when you open throttle or better when you close throttle and open again it chokes a little. But that's hard to say from the sound of a video and some may disagree. It may change with fresh fuel and once you bring the engine up to temperature. Setting the carb right will do the rest.

When you get it sorted buy yourself a set of new tires!


Okey Doke - pulled the little caps off the mixture screws so I could reach them with the airbox on and cracked the rear cylinder's air bypass screw (it was a bit less insistent than the other). Pulled the plug caps and got each cylinder's idle up and back down to balance at ~1000rpm. I think they're about as close as I'm going to get them.
Thanks for the help, guys - now I just need it to stop raining long enough to ride it properly.

After I get new tires, of course. Are the Shinkos still our only real option?


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 Post subject: Re: 750 Weber woes - same same but different?
PostPosted: Thu Apr 27, 2017 9:47 am 
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paso grand pooh-bah
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Joined: Sun Jul 18, 2004 12:00 am
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Location: southern Germany
model: 750 Paso
year: 1987
I have to admit I don`t understand what you did. Did you have your vacuum meter connected? It seems to me you skipped the step of balancing the carb.
Normally you`d ride the bike and bring the engine to operating temperature. Then you`d connect both of your vacuum dials to an Y adapter and connect them to one of the hose nipples of the carb manifold to adjust the dials so they indicate identically. After that you`d connect each one separately. One to the left and one to the right side hose nipple. You close both air bleed screws (carefully to not damage their seat) and adjust the idle speed via the throttle stop if necessary. Then you open the air bleed screw of the cylinder that indicates the higher intake underpressure until both cylinders are the same. Carb balancing done. As jfiore said the other screw remains closed. After that you adjust the mixture screws as he described or better how it is explained here: http://www.carburetion.com/Weber/adjust.htm
It`s possible that after the whole procedure you have to correct idle speed again. The whole adjustment is done at idle so you may notice it may require some tweaking if it doesn`t seem correct when riding the bike.

For tires you have Shinko and Golden Tyre which is actually the same company. GT is just a brand that belongs to Shinko.

Mc tool wrote:
WTH G you selling tires now ...? that's 2 in one day :lol: . I have several new tires available..... original gear ... none of this fake modern stuff :D :lol: :lol:

Yes :) the only way to show Shinko that there is some demand and to keep them producing them is to ride Pasos and occasionally buy new tires. Michelins last production batch is meanwhile 12 years ago.


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 Post subject: Re: 750 Weber woes - same same but different?
PostPosted: Thu Apr 27, 2017 5:23 pm 
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Joined: Wed Mar 02, 2016 2:52 am
Posts: 9
Location: USA
model: 750 Paso
year: 1986
Sorry, I pulled the little shrouds around the mixture screws so I could reach them w/ the airbox on. Set idle to 1000rpm once it warmed up. Raised the rpm with a mixture screw til it leveled off, adjusted idle, did the other. Pulled the spark plug caps to make sure it would run on each cylinder. I suspect the rear cylinder has less intake pressure, so I cracked its bypass screw about 1/8 turn. I know there's no science in that, but my gauges are still buried somewhere.

As it's been the wettest winter/spring on record here in Seattle, I'm just happy that it's ready to ride around the neighborhood to bring it up to temperature and continue tuning if the sun ever decides to stay out for more than an hour at a time.

Thanks for the link, I was looking for just that kind of tuning explanation. Will report back.


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 Post subject: Re: 750 Weber woes - same same but different?
PostPosted: Thu Apr 27, 2017 5:47 pm 
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paso grand pooh-bah
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Joined: Sun Jul 18, 2004 12:00 am
Posts: 5090
Location: southern Germany
model: 750 Paso
year: 1987
I read what you did but still don`t get it ;)
You guessed that the rear cylinder creates less intake vacuum and therefore opened its air bypass screw? You have to open the bypass screw of the cylinder that creates the higher underpressure. If you`d open the one with the lower pressure the carb would be more out of balance. But then you mentioned before that the rear cylinder air bypass screw was less resistant than the one of the front cylinder. That might have been because that screw was seated meaning that the rear cylinder actually could have been the one with the higher intake vacuum this making opening its air bleed screw the right thing to do.
However I'd take the rainy days as a chance to look for your carb balancer!!!


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