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 Post subject: Carb tuning HELP
PostPosted: Fri Apr 08, 2016 12:07 am 
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Joined: Tue Dec 03, 2013 12:12 am
Posts: 68
Location: DePere, WI USA
model: 750 Paso
year: 1988
I'm in (Weber) carb hell on the 88 750.....how many turns should the air and fuel screws be turned out? Also - forgive my stupidity but which one is what? You have one on the upper (when looking at the carb) part of the carb then another on the lower part. Is the lower the fuel and the upper the air?
Thanks


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 Post subject: Re: Carb tuning HELP
PostPosted: Fri Apr 08, 2016 1:27 am 
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paso grand pooh-bah
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Joined: Wed Dec 05, 2007 12:35 am
Posts: 1633
Location: Newzealand
model: 906 Paso
year: 1990
I think the lower screws are for the idle mix and the upper ones are the air balance screws . The balance screws are to equalise air flow in the two barrels and one of them should be closed ( the highest flowing barrel ) and the other open enough to even the flow . You really need a set of vacuum gauges to set the balance and the idle mix screws should be set to get max vacuum and then open 1/8 turn ( ish ) Don't be surprized if the two mix screws need to be set slightly different and the cam timing one these bikes is known ( sometimes ) to differ between cylinders by up to 8 deg , and this is something you should at least inspect if your trying to get a really good tune job...... bit of stuffing about but you only need to do it once ( there are guides in the tech section ... Higgy and Romus did a really good tutorial , Paso cam timing , which sure is worth a look .

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 Post subject: Re: Carb tuning HELP
PostPosted: Fri Apr 08, 2016 2:18 pm 
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Joined: Wed Jul 21, 2004 12:00 am
Posts: 67
Location: Windsor, CT USA
Attachment:
dcnf44_back _view.JPG
dcnf44_back _view.JPG [ 112.61 KiB | Viewed 1940 times ]


Hopefully the attached image is visible - the screws circled in blue are the idle mixture screws, the areas circled in red are the idle air bypass screw housings (screws are recessed within).

Hope that helps!

Start with both bypass screws closed, and the mixture screws about 2.5 turns out. Get the engine started, if it won't idle, first open both mixture screws about 0.5 turns from initial setting, then try closing both mixture screws about 0.5 turns from original setting. Once you get the engine idling, slowly adjust one mixture screw at a time to get the highest idle rpm you can, then adjust the other mixture screw in the same way. Final adjustment is setting the idle speed screw to get about 1000 rpm idle speed.


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 Post subject: Re: Carb tuning HELP
PostPosted: Sat Apr 09, 2016 12:23 am 
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Joined: Tue Dec 03, 2013 12:12 am
Posts: 68
Location: DePere, WI USA
model: 750 Paso
year: 1988
jfiore wrote:
Attachment:
dcnf44_back _view.JPG


Hopefully the attached image is visible - the screws circled in blue are the idle mixture screws, the areas circled in red are the idle air bypass screw housings (screws are recessed within).

Hope that helps!

Start with both bypass screws closed, and the mixture screws about 2.5 turns out. Get the engine started, if it won't idle, first open both mixture screws about 0.5 turns from initial setting, then try closing both mixture screws about 0.5 turns from original setting. Once you get the engine idling, slowly adjust one mixture screw at a time to get the highest idle rpm you can, then adjust the other mixture screw in the same way. Final adjustment is setting the idle speed screw to get about 1000 rpm idle speed.



Yes and thank you! The issue I'm running into is that it's only running on one cylinder. It ran fine all last summer then towards the end of the season it drop a cylinder and would only run at wide open throttle. Checked spark, no issue, pulled the two jets out and blew them out with WD40 and air to make sure they were clean....ran fine for a few days then drops a cylinder again....backfiring through the exhaust and running like crap! I'll try what you've indicated and see what happens. Thanks for the help.


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 Post subject: Re: Carb tuning HELP
PostPosted: Sat Apr 09, 2016 4:48 am 
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paso grand pooh-bah
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Joined: Wed Dec 05, 2007 12:35 am
Posts: 1633
Location: Newzealand
model: 906 Paso
year: 1990
Sounds dumb but try flicking the pass light switch with the engine running and see if that has any effect

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 Post subject: Re: Carb tuning HELP
PostPosted: Sat Apr 09, 2016 1:38 pm 
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Joined: Wed Jul 21, 2004 12:00 am
Posts: 67
Location: Windsor, CT USA
You may have accumulated some ethanol sludge in the float bowl, which can block the idle circuit, since the holes and passages are so small. Try removing the idle jets and carriers, and spraying carb cleaner through the openings. You may also try adding one of the popular fuel treatments for ethanol sludge to your fuel, it can't hurt.

It also wouldn't hurt to remove the top of the carb to check the float bowl for sludge, and at the same time check the float height (should be max of 46 mm) - a float that is set too high can result in a fuel level that allows non-emulsified fuel to spill through the secondary venturis, especially into the left carb barrel when the bike is on the side stand, which will choke that cyclinder (ask me how i know...)


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 Post subject: Re: Carb tuning HELP
PostPosted: Sun Apr 10, 2016 11:27 pm 
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Joined: Tue Dec 03, 2013 12:12 am
Posts: 68
Location: DePere, WI USA
model: 750 Paso
year: 1988
Well I took everyone's advice and at least it's running on both cylinders now so thanks for all the help! With the temp only being in the mid to upper 30's I'll wait to take it for a ride and see if it runs at speed...but at least it's running :lol:

I'll report back and let the group know in a week or two.

Thanks again everyone!


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 Post subject: Re: Carb tuning HELP
PostPosted: Mon Apr 11, 2016 9:30 am 
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Joined: Sun Jan 16, 2005 12:00 am
Posts: 257
Location: The Netherlands
model: 750 Paso
year: 1989
If you have a go at the Weber, be sure to write down the current setting. I mean size of mains, aircorrectors and also how many turns on the screws on both sides of the housing.. Just in case it's an electrical problem after all, you can reset the Weber to last known working setup :thumbup:

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