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VHB 30 Altitude Carb Adjustments

G

Gorpon73

Just got it firing!
Joined
Apr 21, 2015
Messages
2
Location
Bend, OR
Hello,
I'll ask you to pardon me in advance if the information I'm after is located elsewhere on the forum as I am admittedly a noob to the site (but now Guzzi's).
I've recently acquired a '75 850 which comes from sea level altitude and I live at approximately 3,500 feet. The bike was running very poorly so I went through and installed electronic ignition and refreshed everything else on the ignition side (plugs, wires, coils). This always seemed to solve the problem with previous bikes, however, I've never lived at altitude with previous bikes. With the timing slightly advanced and mixture screw adjusted, the bike runs ok, but not very well (no power from idle to about 3,500 rpm and generally rough and weak thereafter).
My next step is to re-jet the carbs to a smaller size due to the altitude. However, I'm not sure how small to go. Is there any reference for altitude vs. jet size for these carbs? Any recommendations are appreciated.
Cheers
 
Adjustments are pretty much trial and error while checking plug condition. Unless someone has a similar bike at the same altitude and has already figured it out. I'd start by changing only one circuit at a time. First set the timing to stock specification. Then if the idle mix screw is screwed out less than 2.5 turns, go to the next smaller pilot. Keep going down until you get a good idle at about 2.5 turns out. For transition, start by lowering the needle, that will lean the mix some. If you go all the way down and it still isn't good you may need to go to a fatter needle. There are resources online for the carb needle profiles. Main jet only comes fully into play at wide open, that will be the last jet to change.
 
Firstly, I would get a complete carby kit, replace every 'O' ring gasket etc. Stop all the air leaks.
2nd. If the needles and or needle jet are worn, replace both sets. Otherwise runs real fat in the mid range.
3rd. raise/lower needles.
4th. Square slide 30m Dellorto's had no alternate main jets. Thread pitch was different to other Dellorto's, but
the Yamaha XJ series 550/650/750/900 have the same sizes, so you can get any main jets you need.
 
From my experience the square slide Dellorto's mains are the same pitch and diameter as the pilot jet. They are available from dealers, and if that fails, http://www.herdan.com/ Just tell them it is for a square slide.
 
Hi,

Many thanks for all the replies! Very helpful. It was my feeling this would be a trial and error process.

My idle seems ok at about 2.5 rotations, so my strategy will be carb rebuild with adjustments to the needle first to evaluate whether new jets will be required.

I'll post the final settings in case its helpful to anyone else with a similar setup/elevation.

Thanks again for the recommendations.
 
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