Topic: carb sync by the book? pt.2

[carl550k]

posted last week that i got my new carb sync tool(vacuum type) and went at it. it was a little off. i got all the cylinders to read the same, but low around 5. (the outer numbers on the gauge). i went for a ride that eve, about 40 miles . yesterday i hooked up the gauges again and to my surprise the vacuum was lower than before.the clymer manual states between 16- 24. mine are at 6, cant get the any higher.some on the previous post thought a air leak. but all 4 carbs read the same. the main reason for the carb sync was because of a racing idle. it either revs to 3000 or dies under 1200 rpms, i have to always mess with the main idle screw at stops. the bike now seems uneven at lower rpms but over 5000 it sounds great and pulls hard. i know its not the motor since the original lost a cam chain last year. the current motor is a used junkyard with only 3300 miles on it and the racing idle is still there. the carbs have been apart for cleaning and they dont leak a drop. could the carb gaskets themselves cause the air leak? cant see what else it could be. btw the bike is 76 cb 550k w/ mac 4/1, and a uni air filter. the carbs are stock original. btw mixture screws are 1/2 turn out each. thanx in advance carl

[motorcycleclaude75]

Hey carl500K,

Are you looking at the right numbers on your gauges.  What is the unit of vacuum.  Remember that the manual says 16-24 cm of Hg.

I just did my sync and the gauges indicated mm not cm so it would be 160 mm to 240 mm, usually there is another scale on the face of the gauge.

It could be 5 but a different unit.  If 5, is actually 5 cm Hg, that usually indicates a leak or something else.

I hope this makes sense.

[carl550k]

 im starting to think i am reading it wrong. the sync tool is the j c whitney vacuum type.pretty standard from what ive seen.the probabilty of an air leak is possible, but all 4 carbs the same? btw i had the gauges hooked up and bliped the throttle and the gauges move together all the same readings. now all the carbs read 6 on the outer numbers. i must be reading it wrong.

[82CB650SC]

The outside numbers are inches and the inside numbers are mm on this gauge
http://www.jcwhitney.com/autoparts/Product/tf-Browse/s-10101/Pr-p_Product.CATENTRY_ID:2004169/p-2004169/N-111+10211+600014252/c-10111

[82CB650SC]

a 6 would be about 15cm or 150mm

[carl550k]

 thanks for the replys guys. im sitting here looking at the gauges on the carb sync tool and bob you are right i was looking at the wrong numbers. the numbers on the inside read mm vacuum.100,200,300,etc.is there a formula to convert this to cm/hg or just lose a 0?

[cmorgan47]

drop a zero. 
that's the beauty of metric.

i appologize on behalf of my generation as we were supposed to be the ones to switch over and we apparently dropped the ball.