strange vacuum reading

at idle the vacuum straight off of one of the carbs bounces around from 0-10 psi does this indicate a vacuum leak"?

Don't know if this will help, but believe it or not that when I am synchronizing carbs I find that low mercury level (low pressure) or bouncing levels usually tells me the bike is running lean. Not sure why but it just seems that way.

Usually a fluctuation in vacuum readings will indicate a valve problem ie: broken spring.

As the slide opens for a carburetor, less restriction occurs in the vacuum path. This decreases the the mercury level. The slide is open too far if you have real low vacuum levels... But.... It should be pretty even level, not bouncing. The bouncing occurs when there is a vacuum leak somewhere; generally the pilot system. If you have o-rings on your mixture screws, that would be a good place to start looking. You might also have problems with the intake manifolds.

Someone mentioned "lean" and this is a true statement. The lean can be caused by the slide being open somewhat too far relative to the others or potentially a misadjusted air mixture screw (badly misadjusted), or even the pilot fuel mixture screws not screwed out far enough... If you set the mixture screws per factory recommendation prior to synchronizing your carbs this generally isn't an issue.

The last comment was regarding broken valve springs causing vacuum fluctuation... hmmm if one of the two valve springs were broken, I suspect it would set up a racket for sure. It would also have the effect of not closing the valve quickly and some loss of comprssion and thus vacuum and a low level would surely result. All great tips/pointers.

Having been an auto tech for almost 20 years, I have experience vacuum fluctuation caused by a broken or weak valve spring. I did not hear any sort of noise resulting from one of the two, but did notice a rapid fluctation on my vacuum gauge, especially when the engine was above idle. Low compression readings are not always common, but are very possible due to the valve not seating completely.

Are you using a round dial vacuum gauge? If so, is there a restrictor in the hose between the vacuum gauge and the intake manifold? The vacuum reading will pulsate unless there is a restrictor installed in the vacuum hose. A small brass aquarium valve can also be used to dampen the pulsation.

Ron



Post edited by: ron g, at: 2005/10/25 20:40

One more picture of the hose restrictor.

Ron

hmm that would explain it i am using a 10 dollar gauge and regular vacuum tubing.

could anyone give me a sure fire way to adjust the air screws?

I have a spare o2 sensor and a multimeter sitting around could i just shove it in the exhaust pipe and tune it that way?

I have a spare o2 sensor and a multimeter sitting around could i just shove it in the exhaust pipe and tune it that way?

I'm new to bikes after some 35 year hiatus but from my autocentric mechanicals POV, no.
Unless you understand how the sensor you plan to use works.
I doubt you could get any meaningful information from the older 'unheated' sensors.

could anyone give me a sure fire way to adjust the air screws?

You're adjusting idle mix, right?
You have a twin, right?

Back in the day we would....
Pull one spark plug. Get her idling on the other cylinder and adjust it's mix for fastest idle then make it just a hair richer. Do the same for the other jug.

On fours, I have found the only method that satisfies is as follows.
1)Sync carbs using a round guage pin that opens butterfly about 1/4 to 1/3 of the way. Set linkage screws until they all have the same clearance.
2)set the idle screws at factory recommended or forum recommended number of turns. Turn 'em all in and out at the same time in 1/4 turn increments until idle speed peaks and then adjust 1/4 turn in rich direction.
3)check vacuums. If they are not even, then I have another problem to solve.
4)let her idle a while and check plugs to see if even and not too rich. If plugs are not even, I have another problem to solve.

I know this diviates somewhat from forum recommended practice, but it's been working for me.

-Duck

Post edited by: Duck, at: 2005/10/26 08:12

Post edited by: Duck, at: 2005/10/26 08:12

Post edited by: Duck, at: 2005/10/26 08:23

what can I use to make my current gauge work?

dustinleavitt wrote:

what can I use to make my current gauge work?

the aquarium valve as ron suggested above would work fine. another idea is a pneumatic flow control valve from an industrial supplier...thats what i used on a 4 gauge setup i made... they swung wildly without any kind of damper in the lines....steve