Vac measurement in cmHg or mmHg

I didn't see an actual value in the service manual for this. What is supposed to be the stock range, in mmHg or cmHg that you should see when doing the sync.

I think off the top of head I was getting 18 cmHg or 180 mmHg last sync. Some googling turns up that 22-24 cmHg is the sort of norm on some bikes.

Just curious if we might be able to track down leaks based on this value.

Thanks!

 

my Haynes manual shows 185mm Hg plus or minus 10. To tell you the truth, just get them to measure the same. If you cannot get them all close I would believe the outlier is your leak.

 

I can get them even, and I know it's common practice to ignore the value and just get them even. I was just wondering more/less what should be an average measurement.

Can anyone else confirm 185 to be an average reading?

Thanks for your input dmlyster!

 

So I'm pulling 18 - 20 cmHg so it's about right.

 

so what do you do if you are pulling very low??? like 75mmhg?

 

Personally I never pay one bit of attention to the actual readings; the goal is to match them.

 

there's a lot of variables, rpm, valve condition, yics tool in or out or leaking, compression, needle bounce. ball park could be a big ball park.
how could you find a vacuum leak if it's adjustable, you would just adjust it away and not know it.

 

I find that well running 4 strokes generally pull about 10-12" (25-30cm)Hg.

If you're much below that it's not running well. Could be valves, dead cylinder, poor compression, carburetor trouble etc.....

 

Having worked on vacuum operated equipment professionally I'll chime in.

The actual amount of vacuum the engine is pulling only has worth (from a diagnostic standpoint) if you are using it to guage piston ring wear, valve sealing, balancing the carbs, and to diagnose vacuum leaks (including case seal leaks on two-strokes).

We have the ability to gauge ring sealing and valve sealing via a compression and leakdown test, which leaves diagnosing vacuum leaks and balancing the carbs as the only functions that we need to look at vacuum readings for. Both carb balancing and diagnosing vacuum leaks are done via measuring the differential between two vacuum ports, so the absolute value of the vacuum being pulled isn't needed.

The material handeling systems I used to work on had a maximum value, and a normal range, of operating vacuum listed in the service manual. The only time we ever were concerned with those values was to make sure that the system didn't pull enough vacuum to collapse the silos. Once that was set we only used the vacuum readings to diagnose for a holed material pipe, or a blockage, via differntial readings.

 

"Material handling" are you talking GRAIN SILOS or some other HUGE iteration of the concept?

Spaces tens of feet in diameter, 40 feet or taller, displacing/containing maybe just a tad more than oh, 650CC?

Same idea, agreed, but you also have to agree that it's an apples vs. small planets of a comparison...

 

I was maintianing recycling machinery. The silos were 4-6 ft in diameter, 12 feet tall, and holding crushed/shredded containers. The numbers involved are not bigger (crushed glass is heavy, but we were only running at 56 mm of Hg to move it through the pipes), and the actual physics are the same. Pull vacuum to move a material into a confined space...after that the two processes go in very different directions (crushed glass will not ignite, or compress very much). The higher vacuum on an engine has to do with the piston speed, and that it needs to move the incoming fuel/air fast enough for the carbs to do their job. Crushed glass just needs to move fast enough to keep up with the crusher. The one number that is a lot bigger is the volume of air moved. I could plug the intake to a silo and run it out of air in about three seconds (and if it was a plastic silo it would collapse, and I would be in big trouble).

 

I was thinking more of finding vac leaks, because I have a fluctuation on #2-3 when idling (NOT adjusting) along with an idle speed drop. So I thought maybe it was sucking air "sometimes" and that would reflect in the mmHg.

 

i can't quite wrap my head around this.
"fluctuation on #2-3" as in, #2 fluctuates and #3 fluctuates also individually. or #2 fluctuates in relation to #3?
your using 4 individual gauges, 2 gauges or a single gauge ?
idle speed drop, like when you spray something around the carbs or is it tied to the vacuum drop? like hear the rpm drop and see the vacuum change all by itself
any other signs of a vacuum leak? like loss of power or hot looking plug?

 

You mean the vacuum is good, then idle and vacuum drop?

I've seen that where the carburetor primary was plugged. It's lean but at the higher revs pulls some fuel from the main. As the revs come down the fuel quits feeding from the main and the rpms and vacuum drop off a cliff when the cylinder cuts out.

If your vacuum is only dropping on some of them it is probably out of sync.

If everything is in good shape (and synced) the vacuum should stay up as you turn idle down, well below normal idle speed, until it stalls.