GS 400 vacuum

[jpettigrew]

Does anyone know who many inches of vacuum a GS 400 in good condition should have? I could not find any listings in the manual. Mine is showing about one inch right now and I think that sounds way low. The compression is about 120 to 125 lbs. If one inch is way low, what would cause this?

 

[Steve]

Vacuum readings vary on throttle position and load. Even at idle, there is no easily-repeatable number, the main concern is that all cylinders are pulling the same amount.

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[jpettigrew]

Normal vacuum in a car is around 14 to 22 inches. Isn't there a number for bikes? I was watching a You Tube on carb syncing with a vacuum gauge and his Honda was running about ten inches

 

[Steve]

You can't compare a car's vacuum to a bike's vacuum unless your car has one carb per cylinder or your milti-cylinder bike has just one carb.

May I enquire why you are asking the question? If you connected a vacuum gauge and saw a low number, that doesn't really mean much. If you are trying to do a carb sync, the actual number still does not mean much, just get all cylinders to the same level and you will be fine.

To help illustrate how important the vacuum number is, my first carb sync gauge did not even have numbers. Just six columns going into the rerservoir of Mercury. Yes, there were marks to make it easier to see that all the columns were even, but no numbers.

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[Gorminrider]

Does anyone know who many inches of vacuum a GS 400 in good condition should have? I could not find any listings in the manual. Mine is showing about one inch right now and I think that sounds way low. The compression is about 120 to 125 lbs. If one inch is way low, what would cause this?

That's incredibly low. Mine was about the middle of the gauge when I tried it. You will want/need a special vacuum gauge on a one carb -(one manifold)per cylinder engine.
Due to THE BOUNCE. It's going to be hard to sync by watching the upperpeak! It bounces so badly...tho I know someone who borrowed my cheapie gauge and thinks he synced them. Better man than I!

as to "what's wrong" I'd suspect the gauge, whichever type it is (special or cheapo) if you do have it attached properly to one of motorcycles manifolds. If you look into carb syncincing in the manual, you may find expected vacuum in mercury (hg) but I don't keep all this in my head accurately so treat it as an idea
...on a car, intake manifold vacuum indicated on a normal cheapo vacuum gauge is steadier due to the multiple cylinders sucking at different times,so it's the average you see at a manifold because it's common to several cylinders.