Topic: No Vacuum Reading

[BrickWoll]

Hey all,

Still trying to get my '77 CB550 to run well.  I've been on a lot of rides and have made it back exactly ZERO times.  Lots of pick-ups in a buddy's trailer.  I won't bore readers with all the rebuild details - carbs and top end, etc.  I'm working my way through systems to eliminate each piece.  Finished both static and dynamic timing.  Looks OK.

Issue:  I've hooked up a vacuum gauge and I'm getting no readings on ANY carb.  My first thought is it's possible I've got a garbage set of sync gauges.  But not even a blip on any of the four ports?  Just doesn't seem right. 

Is there another way to test the gauges to confirm they work(literally sucking on the hose)?  How about testing the carb vacuum without the gauges?  I realize if I'm legitimately not getting vacuum it must be a leak in either the boots (brand new and tight) or the intake manifold o-rings.  I just can't believe all 4 needles wouldn't budge from zero at startup.

Regarding the general running condition, I've had many ups and downs.  Currently fires right up and settles to a bumpy idle.  All pipes hot, not a drop of oil or fuel overflow on the ground (meticulous clear tube adjustment on carb bowls).  PD46As.

Thanks for any thoughts.

[goodtryer]

maybe your gauge is the wrong range/strength/sensitivity (i can’t think of the right term) for reading carbs. the clymer  manual says the carbs should read between 16-24 cm-hg for my 73 500.

[HondaMan]

If the engine runs, there is vacuum. Can you put up a picture of your gages? Many of them have a "valve" of one sort or another (usually a needle valve with a knob or screw to adjust) at the base of the meter, or an inline valve of some sort. If it is closed there won't be any readings.

[BrickWoll]

Thanks, guys.  Model is Alpha Moto (see attached pic).  Pretty sure it's made for motorcycles.  HondaMan, this model has nuts below the gauges that spin open to control needle shake.  I see no other valves or controls.  I tried open and closed.  Nothing moved the gauge needles.  Must be a bad unit.

[newday777]

Test each individual gauge with a Mighty Vac to see if the leak/verify each read the same vacuum level. Pump the Mighty Vac to the same level suction. There should be an adjustment screw on the back of the individual gauges.

[BrickWoll]

Still at this, but I've done a crude test and I'd like some help with diagnosis.  I took the pods off the carbs and physically covered each carb opening with my hand to confirm vacuum. Result:

Carb 1:  3 sec engine begins to stumble to a stall
Carb 2:  3 sec engine begins to stumble to a stall
Carb 3:  3 sec engine begins to stumble to a stall
Carb 4:  Idle INCREASES a la runaway!

I feel like I'm slowly honing in on why my bike runs strong for about 15 min, then dead.

Any thoughts on what would cause the opposite behavior on 4?

Thanks all.

[bryanj]

From what you say 4 is weak as when youchoke it withyour hand it picks up and enginerevs

[goodtryer]

Yep, it’s getting air from some other path. Check the carb-to-intake-runner seal as well as the gasket between the intake runner and the cylinder head.

[BrickWoll]

From what you say 4 is weak as when youchoke it withyour hand it picks up and enginerevs

bryanj - can you clarify what you mean by "weak?"   Thanks.

[BrickWoll]

Yep, it’s getting air from some other path. Check the carb-to-intake-runner seal as well as the gasket between the intake runner and the cylinder head.

Thanks.  Seals from carbs to intake are new and pliable.  Cinched tight.  I've sprayed them to see if the idle changes - it doesn't.  Pointing to that header seal, huh? Also new, but I've read the aftermarket fitment isn't the same.

[bryanj]

Weak as in fuel mix, when you close off the air there is more fuel sucked in