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QUESTION: 1986 KAWASAKI EN450A1 WHAT VACCUM READING SHOULD I GET AT VAC PORT ON FRONT TOP OF EACH CARB INDIVIGUALLY&TOGHTER IF INTERNAL ENG VALVE TRAIN IS OK? THE READING I HAVE AT IDLE IS RAPIDLY FLUXUATING BETWEEN 3HG TO 14HG.THE BIKE STARTS&IDLES FINE BUT BOGS BAD IF YOU TRY TO GO PAST ABOUT 1/4 THROTTLE.CARBS JETS&ALL ARE CLEAN.VACCUM DROPS TO NEAR 0&STAYS LOW TILL YOU LET IT IDLE AGAIN.EXAUST IS CLEAR&NOT RESTRICTED.NO VAC LEAKS BETWEEN CARBS&ENG.JUST SEEMS MY VAC READINGS ARE POSSIBLY INDICATING A STICKY VALVE OR CARB PROBLEM.BUT DOES 1 SIDE AFFECT THE OTHER? SAME READING ON BOTH VAC PORTS OF EACH CARB???

ANSWER:
Hi Robert,

The vacuum reading should hold steady but there are many
factors that affect it. Altitude and the gauges you use can affect
the readings. Also a sticky or leaking valve will cause a regular
drop of the gauge reading like a cycling effect.

IT is normal for the vacuum to drop when you crack the throttle.
It will drop down to 3 or so and then go back up usually
a bit high and then settle down to about 17hg depending on altitude
and gauge flutter etc. Your gauges should have a hose flutter control
unless they are liquid vacuum sticks.

Intake and headgaskets leaks will cause the gauges to go up and down.

Check your compression as well, if the compression is low then your rings need service or
the valve clearance might need checking.

See if it helps to choke the carbs or take a spark plug color
reading to see if the bike is running rich (black) or lean (whitish)
colors. See if the plugs are wet or sooty or dry.
Use a good battery to prevent weak spark.

With good rings the vacuum reading will rise when you
crack and shut the thottle to above the normal reading and then
settle down again to a more steady reading.

Normal readings are between about 14 and 22hg depending
on the gauge and altitude as I mentioned. High altitudes
result in lower readings. 17hg is about average and
a steady reading is better.

WS
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---------- FOLLOW-UP ----------

QUESTION: Gauge fluctuates majorly at idle???

Answer
Your gauge must have flutter control on the hose or they will fluctuate alot.
or
You are losing vacuum somewhere, intake/head gasket
or carb diaphragms cracked if it has the rubber type.

Badly leaking rings/valves might do it but compression check
should show that. Head gasket blown between cylinders
might do it.

A cylinder pressure or leak down test would tell you what's
leaking.
http://motorcyclespeciaist.blogspot.ca/2013/02/compression-and-leak-down-test.ht

WS
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Wayne S.

Expertise

Licensed Motorcycle Mechanic, Knowledge of motorcycles from 1960 up, Japanese, British and most other brand motorcycle repairs.

Experience

Worked for Yamaha, Honda, Suzuki, Kawasaki British: Triumph, Norton, BSA Other: Most Scooters and Mo-Peds

Publications
Canadian Motorcycling magazine article

Education/Credentials
Licensed Motorcycle Mechanic

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