Topic: Compression Test: Do carbs apply?

[Tamothy]

I have read that during a compression test the throttle should be completely open, choke on, and air box removed.  Can a compression be successfully completed without the carbs?  What about without the carbs but with UNI pods over the intake manifolds to protect from contaminates?

Also, is it possible to get accurate results by turning the motor over with the kick starter opposed to the starter motor?  It seems that it would be, provided you spin the crank enough times. 

Thanks!

[Randawg77]

    You just want to ensure the cylinders are getting their full volume of air on the intake stroke to get accurate comp.  #s.  I have heard of several ways to read your compression numbers,   some say it should go to 3/4  of peak p.s.i. on the first compresion stroke and then plateau on the next one,    some just crank until they get the highest #.     Either way you want to be within  10% to 20% across the board  for a good running engine.     

[TwoTired]

Choke on would block the air intake.  You don't want that.  You want unimpeded air to enter the combustion chamber for compression.

Carbs off, inlet open is fine.  Just don't suck in nuts and bolts.  Kick until the number stops increasing.  Move on to the next cylinder.  It is easier if all the spark plugs are out.

Valve lash variance can effect compression numbers.

[Rob69]

I agree with the poster above. If I were you, I'd make sure the  valve clearances are correct before doing a compression test. It's not necessary to have the carburetors installed, though ideally a compression test should be performed when the engine is hot for the most accurate readings.  Still, testing a cold engine will give you a fairly accurate analysis of how healthy the top-end of the motor is. Compression readings should not vary more than 10% across the board.

[bollingball]

I agree with TT choke OFF.

Ken

[lucky]

I have read that during a compression test the throttle should be completely open, choke on, and air box removed.  Can a compression be successfully completed without the carbs?  What about without the carbs but with UNI pods over the intake manifolds to protect from contaminates?

Also, is it possible to get accurate results by turning the motor over with the kick starter opposed to the starter motor?  It seems that it would be, provided you spin the crank enough times. 

Thanks!

The choke must be OFF otherwise the engine cannot get air to make compression.
Battery must be fully charged.
All spark plugs out.

[luap]

Jump the starter, hear it gets to the number a little faster an easier on the leg, an your going to get a more acurate reading using a small displacment compression tester compared to a auto one

[Tamothy]

TwoTired, it makes complete sense to have choke OFF.  I understand why valve clearance must be correct.

lucky, the battery being fully charged is not required in regards to my inquiry about testing using the kick starter opposed to the starter motor.  The reason I would check with the kick starter is because a lack of juice!  Not a desire to kick, kick, kick away!

Rob69, I understand why the most accurate reading would be from a "hot" test.  I am wanting to find out "how healthy the top end is" so thanks for letting me know that a cold test will do that.

luap, glad to know about small displacement testers.  Hopefully the auto type will be sufficient as I plan to "rent" one from the auto parts store for free.

Thank to everybody for their quick advice!

[70CB750]

I done it once on hot CB engine, but since I dont really enjoy burning myself, I dont do it anymore:)

[Xnavylfr]

I would set the valves then do comp check!!
You don't even need carbs on the bike to do check!!
About 3 or 4 kicks on each cylinder should be enough to pump them up..

Where in Western NC are you??? If you notice I live about 10 miles west of W/S.


Xnavylfr(CHUCK)