Re: [omc-boats] new member

On Fri, 9 Dec 2005, Ryan wrote:
> Ok..I got a compression gage. The guy who loaned it to me said I should
> check not only the compression but also the tolerance between
> cylinders??? I assume that means make sure one cylinder isn't way more
> or less the than others. Any idea what the tolerance is + or - ? Is
> this necessary?

I recently picked up the factory service manual for the 1970 155 HP OMC
V-6. It specifies the following procedure for compression testing:

(a) Remove all spark plugs
(b) Put throttle and choke in wide open position
(c) Remove wire from coil to distributor cap and ground securely to engine
(d) Attach remote starter switch to starter assist solenoid
(e) Insert compression gauge in spark plug port. Crank engine through
    at least 4 compression strokes to obtain the highest possible reading.
(f) Check compression of each chlinder. Repeat a second time and record
    the highest reading for each cylinder
(g) The recorded compression pressures are to be considered normal if the
    lowest cylinder pressure reading is more than 75% of the highest
(h) Example: 129,135,140,121,120,100 - 75% of 140 is 105. Thus, cylinder 6
    is less than 75% of number 3. This condition, accompanied by low speed
    missing, indicates an improperly seated valve or worn or broken piston
    ring.
(i) If one or more cylinders reads low, inject about a tablespoon of engine
    oil on top of the piston in low reading cylinder through the spark plug
    port. Repeat compression check on these cylinders
    (1) If compression improves considerably, rings are worn
    (2) If compression does not improve, valves are sticking or seating poorly
    (3) If two adjacent cylinders indicate low compression and injecting
        oil does not increase compression, the cause may be a head gasket
        leak between the two cylinders. Engine coolant and/or oil in
        cylinders could result from this defect.

Also, for Mark (the guy with the boat that won't go), the shift coil
resistance should be between 4.5 and 6.5 ohms, not the 2.xx ohms I
quoted earlier. The current draw is specified as 2.2 +- 0.2 A.

Ethan
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Received on Saturday, 10 December 2005