Re: 1968 55 hp johnson
The compression is okay.... it varoies somewhat with the speed in which the engine is being cranked over.
I'd just replace the impeller for now BUT seal the impeller plate to the lower unit and the housing to the plate to avoid having the pump draw air.
(Gasket Sealers)
(Usually available at any reliable automotive parts type store)
(J. Reeves)
Do Not use any type of sealer on outboard carburetor or intake manifold gaskets as this would cause blockage of various fuel/air passageways. Also, there are new type gaskets, including head gaskets, which are used through out the engines that are coated with a substance that gives a shinny appearance to them. These gaskets are coated with a self sealer and are to be installed as is.... no sealers! Older style head and fuel related gaskets that are not coated with this substance are to be simply coated with oil.... nothing else.
All other gaskets (hopefully I've not overlooked anything above) should be coated with Gasket Sealer (the type that comes in a can that has a small brush in its screw off cap). This also pertains to some water pump installations that use a gasket between the plate and the lower unit housing.
Marprox Sealer 1000 or OMC Adhesive or Boat Armor Sealer 1000 (same substance)[See Below Note] should be used on any metal to metal (Unless GelSeal Is Called For) or metal to plastic surface (such as impeller hsg to plate, and plate to lower unit surface) to eliminate air leaks. It should also be used on the Spaghetti tubing that is used on many powerhead crankcase halves, lower unit skegs, etc. I have found that this substance works quite well on various "O" Rings such as what are found on lower unit carrier and upper driveshaft bearing housings (Note that this substance is obviously not to be used on all "O" Rings). This substance also works quite well in sealing the surfaces between the lower unit and the exhaust housing to which it attaches, eliminating the unsightly water spray between the two after completing a water pump installation.
NOTE 1: I am under the impression that Marprox Sealer 1000 and Boat Armor Sealer 1000 is no longer available.
NOTE 2: OMC Adhesive is now Bombardier 3M Product #847 .
DO NOT use any sealer on the large "O" Rings that are used on the upper and lower crankcase heads (The housing/heads that slide over the top and bottom of the crankshaft). On those "O" Rings, use only a coating of grease.
The exception of surface to surface contacts pertains to the later model matching crankcase (block) halves (which does not incorporate spaghetti tubing) which call for "Gel-Seal", or any other surface that calls for Gel-Seal. This substance seals in the absense of air, and if called for, must be used. One other form of Gel Seal that can be substituted is "Loctite #518". Nothing else that I know of will do!
I advise against using silicone or permatex on a outboard engine. It simply will not function properly. The exception to this (as per OMC's recommendation), is to use the black Permatex (heavily) on the gasket 306242 which seperates the inner exhaust tube from the exhaust housing on the 1959 35hp and 1960 to 1970 40hp OMC outboards.
