Help with Merc 70 Outboard

We have a early eighties merc

We have a early eighties mercury 70hp outboard. The number 2 cyclinder is not working. The spark is fine and we can't see any issues with the electrics. When we take the plug lead off the second cyclinder whilst the engine is running it does not make any difference at all. We thought it might be the piston seal/rings but the compression is fine. We think it might be a problem with the reed valves but are not sure? Any thoughts or information on inspecting the reed valves?

Thanks

How did you determine spark w

How did you determine spark was fine (5/16" gap)or laying plugs on the block????????. Laying plugs on the block is a waste of time and in noway a good test for spark.The reed valves are internal and not easy to inspect.

" We tested the plugs against

" We tested the plugs against the block, changed the leads over to make sure it was not the leads. When we ran the engine the last time plugs 1 and 3 were dirty (new before run) and the number 2 plug was clean. The plug was wet as if it was getting fuel but there is no difference to the engine running when we remove the lead. It came about as we are struggling for power at the top end, we had it once or twice so we know it can go but it is definately only working on two cyclinders. "

" If you tested the plugs agai

" If you tested the plugs against the block , you realy have not done a good job of trouble shooting. Good trouble shooting is the way you save money.Post the results of a proper spark check. "

" Sparky,
Who do you think yo

" Sparky,
Who do you think you are, his teacher, "Try Harder", "Must do better", springs to mind.
The way I read it Richard is asking for help!!. I can't say I'm surprised that he has not answered. If you are such an expert, why don't you elaborate on the correct way to 'spark check', rather than so rudely shoot him down in flames. If Richard knew exactly what to do he wouldn't be asking for help.
John Stewart "

"Richard.... Mercurys are not

"Richard.... Mercurys are not my line of expertise, however trouble shooting must start with the basics.

First, check the compression. Other than the compression being "fine", I saw no mention of just what the readings were. Double check those readings and let us know what they are.

Second, with the spark plugs removed, have some kind of spark tester rigged up whereas you can set a gap of 7/16" (I assume you have electronic solid state ignition). If you have magneto or automotive ignition, set the gap to 1/4".

If you can't figure out how to rig a spark tester, stick a proper size philips screwdriver into the rubber s/plug boot, all the way into the plug terminal. You can then hold the shank of the screwdriver that approximate distance away from the block while cranking the engine.

The spark should jump that gap with a strong blue lightning like flame (really a strong wide blue snap).

If the above two items check out okay, the usual cause is a fouled carburetor(s). If this is the case, the cure would be to remove, clean, and rebuild the carbs (all carbs).

Note that if a carb is fouled, clogged, gummed, hitting the choke momentarily would usually result in a increase of power for a few seconds.

This is assuming that fuel has entered the clogged carburetor but is not flowing through it. Removing or simply loosening the drain screw from the carburetor float chambers would reveal that one way or the other.

Keep in mind that if a reed plate (leaf valve) were faulty (broke, split, stuck open) the pressure within the crankcase would be forcing the fuel/air mixture out the front of the associated carburetor on the pistons downstroke when the engine is running. If you see no variation in the intake area of any the carburetor throats (they all appear alike), it would seem very unlikely that a problem exists in that area. Good luck and keep us informed.

Joe "

" Richard;
Check the spark wi

" Richard;
Check the spark with a spark tester, if available. Mercury's need a hot spark to light the fuel mixture. Also check the carbs. Sounds like a fuel problem. The reeds are located around the crankshaft and are very hard to get at. Good luck and have fun on the water. "