5 and 6 cylinders not working correctly

" It seems 5 and 6 not working

" It seems 5 and 6 not working correctly, cannot get the rpms and horses we are looking for. Took out the plugs, 1,2,3,4 plugs look the same, 5 and 6 fairly clean looking. This is a old circa 1970 merc 100. Any ideas gang? "

" Greg,

No idea even

" Greg,

No idea even what that engine looks like exactly (L6?).. I'll post this up until someone who really knows comes through - If the plugs are clean, you are either getting no spark or no fuel on those plugs. Are they wet w/ fuel or totally dry? Check for spark, if the spark is good, you are probably missing fuel.

If the spark is fine & it's a fuel problem -is that a 3 carb? If so, the carb feeding those plugs is probably gummed up. Get a rebuild kit, disassemble, clean, rebuild.

If it's a spark problem - just the obvious -what do those two plugs share in the ignition system? That will probably lead you to it. Usually you can re-wire things to isolate bad components - come up with a theory & try to make the problem switch cylinders.

Another idea - check the compression, if there is something wrong there, messing w/ ignition & fuel trouble-shooting won't get you anywhere.

Jon "

Likely a problem with the low

Likely a problem with the lower carburetor of the 3 or a problem with the internal reed valves and "bearing" . Problems with the reed valves will require engine teardown.

" Greg...You have an inline si

" Greg...You have an inline six there. These motors were notorious for leaking water through gaskets seals and side plates. This water leaks into the powerhead. Before you go looking for water though you should compression test the engine, check for spark on all six plugs. Its unlikely that you will have an ignition problem as your motor uses a battery CD type ignition with distributor. Next step would be to eliminate the carb that feeds the lower 2 cylinders..this is the bottom one. You can check this by starting the motor and spraying some starting fluid straight into the carb throat. If the engine picks up revs the chances are that the carb is at fault. You should also ensure that your distributor is rotating when you advance the throttle. To check for water in the engine put 2 dirty spark plugs into the suspect cylinders then run the engine. If the plugs come out clean this indicates that your engine is taking water "

" Thanks all for the info, i&#

" Thanks all for the info, i'll let you know how it goes.
Thanks again,
Cheers "