1978 Merc. RPM drop

[ptest]

Outboard runs fine for a while at WOT. Occasionally RPM will suddenly drop to half or less RPM for a few seconds or longer and then return to WOT. It may do this repeatedly. Drop in rpm is smooth and instant, (no stumbling, engine runs smooth even if rpm drops to near idle) I replaced primer bulb and squeezing it has no effect.<br />Compression good, <br />Any ideas...

 

[petryshyn]

Re: 1978 Merc. RPM drop

Sounds like an ignition problem. First try new plugs. Then troubleshoot ignition if not cured....<br /><br />Good luck!

 

[kevlar3000]

Re: 1978 Merc. RPM drop

Is that Distributor or distributor-less ignition? My friend has a 78 115 with a distributor that does almost the same thing.<br />I am curious to hear wht the pros say...

 

[petryshyn]

Re: 1978 Merc. RPM drop

If thats a 70hp, I'd be pricing out a power pack. Or cut it open and repair the circuit board as I did....<br /><br />good luck!

 

[mbb]

Re: 1978 Merc. RPM drop

sounds like it could be an air leak in the fuel system. When the engine is at full throttle it sucks air in the leak instead of pumping fuel, then the bowl level drops, the engine bogs, carb suction drops, fuel fills bowls back up.If it catches instantly when you throttle back thats a sure sign also. Put a section of 1/4" clear tygon tubing between fuel pump and top carb and look for tiny bubbles. There should be none at all. You will usually see them at idle even if problem doesnt show up but at wot.