1980 Mercury 70 HP stalls at mid rpm range

[mercuryengine]

Any help greatly appreciated. I read others in this forum had the same problem, and I tried their suggestions. I had a mechanic replace the water jacket, the water pump, check spark and timing. Compression is good. The rectifier, stator, coils, switch box (power pack) all test good. Fuel pump good, fuel lines good and clear, new primer bulb and fuel lines installed, fuel filters good, carburators clean as a whistle. Engine cowling tight, does not allow salt water onto engine block, no salt on engine. The problem started when one of the 6 gallon tanks was not vented, and negative pressure was created in the fuel tank, possibly throughout the fuel system. That is why I thoroughly checked the fuel delivery system for any debris knocked loose. Spark plugs good. All carburator plugs (from manufacturing process) are tight. The engine works well at idle up to barely on a plane, good at WOT, stalls in between. In stalling range, pushing in choke helps but not fully. It still feels like a fuel problem, but neither I or my mechanic can figure it out.
Any help greatly appreciated.

 

[achris]

Re: 1980 Mercury 70 HP stalls at mid rpm range

Hello mercuryengine, and :welcome: aboard...

It does sound like a fuel pump problem to me (or at least fuel related). Was the fuel pump rebuilt or just looked at? If the diaphragms were not replaced, I suggest replacing them... And check the pump vacuum hoses too (one may have been sucked flat), often overlooked, particularly at service time. The other (remote) possibility is that a seal or gasket has been sucked in and is leaking air. I would replace the hose from the pump to the carburettor with a section of clear and see what you see... You see bubbles, not good....

Chris.......

Pssst, Mercs have switchboxes, jonnyrudes have power packs..

 

[mercuryengine]

Re: 1980 Mercury 70 HP stalls at mid rpm range

Thank you Chris. I did replace both the fuel pump diaphram and the two seal/valves, I even swapped carburators from another engine which ran well but literally wore out and had to be scrapped because not cost effective to repair. I llike your idea of a clear fuel line and will try it.

 

[mercuryengine]

Re: 1980 Mercury 70 HP stalls at mid rpm range

Chris, took your advice, saw bubbles in hose from pump to carburators, changed pump gasket again, this time there were no bubbles from pump to carb, but now bubbles seen from fuel line connector to motor to pump. Changed all fittings and fuel line from tank to motor, still have same problem. All vacuum hoses good. Can you think of anything else? Thank you.

 

[achris]

Re: 1980 Mercury 70 HP stalls at mid rpm range

If you still have bubbles in the line, change to a different tank... The pickup in the tank could be cracked..

Chris..........

 

[quicktach]

Re: 1980 Mercury 70 HP stalls at mid rpm range

Are carbs bolted tight to block and with good gaskets? Sounds like a fuel problem ....but you could also have an advancing problem. Check to see that your trigger plate isn't binding or the plate movement isn't hindered by a wire bent or catching on something. Done alink & sync on it?