Re: Electric fuel pump at tank for 90 merc? Possible?
Yes you can do it. No, you shouldn't do it.
As for coast guard approval... Yes they do make pumps that are Coast Guard approved.
Why you should not do it -
The stock pump is a puller and is pulse operated via pulse signals sent to it by the crankcase. The diaphram pump produces progressively higher pressure and volume as engine RPM increases. The diaphram, pulse tubing size, carb jets and passages are all designed to work with this configuration to give precisely the right amount of fuel to the engine.
An electric fuel pump pushes a constant volume and pressure to the carbs often overpowering the needles and seats. The pressure, even at 3 or 4 lbs forces more fuel into the engine at lower rpms than is needed. The results can often be seen as hard starting, flood starting, backfires, rough idle and the need for excessive intial advance.
The pusher style pumps also have limitations as to lift. I've found that a Holley Red Marine rated at 5-7 lbs barely supplies 3 lbs to the engine when deployed with a below deck tank. 3lbs is still too much at idle or for starting.
Imagine that under normal conditions, you squeeze the bulb then go hit the choke. The bulb looses some pressure and the right amount of fuel flows to correctly enrichen the enging for a cold start.
Now imagine a constant 3-4 lbs of pressure from the electric pump and hitting the choke and blasting a ton of fuel into the engine.
If you try to use a pressure regulator you can limit the pressure and make it start and idle decent but that regulator has no way of sensing the need of the engine and when your demand goes up at higher RPM's you'll lean the engine out and burn a piston.
So you are now obviously thinking this isn't such a good idea..... But wait, there is an approved Merc method. That method is to use a high volume low pressure pump.
The pump is installed in the following manner:
Main tank fuel line ->> 3/8" inline fuel filter ->> 2lb high volume fuel pump ->> Fuel/Water separating filter ->> Stock Pulse Fuel pump -->> Inline fuel filter -->> Carbs.
The draw back to this is that all these filters create way to many trouble spots. I've also never found a pump that was rated at 2lbs with anything more the 1/4" outlets so the volume is not sufficient and the final stick in the mud is that all these low pressure pumps are thumpers, noisy as heck and must be mounted vertically.
Still think an electric fuel pump would be easier than a couple early morning squeezes..... Yeah, didn't think so....
Just to qualify all this, you'll find other posts of mine where I sing the praises of an electric fuel pump. If I could remember where they all are, I'd go back an put a big red X through them.
