OK, thanks guys for the replies. The problem is the engine is very hard to start when cold. I have to pump the throttle 20-30 times to get it to start. Once it's started and warmed up, it's not a problem for a day or two.
1994 Mercruiser 4.3 L alpha 1 gen 2 outdrive. 4 BBL weber carb.
The engine was just replaced with a Jasper rebuilt.
The Weber carbs have an 'unacknowledged' (by Merc) problem... When you shut them down, they dribble fuel down through the main venturi until the bowl is empty, hence your hard starts. I also noticed that the fuel pump seems to lose prime as well. I haven't had a chance to do any proper diagnosis on that part of the problem, but I do suspect they are related.
timjet said:
Because it's so hard to start cold, I am assuming it is a lack of fuel hence the momentary switch to start the fuel pump. However like Chris said it will not work if the engine has to be cranking, so I could bypass everything and run it direct from the battery. But I'm open to any suggestions.
That's not such a good idea. The reason the oil pressure switch is in the circuit is so if the engine stalls out for any reason (without the key being switched OFF), then once oil pressure drops the power to the fuel pump is shut off. Boats are particularly nasty places to have an uncontrolled fuel leak. It stays in the boat, and has caused deaths in the past.
Chris, I just looked at the wiring diagram. There is a yellow/purple wire from the starter solenoid to the fuel pump, but there is also a purple wire from ignition key to the oil pressure switch and then to the fuel pump. As I look at the diagram it seems to me that voltage is supplied to the pump when the starter is engaged and also to the pump when the oil press switch closes. This is consistent with what you said -
"On the starter motor there is a terminal that only get energised when the engine is cranking. If you have a look at the circuits you'll see it. That terminal is connected to the fuel pump, and while the engine is cranking, the pump should be getting power..."
OK, yes but if that were the only circuit then voltage to the pump would stop when the key is released from start to run, hence the purple wire from the oil switch. If the oil pressure switch is bypassed then voltage is always supplied to the pump when the key is on, engine running or not, not a good idea unless it's just momentary on a cold engine.
Am I misreading the diagram?
No, you have it spot on.
The problem with what happens with the carb dribbling and the pump losing prime is that the pump takes forever to pick up fuel again. One way around that is to install an outboard style fuel primer bulb in the line between the tank and the engine. Just before starting the engine, give the bulb a couple of squeezes and fuel is available to the carb. That used to work really well with the old mechanical pumps as the fuel flowed right through the valves and you could fill the carb bowl up and the engine would start right up.... But the electric pumps don't valve the same way, and unless the pump is running, you can't push fuel through it... So, how to work around that? Most people simply short the oil pressure switch (as you suggested), switch the key ON, give the bulb a couple of pumps and away they go. But, as pointed out already, that bypasses the safety feature of the oil pressure switch, and leaves you open to some 'unpleasant' consequences.
Two ways around the problem as I see it would be a) a latching relay initiated by the operator pressing a button on the dash that would allow him/her to then go back to the engine to prime it, with the relay being de-energised by either a timer (30 seconds would be enough) or the action of cranking the engine...
Option b) would be easier to implement. That of putting a momentary switch inside the engine bay (obviously a flame proof one!) that the operator holds while priming the fuel bulb....
Hope this helps...
Chris..........