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Engine or Stern Drive Problem?

M

moresparks

Regular Contributor
Engine rapidly loses power under load after just a few minutes.
I have had a number of problems with an aging Volvo AQ171C engine. The previous problem would be the engine would die after a few minutes under load. This time it does not cut out but has very little drive.
Having overhauled the fuel, ignition and carburettors this looks like a different problem on the face of it, so I am not sure where to look next. I now am thinking that maybe the previous problem was just another symptom and not the cause.
I do not have a rubber bushing on the single prop and the only clue this time was that the trim gauge indication had moved slightly.
To recap the work so far, I have replaced:-
Fuel pump, fuel filter, spark plugs and leads, rotor, distributor, ECU and coil overhauled the carburettors and balanced and set idle and mixture etc. In fact I don’t think there is anything left to check. (Advantages of twin engines).
Compression is good and no vacuum leaks, it starts no problem and revs smoothly through the range, I can leave the engine running in reverse (idle speed) tied to the dock and drive exists. Oil pressure and temperature are normal and I can leave it in running in neutral for ages with no problem .
So where to look next? The stern drive is the SP290A hopefully looking for some pointers but now fearing the worse that the season is over and a big bill.
Maybe time to ditch these old engines and start anew!
 
Is the exhaust manifold getting too hot to touch when you rev the motor up after a bit? Check the ignition timing as you rev the motor up it should be a steady rise in spark advance as the rpm increases? If not pull the plate in the distributor and put a couple drops of engine oil on the pivot points see if you can get it moving again also put some oil in the top center of the shaft where the rotor fits on.
 
Don't give up on the engines yet! If compression is still good, unlikely it's a fatal problem.

BTW... prop should have rubber bushing if its OEM, it's just not obvious.
 
Talking around to technicians, it seems the finger of suspicion still leans towards a fuel supply problem, could be a fuel pick-up from the tank. It’s a single tank but two copper pipe feeds, one for each engine to each fuel filter. In any case I need to omit this from the list, so I am sourcing a small outboard style fuel tank and run the engine from that also replacing the short flexible fuel hose.
The distributor has no plate, as this has an ECU - all replaced complete with rotor and greased according to the handbook.
I didn’t notice anything untoward with the exhaust manifold but I will double check.
Many thanks for your suggestions
 
I agree with your technicians. My next search would also be for a pinhole leak in a fuel hose between the fuel tank and fuel pump. A pinhole leak would allow air bubbles into the fuel flow.
 
Just an update.
It was the flexible fuel hose - seems it was partially blocked.
I have replaced and all is good.
Many thanks for your suggestions.
 
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