Yanmar 1GM 10 Stalls Under load Ahead not Astern

[DD1871]

Engine starts in neutral ok. Revs to full revs no problem. When forward gear engaged on full throttle engine revs decrease and engine stalls. When going astern and full revs engine runs normally under load. New fuel filter fitted fuel sight glass checked and cleaned. Fuel system bled. Gearbox oil looks fine. Can’t see anything on prop. I’ve seen similar threads but nothing which explains why the symptoms would be different depending on whether astern or fwd gear engaged. Has anyone witnessed anything similar or any pointers on what else to check?

 

[scottie]

Different reduction ratio ahead/astern plus alternator taking too much power
Try running with belt off if that’s all it’s driving

 

[DD1871]

Different reduction ratio ahead/astern plus alternator taking too much power
Try running with belt off if that’s all it’s driving

Thanks Scottie I’ll give that a go

 

[RichardS]

Different reduction ratio ahead/astern plus alternator taking too much power
Try running with belt off if that’s all it’s driving

It could also be a difference in gear ratios combined with a blocked exhaust elbow? Worth checking it.

Richard

 

[lw395]

All of the above plus:
Prop more efficient ahead, so slows the engine more?
engine moving forwards under thrust so prop rubbing on P bracket?, Possibly abetted by fouling or bits of rope around shaft?
Worn folding prop?

Does it run Ok at low speeds ahead?
Or does it stall at any speed?

Is full RPM astern the same as ahead?

Fuel starvation?

 

[Graham376]

All of the above plus:
Prop more efficient ahead, so slows the engine more?
engine moving forwards under thrust so prop rubbing on P bracket?, Possibly abetted by fouling or bits of rope around shaft?
Worn folding prop?

Does it run Ok at low speeds ahead?
Or does it stall at any speed?

Is full RPM astern the same as ahead?

Fuel starvation?

Has a rope cutter been fitted recently?