Prop Recommendation: '85 Merc 50hp 2-stroke - 16' Alumacraft

[joewg3]

Hi All - I just bought an '85 16' Alumacraft Maverick with a 50HP Mercury 2-stroke. Boat hadn't been run since 2017, but I've given a restoration/rewire and went over the motor. Prop has a chunk missing. Motor gets up to about 3500 RPM at WOT. It will hit around 6K without drive engaged. Stalls after a bit when idling cold, but I'm about to go through the carbs and replace fuel lines. My friend thinks it's the carbs holding back the RPM, but I can't help but think the culprit is the prop, so I'm changing it especially as there is damage. I can't find any info on the current prop nor what specs should be OEM. I also tried searching the forum. I need to know some recommendations please.

Prop:
Aluminum. Diameter appears to be 10". Number stamped says 48-73138-14. 13 splines.

Boat:
16.33". Weight loaded with people maybe 1000lbs.

Thanks All!

 

[Scott06]

Hi All - I just bought an '85 16' Alumacraft Maverick with a 50HP Mercury 2-stroke. Boat hadn't been run since 2017, but I've given a restoration/rewire and went over the motor. Prop has a chunk missing. Motor gets up to about 3500 RPM at WOT. It will hit around 6K without drive engaged. Stalls after a bit when idling cold, but I'm about to go through the carbs and replace fuel lines. My friend thinks it's the carbs holding back the RPM, but I can't help but think the culprit is the prop, so I'm changing it especially as there is damage. I can't find any info on the current prop nor what specs should be OEM. I also tried searching the forum. I need to know some recommendations please.

Prop:
Aluminum. Diameter appears to be 10". Number stamped says 48-73138-14. 13 splines.

Boat:
16.33". Weight loaded with people maybe 1000lbs.

Thanks All!

A prop missing part of a blade would allow it to over rev rpm not be 2-3 k lower than recommended WOT RPM. I would go through the fuel system and rebuild carbs first, verify your ignition is up to snuff and good spark on all cylinders, then start playing with props. You can not asses what prop is right until the engine can get to WOT rpm

 

[joewg3]

A prop missing part of a blade would allow it to over rev rpm not be 2-3 k lower than recommended WOT RPM. I would go through the fuel system and rebuild carbs first, verify your ignition is up to snuff and good spark on all cylinders, then start playing with props. You can not asses what prop is right until the engine can get to WOT rpm

A prop missing part of a blade would allow it to over rev rpm not be 2-3 k lower than recommended WOT RPM. I would go through the fuel system and rebuild carbs first, verify your ignition is up to snuff and good spark on all cylinders, then start playing with props. You can not asses what prop is right until the engine can get to WOT rpm

Thanks Scott. Definitely looking forward to cleaning the carbs. It really doesn't run bad except for not wanting to idle all day when cold. No misses and revs right up like a weedeater except for when I'm actually trying to scoot!

 

[Scott06]

Thanks Scott. Definitely looking forward to cleaning the carbs. It really doesn't run bad except for not wanting to idle all day when cold. No misses and revs right up like a weedeater except for when I'm actually trying to scoot!

Did you check out the ignition system too? this sounds similar to my neighbors issue a few years back, wouldn't get full revs under load, but ran basically ok otherwise. He had a cylinder that wasn't getting spark did have enough azz to get going with a dead cylinder.

 

[joewg3]

Did you check out the ignition system too? this sounds similar to my neighbors issue a few years back, wouldn't get full revs under load, but ran basically ok otherwise. He had a cylinder that wasn't getting spark did have enough azz to get going with a dead cylinder.

Yeah, I'm getting spark on all cylinders, plugs don't look too bad, and compression is around 105 between the 4 with no real variance.

 

[airshot]

Forget the prop until motor runs perfect, then see how close to max operating rpm is....then go from there !!

 

[jimmbo]

Can the Spark jump a 3/8" gap, on all Cylinders?
Get the proper Factory Service Manual for that Motor
Timing and Pickup Timing don't go out of adjustment, unless someone has fiddled with it
Important that both carbs are feeding fuel, as the Oil is mixed with the Gas, no Gas, means NO Oil. If it has sat a while the Carbs are likely gummed up and need to be disassembled and cleaned thoroughly. Once reassembled, you will need to check the Sync between the Spark Advance and the Carb open, referred to a 'Link and Sync'. The procedure is covered in the Service Manual

FWIW, the current Prop is a 14 in Pitch, which to me is a bit large
There was no OEM prop spec, every on Mercs over 18hp was a custom pick by the Dealer/Owner

 

[airshot]

My boat is a 16' Sylvan SC model, weighs around 1000 lbs and has a 4 cylinder 40 up two stroke Merc OB that turns a 10.5 dia x 13 pitch prop at 5500 rpm...his 14" pitch might be close....gotta get that motor running right first !!

 

[QBhoy]

Hi. I don’t think it’s the prop…unless it’s something daft like a 16+ pitch. But anything from 10-13” should be normal. If this is the classic 50hp Merc…they are actually not even 50hp. They are closer to 40/45hp. Anyway….
Personally I’d be back to looking at fuel and ignition. Plugs etc. an engine even unhealthy will rev all day to the sky, when not under load. Different ball game when it is.

 

[airshot]

Hi. I don’t think it’s the prop…unless it’s something daft like a 16+ pitch. But anything from 10-13” should be normal. If this is the classic 50hp Merc…they are actually not even 50hp. They are closer to 40/45hp. Anyway….
Personally I’d be back to looking at fuel and ignition. Plugs etc. an engine even unhealthy will rev all day to the sky, when not under load. Different ball game when it is.

My mechanic claims the Merc Classic like my 40 up is actually under rated, the actual power output is closer to 45 hp. I know for sure it us much faster and more power than my 1988 40 hp Johnson OB..was. I think the poster has an engine issue as well.