Prop help, 90hp Merc

Thirsty Endgrain · Aug 5, 2022

Hi all, I've never yet had to deal in prop choice until now.

Done a bunch of research, and just got my new to me engine out for a test last night. 2002 90hp carb'd Merc 4 stroke. Pushing a '64 17.5' Glasspar Seafair Sedan.

Currently has a 13" x 19"P alum. prop. Couldn't get over about 4400 RPM's, and range for my engine is 5k-6k. I know if wasn't properly trimmed while doing tests (it was pretty choppy, and I also forgot) so with trimming I probably could have gained another 100 RPM's or so? As I understand it, 1" decrease in pitch corresponds to a 150-200 RPM increase in RPM. And a same size 4 blade vs a 3 blade will also drop RPM's?

One reason I'm asking is because there is a 13 3/4" x 17"P 4 blade stainless steel Mercury Trophy Plus prop for sale locally for a good deal, but obviously I'd only want to get it if it would work for me. But it seems that it might still be too much prop for my application? This is the part I need help with.

Texasmark · Aug 6, 2022

You have a problem somewhere. If that 4 stroker has the 3.3 gearbox as my 2002 90 2 stroker, running on an aluminum 17.5' Fishmaster bass boat, just me, fuel and gear, I was running a 23P Ballistic 3 blade SS.....making the wake in my avatar....rpms in the 5600 area....right at 50 MPH.

QBhoy · Aug 6, 2022

Is that trophy that size for your gearbox ? Or the bigger one ?

Thirsty Endgrain · Aug 7, 2022

Texasmark said:
You have a problem somewhere. If that 4 stroker has the 3.3 gearbox as my 2002 90 2 stroker, running on an aluminum 17.5' Fishmaster bass boat, just me, fuel and gear, I was running a 23P Ballistic 3 blade SS.....making the wake in my avatar....rpms in the 5600 area....right at 50 MPH.

Not sure the comparison would work here, but I don't have enough experience to know that. Yours will have more torque than mine. Also I don't know what you mean by the 3.3 gearbox, is that the gear ratio? Mine I believe is 2.33:1 if that's what you're referencing. Also I'm in a deep vee cuddy cabin with gear and sometimes two people.

I have many problems, but if you're referring to the engine, I'm not ruling that out. I did just clean the carbs, twice, with an ultrasonic cleaner etc. All synced and balanced. New CDI. full service. If there's an issue at this point it evades me. Still wouldn't rule it out though.

Silvertip · Aug 7, 2022

Commpression test is probably in order.

Thirsty Endgrain · Aug 7, 2022

Silvertip said:
Commpression test is probably in order.

Leak down showed ~ 10% across all 4 cylinders. Would there still be a reason to do a comp test?

Thirsty Endgrain · Aug 8, 2022

The seller of that prop is allowing me to test it out. A couple days ago I had it out for a full day of fishing. RPM's were in the 4800 range at WOT. Still under the minimum range for this engine. I am wondering if there's an underlying problem. Fuel pump? Can they be tested or not fully deliver fuel? There is a slight hesitation/shake when getting up to speed in the lower rpm range. I'm wondering if there is some little issue somewhere that's not allowing me full power, of if it's prop or something else related.

Again carbs are freshly rebuilt and thoroughly cleaned. Spark easily jumps 7/16" on all cylinders. Plugs are new. Leakdown 10% on all cyls. I must be missing something.

Texasmark · Aug 9, 2022

Thirsty Endgrain said:
Not sure the comparison would work here, but I don't have enough experience to know that. Yours will have more torque than mine. Also I don't know what you mean by the 3.3 gearbox, is that the gear ratio? Mine I believe is 2.33:1 if that's what you're referencing. Also I'm in a deep vee cuddy cabin with gear and sometimes two people.

I have many problems, but if you're referring to the engine, I'm not ruling that out. I did just clean the carbs, twice, with an ultrasonic cleaner etc. All synced and balanced. New CDI. full service. If there's an issue at this point it evades me. Still wouldn't rule it out though.

Sorry for the typo....yes its 2.33. Your boat is apples and oranges from mine. You are running a much different rig from mine. I can see where you need to drop pitch significantly to get positive response from that hull/load.

You mentioned fuel pump, yes its time for a repair kit as the diaphragm hardens over time and doesn't pump properly...BTDT. Also, don't ignore a clogged fuel filter as being the problem since the grey OEM fuel lines have a nasty history of generating crystalline, tan particles, especially when using E10...BTDT too. Also if E10 has been the fuel over the years, the engine fuel lines may have started to degrade generating small black particles of degraded rubber...fuel filter cut open tells the story....BTDT.

Thirsty Endgrain · Aug 9, 2022

Texasmark said:
Sorry for the typo....yes its 2.33. Your boat is apples and oranges from mine. You are running a much different rig from mine. I can see where you need to drop pitch significantly to get positive response from that hull/load.

You mentioned fuel pump, yes its time for a repair kit as the diaphragm hardens over time and doesn't pump properly...BTDT. Also, don't ignore a clogged fuel filter as being the problem since the grey OEM fuel lines have a nasty history of generating crystalline, tan particles, especially when using E10...BTDT too. Also if E10 has been the fuel over the years, the engine fuel lines may have started to degrade generating small black particles of degraded rubber...fuel filter cut open tells the story....BTDT.

Good to know about all of that. The grey fuel lines I've been wondering if I should replace since they're likely 20 years old now. May do the pump rebuild for the same reason. The engine mounted fuel filter looks pretty good, though I've been meaning to change it. I have a 10 micron fuel filter/water separator just before the engine, fresh filter. The little fuel strainers at the carb inlets are brand new, and the old ones that came out were pretty clean. So I'm not suspecting the Grey lines at this point, but again, I may change them as a matter of course.

Perhaps a 13" 17p prop would be better for me here? I think that's my next move. Will likely return the SS 13 3/4" 17p trophy. It's pretty, but it also needs to work

Thirsty Endgrain · Aug 10, 2022

Well I ordered a Turning Point 13.5" x 15" 4 blade. Hoping that gets me into the sweet spot. If the rpms are good I may exchange it for an SS, but if it runs well enough I'll probably just ride it for a while. Also got a hydrofoil. My engine is heavy for the back of my 17.5' glasspar, so hoping that will help with planing time and behavior.