Prop & Motor RPM question on Mercury 1150

[sportyinc]

I have a 1972 1150 Mercury mounted on a 1972 Glastron V176.
When I bought the boat, I installed a tach and the motor (with a 21 pitch prop) was running at 3800 rpm's WOT but 38 MPH. Not good.
I changed to a 19" Pitch and the RPM's went up to 4000 with MPH of 34. Still not good. Range is 4800 to 5200 RPM's.
I changed to a 15" S.S. Prop and now get 4500 rpm's @WOT @ 34 MPH.

Anyone have an opinion about whether to leave it as is (close enough) or scour for a 13" pitch S.S. prop?
It runs really well but I'm not sure 4500 rpm is close enough to 4800 or that the low RPM's isn't an indication of a problem somewhere else.
Compression is good and equal, new plugs, new plug wires, and fresh fuel. The only thing I haven't checked is timing.

Thanks

 

[Moody Blue]

Re: Prop & Motor RPM question on Mercury 1150

Sure looks like something is out of whack. You should be running at more than 4000rpm with a 19" pitch and certainly more than 4500 with a 15" pitch.

What tach do you have installed and how is it set?

WHat is the gear ratio of the lower unit?

 

[sportyinc]

Re: Prop & Motor RPM question on Mercury 1150

Thanks for reaffirming my suspicions that something is not right. I don't want to go any lower since the 15" Mercury S.S. Vengeance is a great prop.
The tach is a brand new Teleflex Sahara.
The lower unit is 2:1.
The most puzzling to me is the fact that it idles great, starts right up, especially when warm and is really smooth at WOT.
I pushed the throttle all the way forward and the carb butterflies are all the way open.
The only thing I can think of is the timing. I need to verify it is 21 degrees BTDC.
I just checked with Mercury Props and they recommend a 19" pitch.

 

[arsenalpsu]

Re: Prop & Motor RPM question on Mercury 1150

Assuming you have compression, spark on all 6, and the correct diameter prop, check your trim, I bet you have it too much in= pushes the bow down and slows the boat. A LOT of people over look how important trim is in performance and top end speed. Case in point, cursing at 3/4 WOT, 4200 RPM i can get 35mph to 40 mph depending on how she's trimmed. Or at WOT i can go from 5000 to 5800. Read this... http://www.boatsafe.com/nauticalknowhow/trimtilt899.htm

ALSO- the in-lines like to spin and hate to be lugged, prom them for the high range of WOT, and make sure when pulling a full load or toys you drop 2 pitch sizes. I have a 19p and a 17p depending on what I?m doing (this is on a 17? starcraft 1700) Each drop in pitch is about a 200rpm increase at WOT

 

[sportyinc]

Re: Prop & Motor RPM question on Mercury 1150

Thanks for the input. I missed that point of the trim. I will try trimming it out a hole or two and see what happens.
Does anyone know a relatively easy way to test the fuel for oil content? Just to eliminate that possibility.

Thanks