Compression for 50 HP Merc 4 stroke

[usraptr]

Hello,
Recently purchased a 2000 16 Sylvan aluminum fishing boat with a 50 HP Mercury 4 stroke engine, model # 50 ELPT 4s.
Engine starts easy and runs smooth. However, it is extremely slow out of the hole and only has a top speed of 27 MPH according to boat speedo when trimmed out. I'm not overly concerned about top speed but am concerned about taking so long to get up to speed. My son suggested it might be low compression. I ran a compression check and all four cyclinder registered between 110 and 115 pounds. Does anybody know what the compression on this engine is supposed to be.
FYI, boat has stock prop. 12X22
thanks
Brad

 

[longshanks]

Re: Compression for 50 HP Merc 4 stroke

Hi Brad

I have an '06 50, and compression reads at ~180 on all four. It is my understanding that even compression is most important. I'm sure that others will have input.

Do you know what rpms you are running at @ WoT? Unless you are running close to max rpms (close to 6000 for that motor, I think), it will be extremely sluggish. For reference, I am running a 12 pitch by 11" diameter, and turning around 5900 RPMs, getting around 31 mph.

you may want to re-check that you are correctly propped.

hope this helps.

 

[oldrock]

Re: Compression for 50 HP Merc 4 stroke

I have an 01 2 stroke 50hp merc so i know it is a bit of apples to oranges but I would think you should get more spped than that on a 16foot boat. I was told by the dealer that 2 stroke and 4 stroke are just about equal top speed wise and the only benefit of 2 strok is a bit faster plane. My 17 1/2 foot tracker gets 32mph with 3 passengers and full load of gear and I can get 35mph if I trim it out for max speed with a light load. I did change out the prop though so maybe prop is something to look at. Also, my boat takes about 5 seconds to get up on plane from idle so not sure if that is considered fast or slow to you but that is what I get.

Whale tail will help on getting up on plane btw.

 

[JB]

Re: Compression for 50 HP Merc 4 stroke

Sounds severely overpropped to me, but there isn't enough info to be sure.

RPM at top speed is critically important. It should be at or near 6,000rpm and I don't see that happening at 27mph. With a 22" pitch I would expect near 40mph at 6,000rpm.

To resolve it, you need a tach on your engine, at least for testing.

110psi seems low for a 4 stroke, but guages and technique vary too much to make a judgement based on that. They are even, which suggests that it is okay.

 

[usraptr]

Re: Compression for 50 HP Merc 4 stroke

Just found out that my prop is not a 22 pitch afterall. I was misreading a label on the outdrive. Unfortunately I can't find any writing on the prop whatsoever to indicate what size/pitch it is and the boat doesns't have a tach so I don't know what RPM I'm running at WOT. My son, the mechanic, said that if the compression is low, that would be a factor in why it takes so long to wind up, so to speak. That's why I'm curious as to what the compression is suppose to be.

Thanks for the input and if anybody has an anser on the compression issue I'd appreciate it.

Brad

 

[JB]

Re: Compression for 50 HP Merc 4 stroke

I gave you an answer.

Get a tiny tach and tell us the rpm at top speed.