76 Mercury 1500 - 5000rpm @ WOT regardless of prop

[djamloops]

Hey everyone, new user to the board. Can't seem to figure this out so I thought I'd see if anyone here can help. I have a 76 Mercury 1500 (150hp) on a 79 Tahiti 18' runabout. My mechanic has tuned, timed, synced, and tested my motor. It has great compression in all 6 cylinders, everything has been gone through, new wiring harness, new spark plugs and wires, etc. etc....So here's the problem. I have tried 3 different pitch props. 21, 19, and 17, and it will not rev past 5000 rpm @ WOT. It pulled to 5000 rpm with the 21 and topped out at 44 mph. I dropped down to the 19 and again it pulled to 5000 rpm, and topped out at 42 mph. With the 17, it pulled to 5100 rpm and topped out at 41 mph. Now according to what I know about prop pitch, I should gain about 200 rpm/ inch dropped. So theoretically, with a 17 pitch prop, (assuming the motor could not pull past 5k with the 21) I should be able to reach the engines listed redline of 5800 rpm. Right?? any thoughts on what could be limiting my power or rpm at WOT?? is the engine not getting enough air or fuel? distributor problem? help!!

 

[Silvertip]

Re: 76 Mercury 1500 - 5000rpm @ WOT regardless of prop

Using 5000 RPM and a 2:1 lower unit ratio (10% slip) I ran the numbers through a prop calculator and it seems to agree with your numbers so I would say the engine is not making the power it should. Is the boat water logged by any chance which has made it much heavier than it should be? I suspect however some additional diagnostic work on the engine is in order. Wide open throttle timing is one thing that may not have been done. Push the throttle lever fully forward then check the carbs to make sure the throttle plates are all perfectly horizontal in the carb bores. If they aren't, link&sync was not done properly.

 

[djamloops]

Re: 76 Mercury 1500 - 5000rpm @ WOT regardless of prop

It's possible that the boat is heavier than it should be. The timing and carbs were just done at a mercury repair shop. But what doesn't make sense to me is that if I can get to 5k with a 21 pitch prop, why would I only gain 100 rpm dropping 4 inches in pitch?

 

[Clams Canino]

Re: 76 Mercury 1500 - 5000rpm @ WOT regardless of prop

I'd guess the switchbox is limiting you because it's somehow failing. I'm assuming the carbs are cleaned and opening all the way at WOT.

PS: I don;t trust Mercury dealers with the oldies... try an independant repairer that over 45 years old to actually know that motor.

-W

 

[Silvertip]

Re: 76 Mercury 1500 - 5000rpm @ WOT regardless of prop

Checking to see if the throttles are wide open is simple. Just remove the air box and look. If the engine is incapable of reving over 5000 RPM it doesn't matter what prop you have on it.

 

[Texasmark]

Re: 76 Mercury 1500 - 5000rpm @ WOT regardless of prop

I thought cables weren't going far enough too, and was going to suggest it, but when you drop 4" in pitch you should be unloading the engine significantly so that it will make more rpm'sat the same throttle setting.

Carb butterfly, timing, all that stuff is dependent on engine load and unloading the engine unloads it and it should rev higher at the same throttle setting...........like look high high (and how easily) it can rev in N (normally) if you let it go.

Don't know if you had a rev limiter on that old an engine, but if you did that makes sense as that is independent of (or should be) anything but rpm's. I'd go after that.

Mark

 

[jddenham]

Re: 76 Mercury 1500 - 5000rpm @ WOT regardless of prop

Try a different tachometer.

 

[Texasmark]

Re: 76 Mercury 1500 - 5000rpm @ WOT regardless of prop

Now that's a novel idea. You may well have hit upon the problem.

Mark

 

[Silvertip]

Re: 76 Mercury 1500 - 5000rpm @ WOT regardless of prop

I doubt its a tach issue because regardless of prop the speed only changed 3 MPH. As for loading -- in neutral there is zero load (except for exhaust back pressure). With "any" prop and in gear there is substantially more load. More than enough to cause a poor running engine to display its inability to make power. I'm thinking more like weak fuel pump, lazy timing advance, or as was mentioned, ignition system failing at the high end.

 

[Texasmark]

Re: 76 Mercury 1500 - 5000rpm @ WOT regardless of prop

Looks like I missed a tidbit of info. :%

Mark

 

[j_martin]

Re: 76 Mercury 1500 - 5000rpm @ WOT regardless of prop

I'd temporarily T in a fuel pressure gauge after the fuel pump and filter, and before the carbs. You'd be surprised how much fuel pressure you can lose through that little filter. You'd also be surprized at how expensive it can get if you're running lean. Figure it out before you blow a hole in a piston.

 

[Silvertip]

Re: 76 Mercury 1500 - 5000rpm @ WOT regardless of prop

Have someone pump the primer bulb while at WOT. If the engine perks up a bit, its fuel pump/filter time.