135 Black Max low rpm

[Officerelmore]

I have a 1991 Regal Valanti 17 ft with a Mercury 135 Black Max. The motor runs great. The boat is rated for 1100 lbs. The motor has a 14 x 19 stainless prop on it.

The problem I'm having is when the boat is loaded down(about 800 lbs) it is having trouble getting on plane. The tac is showing the motor running at 4900 RPMs. I know that's low, I just don't know what to do to get it up. I've been told to get a different prop with a different pitch, but I'm not sure what pitch to use.

Any help is appreciated.

 

[racerone]

Step # 1 is a compression test.----Then check for strong spark on each lead.----These motors run quite well on 5 cylinders.

 

[Officerelmore]

Sorry I should have mentioned that I already checked comoressiom. 130 I'm all 6 cylinders. Spark is good and strong and has new plugs.

 

[Georgesalmon]

Going down to a 17 pitch or maybe a 15 would help. I'd start with a 17 pitch 4 blade. Merc makes a good one that's inexpensive. Called a spitfire I think. I'm real happy with mine. Your top speed will suffer a bit. I have to change props, have one for playing and one for heavy loading/water sports.

 

[Chris1956]

A 17"Pitch prop would give you 5300 RPM in theory. You may get a few more RPM as you will be more in the motor's powerband. You are trimming that motor, when on plane to get max RPM/speed? I like the Turning Point Hustler's in 3 blade.

 

[Officerelmore]

Yes I'm trimming to get the max speed out of it. A friend of mine told me to go to a 17 pitch as well so I think I'll try that. Thanks guys I'll let you know how it turns out

 

[Officerelmore]

Quick update. I swapped the prop out for a 14x17 and there was little to no change in rpms. It actually started rinning lower about an hour into the boat ride. It was running 4200 rpms. I swapped the prop a back out again and it was still runNing 4200. About ab hour later it jumped up to 5000 for a few seconds then fell back down to 4200. If did this 3 to 4 times and onot the last time stayed at 5000. Now it is running consistently at 5000 and every once in a whole it will jump to 6800 for a few seconds and then fall back to 5000

 

[Officerelmore]

Also it turns out the reason it was having trouble getting on plane was because of the stupid whale tail fin on the back of the motor. I removed that and it no longer has any trouble getting on plane. The rpms are still lowe though.

 

[Chris1956]

Is your tachometer is working properly? RPMs should never jump from 5000 to 6800, unless the prop is sucking in air or the prop hub is faulty. When the tach reads low, do you hear the motor slow down? You may have a bad spark plug, plug wire or coil.

 

[Officerelmore]

I'm sorry that was supposed to say it jumps from 5000 to 5800. Not 6800. There is a define difference between the two. When it jumps the boat gains a lot of speed for the few seconds that it stays up and you can hear the difference too.

 

[Chris1956]

OK, it sounds like you are running on 5 cylinders most of the time, and then 6 cylinders, when the RPMs jump up. Since compression is good and spark is good, the issue is either fuel or perhaps water intrusion. Inspect the spark plugs and see if one looks different that the others. That motor has one carb throat for each cylinder, so it is possible one has a dirty main jet. Alternatively, it could be a fuel shortage at high speed. Does pumping the primer bulb help it to stay at high RPM?

 

[Faztbullet]

Its not a fuel issue as if lean/water or jet stopped up it would have cylinder damage by now. From cowling in picture it about same year as boat, it sound like a wiring/switchbox or bad plug as have had several here lately bad out of box(NGK)