Low RPM..only reaches 3k

[Tinlizzy]

Hello yet again

This is in regards to a 1971 1150 (115 hp) Mercury outboard. On a 19' closed bow with a 17" pitch prop (3 blade).

When I ran the boat in the lake a few weeks ago it would only reach 3000 rpm at WOT. So I decided to do a link and sync. I finished that today, primary pickup is set to 3 degrees BTDC and max advance is set to 21 degrees. The carbs are opening fully at WOT. I have verified all of that.

I ran the boat again today and I am still stuck at 3000 rpm at WOT. I checked the timing on the lake at WOT and it was firing at 21 degrees like it is recommended. So I do not think its a timing issue.

I installed an inline fuel filter a few weeks ago, and the few times I have ran it maybe 30 mins all together there seems to be a lot of sand/black particles in the filter area. (clear inline filter) Also the filter does not stay completely full of fuel when the motor is running. Even when primed it allmost seems like air is getting in there.

Also it looks like what seems to be a bit of fuel sprayed/dripping around in the carb areas. Over the three of them. Is that normal?

It would not start a few times and died a couple of times when we were on the lake. I removed the filter, tried priming the bulb and the fuel was not going into the filter but the bulb was getting hard a few times, then soft. Allmost like an air lock?

I was not sure if it could be a fuel problem because everytime it stops at 3000 rpms. not like 2500 a few minutes then 3500 etc. There is a consistency to a point.

Also I have no reverse gear, as I posted in another thread I lost it when changing the impeller. I only have forward, would that cause this problem?

So with that information (sorry for the long post) why am I only reaching 3000 rpm? The prop pitch should be under propped I would think. I thought I should be running a 21" pitch for this boat. This is a new to me boat/motor. And the owner before me put this motor on it and never ran it in the water with this boat. What do I do next?

Thanks in advance all.

 

[Tinlizzy]

Re: Low RPM..only reaches 3k

/bump anyone?

 

[Chris1956]

Re: Low RPM..only reaches 3k

Tin, Your inline fuel filter is behaving normally. If you do not get an increase in RPM at WOT when you pump the primer bulb, the fuel pump and inline filter are likely fine.

I disagree about the pitch on that prop. On a 19' boat, a 21" pitch prop will likely be quite overpropped. The 17" pitch prop may be optimum, depending on the weight of the boat, and hull type.

Check your spark plugs for fuel and water fouling. It sounds like the motor may be running on 5 cylinders. It is usually the #6 cylinder that doesn't fire, provided compression is good on all six. No. six is prone to fouling due to a leaking fuel pump, spark plug wire arcing to ground or water infiltration.

 

[Tinlizzy]

Re: Low RPM..only reaches 3k

Thank you so much for the reply Chris. Now I know where to start.

I will start by checking for spark on all of the cylinders today/wire conditions. Will there be a difference in spark from idle rpm to WOT? In other words, is it ok to just check for spark while the motor is idle?

Before I purchased this motor the previous owner replaced the powerhead. It still has the sharpie marks on it from where he purchased. 140 # on all of the cylinders. I have not checked it myself personally.

Thanks for the prop advice as well, I am a newbie yet

Is there a way to check for a leaking fuel pump? It looks to me like he may of rebuilt the upper fuel pump, the gasket on the fuel pump base looks new/changed recently compared to the other pump. The top pump is near the number 3 cyl. if I remember correctly. And the lower is near the number 6 cyl. Which has me thinking about what you said in regards to the fuel pump leaking and fouling out the number 6 cyl.

I will check this out today.

 

[Chris1956]

Re: Low RPM..only reaches 3k

You can check for arcing of the spark plug wires at idle, at night, or you can listen for the "snap-snap" of the arc. I also run my fingers gently over the wires to feel the tingle.

I do not have too much experience with the dual fuel pumps. It is likely that raw fuel will leak into the cylinder that the pump is mounted on, if the diaphram is bad. If you see a spark plug loaded up with fuel or oil, and it matches one that has a pump mounted on it, it could have a bad diaphram.