1985 50 HP 4 Cylinder Mercury losing power at WOT

[Alw1]

I have a 1985 50 HP mercury 4 cylinder SR# 0A103332 on a pontoon boat.
I purchased it from a friend about a month ago and he said that it ran but not very fast. It had been sitting for about 2 years under a carport. I have rebuilt the carbs, fuel pumps, replaced spark plugs, checked plug wires, replaced bad stator, ohmed out trigger and adjusted floats back and forth to see if it was a flooding problem. Also replaced primer bulb and hose. I have also replaced the rectifier that was bad. It runs and idles great but bogs between 3/4 and full throttle. I took it out Sunday to see if my last adjustment of the floats had any effect and it idled and ran good up to 3/4 then bogged at WOT again. I backed it off to just before 3/4 and let it run for a while. All of a sudden it opened up and took off and ran good for about 10 mins then went back to bogging again. I have ordered a compression tester and it should be here today.
I also just received a DVI adapter to use with my fluke meter to test voltages. I have downloaded specs for all electrical components and will be testing Trigger, Stator, and coils this afternoon. Is there specs to test the switch box? I am trying to locate the problem without just throwing parts at it hoping to get lucky. Any info would be greatly appreciated.

 

[Texasmark]

You replaced the primer bulb and hose but you said nothing about the internal rubber; hoses and the fuel pump diaphragm. Bet your problem is a black piece of internal hose that fell off a piece of degraded hose and got past the fuel filter and into your high speed jet in one of your carbs.....if you have one on that engine....don't remember the old classics having internal filters in the fuel lines.

Since there is gas in the carb and bouncing around and various fuel pressures at different speeds, at one point your piece of crud became dislodged and you had stellar performance till it found it's way back where it was, blocking the high speed jet on one of your carbs.

Get your carbs cleaned out and include 100# compressed air and if the jets come out pull them and look into and through them. Put your air on tubes and in holes and with the bowl off and the carb casting in your hand, not on the engine, stick the air in a hole and ensure it comes out somewhere....if not find out why not.

 

[jimmbo]

A 1985 probably would only have a screen on the Fuel Pump(which is likely part of one of the carbs) as a filter.

 

[Alw1]

It has a fuel filter after the motor quick connect with a plastic screw off cover and round cylinder shaped screen filter. I took it apart and checked it for trash did not see any. I did rebuild the fuel pumps when I did the carbs forgot to mention that. Before the carb and fuel pump rebuild the primer bulb would not pump up hard but now it does, Everything checked out yesterday with ignition getting good spark on all 4 cylinders. Did not receive compression tester yet so have to wait on that one. I bought new fuel line to replace everything from the engine quick connect to the carbs so all fuel lines will be new by this evening. I do hope that it is a fuel line separating that would be so simple a fix. I have tried pumping the primer bulb while it is bogging at WOT but I see no change other than it sounds like It is starting to flood if I continue to press after the primer bulb gets hard. I have noticed fuel squirt out of the bottom carb's top left port if I continue to press hard after the bowls are full. I will pull carbs while replacing fuel lines today and go through them again.

 

[Alw1]

Well I replaced all the fuel lines and retuned the floats one more time and took it to the lake which is only 4 miles from my house. When I tried it out it was still bogging down at wot. I got my son to drive while I looked at it and noticed that the choke plates were not level when wide open. There was a spring that would not let it open all the way up that was attached to the choke linkage. I have taken it off and put it back every time I worked on the carbs not realizing that whoever worked on it before I bought it had it on wrong. Anyway it hit 5200 RPMs with no surging or bogging out and ran fine after that. I would like to thank y’all for the advice and suggestions they certainly helped out and pointed me in the right direction.

 

[Texasmark]

Glad you got her fixed. Yes it is a problem trying to remote problem solve. Little things go overlooked and as in your case may be the key to the city. Whatever repairs you did in your search for the solution weren't time wasted........you would have been there sooner or later anyway and now those chores are behind you.

 

[Alw1]

I ran the boat this weekend and had a good time running around the lake. The motor started and ran good up to the very last push of the throttle leaver. I took note of a few things that I have some questions about.
First I ran the boat with just me and my wife and it ran about 5200 rpms , when I added 2 more people it ran 4200 rpms and was working noticeably harder. With 6 people it went down to 3600 rpms. Is this normal? This is my first pontoon boat so I have nothing to compare it to. Also noticed that when running all 3 scenarios the engine would seem to bog down similar to the way it did before when I would give it the last push of the throttle leaver. I’m talking about the last 1 1/2 to 2 inches of throttle lever it would seem to try to bog then I would back off and find the sweet spot and leave it there. It seems to me that maybe timing advance or a coil might not be hot enough for that last push of WOT. I have not touched the timing adjustment on this motor but I did check the flywheel to see if it was at zero when the #1cylinder was at TDC and it was. This is the numbers off the prop. 48 8 818 A10 9P this exactly how it looks spaced and all. This is a 20 ft bass buggy pontoon boat. Is this the correct prop or do you think the previous owner changed it?

 

[Texasmark]

2 strokers have to get their power from RPMs since they run around 1 cu. in. per hp and HP is (torque x RPMs)/5252 and low cu. in displacement 2 strokers don't have the makings for much torque.

Once you get at the peak of the torque curve, on the way down in RPMs with your excessive loaded boat, rather than torque increasing as RPMs drop which is the usual case when you are running at the top of the RPM range, the torque drops as the RPMs drop and you get to a runaway till it just croaks and quits.

Decide on where you boat the most and prop for the upper limit of the RPM range for that. That will give you some latitude when you carry your 6 folks and you can "restrain yourself" when lightly loaded.....keep the throttle backed off as necessary to keep the RPMs in reason.