Major power loss under load

[sandking]

Hello all. I just finished a restoration project on a boat that sat for about 6 years. The motor is a 1972 Merc 1400 inline 6. I have drained and cleaned the fuel tank. Rebuilt all three carbs. Changed the water pump and base. Put in new spark plugs, lower unit oil, etc, etc. The motor purr's like a kitten. I just got back from the first trip with it. Started out great. The motor starts in about 1 second even when cold. When I took off, the motor cavitated (sp?) while trying to plane. So I backed off a little till the boat planed. I was then cruising at about 4800-5000 rpm only for a couple of minutes taking it easy, then it just stalled while under power. It started right back up, but now it will not run past 2000 rpm at WOT. I checked all fuel lines, filter, carbs. The carbs are opening all the way. The choke is ok. We tried to advance and retard timing while driving, tried to pump the primer, we checked all cables to ensure full throw..................................Everything was just where it needed to be except for the speed. The high idle lever will rev the motor upto amost 4000 rpm, but no luck getting that under a load. Any suggestion will be greatly appreciated, since I had spent numerous hours trying to get this boat ready for this past weekend only for it to fail. Thanks in advance, Joey

 

[akita8]

Re: Major power loss under load

Do a compression check to see if you blew a cylinder.

 

[sandking]

Re: Major power loss under load

If it was a blown cylinder, would'nt the motor run like crap? It seems like it would run like it has a miss or something. But I will check anyway. Thanks.

 

[studlymandingo]

Re: Major power loss under load

Welcome to iboats. A compression check will give you an idea of the overall health of the engine. The cylinders should be within 10% of each other.

Have you tried pumping the fuel bulb while you are trying to plane off? If this helps then you have a fuel issue such as the fuel line, bulb, or fuel pump. The line and bulb are an obvious easy fix, the fuel pump can be rebuilt fairly easily, and inexpensively. It is also possible that you got some trash from one of the lines that has stopped up one of the carbs.

After that, you need to check your spark. You can pick up a spark gap tester from your local auto parts store for less than $10.00. You need to make sure each cylinder has a consistent strong spark at 7/16" gap. You could have an issue with spark at higher RPMs which would be indicative of a switchbox or stator.

 

[Chris1956]

Re: Major power loss under load

Sandking, The inline sixes run pretty smooth on 4 cylinders. When you do your compression check, look at the plugs, especially on the bottom two cylinders. A bad fuel pump diagram will flood out the bottom one or two plugs. If the compression is good, you likely have a fuel starvation issue, or a water ingestion issue.

Another test is to run the motor on the muffs and run you fingers over the spark plug wires. If you feel a tingle, they could be arcing to ground.

 

[sandking]

Re: Major power loss under load

Welcome to iboats. A compression check will give you an idea of the overall health of the engine. The cylinders should be within 10% of each other.

Have you tried pumping the fuel bulb while you are trying to plane off? If this helps then you have a fuel issue such as the fuel line, bulb, or fuel pump. The line and bulb are an obvious easy fix, the fuel pump can be rebuilt fairly easily, and inexpensively. It is also possible that you got some trash from one of the lines that has stopped up one of the carbs.

After that, you need to check your spark. You can pick up a spark gap tester from your local auto parts store for less than $10.00. You need to make sure each cylinder has a consistent strong spark at 7/16" gap. You could have an issue with spark at higher RPMs which would be indicative of a switchbox or stator.

We did try pumping the fuel bulb, but it would just bog the motor quite a bit, maybe too much fuel. I know this sounds like a typical lack of fuel problem, that was my first guess along with about 6 other people, but all the fuel looked ok. I pulled the cover of the fuel pump, and the diaphram looked good. But I will rebuild it anyway just to be safe.

It almost sounds as if there is something in the lower unit that is broke and putting too much drag on the motor. Is that possible?

I am going to check the compression and plugs later this evening (I hope). Thanks for all the replies.

 

[sandking]

Re: Major power loss under load

The only thing that keeps confusing me is that it will jump upto almost 4000 rpm with the high idle lever, but not in gear.

I am really keeping my fingers crossed that the compression is ok. I dont know what it would cost to fix, but I dont think it would be worth it on this motor.

 

[Chris1956]

Re: Major power loss under load

Sandking, Jumping up to 4000RPM under no load is normal for a damaged (or good) motor. They can make 4000RPM with no load with two bad cylinders. Actually running a good motor at 4000RPM with no load will make it bad!

 

[sandking]

Re: Major power loss under load

Well....................I have not checked the compression, (I dont have a tester yet) but I pulled the plugs out. #1 plug has aluminum splatter on it. This would be the problem. Numbers 2-5 look normal and #6 was dry and clean. I will still do a comp check to see what the rest of the cylinders look like. What would be the most common cause of this? The rest of the plugs were not lean. Could it be from a possible over-rev while I was trying to plane and it was cavitating?

Anyone have any hints on the best way to perform the fix? Is it even worth it for this old motor? I would be doing the repairs myself by following my Clymer manual. I dont know much about these two strke outboards, but it seems to be more time consuming then anything else. Would I be better off looking for a different motor? Again, thanks.

 

[Chris1956]

Re: Major power loss under load

Sandking, I do not believe that overreving while planing (ventilating) was the issue. More likely a lean mixture, overadvanced timing, bad waterpump or loss of cooling water was the issue. The aluminum deposits on the plug are a bad sign. If you do have one or two bad cylinders, you will need to tear down the motor until the block is vacant of pistons and crankshaft, buy oversize replacement pistons, and have the bad cylinders bored oversize. Pistons are about $95 each and boring a cylinder is about $30. Gaskets will cost a few dollars. You wll also need to determine the cause of the failure and fix it. I would think a carb overhaul, new waterpump impeller ($40) and a link and synch would fix it for you.

 

[roger3645]

Re: Major power loss under load

I'm with Chris on the deposits on the piston caused by overadvance timing. This may have happen when you were advancing and retarding the timing while underway. Check your prop to make sure it is not slipping.

 

[roger3645]

Re: Major power loss under load

Disregard checking the prop, it would rev high and not gain speed if the prop was slipping. What was I thinking?

 

[sandking]

Re: Major power loss under load

Well ,I am kinda giving up on this project for now, just too frustrated.

When I serviced the motor and carbs, I did not mess with the timing at all, I left it where it was from when the last time it ran. Same with the idle screws on the carbs, they were two turns out, and thats where I set the new ones.

After the motor blew the piston, thats when I was messing with the dist to see if there was something wrong there. I dont know how the motor could have leaned. Unless there was some debris floating in one of the fuel lines somewhere. But wouldnt that cause two of the cyclinders to have a lean mixture since there are three carbs for the six cylinders?

 

[Chris1956]

Re: Major power loss under load

SandKing, The top cylinders of a carb pair (# 1,3,5) run a bit leaner than the bottom cylinder of a pair. In additon, cooling water has to be pumped all the way to the top of the motor. This causes #3 cylinder to be succeptable to problems, followed by #1 cylinder.