1970 4 hp Yachtwin rough after 20 minutes

[KC49111]

I serviced the engine recently with helpful advice from Chinewalker and Paul M. <br /><br />New impeller and housing<br />Carburetor clean/rebuild<br />Plugs, wires,coils (left condensers and points)<br />New head gasket and cleaned passages<br />Cleaned and reassembled the fuel pump<br />Decarbed with Engine Tuner<br /><br />The motor runs well in the tank. This weekend was it's first endurance test. It runs fine at various speeds for about 20-25 min and then starts to run rough, sort of a lurching combined with an intermittent miss. It doesn't quit and once seemed to resume power, but it doesn't sound healthy. After being shut down for a while it would repeat the same behaviour of run fine and then rough (4 times).<br /><br />-Squeezing the fuel bulb didn't make a difference<br />-Plugs looks equal, a bit on the oily side<br />-Coil wires shouldn't be rubbing on the flywheel<br />-Pistons and rings looked OK when I had the head off.<br />-The armature plate has some slight play, but I would think this would create a persistent problem if it was an issue.<br />-Cooling seems fine. The head is hot but not boiling.<br /><br />It's hard to tell where the problem is. I'm considering a new fuel pump, but any advice would be welcome. Thanks.

 

[Paul Moir]

Re: 1970 4 hp Yachtwin rough after 20 minutes

When you pulled the head, did any water come out? Is the head still off?

 

[KC49111]

Re: 1970 4 hp Yachtwin rough after 20 minutes

Hello Paul, I have not had the head apart since putting it back together a few weeks ago. At that point there was the usual damp salt residue to clean out, with no sign of rust on the cylinders. The flow from the relief holes is much stronger and cooler than previously, so I'm assuming the head is getting sufficient water.

 

[Paul Moir]

Re: 1970 4 hp Yachtwin rough after 20 minutes

You see, there's two parallel cooling paths so you can have good flow through the block, yet still have overheating problems. There's a number of small passages that could be blocked or partly blocked.<br /><br />Looking at the back of the engine, the water comes up behind the exhaust plate (forward 1/4 or so) cuts across the top of the block through the rounded 'pipe' section, and down the intake side of the block through the 'pipe' cast into the right hand side. From there it goes out with the exhaust.<br />That's 1/2 of the cooling system, and most the water goes through that way.<br />The cylinder jackets and head are fed from where the 'pipe' running across the top of the block cuts into the cylinder jackets. It's emptied through a small hole at the bottom of the cylinder jackets, just to the left slightly. It then goes through a narrow passage between the block and the base gasket, and out with the exhaust.<br /><br />I suspect you have a clog or restriction somewhere in the second circuit. Unfortunately, it's most likely (IMO) in that passage under the block, which necessitates removal of the powerhead from the outboard. Did you pull the powerhead when you replaced the head gasket?<br /><br />I don't think it's the fuel pump that's the problem. If it were, pumping the primer bulb would have made it run better or worse (in the case of a broken diaphram).

 

[KC49111]

Re: 1970 4 hp Yachtwin rough after 20 minutes

I'm likely fixated a bit on the fuel pump because it's the only external part I disassembled but did not replace, and I've seen it listed as a replacement part only.<br /><br />I had the leg and exhaust tube off leaving only the lower case attached to the powerhead while freeing the driveshaft. The intake tube is clear.There was one visible water outlet hole when I removed the exhaust tube and compressed air seemed to flow out of this easily when blown from the intake side. Am I understanding correctly that this is the end of the 2nd circuit, and the first circuit ends further up in the head and flows out with the exhaust?<br /><br />While cleaning the jackets I could see the figure 8 shape around the cylinder interersected by the water tube passing across at the top. I was as careful as possible, but don't recall seeing/feeling any exit holes visible at the bottom of the figure 8.<br /><br />I assume you're thinking of a heat problem. Anything in your experience using temperature crayons or the like? I'd like to avoid disassembly if I can. Thanks again