Mercury 650 Dead Cylinder

[pgoodger04]

I have a '74 Mercury 650 3 cylinder that I am pulling my hair out on! Sorry this is going to be long but I have tried everything I have thought of as well as scouring the net and service manual for this engine for ideas.

The motor sat for a couple of years before I started using it again.

I started off going through the carbs. Stripped the carbs down to bare bodies ultrasonic cleaned, all new gaskets, needles and seats. All orifices have been shot with carb cleaner and then air shot down each bore. All passages are clear and clean. Checked floats made sure that they both floated and then reset the primary and secondary levers. Then bench tested with fuel and primer bulb. Needles to say the new seats leaked like a sieve. So the Old ones went back in.

I checked the compression.

Cylinder 1 130 PSI
Cylinder 2 130 PSI
Cylinder 3 128 PSI

I put a new carbon button and spring in the distributor and new plug wires and coil pickup wire. New Champion L77V plugs. With the plugs out of the cylinder head and grounded all three plug have a good spark. Putting a steel drift in the end of the plug wire you can get a spark to jump to a ground from 3/8 to 1/2 inch for each plug wire.

The top cylinder is running great, Cylinders 2 and 3 have a miss or are dead. You can remove the plug wires from these 2 cylinders and there is no change in rpm. Every once in a while there will be a surge in rpm indicated on the digital tach

The bottom 2 cylinders the plugs are wet every time I take them out.

I have checked the timing and synchronization of the engine according to the service manual. The power-head will not run well enough to get the timing and synchronization properly set. I have to jack the primary pick up to about 8 to 12 BTDC to get it to even stay running @ arround 1000-1200 rpm in a body of water

Both carbs appear to have the same amount of vacuum coming into the throat of each carb. I dont have a way to accurately test this. It doesn't matter where the idle adjustment is set on the bottom carb nothing seems to work.

I am going to take the floats back out tomorrow and weigh them individually on a gram scale. The bottom carb is getting a small pool of fuel right infront of the main nozzle.

I am thinking the smoking gun is the bottom carb float level is high and I am flooding the bottom 2 cylinders or is there something else that I need to be looking at that I have missed?

Any help would be greatly appreciated.

 

[Chris1956]

A couple of things. There were different inlet needles and seats made for mercs over the years. Older were brass seats with rubber inserts and nylon needles. Newer were solid brass seats stainless needles with neoprene tips. The newer ones were better, but you cannot mix 'em.

Old floats were foam and tended to dissolve in alcohol gasoline. Newer floats were hard plastic. newer is better.

Older Mercs can suffer from water infiltration. The water puts the spark out, and they won't run on a cylinder or two.

Bad lower crank seals cause water into the crankcase and cylinders. Leaky exhaust baffles and water jacket covers also cause water to get into the crankcase.

Crank the motor on a flusher with the plugs out and see if you get water on a rag. Inspect the spark plugs for water droplets.

 

[Faztbullet]

Pullthe exhaust cover andinsecpt it for leakage along with cylinder inspection...lower seals maybe bad allow water into lower cylinders or cylinder maybe scuffed on #2 an 3

 

[pgoodger04]

I have replaced the floats with the newer plastic floats a couple of years ago when I had the motor running. I had one flat that was saturated. As for the needles and seats. I put new needles and seats in and the lower carb I never could get the newer style to seal correctly. So the lower carb has the older style brass seat with the rubber inserts.

I pulled the plugs and put the flusher on and cranked the engine. The only thing on the rag was gasoline.

I am going to bore scope cylinder 2 & 3 and see if there is any scuffing.

By the way I forgot to mention the water pump was just replaced.

I am at a loss on this project

 

[Chris1956]

You can remove the transfer covers and inspect the cylinders.

 

[pgoodger04]

Well I think I found the culprit, I pulled the transfer covers for the cylinders. The cylinders didn't show any signs of scuffing, However the lower valve type main bearing assembly. between #2 and #3 cylinders there is a chip on the leading edge of the top reed closest to the transfer plate side.

I can only assume that this is causing an issue with crankcase pressure/vacuum issues between the 2 cylinder that are dead.

I am going to see about pulling the powerhead and tearing down the engine to put new reeds and stops in.

Are there any other parts/pieces that I need to look at purchasing other than gaskets/seals.

Thanks for all your help and any suggestions are greatly appriciated!

 

[Chris1956]

You can see the reed valves from the transfer ports? I didn't think that was possible. Well, maybe your motor is different from IL4/6 motors that I worked on.

 

[Linkedit]

How long would you crank a motor over for to see if there was water on a rag? I imagine water would be quite apparent. Or would they be minuscule?

 

[Chris1956]

For water to exit the cylinders via the spark plug holes, it would need to be pressurized by the impeller, fill the chamber below the end cap of the motor, pass the seals of the endcap and find it's way into the cylinder and out the spark plug hole.

Of course this presumes no water is already in the crankcase.

It is easier to detect the water after she has run on a flusher, as she will have residual water in the crankcase.

 

[Chris1956]

Since oil and water don't mix, the water beads up on the oily rag.

 

[pgoodger04]

Just to give everyone an update, the power head has been pulled off and striped of all the accessories. When I get to tearing into the power head hopefully this weekend I will post pictures.

This particular engine has what the parts manual is calling valve bearings. The main bearing journals between cylinders 1/2 and 2/3 have 2 reed leafs and reed stoppers located on the to and bottom of each bearing.

After pulling the power head I pulled the exhaust manifold/exhaust tube and made a plate to seal the exhaust port to perform a leak down test on each cylinder. Each Cylinder was rolled through to their compression stroke and pressure was applied Cylinders 1 and 3 pass a leak down test. Cylinder 2 failed miserably. There were no other bad gaskets or seals found on the power head.

With the chip in the reed on the top 2/3 cylinder bearing. This allows the engine when running to cause a lean condition on the engine due to the reed allowing the air/fuel mix in cylinder 2 to be forced into cylinder 3 and back out of the carbs. It also Loads up cylinder 3 where it causes a super rich condition. I hope this explanation helps anyone that is searching for more info on this.

This is only pic I could find on the web. This reed bearing is out of a 4 cylinder model

 

[Chris1956]

Reed valves are pretty reliable. I would be interested in seeing the reed valve damage.