Good spark and compression, one cylinder only

[Pianh]

I have a 1970 40hp Johnson. A few weeks ago while using her, one of the cylinders stopped firing upon trottling back. I replaced the plugs, checked the spark, adjusted the points, replaced the condenser, and still she is only firing on one cylinder. The compression is 118psi in one cylinder and 120psi in the other. It is the lower cylinder that is not operating. Could it be a reed problem, or something else fuel related? It is puzzling to me in that the symptoms appear spark related but the spark is there, and appears strong.

 

[OBJ]

Re: Good spark and compression, one cylinder only

Hi Newfie...welcome to the forum...<br /><br />When you pulled the plug, was it wet?<br /><br />How much spark do you have? I know it sounds like a stupid question. But....If you can rig a spark tester to measure the spark, do so. The spark should jump a 5/16 to 3/8 gap and be very hot. If it don't, then basically, no spark on the cylinder. <br /><br />Now...help my brain out. How many coils under the flywheel?

 

[Joe Reeves]

Re: Good spark and compression, one cylinder only

Newfie.... That problem is usually caused by one of two things (or both) assuming that the points, condenser, and coil are okay.<br /><br />The first is that one of the spark plug wires has chaffed through which allows it to ground out as the armature plate turns.<br /><br />The second is that too much rocking type motion exists in the seating of the armature plate (the plate the coils etc are attached to). The solution to this problem follows.<br /><br />********************<br />(Magneto Armature Plate)<br /> <br />If the armature plate has a a loose fitting, wobbling motion (the plate that the points, coils etc are attached to), it is usually caused by a slightly worn support ring. This allows the point setting to change erratically as the armature plate turns.<br /> <br />The cure is to remove the armature plate so that the support which is attached to the powerhead with 4 screws is visible. Then, with a screwdriver and hammer, or some tool of your choosing, and looking straight down at the support ring, make an indentation at what would be called the 12, 3, 6, and 9 O'clock positions. Install the armature plate and check for a smooth turning wobble free movement.<br /><br />You may need to do that procedure a few times to obtain the proper fit (not too tight, not too loose), but it's worth the effort. <br />********************

 

[Pianh]

Re: Good spark and compression, one cylinder only

I have two coils under the flywheel.<br />With the engine running (on one cylinder) I am gettin spark with the other. (I attached a third plug to the wire and the spark with that plug is bright and strong and has given me a good jolt.) I was next planning to attach a timing light to the lower plug wire to ensure it is firing through the plug.

 

[Pianh]

Re: Good spark and compression, one cylinder only

No the plug is not wet with water.<br /><br />I do have a little rocking of the armature plate but since I was getting a spark with it the way it is, and since the problem appeared suddenly and not intermittent I wouldn't have thought this the cause. I will try to tighten though.

 

[Pianh]

Re: Good spark and compression, one cylinder only

Joe,<br /><br />Is the support ring (for the armature plate) you are referring to, the removeable one with the notches for the four bolts, or is it the threaded ring recessed in the block.

 

[OBJ]

Re: Good spark and compression, one cylinder only

Newfie....inspect the coil for number two cylinder for cracks in the coating. If cracked, look under it for a black spot. This would indicate that the coil is firing to ground. If both are cracked, wouldn't hurt to replace them both.<br /><br />I also meant wet with gas.

 

[Pianh]

Re: Good spark and compression, one cylinder only

I put a timing light on the plug wire and it shows firing of the plug in the number two cylinder. With the engine running and the timing light inplace, if I pull the spark plug wire the timing light stops. It looks like the ignition is OK. The plugs are black with oil on them indicating poor spark or possible rich mixture so I am going to replace the fuel (although it was fresh when the problem first started and had new quality rated outboard oil) and run the motor in the lake. Since I have adjusted synchronization and gap it may need a good run and adjustment of the low end.

 

[Joe Reeves]

Re: Good spark and compression, one cylinder only

The ring I refered to is the one that is attached to the block with four screws.

 

[G DANE]

Re: Good spark and compression, one cylinder only

Have you checked if the reed blades are OK. I lost one and had a dead cylinder although compression and spark was fine. Pull carb and look at the reeds behind. All blades should be there and none chipped.