Running on one Cylinder

[BobRip]

I had a minor problem with the PT422 and Robin engine yesterday. I started it up and it ran rough. I pulled one plug wire and there was no change in engine running. I pulled the other and the engine stopped. It would not run at the normal idle setting. I pulled the plug and it was wet. I checked the spark and it was OK. I replaced the plug with a used one in good condition. Same problem. I looked at the air filter and the foam prefilter was seriously dirty. I cleaned that. The filter looked good. I spun the engine over with the plug removed and replaced the plug again. I then ran it for about five minutes and the plug starting firing again and it ran fine. I then mowed for about 30 minutes and everything was OK. I guess I need to check the prefilter more often.

 

[cqaigy2]

Thanks for the heads up!

 

[BobRip]

I just wonder if my diagnostics are correct. Any suggestions on this. I used to run platinum plugs and should probably go back to them.

 

[3238dpw]

Yeah, check that air filter often! I know a guy who owns a large lawn service. He said they used to change the oil in their mowers once a week, but over time and experimenting they figured out that they could extend the service intervals longer than recomended without doing any harm to the motors as long as they cleaned the air filter every day. Of course this is in some of the toughest conditions like hot dry and dusty. When it was real dusty he even went as far as to give the crews extra filters to change during the day.

 

[BobRip]

Yeah, check that air filter often! I know a guy who owns a large lawn service. He said they used to change the oil in their mowers once a week, but over time and experimenting they figured out that they could extend the service intervals longer than recomended without doing any harm to the motors as long as they cleaned the air filter every day. Of course this is in some of the toughest conditions like hot dry and dusty. When it was real dusty he even went as far as to give the crews extra filters to change during the day.

Good info, thanks. I need to get several of the foam prefilters and clean a bunch at one time. Then swap out often.

 

[J_J]

Good info, thanks. I need to get several of the foam prefilters and clean a bunch at one time. Then swap out often.

Bob.

I don't think the filter had anything to do with the one cylinder not firing. If for some reason the one cylinder does not fire, and gets wet from fuel especially with full choke setting, it will stay wet and and bleed the spark to ground. If that cylinder had spark, the best thing to do is to dry the plug and try again with out choke. They make an in-line light that will flash if the coil is putting out voltage. Take the plug out and connect wire and crank over and see if the plug fires. You can definitely tell when a cylinder is not firing.

 

[BobRip]

Bob.

I don't think the filter had anything to do with the one cylinder not firing. If for some reason the one cylinder does not fire, and gets wet from fuel especially with full choke setting, it will stay wet and and bleed the spark to ground. If that cylinder had spark, the best thing to do is to dry the plug and try again with out choke. They make an in-line light that will flash if the coil is putting out voltage. Take the plug out and connect wire and crank over and see if the plug fires. You can definitely tell when a cylinder is not firing.

You have some good points. I checked for spark with a screwdriver and of course tried a torch to the old plug, and then new plug. I did start with full choke and did not try starting with no choke (probably the problem). Will the light tell when the plug is bridged by gasoline or whatever? I have a spark gap device like this and did not use it here. I will look for a lite today when I get platinum plugs. I was kinda surprised when I had to let it run for several minutes before it finally fired. It might be a weak coil (if that is possible).

 

[SnowRidge]

If you have a badly clogged air filter, the engine will pull a high vacuum, which will tend to suck gas and/or oil into the cylinder. The oil either gets past the rings or the valve seals. The gas gets sucked out of the carb. If there is insufficient air flow due to the clogged filter, the gas won't atomize properly.

If any of it gets deposited on the plug in a liquid form, the plug won't fire, so it could have been the clogged filter.

Once you changed the plug, the cylinder probably wouldn't fire because it was wet with gas. The fact that it took five minutes to dry out and fire is very unusual. Possibly, there is something wrong with that cylinder.

I assume the Robin has an automatic compression release like the Kohler. Those things prevent you from accurately testing the compression with a simple compression tester. A leak-down tester being required to accurately test the compression.

However, you can still do a relative test with a simple compression tester. If one cylinder is markedly lower than the other, it will be bad. Test at WOT with both plugs pulled.

There is such a thing as a weak spark. It usually isn't the coil that causes it, but it isn't out of the question. Bad spark plug wires are the most likely cause.

 

[J_J]

You have some good points. I checked for spark with a screwdriver and of course tried a torch to the old plug, and then new plug. I did start with full choke and did not try starting with no choke (probably the problem). Will the light tell when the plug is bridged by gasoline or whatever? I have a spark gap device like this and did not use it here. I will look for a lite today when I get platinum plugs. I was kinda surprised when I had to let it run for several minutes before it finally fired. It might be a weak coil (if that is possible).

Bob.

The ignition light only tells you that you have high voltage going to the plug. If the light is flashing and the engine is not running right, it means that the voltage is being sent to ground to ground through the plug, Bad plug, carbon-ed up or wet. If the plug will fire outside the cylinder, you should be good to go. I used to make ignition testers out on those NE2 bulbs, just spread the leads and wrap around the spark plug cable. I also used the old transistor radios to time the engines. You could rock the flywheel back and forth and hear the static when points opened and the coil would dump that high voltage.

 

[BobRip]

Snow, J_J, The engine is running a starting fine. I did put new platinum plugs in it today. It is my intent to just use it and not do any more test. The compression test would be easy and I will try it soon, I hope.