Cub Lo-Boy intermittent misfire

[lhfarm]

64 Cub Lo-Boy runs perfectly most of the time, but has started to run rough, I think because of a misfire now and then. Some times it will start, but run rough. It may or may not smooth out after a short period. I put it away running rough and when I went to start it yesterday, it ran perfectly without a misfire. I've purchased new plugs, points, condenser, rotor and cap. I don't remember the last time it had a tuneup.

I'd like to know what is causing the problem, so am trying to figure out how to approach this. I am thinking of replacing one thing at a time (I'd do all the plugs at once) just to see what "fixes" it. Or should I just do the tuneup and forget trying to solve the mystery?

Does anyone have a suggestion of a probable cause? I didn't buy new wires, so I figure there is a good chance that will turn out to be the problem. How would you approach this?

On a related note, the tuneup kit cost $88. The cap was $31! That seems crazy to me.

Thanks,

 

[Soundguy]

i like to start troubleshooting before I start buying parts.

otherwise you can get down to repalcing the entire tractor if you keep guessing the wrong part.

1, are you sure it wasn't bad gas / water in the gas?

2, assuming battery ignition, I'd jump the ignition switch .. and in fact jumper out anything but any resistors inline with the coil.

next check spark.

if spark is good and regular as viewed from the coil wire to the block, then I'd move on to fuel and intake.

an intermittant miss or put-put can even be a sticky valve issue. especially easy to check on a flatty.

soundguy

 

[Argosy]

You've got the parts, you don't know when it was last done, tune it up. Pay attention to the old plugs, they can tell you a lot about what the problem is.

 

[lhfarm]

i like to start troubleshooting before I start buying parts.

otherwise you can get down to repalcing the entire tractor if you keep guessing the wrong part.

1, are you sure it wasn't bad gas / water in the gas?

2, assuming battery ignition, I'd jump the ignition switch .. and in fact jumper out anything but any resistors inline with the coil.

next check spark.

if spark is good and regular as viewed from the coil wire to the block, then I'd move on to fuel and intake.

an intermittant miss or put-put can even be a sticky valve issue. especially easy to check on a flatty.

soundguy

The tractor was retired as my primary mowing machine about 6 years ago. So it doesn't get run every week. Since I'm sure it hasn't had a tuneup in at least 6 years, I figured it was past due.

The last issue I had with this tractor was fuel related. It would just stop running, like it was out of gas. But when I checked, there was fuel in the bowl and plenty in the tank. The screen inside the tank was clogged with rust particles and was filling the bowl so slowly that sometimes it would just die. If I waited a minute or two, it would run for a while before it stopped. I cleaned it as best I could without taking the tank off and the problem went away. I did check the fuel bowl and drained the carb bowl the past week, but the problem continued. As far as bad gas, I know I've filled the tank once or twice since this started last spring.

The plugs are hard to get to, without getting burnt, but I could pull the wires at the cap to see if one is not firing. I would like to know cause, so I can fix it quicker the next time.

Thanks for your suggestions. I'll start changing parts later this week.

 

[Soundguy]

good luck on the parts swap.

soundguy

 

[lhfarm]

good luck on the parts swap.

soundguy

I'm thinking more of a phased implementation of the tuneup. I'll start with the plugs, then the cap and rotor. Then the condenser and points. I'll run it a while and see if there is any change. If none of that works, I'll buy plug wires. After that, I'll be back asking how to check for a stuck valve .

 

[Soundguy]

as obi-wan would say.. 'use your thumb luke...'


soundguy

 

[lhfarm]

Drove the tractor into my garage (it lives in the barn). It was running great. I pulled one plug wire at the cap. RPMs dropped but it didn't cause the rough running I had been seeing. I also had good spark from the cap to the wire I had in my hand.

Let it cool down and changed the plugs. I'm posting a picture of the old plugs - No 1 is on the left, then 2, etc. They aren't grey, but there isn't much carbon either. They weren't wet. Maybe running a little rich? Don't have the experience to read much more from them.

Mowed for 20 minutes or so in the heat. The Cub ran perfectly. I'll run it some more before changing the cap and rotor.

Anyone use an additive to the gas for sticking valves?

Thanks,

 

[Soundguy]

a hair rich.

buy a cheap quart of plain atf.. walmart brand.

dump half a quart in the crank case.. add the other half quart to a full tank of gas... then go fog the skeeters.

soundguy

 

[Argosy]

Gotta agree it's a little rich. No. 4 looks like the gap is less than the other 3. Or maybe it's just the angle.