851 Mystery continues

Andy in Manteca · Jun 2, 2017

I'm at my wits-end. I have a 851 Power Master that is my working tractor, mainly mowing a ten acre pasture. The tractor was acting like the carb was fouled so I rebuilt it, keep stumbling every 5 mins of run time. I replaced the carb with a new
one - same issue, stumbles to the poling of shutting off the engine. I noticed rust in the tank and foul fuel in the filter - replaced the fuel lines, fuel shut off and cleaned the tanked, now the fuel runs clean, still stumbles after five mins of run
time but stays running but will continue to stumble every 5 mins. Perfect excuse for Pertronix ignition system (I converted to 12 v system last year with an alternator). After all this, still stumbles after 5 mins of run time. The symptom feels like
its running out of fuel with some backfiring. At one point I touched the coil and noticed that it is very hot - too hot to touch. The coil has the resistor inside. I'm at a total loss. Any ideas?

old · Jun 2, 2017

#1 coil does not have a resister in side of it it is just wound different then a 6 volt coil.
Trouble shoot before parts always.
#2 check that you have a good blue/white spark at the center wire of the cap and at all the plug wires that will jump a 1/4 inch gap or more.
#3 most electronic ignition have to have resister plug wires of you let the smoke of it them.
#4 a coil should get pretty warm but not so hot that if you spit on it the spit turns to steam fast.
If you have a good spark and the other things are correct go to the carb and do this.
Pull the carb drain plug and make sure you have a good steady flow of gas that will fill a pint jar in less then 3 minutes and yes catch the gas.
Did you by chance route the fuel line in such a way it is to close to the manifold and gets hot so the gas boils in the line?? If you did that can be the problem also.
Or try running it with out the gas cap on the tank vent maybe clogged so it pull a vacuum in the tank

Andy in Manteca · Jun 2, 2017

thanks Old - will do.

Jimmyjack · Jun 2, 2017

Check and see if the fuel line goes behind the manifold;. If it does you may be getting a vapor lock when it heats up. I rerouted the gas line around the front of the engine and it solved a similiar problem for me.

Andy in Manteca · Jun 2, 2017

Thanks JimmyJack,

Yes, I re-routed the fuel line in front of the engine/manifold and it is well clear of the exhaust. I thought the fuel cap vent was plugged but it is not. The carb is new and all fuel lines and shut off/valve have been replaced. The fuel bowl shows super clean fuel and it runs as quick as Old suggested. I spoke with a Tech at Pertronix and I'm going to check out the coil to make sure 12 V is going in and at least 9 V is coming out. The coil does get hot but not enough to vaporize water. If the coil spits out anything less than 9 V then that could be the problem. Fingers crossed.

cheif · Jun 3, 2017

I had a similar problem on my 850,turned out to be a cracked intake manifold.

JMOR · Jun 3, 2017

"I'm going to check out the coil to make sure 12 V is going in and at least 9 V is coming out." What does that mean???

Sean in PA · Jun 3, 2017

I think that's what the Pertronix tech support person told him to do. Maybe with the electronic ignition module wired in place of the points and condensor the "ground" side of the coil primary is always supposed to sit at 9 volts above ground when the hot side is getting 12 volts and the engine isn't running? I don't know for sure but that's what I assumed from what he posted.

JMOR · Jun 3, 2017

(quoted from post at 15:51:07 06/03/17) I think that's what the Pertronix tech support person told him to do. Maybe with the electronic ignition module wired in place of the points and condensor the "ground" side of the coil primary is always supposed to sit at 9 volts above ground when the hot side is getting 12 volts and the engine isn't running? I don't know for sure but that's what I assumed from what he posted.

hat would be an interesting affair, with 3 volts across the coil and 9 volts across the switching device.

Sean in PA · Jun 4, 2017

(quoted from post at 17:06:31 06/03/17) That would be an interesting affair, with 3 volts across the coil and 9 volts across the switching device.

I assume that is only until it switches and the "ground side of the coil goes to (or close to) real ground. As I said, I don't know for sure but that's what he said as part of the same sentence where he said that he talked to the Pertronix tech, so I assume that was what he was told by them.

WellWorn · Jun 4, 2017

I am using an 851 that had a similar issue, but it seems to be the fuel cap. Ran with the cap loose to verify, replaced the cap and solved the problem.

Yours sounds like the coil isn't right. Warm is ok, but too hot to touch is not. If you replace that and still have a problem, I can only assume you checked the carb float and needle for binding. I just had a small engine that had that problem - chrome plating on the side of the needle was coming loose and bound it up so it wouldn't drop as the bowl emptied, yet let it sit a few minutes and enough would leak in to allow it to run again for a few moments.

Andy in Manteca · Jun 4, 2017

Thanks all for the comments and recommendations. In the end, I believe it was a bad coil. I replaced it today and she runs like a top.
Cheers all!

JMOR · Jun 5, 2017

(quoted from post at 19:22:08 06/04/17) Thanks all for the comments and recommendations. In the end, I believe it was a bad coil. I replaced it today and she runs like a top.
Cheers all!

ny think further on that 9 volts?

851 Mystery continues

Andy in Manteca

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