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Honda 12 hp engine dies when warm


rp777's Avatar
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11-23-04, 11:46 AM   #1  
rp777
Honda 12 hp engine dies when warm

it starts easily when cold, runs strong for about 10 minutes, then backfires a few times and stops. starts again when it cools for a few minutes, and will, on a cold day, run for a long time. i can't believe there's anything wrong with carb or spark, because each time it cools a bit and i restart it, it will run strong and steady until it gets warm(not hot).

any ideas?

 
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11-23-04, 12:02 PM   #2  
Sounds like your coil is bad. Usually that is the case when a bad coil goes bad, it will stop giving spark when it gets hot, but will when cold. Check for spark aftor engine dies to see if it is getting spark or not.

 
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11-23-04, 12:57 PM   #3  
You don't mention what this motor is mounted on or it's age, but I suspect that Terminator is right. The other possibility is a clogged fuel tank vent not allowing air in. As fuel is used, if the tank is not properly vented, it will creat a vacuum inside, starving the engine for fuel. During the few minutes it is "cooling" off, enough air may be leaking into the tank to allow a restart. It cost's nothing to check this, just try operating with the fuel tank cap loosened, try to operate the engine and see if it still stalls.

 
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11-23-04, 02:51 PM   #4  
rp777
Honda 12hp dies

thanks, Bontai and Terminator,

The motor is mounted on a Billy Goat mower - a self powered mowing deck, and is about 2 years old(out of warranty)

I have observed this for some time, tinkering with it, and trying to rule out fuel & spark(knowing that most problems are one or the other)

OK... the motor dies when running in the hot sun, but not on a cool day, in the shade, etc. When it dies, it backfires, and will recover and start running again if load is quickly removed - it will idle, just won't power anything. Also, when it is backfiring and sputtering down, if the ignition is turned off, the backfiring stops... so i figure it is getting spark.

Also, i have tried force-feeding it fuel with it is warm, but this makes no difference. There is no black smoke until the moment it dies, and mixture seems to be neutral(again, until it dies).. I tried fuel starving it, by turning off the fuel valve, and it speeds up, runs rough and dies with no backfire.... nothing like what it is doing when it gets to temp and dies when working.

I keep thinking that head temperature is controlling things, and if it could be kept cold, the engine would run fine. It is not really hot when it stops - no smoking or unusually high temp, just warmmed-up.

How about cracked head, valve seat problem or gasket? Are there any good ways to rule out those problems? What else could it be?


thanks so much, rp

 
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11-23-04, 03:33 PM   #5  
When was the last time you checked the oil? What kind of oil are you using? You should be running SAE 30 weight engine oil. And you should have changed the oil to use it for year 2004. If you did not change the oil, go ahead and get the oil mentioned above and change it for year 2005.

 
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11-23-04, 03:37 PM   #6  
Azis
It sure sounds to me as well that you have a coil or related problem. The black smoke is most likely the extra fuel that is drawn in but not burned during the time the spark plug is not firing, when it does fire, it has an added amount of fuel to burn which it can not burn it all producing black smoke.
It is real iffy finding an intermitten coil, sometimes a coil will show good but when replaced the sypmtom disapears.

A few things you could try.
1. Try running it with the kill wire disconnected at the coil. This will eliminate any saftey or key switch.
2. Check the resistance of the coil. I don't have exact numbers, but when checking at the Hight tension lead (spark plug connector) to the stack on the coil, (ground terminal on the coil) you should get between 2K and 10K ohms, if so the coil MAY be good
3. Check the high tension lead for shorts or opens, connect a continuity meter as in step 2, and wiggle the spark plug lead trying to find movement in the reading.
4. Place the coil in a heated oven around 150 degrees for about 30 mins and perform checks 2 and 3.

Again unfortunately these tests are not full proof, however if you purchase a new non-returnable coil or you can locate a known good one you should then be able to accuse or clear the coil as the culprate.
I have also changed untestable electronic parts with new and had the same problem only to try yet another part and fix it, in other words I bought a bad non-returnable part

 
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11-24-04, 12:49 AM   #7  
I agree that it does sound like a coil...but...I also, like you, prefer to positively identify the problem before replacing a part. Check valve adjustments as well as the other things mentioned. Let the engine die, then quickly pull the plug and check for spark. If none, but when it cools the is spark, then the coil is even more suspect. Before replacing it I would try disconnecting the kill wire at the coil and see if it quits. If so, bad coil...if not, probably a bad low-oil sentry switch or the oil level is low (letting the oil-sentry kill the engine).


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11-24-04, 11:46 PM   #8  
rp777
Honda engine dies when warm

Thanks, folks. Have a great Thanksgiving. I'll try to get to changing oil and looing for spark when hot after turkey! Bless you all

 
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12-02-04, 09:46 AM   #9  
Well, the turkey is gone and so is the pie, so how are you making out on your Honda?

 
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12-02-04, 12:24 PM   #10  
rp777
Honda 11 HP dies when warm

Thanks for asking!! We have had so much rain the last weeks it has been hard to get out and test it, but after changing the oil, I confirmed that all the same symptoms are there, and it still dies when it warms up some, even after loosening the fuel cap.

I still have my doubts about the ignition theory, as well, because it dies normally when i switch off ignition(no backfires), but when it dies during mowing, after warming up, it backfires - which indicates to me that there is spark. The head is not hot enough to make it diesel - and if it starts sputtering and backfiring, i can stop it by turning off the ignition.

I just wonder what could be wrong with head or valves, etc. that would allow this motor to run strong for 5-10 minutes, then predictably die every time it reaches a certain temp??

Many Thanks, Ron

 
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12-02-04, 07:39 PM   #11  
I'm inclinded to be in the 'ignition system' camp on this one. My theory is as follows:
A) Engine warms up...ignition partly fails.
B) There's a weak spark....but some spark.
C) Weak spark only partly burns fuel in combustion chamber.
D) Unburned fuel exits combustion chamber .
E) Unburned fuel ignited in exhaust system causing backfiring.

Just a good guess, but it's a very plausable explaination for the symptoms. That would explain the fact that you can shut the engine off with the switch when it's backfiring. The lack of engine power would also be explained as well because you can't burn enough fuel to make any HP. I don't have any sneaky tricks to narrow things down a bit more. If you have one of those Briggs spark testers you could compare the way the spark looks when the engine is acting up with how it looks when normal. A dark cloudy day would make that easier. Before you did anything I'd take a good look at the engine air cooling system and make sure that the air channels aren't obstructed by oily grass. I've seen engines almost completely plugged it's a wonder that they don't burn up. Your coil and/or ignition electronics could well be affected by an engine block that's getting way too hot although I'd say that it would get almost as hot (on a percentage basis) on a cool day as a hot one. Hey, we've go snow here, what are you doing still mowing grass??


Last edited by jughead; 12-02-04 at 07:52 PM.
 
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12-03-04, 09:55 AM   #12  
rp777
Honda 11hp dies when warm

Cooling fins are clean. I doubt it has more than 40 hours on it...

I checked on prices of ignition parts. LOOK OUT!! and, I suppose a spark-with-heat problem could really be any of these...

http://www.parts-ol.com/liteshop/car...ategory_id=313

H2324176 - rectifier $94.45 + Shipping
H4542528 - coil assy $137.19 + Shipping
H3803467 - noise suppressor cap $16.06 + Shipping

So i got to check the spark theory before going that way. I have a scope that i use to repair recording equipment. maybe i can work out an inductive probe of some kind....


Thanks, Ron

 
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12-03-04, 12:02 PM   #13  
I used to work on a lot of ignition scopes (Sun) many years ago. The most valuable information was the difference of voltage between cyclinders. Since we are talking a single cylinder engine here I think that trying to use a scope would be a large effort with little real information in return. Briggs makes a nice little spark tester that allows you to see the quality of the spark directly. I think that information would be more useful, and that opinion is from a techno-freek. If you were working on the exact same engine model every day with the same scope then you would be able to tell if what you are seeing is normal or not. I might be inclinded to try a scope myself if desparate enough though. If you could be sure to have the inductive pickup aligned exactly the same way every time you could get a baseline reading when the engine is working OK and compare it to the reading when it's not. Ignition parts seem to be pricey but ultimately the only real way to tell if that's the problem would be by direct substitution. Not a pleasant thought if you are wrong and the real problem turns out to be something simple and/or cheap like a defective spark plug.

 
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12-03-04, 11:29 PM   #14  
The reason the engine shuts down nicely when you switch the switch off is you just gave the coil a complete path to ground with no resistance. When the coil goes bad, it almost never just stops like when you kill the switch. The coil has a fine wire wound around and around the armature core. If it ever develops a break in the circuit on that fine wire, it can still work. When the coil warms up, the wire inside expands just enough to start breaking that connection. Then the spark becomes intermittent and causes backfiring until the connection is broken enough to lose spark entirely, or cause enough resistance to cause the spark to be erratic and untimed.

You are hesitant to think it is an ignition problem because of the symptoms. The symptoms are exactly what makes me think it IS an ignition problem. It's doubtful that the rectifier or the cap are the culprit here. I believe it's the coil.


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12-03-04, 11:35 PM   #15  
rp777
Honda 11hp OHV dies when warm

You're right, Jughead, I'm liable to chase my tail for hours with the scope. I decided to pull off the cover and take a peek at the coil. Could not find rectifier or capacitor. Maybe those are only present on model with electric start? I put the recoiler back on and ran it until it quit. then i checked the temp of the coil - about body temp, I'd say. Warmth was just starting to come up from the mounting castings. After a few minutes of cooling, the coil had actually warmed up... picking it up from the warm head. Of course it started/ran/quit again, and i did this several times, to the point where the coil was MUCH hotter than body temp - too hot to hold onto... and it still starts fine and runs for a few minutes. Remember the first time it quit the coil itself was not warm at all....

My thinking is that this has something to do with temp alright, but not the temp of that coil - it's head temp that matters(?) ..... i'm thinking about replacing the spark plug tomorrow. Thanks for your help. Any other ideas you have are certainly welcome!

best, Rp

 
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12-06-04, 11:02 AM   #16  
rp777
Honda 11HP fixed!

It was the spark plug. Thank you all for your help, and Jughead especially, reminding me to always check the $2 item before spending $$$

Happy Holidays!

 
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12-06-04, 11:55 AM   #17  
Glad you got it fixed. I have a tendency to 'break out the heavy artillary' on some of these problems myself and feel a little disappointed when it's something easy like a spark plug. Who da thunk it.

 
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12-06-04, 11:33 PM   #18  
Good grief!

Well, I guess it was ignition, but far from the general concensus here. That's great that it was just a $2 fix! Thanks for letting us know what it was!


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12-07-04, 05:11 AM   #19  
I guess the lesson learned here is that you should allways have a pack of 12 sparkplugs on hand when needed for replacing. If your unit is for residental use, you only have to replace them once a year. Which will make 12 sparks go long way.

 
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