Cause of sooty plug under load on Honda mower?

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JonD
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Hi all,
I'm looking after a shared Honda mower with a GCV135 engine, and recently changed the plug for a new one. It ran beautifully, and after sitting at full speed but under no load for ~15 mins the plug was still looking brand new.

1 day later after some of the other users have been mowing long grass the plug body is looking quite sooty, and the insulator, although mostly white at the nose, has a sooty "shadow" on the part under the earth electrode.

My concern is what can I change to fix this?
I recently cleaned out the carb and replaced the gaskets. The inside of the carb was spotless even before I cleaned it.

The only adjustment I can see is the pilot screw, which is locked, and in any case the idle mixture isn't a problem.

Any tips very welcome!
robo
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Is it the right plug for the engine
JonD
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Hi,
yes, I checked the service manual. NGK BPR6ES

Best regards
Jonathan
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oldherbaceous
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Afternoon Jonathan, i wonder if it is just cutting the long grass that is causing the problem! Maybe if the mower is trying to work too hard, to let it run at the right revs!!!!

Another thought was, you don't think someone else has had it running, with the choke slightly on, maybe?

You could always clean the plug, and run it on short grass for a while and see what happens, before you start delving deeper.
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Ricard with an H
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I'm going with OH on this but on a slightly different tack, when your carburettor filter gets bunged up with chaff or fuel-oil leaking through you get a rich mixture.

Easy to test. take-off the filter and run the engine but after removing the plug and letting the combustion chamber clear itself.

I had a similar problem to this from storing a brushcutter with the fuel tank uphill of the carburettor, fuel and oil had soaked the filter to the point air couldn't get through.

Another point I managed to prove as part of ''Murphies-law" if your machine doesn't start quickly whilst it's cold then pull the plug and find something else to do for the next hour. Then try again.

Even though I'm often referring to two stokes starting as a black-art I firmly believe that the problems do transfer to four stokes if we're not attentive enough to the needs of these engines.

The idea that four strokes are happy with old fuel is a convenience, yes they'll cope better. But they don't like old fuel. Two strokes are more sensitive so this time of year I always use a fuel-preserver.

Keep this thread going and someone will probably come up with an answer, it'll probably be Sally. :D
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JonD
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Hi guys,
thanks so much for your suggestions. A few had occurred to me already, so I can answer a few of the issues raised right away:

1) Long grass seems to be the issue :-) Not sure if it's just something we'll have to live with, but as you say hopefully lighter loading will allow the soot to burn off. Not too confident on that though given past performance.
Is high loading expected to cause over-rich conditions?

2) Choke being left on. I can't rule it out, despite the big label I put on the throttle lever warning not to leave on choke! I found when I had a quick go it was *just* slightly on choke when I thought it was on fast. I'm guessing very slight closure of the choke butterfly shouldn't affect it too much?

3) Air filter. Had that problem previously. It got soaked in oil when someone tipped the mower up. It's sporting a new (and clean) filter as a result.


The issues started after the oil-soaked air filter situation. The plug got badly fouled. I replaced the filter, and cleaned the plug, but a while later it went wrong again when people started using 2 stroke fuel in it because that's all they could find. It seems to have oiled up the plug when the first load was added to the tank, and then the carb wouldn't draw the thick 2 stroke fuel any more.

I cleaned it up and got it running, but the plug just seemed too fouled to weather any abuse, so I changed it. I'm now worried that this new one is going south too.

Thanks
Jonathan
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peter
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Did you clean all trace of the two stroke from fueltank cap through to inlet port?

How long has it been run since then? In other words, could it be the sticky residue of two stroke plus the slight choke-on that's causing it?
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robo
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I would give it a good spraying with carb cleaner to remove any 2 stroke residue as well as changing the plug for a new one if you have not done already, the fact that the problem occurs when cutting long grass is a bit strange as that is when the engine will be working the hardest it should get hot and normally clear any oiled up plug, it sounds like a partly blocked airfilter or residue of 2 stroke oil
sally wright
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Dear Jon,
I would suspect that having 2-stroke petrol mix running through the engine has caused the spark plug to deteriorate and a replacement is called for. Yes I know it is a new plug but the added oil will have caused the engine to run hotter than it was designed to do and it might have damaged it. It may be that the plug was a dud but they are not expensive and it is an easy fix if this is the problem (if not then you have a spare).
Other things to check are the throttle cable for wear and loose connections and also the plug cap and cable for damage (there may be fraying in the wiring). Sometimes running the engine at low revs can cause sooting, I am not familiar with the exact model you have but if the speed is directly related to the engine speed; going slowly through long grass to avoid choking the blades might result in a choking motor instead.
If the problem persists then the only thing I can suggest is to run the machine at full revs for a minute or two before shutting it down to put away. This will get rid of most of the sooty deposits that cause poor starting. Don't forget to use only a brass bristled wire brush for cleaning the spark plug.
Regards Sally Wright.
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Ricard with an H
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I'm Also wondering now if this machine is up,to the job it's being asked to do, I have the same engine but on a Klippo side-eject. This engine is not big enough for long grass or wet grass and is why I also have a 6 HP machine. The 3.5 is only ever used for trimming, yes it will cut long grass but only with a freshly sharpened blade and regularly scraping the stuck on grass on the underside.

My neighbour also has the Honda 3.5 on the basis of the performance of my machine, he doesn't cut long grass but he does leave it until it's a little to long. I can tell by the tone of the engine that the machine is labouring, it would help if he kept the blades sharp though like many they are required to smash the grass rather than cut it.

If you're the only user out of multiple users who takes interest and responsibility this might always be a problem.

A sharp blade makes a massive difference.
How are you supposed to start and maintain a healthy lifestyle if it completely removes a wine lover’s reason to live?
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JonD
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Hi,
again, thanks to all for your input. Most useful.

The mower is in fact for an entire allotment, so it's anyone's guess how badly abused it is. Interesting that a 135 might not be up to the job, particularly on long grass. I can believe that. I know when I tried it after after after noticing the soot, it was indeed labouring very hard on the long grass in places, and in fact stalled several times, despite me having just sharpened the blade.

The plug was brand spanking new once the 2 stroke issue had been solved, and solved several tanks ago. I no longer see puffs of blue smoke on startup that I saw for the first few tanks after I drained the 2-stroke.

The new plug stayed absolutely spotlessly clean when I ran it at full revs but under no load for about 15 minutes, then within 1 day back on the allotment the soot had reappeared.

I did adjust the throttle cable according to service manual, so I get choke after the detente, and it goes all the way from throttle to idle just fine, but I did notice that there wasn't as much "gap" between full throttle and partial choke when I came back to it after that day's use. Maybe the cable is slipping.

Looks like I will just have to keep an eye on it and have several spare plugs to swap in at a moment's notice.

Best regards
Jonathan
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oldherbaceous
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Afternoon Jonathan, i think these sort of problems always seem to occour more on shared machinery. With different people putting their own fuel in, it is a recipe for disaster, with old fuel or mixed fuel going in, but i don't suppose there's a easy way to get round it.
Kind Regards, Old Herbaceous.

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