Chipped ceramic on spark plug
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    Chipped ceramic on spark plug

    I replaced the spark plugs in the B48 of my 448. Both had oil on the threads. On one the porcelain insulator was chipped away. I've thought at times I heard detonation while mowing so I have been using 93 octane gas.

    The PO thought this motor needed new valve guides soon. It does smoke a little bit when you up the throttle abruptly but doesn't use enough oil to see evidence of it on the dip stick. Any thoughts?


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    Super Mod dundee222's Avatar
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    I don't see any porcelain chipping, which is a good thing. Looks more like carbon build up that fell off [from the pics]. How many hours has these plugs been run, and does it include a lot of time idling?

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    Quote Originally Posted by ovlov760 View Post
    I replaced the spark plugs in the B48 of my 448. Both had oil on the threads. On one the porcelain insulator was chipped away. I've thought at times I heard detonation while mowing so I have been using 93 octane gas.

    The PO thought this motor needed new valve guides soon. It does smoke a little bit when you up the throttle abruptly but doesn't use enough oil to see evidence of it on the dip stick. Any thoughts?

    The Onan B43/48M run rich so you should expect a bit of smoke when throttle up quickly.
    Now if your B has a marvel schriber carb you may be able to tune the high speed jet, but my Onans do the same thing.
    So it's not something I'd worry about, what I'd be more concerned about is carbon buildup in the combustion chamber.
    Also since the Onan is a low compression flat head it doesn't require high octane fuel and predetonation is not a common problem compression is only 6.6 to 1 Remember if it is full of carbon the compression ratio will increase.

    Thanks
    Jim

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    Quote Originally Posted by dundee222 View Post
    I don't see any porcelain chipping, which is a good thing. Looks more like carbon build up that fell off [from the pics]. How many hours has these plugs been run, and does it include a lot of time idling?
    I don't know how many hours they have on them. I bought the GT a couple of months ago. I've slowly been going through stuff and replacing it. The plug gap was beyond .025" but not excessively so. It did at least have the correct plugs in it.

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    I'm not any sort of expert by any stretch, but I don't think using "high test" 93 Octane in our small machines is good for the motor. I would think 93 Octane would be burning way to hot and giving you a bigger internal bang, one your engine isn't designed to sustain, which could quickly shorten/end it's life.

    That said, I let someone with more brains than I weigh in on that. I would just replace the plugs, gapped properly, and put regular gas back in her.

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    Super Moderator jbadman's Avatar
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    Without running a mower, and by holding the lift lever until the hyd relief valve blows, does the engine produce the detonation sound you mentioned? Try this at low / med / high idle when operating temp achieved.

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    I don't see any porcelain chipped either. And, the plug shows no signs of overheating or detonation. Too dark colored for that. But,it wouldn't hurt to check and clean the cooling air path through the engine, tins and fins, just in case short term overheating may be causing the sound you hear. 87 octane should be plenty with the low compression the Onans have.
    edit: almost without exception detonation damage will be on the very end of the porcelain.

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    almost looks like it was oil fouled plug and a bunch of carbon build up on it and not so much of a chip,,,, from the pic anyways.

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    Super Mod dundee222's Avatar
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    Higher octane gas is more slow burning to prevent detonation in high compression engines. The only effect it will have in our Onans is a little less power due to the burn rate/lower peak cylinder pressures.

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