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Discussion Starter #1
Hi all, bought a 18 inch dedicated mulching mower fitted with a 5 HP Briggs, the red top motor. Bought with the intention of servicing/cleaning/ selling on again.

It starts first pull, has good power, no smoke, but is "missing" all the time its running, every second or so. I changed plug, carb is clean, fresh fuel, air filter new..took head off and cleaned off carbon deposits, but nothing has made a difference. Wondering if a valve is a bit leaky?

Seems a good motor apart from this problem, and I feel too guilty to sell it on running like this.

Any tips on diagnosing a leaky exhaust valve? Or tips on what else to look for?

Thank You!
 

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Administrator
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is it posible that there is a kill wire or connection that is shorting out, making it miss?
 

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Discussion Starter #3
Hi, no I've looked just in case, but the kill wire isn't the problem, checked the flywheel key this morning, and that was about 1/8th of an inch out, but still in one deformed piece.

Changed that and it's a little better, but still missing at low revs.
It has the non adjustable carb so I'm picking something might be wrong in there. Have cleaned it out with compressed air but no change.

Thanks for the input!
 

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Premium Member
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If this has the vacu-jet carb. mounted on top of the tank, change the diaphragm and use an extra gasket, so that the diaphragm is gasketed on both sides. I have fixed several of these in this manner with good results.
 

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BOSS
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try a bigger gap
 

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Certified Technician
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Try running some wire down the bowl nut holes...like a bread wrapper tie...
 

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Discussion Starter #7
Hi all, thanks for replies.
Tried another carb off a mint running motor, but just the same result, I reckon it's something to do with valves?
Will sell it as is, I'm not knowledgeable to do a valve grind. Well I am, but just my luck I'll make it worse!!
Cheers
 

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blinged out
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In response to your asking about valve testing! do a search for "leakdown test" you won't have the right tools,but a fitting can be made from an old sparkplug!
on small motors it's easier just to pull it apart, but this is how it's done on large engines! just an FYI!
 
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