Craftsman 6.5 HP B&S Motor Surges, Have I Tried Everything?

upupandaway

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starts, runs, then surges and eventually dies,

Right when it dies, if u manually add\spray gas\starter fluid when it dies does it run?
If no but it still sparks, I would check the valve tappet gaps when it is cold.
 

slomo

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Right when it dies, if u manually add\spray gas\starter fluid when it dies does it run?
If no but it still sparks, I would check the valve tappet gaps when it is cold.
And make sure you have the valve cup/cap deals that ride on the small stem end of the valve. Those cups/caps are pressed on by the rocker arms to actuate the valves. Yes, valve to rocker arm clearance check is wise......

slomo
 

Gilmourisgod

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Thanks, I'll pick up one of those spark testers ASAP. I'm cleaning the gas tank today just to check that off the list.

I took the shroud off so I could see the plastic intake tube that feeds into the cylinder head, looks fine, no cracks or other obvious problems. I tried spraying WD-40 around the carb back face, no white smoke or other indicatiation it's sucking air.

Although its a brand new carb, it's one of the cheap $12 Chinese clones, not OEM B&S, I took it apart anyway and checked all the passages for clear. Cant find anything wrong with it, except maybe the butterfly is bit stickier than I'd like. One thing I noticed, unlike the original carb, this one does not have a rubber seal for the float needle, its just a machined brass insert. The needle is 4-sided instead of three like the B&S carb. Is that how it's supposed to be? The depth of the float needle hole doesn't seem deep enough to accept a secondary seal of any kind and still have a level float, so I assume the needle just seats on the brass? Guess that makes these carbs non-rebuildable? If you guys think I effed-up buying this cheap carb, let me know, and Ill return it and get OEM.

I saw the linkage from the back of the motor to the carb butterfly, seems to move freely, and with the governor spring on, it seems to flp back and forth easily under spring pressure. I see you can bend the spring tab to increase or decrease the amount of tension on the carb spring, any way that's factoring into surge? Is that how engine RPM is adjusted? What's driving the amount of tension on the lever coming out of the engine?

Thanks for all replies and advice so far, as should be obvious, I'm a novice at small engine repair and troubleshooting, but luckily I'm curious and stubborn. I WILL figure this out. :)
 

Gilmourisgod

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Just took the coil off to test with a multimeter, I'm reading that it should be around 2.5 to 5k ohms, getting 4.74 k ohm reading on spark plug cap to the bolt holes on the coil, same from spark plug wire to kill switch contact. I've read that a bad coil can be intermittent, becuse resistance changes when its hot, so maybe it only goes wonky when the mower heats up? That's the primary symptom here, it runs great for about 5 minutes, then starts surging. Unlike before carb replacement, it doesn't die now, it will run long enough to mow the whole lawn without stalling, it surges the whole time, but up and doen in a fairly narrow RPM range, its not almost stalling then revving up, so it's useable, just annoying as hell.

One other thing I noticed was some rust on the flyywheel magnets and some hardened crud on the coil "shoes" that are gapped to the flywheel. I've polished up both the flywheel magnets and coil shoes with a grey scotchbrite, so they are shiny, but I haven't really removed any metal. I've watched YT videos showing 0.012" coil gap, .010", and "a piece of paper folded in half". Maybe I'll shoot for the middle at .011" and see how it goes.
 
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Rivets

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Testing the coil is meaningless, as Hall Effect coils have internal components which affect the readings. Second, the ignition system will not affect a surging problem. Surging is caused by a lean fuel mixture. Surging after an engine warms up is most likely cause by no getting enough fuel into the carb bowl or when a crack in parts expand as they warm up Or a leaking gasket. To test this, start the engine and spray carb cleaner around where the carb bolts to the engine and the outside of the carb. If RPMs change, you will have found a leak. By the way what are the model, type and code numbers for your engine and the part number for the new carb you purchased.
 

Gilmourisgod

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As to the float needle, not sure what I'm seeing, it could be plastic, as it's a different color than the rest of the needle, but I didn't notice if its literally a separate tip on the needle.
 

Scrubcadet10

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I ordered a knock off carb for this same model before.... seems the Chinese are using the brass needle and plastic/rubbery tip now.
 

Gilmourisgod

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The mower model # is 917.388740, a Craftsman 6.5 HP, 21" push mower with the manual primer on the air cleaner housing. The serial # on the engine is listed as 123K02-0181-e1-02051559. E-replacementparts.com lists a replacement carb for this model as #799868. Using that number, I found and bought this carb off Amazon:

If this is somehow the wrong carb, or just junk, let me know, I can just return it and get an OEM B&S carb, probably what I should have done in the first place, but a lot of people posted positive reviews on these. I may just go ahead and do that now since it's not compatible with the standard B&S carb rebuild kits. Since the mower runs great for the first five minutes before it starts surging, I can't see how its the carb, unless maybe the bowl or needle is sticking only when it heats up?

It looks identical to the original B&S carb, with a couple odd exceptions:
1. The bowl jet nut is a 13mm bolt instead of 1/2", jet holes and size appear identical to OEM. All the interior vents and passages look identical to OEM, and I know the primer bulb works, so air is getting through the right passages.
2. There is no red rubber seal in the float needle hole, the needle mates to a permanent brass insert in the carb that looks non-replaceable. The needle is a 4-sided object instead of three sides. I can't tell if the needle tip is plastic or not.

I did try spraying carb cleaner around the base of the carb where it connects to te intake manifold, no change in RPM.I used a brand new gasket set from carb to air cleaner housing, new O-ring from carb to intake manifold. I have NOT tried running the motor with cowl off and spraying carb cleaner around the base of the intake manifold where it's bolted to the cylinder head, but have confirmed there are no visible cracks or damage to the plastic intake manifold. I agree its got to be a carb problem or air leak, and so far I'm not seeing any evidence of an air leak. I checked the coil because I heard they can misfire when hot, and did find a fairly thick layer of hardened crud on the coil magnetic "shoes", which may have nothing to do with it. I polished that crud off, as well as some rust on the flywheel magnets, and gapped it to .010" with feeler guages between flywheel magnets and coil shoes.

Should I take the intake manifold off and check the condition of the gasket? I'm a little wary of running the engine with the cowl off, but will if that's conclusive to find the air leak. Maybe I'll just pull it off for process of elimination, it could be cracked on the bottom, or an obviously shot gasket, and I wouldn't see it from above,
 
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