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So.... Given a Craftsman 6.5 HP mower that was left sitting for at least a couple years. I don't know the model year, I'm guessing it's at least 10 years old. The gas had dried in the tank and carb leaving a varnished mess. Here's what I did:

1. Unpluggd Fuel hose and sprayed carb cleaner in the tank until is started draining, didn't see anything really nasty come out of the tank. Refilled with clean fuel, standard gas station 87 octane with Ethanol. Bad idea?
2. Did a carb rebuild with new float needle and seal, gaskets, and thorough cleaning, fresh gas, new air filter, new plug. Engine started second pull and ran great for about 1 hour, then wouldn't restart. Primer bulb doesn't work.
3. Looking into carb, I see primer isnt pumping gas up through the emulsion tube.
4. Tried replacing carb gasket to air cleaner housing, check primer bulb, still won't prime. Air cleaner housing looks a little warped due to overtightening bolts. Remove carb, clean again, repeat, NFG.
5. Tired of screwing around, I replace carb with Amazon special Chinesium carb, replace air cleaner housing with OEM B&S part with integral primer. Replaced cracked fuel hose. All new gaskets throughout. I used (2) carb/air cleaner gaskets thinking I might have an air leak.
6. Engine starts second pull, runs great for 5 mins, then starts surging, spitting, missing, then dies.
7. Let it cool off a while, same issue, starts, runs, then surges and eventually dies, like it's fuel starved.
8. Gas cap is missing the metal "cone" inside I see on B&S gas caps in videos, I can see the plastic pin that broke off at some point, rubber seal is still there, but no aluminum piece. NFG, engine starts right up, but starts surging after about 5 mins.

From everything I've seen online, this sounds like a fuel blockage or vapor lock, but since it's a brand new carb, brand new fuel line, etc, hard to believe its that. I did double check the bowl nut jet, some reports of metal debris on these carbs, none visible here, Confirmed that float is parallel to carb body, moves freely, gas flows when float drops.

The tank shows no sign of leakage, and I didnt see any debris in it via flashlight, but is there a filter in these plastic tanks that might have gotten clogged?
Advice appreciated, I am stumped!

Try something simple the next time it starts surging. Loosen the gas cap, the vent may not be working. If surging stops replace the cap.

2nd the gas cap.
Also, if you have the engine i think you have i would check the plastic intake pipe or manifold, for small cracks, surging means not enough fuel or too much air.

I did replace the gas cap, NFG. Also tried running it with the gas cap entirely off, it surges with or without the cap. I've read there is a filter inside the gas tank itself, any way to clean that? Is there a way to testy the tank for air tightness? My next idea was to remove the tank and try appying compressed air to the outlet with cap on an see if I have leaks anyplace?

Get some WD-40 in a trigger sprayer, no aresol can.
With the engine running, spray it around the carburetor area where it meets the intake pipe, if there is a change in RPM, or the mower blows white smoke, you have an air leak at the carb.
If you have good constant flow of fuel out of the tank the tank should be OK.

Did you try loosening the gas cap? Sorry but I had to pile on LOL.

Get an inline spark tester. Run the engine on flat ground as long as you can. See if the spark dies on you.


slomo

Have you pulled the fuel line off the carb and checked the fuel flow. I have seen crapped up tanks have very small fuel flow and what happens is the unit sets for a few minutes and the carb fills up and engine starts and runs till float bowl runs out of gas then when it sets for a few mi s it will start again. You can't use air to clean the tank. If it sat and dried and varnished that crap will keep coming loose.

Take an air compressor blow gun and blow the fuel line each way. Remove the fuel tank and fill with 8 ounces of ammonia. That should remove any old varnish slash residue in the tank. Wash the tank out with Dawn soap and water. If the tank is really bad add some BB's for agitation. Drain all the water out and spray down with WD-40. Reinstall the tank and test. Tank should be like brand new.

Get an inline spark tester.

Sounds like you got the carb issue resolved. Tried every carb trick there was including the new fuel cap trick. You need air, fuel, spark and compression. You should have air and fuel. How about the last 2?

slomo

I did take the bowl off the new carb with (new) fuel line connected, seemed to get good fuel flow when I lifted the float, so I think it's getting fuel. The tank is the one thing I haven't thoroughly cleaned or replaced, so that's worth doing anyway. I'll try the WD-40 air leak suggestion as well, since I see no obviuos fule issues, I figure it's air or spark, and since it seems to run great for the first 5 minutes after startup, the spark seems unlikely. I have read that one symptom of a bad coil is missing when it heats up.

Unlikely or not, your mower runs like crap. Fuel is covered and air. Get a cheap inline spark tester. Here you go a whopping 5 bucks.


slomo

How was the spark test?

slomo

Gilmourisgod said:

starts, runs, then surges and eventually dies,

Click to expand...

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.

upupandaway said:

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.

Click to expand...

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

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.

Does the needle have a plastic viton tip?

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.

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.

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.

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

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,

The OEM carb number Is 799868, same as what you ordered, but it should use a solid float needle with a replaceable viton seat. If yours has a solid seat you have a chinese knockoff.

Needle and seat test

Take the "car-bu-trator" off. Remove the fuel bowl. Turn carb upside down. Blow into the main fuel inlet hose barb. Needle and seat should hold back your lung power. Then raise and lower the plastic float. Should hear your air go in and out of the "carb-bu-trator". This is a must if you are cleaning a dirty carb. Verify the needle and seat is working proper. Sucks to reinstall a cleaned carb and find fuel on your mower deck.

Confirm that the carb float is LEVEL with the carb body, where the carb bowl seal is. And that the needle and seat stops the fuel flow when the float is parallel to the carb body.

Make sure you put that thin paper carb gasket on the correct way. Those are easy to install upside down or back'ards.

How was the 15 minute spark test?

If you buy an OEM Briggs carb, buy DIRECTLY from Briggs. Don't get it off Ebay or Amazon. Go directly to the manufacturer. That way you KNOW that carb is proper and will run another 30 years. I say this as there are a ton of Briggs parts being copied from China and other countries.

Running like crap and popping after 5 minutes, that tells me you are pretty lean while running on the main jet. Something in the MJ is starving the engine for fuel. Or like others said, you have a vacuum leak some place. Vacuum leaks allow more air into the cylinder causing a lean running condition.

Page 1 says you ran/run E10 fuel. Get real 100% gasonline if you can. These engines "tolerate" E10 but are really made for 100% unleaded. Run fuel stabilizer year round. E10 is a bad idea.

slomo

It's definitely a Chinese Knocoff, can't imagine it's anything else for $10.99. The 13mm bolt nut/jet indicates its not OEM. Is that the likely source of my surging problem, just a crappy Chinese carb? If so, I'll return the damn thing and cough up the extra cash for an OEM B&S carb. I know they have theirs made in China too, but maybe the QC is better?

The latest update in this little fiasco:
Tried the carb needle test, its definitely working to seal fuel in or out. If its the Chinese carb, its something more hidden, like an improperly drilled air passage. I've cleaned it, wheedled all the passages with a twist tie wire, float nice and level with the carb body, damned if I can see anything wrong with it. To recap, this is what I've done so far:

1. Cleaned gas Tank with Ammonia, let sit overnight. This did liberate a small amount of varnish like gunk, but nothing chunky. Let it dry for 48 hours and then hosed it out with WD-40, which seems to have pushed out the last bit of water that didn't evaporate. If it's an internal filter clog, I can't get to it. I tried blowing through the gas fill opening and didn't get air out of it anywhere but the outlet tube.
2. Aftemarket Carb Replacement
3. New gas cap
4. New Air cleraner housing with new primer
5. All new gaskets, carb to air cleaner, carb to intake manifold.
6. New fuel hose
7. Resistance test (apparently a wast of time) and cleaning of the coil, tested within spec, set magneto gap to .010". Kill switch wire looks good, obviously working.
8. Inspected and removed intake manifold, no cracks or obvious problems with the manifold gasket to cylinder head, looks likes it's never been removed. Having removed the intake manifold (gasket stayed on the head like it's glued on), I should probably replace it.
9. New Spark plug
10. WD-40 test for air leaks around carb while running negative, but I did NOT try spraying around the cylinder head to intake manifold gasket, which requires removing engine shroud while running. I think I'd rather just replace the gasket.

I have a useable mower here, I can get through the whole lawn without it stalling, but it usually requires 2-3 pulls to start, and begins surging after 5 mins. +/-.

From the suggestions posted, all I have left to try is a spark test (which I'll do as soon as I can get over to Harbor Freight to pick one up) and replacing the carb with OEM. I'm returning the Chinese Cheapo.

One question on the gas cap: Neither the OEM B&S cap or the aftermarket replacement has any kind of hole in it anyplace I can see. How does air get sucked into the tank through the cap if it has no openings?

BTW, does anybody know if the E-Replacement part carb is OEM, its around $38 vs. $50+ on the Briggs website. I realize thats the safest bet, but hate paying $20 more for literally the same carb just becuase B+S is cranking up the price.

Briggs caps are now vented through the threads, no more holes, so I doubt that is the problem.

I trust you on pricing but Briggs normally has competitive parts pricing? Shouldn't be too far off other reputable places.

slomo

I'd bet that Chrinese carb is not a 1:1 parts wise carb to a real Briggs. It might work just fine. Is there a pilot screw to adjust on the Chrinese carb?

slomo

$49

This ebay page has a couple cheaper OEM Briggs carbs. Are these labeled OEM Briggs Ebay carbs legit or what? Those Sten carbs should be okay, I've never used one myself. The other real experts here might have.


slomo

It Runs!
I replaced the intake manifold gasket and put in an OEM B&S carb ($38) and it ran for a full hour without any real surging. I did notice a tiny bit of surge when I tilt the front wheels off the ground to turn it, but otherwise runs smoothly. The existing intake manifold gasket seemed pretty tight and fully intact, but it came off in bits, so maybe it had hardened or shrunk. Since I replaced both carb and gasket simultaneously, I can't say for sure which solved it, but I'm guessing it was the carb. Lesson learned, don't bother with the cheap aftermarket carbs, a real crapshoot quality-wise.

Thanks for advice, All.

Thank you for coming back and letting us know that, and how, you fixed it.

Gilmourisgod said:

It Runs!
I replaced the intake manifold gasket and put in an OEM B&S carb ($38) and it ran for a full hour without any real surging. I did notice a tiny bit of surge when I tilt the front wheels off the ground to turn it, but otherwise runs smoothly. The existing intake manifold gasket seemed pretty tight and fully intact, but it came off in bits, so maybe it had hardened or shrunk. Since I replaced both carb and gasket simultaneously, I can't say for sure which solved it, but I'm guessing it was the carb. Lesson learned, don't bother with the cheap aftermarket carbs, a real crapshoot quality-wise.

Thanks for advice, All.

Click to expand...

Very well could of been a float setting or pilot jet setting if adjustable on your carb. Either way, good to know it's running now.

slomo

Congratulations.

Might be missing that small throttle return spring in the linkage. Make sure all the carb linkage and springs are proper.

slomo

Hammermechanicman said:

Have you pulled the fuel line off the carb and checked the fuel flow. I have seen crapped up tanks have very small fuel flow and what happens is the unit sets for a few minutes and the carb fills up and engine starts and runs till float bowl runs out of gas then when it sets for a few mi s it will start again. You can't use air to clean the tank. If it sat and dried and varnished that crap will keep coming loose.

Click to expand...

I had an experience just like this on a craftsman, I replaced the entire gas nine, filter and shutoff, now runs much better, I did notice that the fuel tank had a rubber insert and washer, I do believe it had contributed to this problem, I changed out tanks with a tank that just has the plastic nipple, I would check this as well.