Briggs Carb madness

[hangfirew8]

OK I have a mid-2000's Sears 22", 917.378072 with a "6HP" (yeah, right) flat head Briggs & Stratton that hasn't run right since it was left with gas in it and didn't run it for a solid year (I know, I know). When I decided to get it running again, I pulled off the carb and cleaned a bunch of gritty grey stuff out of it everywhere. That got it to run, but not for long (few or several seconds at a time). Spark is good, even as it shuts itself down (I have a Lisle spark checker, great diagnostic tool). Gas flows freely from the tank, the tank and gas line are free of debris and the inside of the carb is spotless. This Spring I tried to start it, same behavior, so I emptied all the gas and put a lot of Seafoam in it and let it set for a few months.

This weekend I removed and cleaned out the carb for the third time. I checked both holes in the float bowl bottom bolt, and all passages in the carb itself. I've probed them all with a very fine wire, blew it out with compressed air, and replaced the float seat. The passage to the front face of the carb is clear. The bowl is not bent and does not interfere with the plastic float. The float moves freely and I replaced the little red seat. The float is light and not full of gas or holed. The float pivot pin is smooth and shiny.

Since I have spark and it can be made to run, I figure the problem is either a.) too much gas or b.) too little. By its behavior I'm leaning towards b.). It seems to run until the float bowl is empty and then conks out until it has a chance to catch up. After it runs I have to wait a minute and then I can start and run it again for several seconds, it runs strong and then conks out again. When I drop the float bowl after running, some gas comes out but doesn't flow unless I push on the float.

The float bowl doesn't seem to have any adjustment to it. The plastic needle holder is what is and and can't be bent for adjustment.

Engine is 12H802-2640-B1 and Carb B&S 498170 .

I've already given up on the carb and ordered a new one off of eBay. It has been superceded by 799868, so I got that.

Assuming the new one works great, I'll have the old one on my bench where I can fiddle with it as I please. What I want to know is, what else to look for?

Also, does the 799868 have any improvements over the old 498170?

Thanks!
HF

 

[tcreeley]

Same story with my 1 cyl mantis tiller.- Ethanol in the gas and sitting over the winter killed the cylinder when I got it going in the spring- replaced everything - sparked, ran died. I finally took it to a guy who told me what was wrong. I hope your story is better.

 

[RalphVa]

First thing I would have tried would be to dose it with Sea Foam. This stuff has saved me from doing carb work on about a 1/2 dozen carbs.

Ralph

 

[crash325]

Same story with my 1 cyl mantis tiller.- Ethanol in the gas and sitting over the winter killed the cylinder when I got it going in the spring- replaced everything - sparked, ran died. I finally took it to a guy who told me what was wrong. I hope your story is better.

OK TC, what did the guy tell you??
Had a similar B&S engine on my cement mixer, tank rusted up & plugged the carb. Even with the cleaning it still has problems.
Not sure how it works but those engines have some kind of funky fuel pump, that fails to function fairly often. Even when it would run & work every now & then I'd have to choke it to keep it running. Kept the air cleaner off so that I could stick a thumb down the carb every now & then.

 

[hangfirew8]

First thing I would have tried would be to dose it with Sea Foam. This stuff has saved me from doing carb work on about a 1/2 dozen carbs.

Ralph

Ralph, I like Seafoam too, I guess that's why I wrote "...so I emptied all the gas and put a lot of Seafoam in it and let it set for a few months."

When I took it apart again Saturday, the first thing I did was empty all the Seafoam, and when I put it back together I put in Ethanol-free gas.

Crash325, fortunately rust is not a problem on my end, the mower has a plastic tank. There was some small amount of grass debris in the drained Seafoam, so that's out of the tank now and no longer causing problems.

As for the funky fuel pump, from the parts breakdown I don't see a vacuum pump. The tank is above the carb so it appears to be gravity-fed, with the float bowl valve doing the metering and the float bowl itself providing fuel directly to the carb.

 

[hangfirew8]

As a follow-up, I installed the 799868 and the mower (after a few priming pulls) started right up and ran strong as long as I let it. I let it sit and it started up later on the first pull.

 

[dodge man]

Bad float in the original carb?

 

[mostly_gas]

hangfire8, I like to use carb cleaner (immerse in can type) and not sea foam when carb is partially plugged. I would also check for how fast gas is flowing into carb.

 

[hangfirew8]

The float was good. I've used the canned carb cleaner in the past, nasty stuff. I want to avoid it if possible.

If I run into a carb that is impossible to replace cheap, I may invest in an ultrasonic cleaner. At the moment it's fastest and cheap enough to buy a new carb, for most any outdoor power equipment.

 

[Rod Bender]

I work on these a lot. With needle and seat and float in place observe the float with the carburetor inverted. It should be nearly level or very slightly raised. If not, replace the needle and seat. Probably the ethanol has caused the vitron seat to swell and has caused the float level to be too low for fuel to be picked up. Also, make sure the bowl nut is absolutely clean. Also, a test I do to see if the very small carburetor passages are clear is to squirt carburetor cleaner through the two holes along the front edge of the air cleaner mounting surface as well as the passageway leading up from where the bowl nut screws in. You should see carburetor cleaner coming out from another location in the carburetor. Those carburetors list for less than fifty dollars, so do not spend too much time on it.