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Briggs & Stratton 12.5 flooding

Posted by tommymc51 (My Page) on
Tue, Sep 30, 08 at 7:21

I have a 18 yr old Sears tractor w/12.5 B&S motor. It has always run well, but suddenly it wouldn't start. The air cleaner housing was filling with oily gas and the motor was clearly flooding. Long story shortened, I have replaced the plug, fuel filter, rebuilt and cleaned the carb, and changed the oil. (crankcase had filled with gas)
I can generally get the motor to turn over and catch, run poorly for a minute, then flood out. I have done this with the air cleaner off (realizing this affects the mixture) and can see that gas and some smoke are spitting out the intake side of the carb. Although I've successfully rebuilt carbs before, I can't discount the possibility that I did something wrong. The kit had minimal instructions and jets for several (Walbro) models. The one I used looked exact but....?
I'm starting to wonder if the symptoms would fit a stuck intake valve, but don't know enough to be certain. Any suggestions would be appreciated.


Follow-Up Postings:

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RE: Briggs & Stratton 12.5 flooding

"Although I've successfully rebuilt carbs before, I can't discount the possibility that I did something wrong."

Well obviously the carb float needle is not sealing off. Wrong needle, wrong seat, improper installation.

No, the Intake Valve is not causing this problem. It is normal for that model B&S engine to blow back some thru the carb at cranking speed due to the "Easy Spin" cam grind. Trick is to determine normal from excessive.

When you get the float needle fixed, you need to install a gas line shut off valve and use it when the engine is not running.

Walt Conner


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RE: Briggs & Stratton 12.5 flooding

Walt, thanks for the quick reply. I'll double check the float valve, but that's one piece I was pretty sure I got right. Although the kit (B&S #497535) was 'universal' and had different jets, the correct float needle and seat were pretty obvious. The float is non-adjusting. I've pulled the bowl off with the carb in place, and moved the float up and down. It seems to have a good seal, shutting off the gas flow with light upward pressure. I was careful to put the seat in properly with the ribbed side toward the gas line.

Since my first post, I pulled the valve cover and you're right: the valves seem to be working fine, at least when I rotate the motor by hand. I guess the carb is the culprit. I've rebuilt carbs for a 1960 Chevy, a couple of Honda motorcycles, and two old Techumseh motors and never had this problem. Maybe I should try one of the other jets that were in the kit. There was one other that 'looked' identical. To my eye, the oriface was the same size and all ribs and holes lined up. It was painted blue to differentiate it from the others, but no mention in the instructions what the blue was code for. Any thoughts?


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RE: Briggs & Stratton 12.5 flooding

Well I tried to post a follow up which said the float its' self might be bad.

I would take a close look at the float needle, compare it to the one you took out.

Walt Conner


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RE: Briggs & Stratton 12.5 flooding

[Well I tried to post a follow up which said the float its' self might be bad.]

That's a good thought. It's plastic, so not rusted. What's the best way to test it, assuming that there's no obvious gas inside it? Maybe dunk it in water and watch for bubbles? BTW, is it just coincidence that this fuel system issue is happening after the first summer using 10% ethanol gas? It's all we can get around here anymore.


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RE: Briggs & Stratton 12.5 flooding

My guess is that when you had the carb off, you DIDN'T flush the fuel line.
Often, when disconnecting carbs, fuel filters etc., a piece of crud gets broken loose inside the line. Upon reinstallation, that piece of crud immediately gets flushed down to the needle/seat and causes flooding problems.


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RE: Briggs & Stratton 12.5 flooding

"It's plastic, so not rusted. What's the best way to test it, assuming that there's no obvious gas inside it?"

IF it were leaking, it would have gas sloshing around inside.

"BTW, is it just coincidence that this fuel system issue is happening after the first summer using 10% ethanol gas?"

Maybe not, I haven't had a problem but I have heard several people say the Ethanol will cause deterioration of some gas lines and particulars may be flaking off.

Walt Conner


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RE: Briggs & Stratton 12.5 flooding

[My guess is that when you had the carb off, you DIDN'T flush the fuel line.]

Well, you're half right. Before rebuilding the carb, I drained the tank through the gas line and replaced the filter. Not a solvent flushing, but that should have gotten any loose crud. After rebuilding the carb, I started with fresh gas and new filter. The filter should have caught anything that was still floating around. There is only 6" or so of old hose between the filter and carb which is unflushed and unfiltered. Maybe there was something in there. I'm going to pull the carb back off and give it another inspection and blow-out. I'll let you know the results.


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RE: Briggs & Stratton 12.5 flooding

Before hooking the fuel line back up to the carb, let gas run through it a couple seconds. All it takes is the tiniest speck of crud to mess things up.
Been there....


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RE: Briggs & Stratton 12.5 flooding

"The filter should have caught anything that was still floating around."

To get a filter that is really fine enough to ensure this, you have to have an engine equipped with a fuel pump. Filters in gravity feed systems can not be fine.

Walt Conner


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RE: Briggs & Stratton 12.5 flooding

It is obviously a float or float needle/seat prob it can be nothing else


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RE: Briggs & Stratton 12.5 flooding

I had a similiar problem with my 18hp vanguard on my generator. Now my generator is a few years old, but has less than 10 hours total time. One day I smelled gas in my garage and my generator was leaking. I shut the fuel valve and when i got home from work turned it back on and checked to see where it was leaking. Of course it didn't leak even after a few hours. So I reinstalled the air cleaner etc and fired it up. Ran fine. Only thing I can assume is some crude messed with my float needle. When it had happened, my generator had been sitting for a few months so I found it odd that it would randomly start leaking, but from now on I turn the fuel shut valve off


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RE: Briggs & Stratton 12.5 flooding

[To get a filter that is really fine enough to ensure this, you have to have an engine equipped with a fuel pump. Filters in gravity feed systems can not be fine.]
The original filter had a mesh screen. The replacement is 'generic' clear plastic with a paper filter. Is that going to be a problem with gravity feed? (assuming I ever get the beast running!)


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RE: Briggs & Stratton 12.5 flooding

Problem solved, and I just finished mowing my lawn for what I hope is the last time this season. The problem was that I had the wrong jet in there. As I said, the carb kit covers several models and comes with 4 different jets. I had done a close comparison and determined the one closest to the original. Problem was that I didn't put that one in, I must have mixed them up on the bench and grabbed the wrong one. The one I had installed had an obviously larger oriface...no wonder it was shooting too much gas!


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RE: Briggs & Stratton 12.5 flooding

"Problem solved, and I just finished mowing my lawn for what I hope is the last time this season. The problem was that I had the wrong jet in there.....The one I had installed had an obviously larger oriface...no wonder it was shooting too much gas!"

There is a lesson to be learned here. Many of us seem to believe that bigger is better. This isn't true when it comes to main metering jets.

Gas molecules are tiny comparatively speaking, and getting too much gas will flood an engine out very quickly.

I had to learn the hard way with my Farmall Cub. There were multiple sized main metering jets for the Cub's carb, and they were coupled with other parts as to which one was best to run.

I finally fixed my rich condition by going to the smallest jet I could find. Sometimes it is hard to see the forest for the trees. Good trouble shooting tommymc51.


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RE: Briggs & Stratton 12.5 flooding

Reviving an old thread because I have the same issue. I have run through all of the items listed above, even replaced the entire carb with a rebuilt because I broke the housing during one of the removals. So I have replaced the carb, fuel line, filter, checked the intake valve to ensure it works and still have flooding. I have resolved to not spend another dime on this except to have it looked at by a professional unless someone can give me an idea of what else to look at. ANY help would be greatly appreciated.


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