B&S lawnmower engine spewing gas out the fuel tank cap vent holes

[greynold99]

Hi all,
I don't have all the specs on this engine with me but last night after mowing one tank of gas with my 4 yr. old Weedeater 500 lawnmower, when I refilled it - and I didn't overfill it - When I re-started it, the gas started spewing out of the 3 vent holes in the gas tank cap. It was like a very tiny stream out of each of the holes. Quite impressive but I got to thinking if the fumes hit the hot engine or muffler, it's going to get real exciting, quickly.
Well, I shut it off immediately, to avoid a fire and after moving to a level location I restarted it and the same thing happened - just spewing gas like a fountain.

Didn't get a chance to disassemble or anything last night but I was wondering if anyone has had something similar happen to them and what would cause it to happen... Also, how to fix it?

This mower was damaged by hitting a stump earlier this Spring and once I replaced the blade and shear-pin, it's run pretty good a half-dozen times but did develop a little handle vibration that it didn't have before.
However, when I was looking over the fuel tank area, I noticed a hole in the top of the frame, near the left rear engine mount bolt. Must've damaged the frame more than I realized and might account for the extra vibration I noticed.

I think I can either repair the frame bolt-hole by reinforcing with some strap-steel or get it welded. But the gas spewing out of the cap, that's a new one on me.
Any ideas or suggestions?
Thanks,
greynold99

 

[gregjo1948]

Does it have a fuel pump or is it gravity fed? gregjo1948

 

[Rivets]

Deflector in the cap is probably missing. Replace the cap.

 

[greynold99]

gregjo1948,
This is just a plain old 168cc B&S engine on a high-wheel push mower - no fuel pump, plain gravity feed. Like I said I ran a tank of gas, filled it up and when I restarted it, 3 streams of gas came literally out thru the gas cap vent holes about 1ft to 1 1/2ft. high. Never saw anything like that and I thought that there was only suction from the carb acting on the fuel in the tank...

Rivets,
I checked the gas cap and that little 'cardboard-like' element was still up in the top of the gas cap - if that's what your referring to as a deflector.

I didn't overfill up to the filler hole as I sometimes do since I was mowing on a slight grade on a hill side.

greynold99

 

[gregjo1948]

gregjo1948,
This is just a plain old 168cc B&S engine on a high-wheel push mower - no fuel pump, plain gravity feed. Like I said I ran a tank of gas, filled it up and when I restarted it, 3 streams of gas came literally out thru the gas cap vent holes about 1ft to 1 1/2ft. high. Never saw anything like that and I thought that there was only suction from the carb acting on the fuel in the tank...

Rivets,
I checked the gas cap and that little 'cardboard-like' element was still up in the top of the gas cap - if that's what your referring to as a deflector.

I didn't overfill up to the filler hole as I sometimes do since I was mowing on a slight grade on a hill side.

greynold99

It may fired thru the intake valve and pushed fuel backwards up thru the tank. Same thing as blowing thru the gas line. gregjo1948

 

[greynold99]

gregjo1948,

Assuming that's the case, is this fixable and would it be a DIY project or should I have a small engine repair shop do it?

Sounds like the intake valve isn't closing completely and may have resulted from my earlier stump encounter... What I don't understand is why it wouldn't have appeared sooner than the 6 or so times I've run several tanks of gas through it.

I haven't started it since and wanted to look at more closely. I'll give it a look tonight and see if it's still doing the gas streams. Maybe the valve just got stuck with the heat of mowing.
greynold99

 

[gregjo1948]

gregjo1948,

Assuming that's the case, is this fixable and would it be a DIY project or should I have a small engine repair shop do it?

Sounds like the intake valve isn't closing completely and may have resulted from my earlier stump encounter... What I don't understand is why it wouldn't have appeared sooner than the 6 or so times I've run several tanks of gas through it.

I haven't started it since and wanted to look at more closely. I'll give it a look tonight and see if it's still doing the gas streams. Maybe the valve just got stuck with the heat of mowing.
greynold99

It might be delayed action from stump jumping. Check the key on your flywheel. The stump bump may have damaged it and it finally sheared. That would change your timing and tried to fire on an open intake. gregjo1948

 

[greynold99]

I got a chance to look at my Weedeater mower a little more closely this past weekend and it turns out the 'stump' event caused a little more damage than I was originally aware of. And, in fact the damage may have caused the fuel-spraying out the fuel cap... Turns out that two of the engine mounting holes on the deck were broken & cracked. Engine must be vibrating like crazy with only one bolt holding it down securely.
It must've taken the 6 or so times since I replaced the blade and shear pin for the cracked deck holes to completely break from the frame - suppose I'm very lucky the 3rd bolt held.

I think I can fix it with a couple of 1 1/4" wide galvanized flat strap steel about 3" long and liberal application of Epoxy Steel Putty & JB Weld. The holes already in the galvanized flat strap steel are 3/8" wide - same width as the engine mounting bolts.

I was actually hoping to find a 7.25" diameter flat steel ring or some other piece of curved flat steel piece to use at the big box stores that would 'approximately' fit the decks cut-out opening for the mower shaft & blade, maybe even requiring a little bit of modification, but came up completely empty.
If you've ever done this type of fix-it and know of a source for a flat-(thin)steel ring, let me know.

But, I think this was the cause for the gas spewing out of the gas tank - too much vibration forcefully sloshing the gas out.
I'll see later this week when I get a chance to do the repair.
Greynold99