11HP Briggs and Stratton, gas blowing out the carb! - HobbyTalk
 
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post #1 of 8 (permalink) Old 04-14-2009, 01:11 PM Thread Starter
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Unhappy 11HP Briggs and Stratton, gas blowing out the carb!

Going insane!

I've got a Briggs and Stratton 11 HP L head (horizontal) on a mower I'm trying to fix.

When I check the valve clearance at 1/4" past TDC I have the correct clearance .007 (aluminum block), however, during the ENTIRE compression stroke, the intake valve is open a few hundreths and it's blowing the fuel out of the carb. How could this possibly work? How could the engine ever NOT blow its combustion mixture out of the carb?

I've heard of a easy-start system that releases the compression when rpms are low (during crank) but I'm not sure if this engine has that. I pulled it off the mower and opened up the bottom end, and I don't see any mechanism in there that would do any such thing. Also, I thought those mechanisms worked on the exhaust valve. Anyway, on the cam, I only see the two regular lobes.

I've checked to make sure the flywheel key is in good shape, and the two dots on the cam are lined up the way they should be.

HELP, I don't know where to go from here!

Model 252707 Type 0621-05 Code 91081611.
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post #2 of 8 (permalink) Old 04-14-2009, 05:43 PM
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I would take the valves out and check the seats and valves for damage, its normal for some gas to spit out during starting but not while running.
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post #3 of 8 (permalink) Old 04-14-2009, 10:22 PM
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I think that the setting is 7 on the intake and 9 exhaust, when on valve one is open, all the way extended set the other one, do the same to set the other valve, of course do this after you have lapped them in and make sure that the seat is still intact in the block, that should stop that, Light Mechanic
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post #4 of 8 (permalink) Old 04-14-2009, 10:57 PM Thread Starter
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Hey, thanks for the replies guys.

The seats and valves appear to be in quite good condition despite their age.

The valve seats well at some points in the cycle, but the rod is keeping the valve open by just a bit during the compression stroke.

For example, after the compression stroke finishes, and the power stroke downward starts, the valves are sealed tight.

It's like the cam is the wrong profile (with just a tiny bit of lift at the wrong time), but it's the same cam it's always had, and as far as I can tell, the timing is on.

Please keep the ideas flowing! The grass is getting long!

Thanks!
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post #5 of 8 (permalink) Old 04-15-2009, 09:46 PM
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If you have a compression gauge, put it in the spark plug hole, put compressed air on it, use a regulator, start with 5 PSI, turn the engine over by hand with the muffler and carb. off, if the valves have any leak they hiss, you must seat them with valve grinding compound, set the clerance and if there is a delay still move the cam over one tooth, think on that, Light Mechanic
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post #6 of 8 (permalink) Old 04-15-2009, 10:07 PM
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Quote:
Originally Posted by oilstain View Post
Hey, thanks for the replies guys.

The seats and valves appear to be in quite good condition despite their age.

The valve seats well at some points in the cycle, but the rod is keeping the valve open by just a bit during the compression stroke.

For example, after the compression stroke finishes, and the power stroke downward starts, the valves are sealed tight.

It's like the cam is the wrong profile (with just a tiny bit of lift at the wrong time), but it's the same cam it's always had, and as far as I can tell, the timing is on.

Please keep the ideas flowing! The grass is getting long!

Thanks!
Many older Briggs engines used a "bump" type compression release built into the lobe profile (no flyweights or mechanical linkages). This type of release bumped one of the valves open slightly during the compression stroke, it's helps relieve compression at cranking speeds, but above certain rpm's does not affect compression as it happens so quickly that it's no longer a factor on the compression. Compression release can be on either the intake or the exhaust.
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post #7 of 8 (permalink) Old 04-20-2009, 10:09 AM Thread Starter
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All that pain and turmoil, and all it was was too much gas. I will rebuild the carb when I get my rebuild kit, but for now, I just have the needles about 3/4 turn out and it runs pretty well.

Thanks for all the help!
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post #8 of 8 (permalink) Old 04-20-2009, 12:41 PM
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Smile Thanks!

Quote:
Originally Posted by 30yearTech View Post
Many older Briggs engines used a "bump" type compression release built into the lobe profile (no flyweights or mechanical linkages). This type of release bumped one of the valves open slightly during the compression stroke, it's helps relieve compression at cranking speeds, but above certain rpm's does not affect compression as it happens so quickly that it's no longer a factor on the compression. Compression release can be on either the intake or the exhaust.
Thanks a million for this info. I had been wondering the same thing about my 257700 type 0143 10.5 HP vertical! I had heard of the compression release, but didn't know by what mechanism it worked. I too had fuel chugging through the intake, and thought, timing, timing, timing. Problem was, the engine ran fine. But then like now I have starting issues: Runs great, wont start LOL. Actually it does run, but I'm not hijacking this thread

Thanks again for the info, I may just get my grass cut today afterall!

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