Mid-80's 4HP B&S Engine w/ 40PSI compression - Lawn Mower Forums : Lawnmower Reviews, Repair, Pricing and Discussion Forum
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post #1 of 19 (permalink) Old 06-03-2020, 03:32 PM Thread Starter
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Mid-80's 4HP B&S Engine w/ 40PSI compression

I've got a mid-80's 4HP B&S engine that came off a 21" sellf-propelled Snapper. I don't know the history on it - trying to decide if I want to bring it back to life to use on a new Jacobsen project. I'd like to use this engine as it's the higher-end B&S that had the adjustable Walbro carb and nicer round 2-stage air filter.

I started by checking compression and it's, at most, 40PSI. Checked valve clearance and they're pretty much spot on at .012" for Exhaust and .006" for Intake.

Thoughts on next steps?

Last edited by DPDISXR4Ti; 06-03-2020 at 04:48 PM.
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post #2 of 19 (permalink) Old 06-04-2020, 02:39 PM
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Shoot some oil in it and check again to see if the rings are whooped.

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post #3 of 19 (permalink) Old 06-05-2020, 11:53 AM Thread Starter
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Shoot some oil in it and check again to see if the rings are whooped.
I was thinking that. Thanks for the confirmation.

I guess if that doesn't reveal anything I'll pull the head.
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post #4 of 19 (permalink) Old 06-05-2020, 03:41 PM
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Sounds good, keep us posted!

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post #5 of 19 (permalink) Old 06-23-2020, 02:46 PM Thread Starter
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Finally had some time to spend on this project. Pulled the valves and lapped them. Checked valve clearances - now they're right at .010 and .060, right where they should be. Reassembled and compression went up to about 60 PSI. Good, that needed to be done.

Next, disassembled the carb and cleaned everything. Pretty sure I got all the jets; they certainly needed it. Tried to start it - nothing - not even a hint of trying. Verified spark - have spark.

Tried shooting some ether in the carb - no change- nothing at all.

Next step is to pull the flywheel and make sure it's timed correctly.
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post #6 of 19 (permalink) Old 06-29-2020, 09:44 AM Thread Starter
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Pulled the re-coil assembly to reveal that the flywheel key was not sheared and thus the flywheel was exactly where it should be. I decided there was no need to actually pull the flywheel at that point and put the re-coil assembly back in place.

Running out of things to try, I replaced the spark plug with a known-good one. Tried starting, both with fresh gasoline and spray ether. Still, not even the slightest hint of starting.

I think I've exhausted everything I know about making these engines run. Anyone got any other suggestions?
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post #7 of 19 (permalink) Old 06-29-2020, 11:52 AM Thread Starter
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Here's some pics to show what I'm working with. It's obviously a test setup.
Attached Images
File Type: jpg Briggs4HP-5.JPG (1.02 MB, 2 views)
File Type: jpg Briggs4HP-4.JPG (974.7 KB, 1 views)
File Type: jpg Briggs4HP-3.JPG (1.07 MB, 3 views)
File Type: jpg Briggs4HP-2.JPG (1,009.0 KB, 2 views)
File Type: jpg Briggs4HP-1.JPG (1.04 MB, 2 views)
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post #8 of 19 (permalink) Old 06-29-2020, 12:23 PM
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If it's got spark, and the key isn't sheared, my bet is the compression is still low. Briggs flatties with no compression release usually hit at least 90-100 psi. If it had enough compression, it would pop on ether or carb spray. Could be the rings/aluminum bore are worn, the valves needed to be resurfaced before being lapped in, or I guess it could be the head gasket if you didn't throw a new one on there. If you wanna keep chasing this, you might have to look into a ring job or getting new faces cut on the valves/replacing them.

One last thing, did you squirt the ether directly in the spark plug hole? That way you know it's in the combustion chamber 100%. Don't even have to tighten the plug more than hand tight; just get it in there and rip the cord a couple times. If it doesn't even try to start, you're almost certainly not building enough compression.

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post #9 of 19 (permalink) Old 06-29-2020, 01:49 PM Thread Starter
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Quote:
Originally Posted by motorhead12 View Post
One last thing, did you squirt the ether directly in the spark plug hole? That way you know it's in the combustion chamber 100%. Don't even have to tighten the plug more than hand tight; just get it in there and rip the cord a couple times. If it doesn't even try to start, you're almost certainly not building enough compression.
No, I just squirted the ether into the carb with it in un-choked position. I like your idea of spraying it into the spark plug hole. I will report back.

FWIW, a couple years back I had a B&S engine with almost identical symptoms - 40 PSI compression. In that case adjusting the valves got it to 60 PSI and it's run great ever since. It was suggested then that sometimes the full compression number don't produce just by hand-pulling.
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post #10 of 19 (permalink) Old 06-29-2020, 03:05 PM
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Hmmm I'm wondering if your engine doesn't have a compression release. A lot of the newer flatheads do, and they bump a lifter to make it easier to spin and then disengage once the engine is spinning fast enough. Those engines don't give normal compression readings at low rpm. If you don't have one, then low compression is just low compression.

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