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I have an old Snapper Hi-Vac with in my opinion has low compression. Before I put any time into her I wanted to get some info on if she will run with 50 psi. Thanks!!

Being from the 80's I would be checking the valves of that L-head engine. THey tend to close up and drop the compression more as the decompression takes place.

Thanks, I will pull the head off and check her out. At the very least I will clean all the build up off the valves to make sure they are getting a good seal. Updated to come. Once again thank you!!

Compression gauges are not good on most lawn mower engines.
If you are having a weak running mower, hard to start, etc.. You should be looking at the ignition points.

Fish said:

Compression gauges are not good on most lawn mower engines.
If you are having a weak running mower, hard to start, etc.. You should be looking at the ignition points.

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When I can get her outside (mower is currently in my basement) I will see if she runs on gas. I got her at an auction and I don't have a garage to work in so I brought it in my basement for something to tinker on. She has good spark and runs on either but being in my basement I don't want to stink up my house with lawnmower exhaust. Looks like I will wait a couple of weeks and see how she runs when I get her outside. Thanks for your info.

Visible spark on an old Briggs can fool you. Take off the starter cover and look at the coil, see if there is a wire going under the flywheel, that would be to points.
While the cover is off, and the spark plug unplugged but screwed in, rotate the flywheel backward, and see how much resistance you encounter, it will give you a better sense on it's true compression.
If you do decide that the ignition is a problem, just putting on a newer coil, after cutting the wire to the points, will fix things without the trouble of having to remove the flywheel.

Put up the engine's model numbers, type and code for more detailed info.

Actually taking a compression test can help.
1st) a dry test.
2nd) add two squirts of oil in spark plug hole & re-test. Compression will be higher, but if it is very much higher, then valves are not the only issue. Aluminum sleeve blocks do wear easier.

What happens to the valves is they beat themselves deeper into their seats, reducing & /or elimintaing the necessary valve lash clearance, so compression drops. Fix is to remove one valve at a time & grind off a few thou as your situation needs (off the stem head) & replace & check for correct lash spec, w/ feeler gauge. This may take a couple times or so, but go easy, easier to remove a bit more, than to wish you could add some back.

My vintage MTD has a B&S L-head on it, & it still cuts decent with only 50 psi compression. Although mine is slated for replacement this season.

One will find that the block surface warps a tad with time and heat, and the seats will be cocked relative to the valve faces. That is where the compression is lost, and a proper valve job is in order, and not just lapping the valves.
I use the Neway valve seat and valve face cutters.

Fish said:

Compression gauges are not good on most lawn mower engines.
If you are having a weak running mower, hard to start, etc.. You should be looking at the ignition points.

Click to expand...

Points - wrong advise unless the engine is missing on some power strokes. 50 pounds is enough to start and run. Compression varies with speed of rotation. Most small engines have no specs for compression as a basis for such a reading cannot be established. Cleaning the carbon up by taking the head off is fine as long as you remove all that may wedge open a valve and cause a problem you did not have or do not properly re-gasket and torque. A leak down is the only sure way to determine externally if compression is being lost. Turning thee engine over slowly and feeling the resistance is a quick way to assess the likelihood of compression losses. The post on valve seats and rework is relevant if some compression is lost not due to carbon or valve lash settings. If the valve can be turned when it is supposed to be closed, the valve lash is to tight. Any non flat surfaces on the valve or seat and not shinny are sure to be the root cause of any loss of valve lash. Simple carbon under the valve that did not crush can be a cause. A few hammer taps on the valve can crush it to close but still needs to be certain it is sealing and will when assembled. It can be hard to visually see if the valve is hung on carbon as the lift of only .001" can cause some loss of compression. Some carbon issues will disappear when run for awhile. Then we have sticky valves or bent - these tend to be more intermittent and often a trained ear can hear the difference. Now we can add three final valve issues 1) valve seat loose in the block, 2) bent valve, 3) worn valve guide causing wandering seat to valve closures. I am sure if a think awhile, I can come with more such as a burnt valve.

As I said, he should look at the spark, if he is having weak running issues. Likely dirty points, unless someone has upgraded them.

Cannot say that I ever tapped on a valve before.....

7394 said:

Actually taking a compression test can help.
1st) a dry test.
2nd) add two squirts of oil in spark plug hole & re-test. Compression will be higher, but if it is very much higher, then valves are not the only issue. Aluminum sleeve blocks do wear easier.

What happens to the valves is they beat themselves deeper into their seats, reducing & /or elimintaing the necessary valve lash clearance, so compression drops. Fix is to remove one valve at a time & grind off a few thou as your situation needs (off the stem head) & replace & check for correct lash spec, w/ feeler gauge. This may take a couple times or so, but go easy, easier to remove a bit more, than to wish you could add some back.

My vintage MTD has a B&S L-head on it, & it still cuts decent with only 50 psi compression. Although mine is slated for replacement this season.

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Great, I am getting some good direction from everyone. Thank you!! I am enjoying my new part time hobby too. cheers!!!

74eldo said:

Great, I am getting some good direction from everyone. Thank you!! I am enjoying my new part time hobby too. cheers!!!

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Put up some pics and videos, especially of it starting and running.

Fish said:

Put up some pics and videos, especially of it starting and running.

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Will do, just gotta wait until I can get her outside. Looking forward to some warm days to git rid of this snow...

In the 60s today here in Ky!!

I cannot wait!!!

Some engines have compression releases/exhaust dumps to make cranking easier. These will skew a compression reading.

It should crank and run on 50lbs. Probably not make any great power/torque. It's an old Briggs. Sucker might run on 5lbs.

slomo

slomo said:

Some engines have compression releases/exhaust dumps to make cranking easier. These will skew a compression reading.

It should crank and run on 50lbs. Probably not make any great power/torque. It's an old Briggs. Sucker might run on 5lbs.

slomo

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I got a chuckle at the 5lbs remark, thanks. Yea I guess if it isn't broken don't fix it. Looking forward to some weather where I can get her fired up, I may run a couple of tanks of sea foam through her and take it from there. cheers!!!

5 lbs would be very interesting.
Atmospheric pressure is 14 psi ( approx ) so at 5 it had become a vacuum pump, or we have entered the " Twilight Zone " ( cue creepy music )

bertsmobile1 said:

5 lbs would be very interesting.
Atmospheric pressure is 14 psi ( approx ) so at 5 it had become a vacuum pump, or we have entered the " Twilight Zone " ( cue creepy music )

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It was a joke Bert. I meant the old motors are hard to kill. Kill meaning stop running. Stop meaning zero revolutions per minute.

slomo

Aussies do get USA humour, but the reverse rarely seems to happen ( que creepy music )

bertsmobile1 said:

Aussies do get USA humor, but the reverse rarely seems to happen.

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You need to remember that Aussies are on the bottom of the world and hanging on by their toes. All that blood rushing to their head... it's a wonder they can laugh at anything.

But do their toilets flush backward?

no,
but the drains do

bertsmobile1 said:

no,
but the drains do

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I have heard that to be true..