Will an early 80's Briggs run with 50 psi?

[74eldo]

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!!

 

[StarTech]

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.

 

[74eldo]

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!!

 

[Fish]

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.

 

[74eldo]

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.

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.

 

[Fish]

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.

 

[7394]

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.

 

[Fish]

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.

 

[VRR.DYNDNS>BIZ]

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.

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.

 

[Fish]

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