Is 70 pounds of compression too low?

Hi All:

My two 19" LB mowers each tested at 70 psi. with dry cylinders. They both run and start great. Is this pressure good enough to continue using? Should I do a compression test after adding a bit of oil to the cylinder. Of course this may add considerable bit of compression to the reading. Which is correct, dry or wet cylinder for taking a compression reading?

Tia,

Franchi

Wet.

70# is not bad. I have run a little less but 100# would be great. If this is the same carb dripping that could be warped or upper or lower seals will help matters.

Are they D Series engines? Lawn Boy specs are 65# cold PSI on the D-400, and 70# cold PSI on the D-600 engines. If that's the engine you have, you're right on with factory specs.

L8R,
Matt

My bad. I read that too. Don't know what I was thinking of.

Those were just the minimum figures I posted. Normal cold compression would be 70-85 PSI for the D-400, and 90-105 PSI for the D-600. Here's a basic spec sheet from the Lawn Boy Service Manual for comparisons between the D-400, D-600, and F Series engines. Here's the link to that page. Glad that Toro has kept this info available.

https://lookup3.toro.com/ttcGateway/acrobat/manuals/lball64.pdf

Here's the link to all of the documents. I found them quite useful getting through some issues with my two vintage Lawn Boys. It covers Lawn Boy engines from 1950-1988.

https://lookup3.toro.com/ttcGateway/acrobat/manuals/lball.html


L8R,
Matt

The actual numbers in the dial mean diddly squat +/- 20% due to operator & instrument variations.
Compression readings are generally done both wet & dry.
In use the cylinder will be damp with oil.
What is important is the difference between the wet & dry readings
Much more than 10% and you need to think about rings.

What you should be doing is running a compression test on a hot engine every year or so and plotting the readings.
When they suddenly start to drop it is new ring time.
Caught early you will be able to use anything up to 4 sets of rings per piston.

bertsmobile1 said:

The actual numbers in the dial mean diddly squat +/- 20% due to operator & instrument variations.
Compression readings are generally done both wet & dry.
In use the cylinder will be damp with oil.
What is important is the difference between the wet & dry readings
Much more than 10% and you need to think about rings.

What you should be doing is running a compression test on a hot engine every year or so and plotting the readings.
When they suddenly start to rise it is new ring time.
Caught early you will be able to use anything up to 4 sets of rings per piston.

Click to expand...

Very good mate. So when the compression rises the rings are wearing down? Do yall have many lawn boys in your neck of the woods? I am just a just fix it kind of guy. At least that's all I hear from my wife. " Just fix it ".

Got-to-be-old said:

Very good mate. So when the compression rises the rings are wearing down? Do yall have many lawn boys in your neck of the woods? I am just a just fix it kind of guy. At least that's all I hear from my wife. " Just fix it ".

Click to expand...

Opps,
Sorry about that
Should of course been drops rapidly. so I have gone back & fixed it.
I plot the compression readings inversely because an inflection point is easier to pick up on a rising line than a dropping one.
And the wet compression reading are plotted on the same page as the dry readings which gives a second set of triggers as the lines start to diverge at a greater rate.

I was born into a dirt poor family and have been that way most my life so getting the maximum use out of everything is Number one priority.
A set of rings on our local blue smoke is $ 5 , the head gasket is $ 2 & crank case seal is $ 5, so under $ 20 and in 1/2 to 1 hour the mower is back to new.
A piston is $ 15 which doubles the price & adds about 15 minutes + another 1/4 to 1/2 hour for decoking the ports.
Any longer and you end up with a rebore which requires removing the engine & a full disassembly which is a 4 hour job and costs around $ 100