Another thing: Did you have the throttle in wide open position when testing for compression?

baymee I ceecked the valve when i had the head off they looked okay i cleaned them up & seat looked okay.I dont remember if i had throttle wide open. Should it be? Jim

yes, it must be open.

Agree with baymee above. B&S used to say to turn the engine backward until you felt resistance, flip the flywheel forward (clockwise), if the flywheel hit compression and rebounded, it had enough compression to run. 30 psi is not enough.

Walt Conner

How much difference in the compression reading will there be when having the throttle open as opposed to closed? I can't imagine it being that great a difference. Maybe if the engine was being turned over by a fast spinning electric starter but not when being pulled by hand.

Yeah, I'm puzzled by the open throttle recommendation as well. Back to the OP's question - if the engine has a compression release I'm thinking that's gonna reduce the compression guage reading.

Orange , does the Op's Model Designation have a Internal or external compression release mechanism as RB has queried ?

Orange , does the Op's Model Designation have a Internal or external compression release mechanism as RB has queried ?

The open throttle allows full airflow into the cylinders.

I.m not sure if it has compression release the mod no.is 135202 type 0171-01 code 93881907.ITS A 5 HP ON A LEAF BLOWER. When i had the head off i cleaned it up & the valves looked like they were seating. Still cant start it.
I was hoping i would.nt have to do a valve job .
Sometimes it will pop smoke out of the muffler like it wants to start but no go...

Virtually all "L" head, flathead, B&S engines utilize a "passive" compression release that consists of the camshaft lobe for the Intake Valve being ground in such a way that it holds the Intake Valve slightly open well into the compression stroke relieving some compression at cranking speeds. This has little adverse affect at operating speeds according to B&S.

That engine is almost certainly an aluminum bore. I expect the rings are worn out and cylinder wall may be damaged plus you really need to check the valves with Bluing or use a blow down test to see if they are seating. Some people put a series of pencil marks across the valve seat face then turn the valve slightly and see it the marks are all blurred, I have never had much luck trying that.

Anyway, 30 psi compression is not enough for the engine to start. Did you check the valve clearance?

Walt Conner

"The open throttle allows full airflow into the cylinders."

I know that but I don't see it making a hill of beans difference in actual readings at normal start-up cranking rpm's. Does anyone actually think that opening the throttle is going to change a 30psi reading to 70 or above? Let's get real here. I would bet that you wouldn't even see a 5psi increase by opening the throttle, but I could be wrong. Has anyone here actually checked the difference?

Oh Ok Walt you answered the original question that I posed to Orange errantly 3 times....Sorry Guys ! Thanks for the Cam Designed Compression Release Relativity Info . ;)

Sorry ewalk. Glad sax answered your question.

No Problem Orange , I inadvertently (computer hang up) responded multiple times for your slant lol . Walt advised me of the internal mystery decompression principal . Silly Me :)

'Glad sax answered your question'

You have ESP! I didn't post in this thread until I read that! LOL
The old B&S 3.5HP engines didn't usually have the compression release, but I bought a 'commercial grade' Homelite/Jacobsen 20" in 1990 with a B&S 3.5HP that had one. It also had a large muffler and a 'QUIET' decal on the shroud. I never knew by what means they accomplished the compression release before reading this thread - good info. Funny, it was a 'commercial' mower but without iron bore or roller bearings. Also funny is the fact that it is still in use! I had given it to a relative who abused it for several years. I brought it home and fixed it up and they did it again. I finally took it out of there, fixed it up again and gave it to my brother who had just moved back here from Las Vegas and had a very small grass area to maintain. They are taking care of it and I go over and do maintenance. It still has compression and doesn't smoke or knock. Wish I could say that about myself!

On the subject mower, have you injected engine oil into the spark plug hole and pulled it through to oil the cylinder before testing? Also, the 'blow-down' or 'leak-down' test is the real quick way to find out if it seals, but I don't know how to do this with a compression release.

{{gwi:128202}}

Yeah Sax, Orange was quite gracious with offering you some Royalties lol :)

Oops.....sorry Walt. I just gave your credit away. My bad.

'Back in the day', I used to take an old spark plug and
break off the porcelin and knock out the center electrode,
with porcelin and then weld a male air hose nipple on it.
I put the suspected cylinder on top dead center and
hooked up my air hose. If the air blew out the crankcase,
then I had bad rings. If it blew out the carb, I had a bad
intake valve. And, if it blew out the exhaust pipe, I had,
ta-daaa, a burnt exhaust valve. I know, I know, someones
going to tell me I forget to screw the nipple/plug into
the head. I guess you can't remember everything.

Not much has changed. Now you get two gauges to compare input air and holding air.

Did you ever see what happens if you don't hold the cylinder at TDC under 90# of air pressure? Well, it happens so fast, you don't even know what hit you.

Sure I saw it, that's why they call me Stumpy.
Actually, on small, single cylinder engines,
a much lower air pressure is much kinder on
the old knuckles.

I had a 1/2" breaker bar, 18" long, attached and was holding it at TDC. The thought came to my mind that I'd like to see how it sounds when air escapes from the valves and I moved the bar a little and bango, It flew out of my hands and across the room.

Reminds me of the time I had a dado blade in my radial
arm saw and took too big of a bite out of a piece of
1 in.X 2 in. pine, and it flew about 20 ft. where it
put a hole right through a cabinet door that was made
of 1/2 in. plywood. OUCH!! Of course, that was when
I was young and foolish. I'm not young anymore.

Back to the Compression Effects of open vs closed throttle settings . Would 14.5 Atmospheric really really alter the 30 # reading one way or the other ?

IF you go into compression stoke with a light partial vacuum from throttle not being open, what are you going to compress? ALL auto & small engine instructions say to have throttle wide open when doing compression test.

Walt Conner

Someone please try this and give us their actual readings. I would but it's 31 degrees out in the garage. My bet is still less than 5 psi difference at normal, hand pull, RPM's.

By the way, I'm not saying that testing compression with the throttle closed is the proper way to do it, I just don't believe that it makes that much difference when testing at pull start speeds and especially when you are only reading 30psi to start with.

I was going to do the test but I don't have anything here right now.

Ok, I went out and grabbed the 10424 Lawnboy. E series 6.5hp 2 cycle engine. It used to blow a 95psi with a borrowed compression gauge. My new gauge (cheapy from Autozone) was used. Sprayed about a teaspoon of Aero Kroil into the cylinder and pulled it over about 5 cycles. It then blew a 106psi with the throttle wide open after 4 sharp pulls on the rope. With the throttle closed it blew a 104psi and with the choke on it again got about 104. Spinning it with a drill or starter might make more of a difference. Otherwise, the difference is insignificant.

Most OPE Units I have tested Walt are not hermetically sealed lol . Even a minute leak rate which normally aspirated and Carburatored Engines Provide , should give valid Test Results. As Orange stated not that it is GM Goodwrench Approved just did not see where it would affect the bottom line . Perhaps it will ? I have never ensured WOT on any application only partial to allow full atmospheric Pressure within the Intake Plenuem..Thanks for the Insight Dude ! :)

well after all those posts i put some oil in the cylinder & got the com up to 45lbs .no start am ilooking at a ring job?

"Most OPE Units I have tested Walt are not hermetically sealed lol . Even a minute leak rate which normally aspirated and Carburatored Engines Provide , should give valid Test Results. As Orange stated not that it is GM Goodwrench Approved just did not see where it would affect the bottom line . Perhaps it will ? I have never ensured WOT on any application only partial to allow full atmospheric Pressure within the Intake Plenuem..Thanks for the Insight Dude ! :)"

Not sure where you are coming from on this. If you are familiar with 4 stroke engines, you know that they operate with a partial Intake Manifold vacuum lower pressure in the intake manifold at partial throttle otherwise, no air or gas would enter.

I guess you don't recall the days when car power brakes, cruse controls and even windshield wipers operated on manifold vacuum. Slightly different between lawn mowers and 4 stroke autos, same principle. Point was to admit as much air as possible ::)) dude

Walt Conner

"even windshield wipers operated on manifold vacuum"

What a joke they were. I remember climbing a steep hill in a downpour in a '58 Ford with vacuum operated wipers and they just stopped dead.

Walt: I was just advising that perhaps quarter throttle or even half is more than enough to allow sufficient atmospheric pressure to assist with adequate compression testing. Yes I realize that positive and negative crankcase pressures can assist with fuel delivery , thus the diaphram operated fuel pump on numerous 2-strokes vs mechanical or electric fuel pump on 4-strokes. As for the vacuum assisted wipers , yeah reluctantly Walt , had a 58 Impala with this form of dreaded window wipers. Had the same experiences as Bay during heavy rain , poor performance at low speeds lol . Thanks for your example Walt brought back some memories !

My post had nothing to do with crankcase pressure, the subject was whether the cylinder needed to get a full gulp of air for a compression test.

My last word on the subject, IF you want accurate results, follow the instructions for the instrument you are using.

Walt Conner

The Ford shop manual from the days of carburetors states" Engine at operating temp, all spark plugs removed, carb throttle plates wide open, crank until pressure stabilizes".
So the throttle should be open. On any engine with a governor (such as most mowers, generators, etc) the throttle should already be wide open when starting.

"On any engine with a governor (such as most mowers, generators, etc) the throttle should already be wide open when starting."

None of my mowers with a handle mounted throttle is this way. The throttle plate is between the air filter and the main jet and the governor plate is between the main jet and the engine. Therefore you can have the throttle completely closed and the governor wide open at the same time.

Well I said it was my final word but the above is not so.

Walt Conner

Oops. I meant to say choke plate not throttle plate is between the air filter and main jet. Therefore you can still have a similar air restriction with it closed and the throttle/governor plate wide open.

Well ifin the Ford and Goodwrench Manuals instruct to be Fully open than Fully Open it is . Who am I to go against Industry Standards. Although 14.7 Atmospheric Pressure (@ Sea Level) would always ensure a Effective Pressure Differential (Negative Pressure) within the Intake Manifold to ensure a Full Fuel Charge . As I said , Walt if the Manual States this requirement then by all means follow the Manual . It was just a question ? :)

The bottom line is, the OP was getting 30 psi compression. No amount of opening the throttle was going to get the reading up to an acceptable level so there was no sense in running out and doing another test just to make sure the throttle was open.

Is opening the throttle the proper method? - YES

Will it make a large difference in small engines at hand pull RPM's? - NO, not unless 2% is large to you.

Orange : All depends on what 2% your referring to Dude ! Density is a finite item Feathers or Fodder lol :) Ole Walt makes a very Valid Statement , but I agree with the acceptable compression range with throttle open or shut is not going to met with the Op's 30 # Indication. For the Record I normally ensure partial throttle opening , but could be incorrect in my policy ..Bad Me :) I will try to follow WOT positioning in the future Walt .

"what sould the com be on a 5hp b&s engine. i put a new head gasket on it,im getting 30 psi. wasgetting 0 before, the valves seem to be working but i cant start this leaf blower for nothing, im getting gas & spark. How long should it hols the 30 lbs."

Let's circle back to what the original poster was asking about and re-read what he said. He was getting -0- before and 30 psi after a head gasket change. To me that indicates the possibility of a warped head - or a poorly installed new head gasket.

Initially I was leaning towards Valve leakage. But there may be a Head Gasket or Warpage issue after-all RB . I guess a leak down test will be required to validate where the leakage is originating ?

Given that the pressure improved if slightly by changing the headgasket I would inspect the head carefully for cracks and maybe even a stripped spark plug hole. Then I'd check for flatness.

I'm leaning toward rings since he added oil to the cylinder and increased the compression by 50%.

Could be a multitude of issues or a little of all . A leak down will reduce the process of elimination ritual .

how do i do a leak down test? jim

google "leak down test" There's lots of info about it.