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Old 08-05-2008, 12:37 PM   post #1 of 20
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Default Spec'd compression value Briggs 18hp twin - 422707

I have more detail posted in another thread on my struggles with this motor, but I'm looking for a clear answer on this - what compression value is specified for the 18hp Briggs twins? - this is model 422707.

I went to engine(s) last night and took some compression measurements.

Engine1 (the one I most recently put into the JD 116) - cyl.1 = 50psi, cyl.2 = 75psi.

Engine2 (the spare) - cyl.1 = 65psi, cyl.2 = 70psi

I should have checked this before I installed the engine. What is the expected value for these motors? All these values seem a little low to me.

From reading other threads, verifying that the valves aren't leaking are also key to the compression reading. I'll be checking on that in the near future.
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Old 08-05-2008, 02:07 PM   post #2 of 20
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Default Re: Spec'd compression value Briggs 18hp twin - 422707

I don't think that Briggs ever put the compression values out. They do, however, recommend a leakdown test to check an engine for any problems. The only compression check refered to in my Briggs manuals is to spin the flywheel backward quickly. If it rebounds, the compression is good enough for the engine to run.
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Old 08-05-2008, 03:37 PM   post #3 of 20
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Default Re: Spec'd compression value Briggs 18hp twin - 422707

Thanks for the info. No published compression values...well, that should make this more difficult...

I guess I'll try the backup motor next, and then go from there.

Anyone ever measured a known good motor? What values did you see?
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Old 08-05-2008, 04:16 PM   post #4 of 20
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Default Re: Spec'd compression value Briggs 18hp twin - 422707

While B&S doesn't post compression figures, I would expect min. around 100lbs.

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Old 08-05-2008, 09:56 PM   post #5 of 20
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Default Re: Spec'd compression value Briggs 18hp twin - 422707

A good 12hp B&S single I checked tonight showed 75psi. Hmmm...
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Old 08-06-2008, 04:06 AM   post #6 of 20
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Default Re: Spec'd compression value Briggs 18hp twin - 422707

Small engines have compression releases which do function properly if the valves are adjusted to spec. So its normal for a engine compression test to show 60 to 90 PSI cause the compression release kicks in untill the engine is running and then the engine will be doing around 200 or more PSI, thats a proper running engine of course. If you actually have a leak, then your compression will be considerable less and not enough compression to keep running.
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Old 08-06-2008, 09:21 PM   post #7 of 20
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Default Re: Spec'd compression value Briggs 18hp twin - 422707

Quote:
Originally Posted by Termy View Post
Small engines have compression releases which do function properly if the valves are adjusted to spec. So its normal for a engine compression test to show 60 to 90 PSI cause the compression release kicks in untill the engine is running and then the engine will be doing around 200 or more PSI, thats a proper running engine of course. If you actually have a leak, then your compression will be considerable less and not enough compression to keep running.
200 or more PSI?
That's about 14:1 compression/ I don't think so-
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Old 08-06-2008, 09:53 PM   post #8 of 20
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Default Re: Spec'd compression value Briggs 18hp twin - 422707

Nice to see that with the compression releases, you can actually get a true compression value unless the engine is running...which is darn near impossible :-O

So it's a little hit or miss between the two motors I have at the moment. I guess I'll get the second one ready to swap in and see what happens.
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Old 12-01-2008, 02:13 PM   post #9 of 20
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Default Re: Spec'd compression value Briggs 18hp twin - 422707

Well, I skipped the second and went to a third motor I found locally. That one runs, but died at higher RPMs. I'm also thinking I may have a bad ignition switch on this mower, too...it doesn't always kick right over in start - either a bad ignition switch or kill sensor on the brake pedal, transmission lever, etc.

I took apart the original bad motor over the long weekend. Turns out that one of the crankshafts disitegrated, taking out most of the rest of the internals of the motor.

So, over the winter, I'll have to piece one of these 3 possible donor motors together to get a running setup. Oh, and the throttle cable sprung loose from its retainer and stretched the "spring" sleeving. yeesh.
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Old 01-23-2011, 05:53 PM   post #10 of 20
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Default Re: Spec'd compression value Briggs 18hp twin - 422707

not that im any help but i have an 82 craftsman with a 16 hp its original with low hrs and im getting about 60lbs per cylinder i know a guy here who deals in briggs and has been in business for 30is yrs and said that those were satisfactory. ive always been told as long as both cylinders were close in ratio then you dont have too much to worry about. i do have a good 18hp that i need to get readings off of and for me to even use this motor i have to switch my sump over and intake etc since its a horizontal not vertical.
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Old 01-23-2011, 06:08 PM   post #11 of 20
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Default Re: Spec'd compression value Briggs 18hp twin - 422707

Quoted from the Briggs repair manual....

Check Compression
Briggs & Stratton does not publish any compression
pressures, as it is extremely difficult to obtain an
accurate reading without special equipment.
It has been determined through testing, a simple and
accurate indication of compression can be made as
follows:
Remove both spark plugs and insert a compression
gauge into either cylinder (one cylinder at a time). Turn
engine over with engine starter until there is no further
increase in pressure. Record this reading. Repeat
procedure on other cylinder and record that reading.
The difference between both cylinders should not
exceed 25%. More than 25% indicates loss of compression
in the cylinder with lower pressure. See example.
Example:
Cyl. #1 Cyl. #2 Diff. % Diff.
Eng. #1 65 PSI 60 PSI 5 PSI 7.6%
Eng. #2 75 PSI 55 PSI 20 PSI 26.7%
If compression is poor, look for
1. Loose cylinder head bolts
2. Blown head gasket
3. Burned valves, valve seats and/or loose valve
seats
4. Insufficient tappet clearance
5. Warped cylinder head
6. Warped valve stems
7. Worn bore and/or rings
8. Broken connecting rods
Cylinder Leakdown Test
The cylinder leakdown tester, Tool # 19413, may be
used to test the sealing capability of the compression
components of each cylinder and quickly identify the
problem component.
Cylinder Balance Test
If the engine is hard starting, runs rough, misses or
lacks power, perform a cylinder balance test to
determine whether both cylinders are operating to their
full potential.
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Old 01-23-2011, 11:23 PM   post #12 of 20
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Default Re: Spec'd compression value Briggs 18hp twin - 422707

Flathead motors don't have high compression ratios so 75 to 100 psi would seem about right.
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Old 01-24-2011, 12:14 PM   post #13 of 20
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Default Re: Spec'd compression value Briggs 18hp twin - 422707

larrybl hit it on the nose thats how i went about checking the compression on the motor. as i said i know that cylinder 1 exhaust valve needs work i think it is worn after not running for yrs and after its first start in yrs it might have stretched it out since theres a tapping on that cylinder.
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Old 01-24-2011, 01:47 PM   post #14 of 20
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Default Re: Spec'd compression value Briggs 18hp twin - 422707

my old 85 42... puts out 90 on each cylinder
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Old 02-14-2011, 03:55 PM   post #15 of 20
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Default Re: Spec'd compression value Briggs 18hp twin - 422707

I got 78 psi on a Tecumseh OH160 (cold)and 120 PSI on my Skidoo Mach 1 im thinking any more than 150 or so would run on something other than pump fuel. I'll have to check my Kohler Command some time and see what that has.
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