Briggs Engine low compression

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  #1  
Old 04-20-14, 03:49 PM
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Briggs Engine low compression

I have dragged out my old Briggs 16hp opposed twin and tried fixing the low compression problem. Again.

History:

402437-0666-01 complete rebuild: machine shop bored .20 over. new rings, pistons, valve job. Cold compression on starter. #1 30lb. #2 50lb. I have a harbor freight leak down kit. testing showed moderately good sealing, better by 10-12% on #2 cylinder.

Today:

the exhaust valve on the lower cylinder leaks enough to hear it, when valve is off lifter. I don't know if it needs more lapping, or if it might be a seat problem. the guide is somewhat sloppy, but I doubt this is the cause of only 30# compression, even on a cold engine. I know this needs fixed.

Here is my question: why does Briggs Stratton tell you to check valve clearance at 1/4 " past TDC?

When I check #1 cylinder, the intake valve does not seal until 1/8" before TDC.

I cannot put a feeler guage into the intake lifter/valve-end space until 1/8" AFTER TDC. Is this normal? to have only 1/8" of both valves closed during the compression stroke?

The compression stroke is about 2 1/8". I thought both valves would be closed during most of the length of the compression stroke?

I would appreciate help to understand this.
 
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  #2  
Old 04-20-14, 09:39 PM
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If I recall correctly, the 1/4" past TDC thing is for ohv engines with compression release, not your engine. I'll have to dig out a book and re-check, been a while. Checking compression on small engines with a gauge is not really the best method anyway due to compression relief methods. Do the leakdown test and remedy any excessive leaks and rotate the engine backwards to get a better feel for the compression.
 
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Old 04-20-14, 10:10 PM
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Happy Easter (night) Cheese!

my manual says to check that way. I found an old thread somewhere that described a compression release on the intake valve that seems to be the way my engine works. valve opens just a teeny bit until just before TDC.

I saw the backward engine spin suggestion but found I could get no "spin" by hand with this twin engine. I put the end bolt in the PTO shaft, and hooked up my electric drill with compression guage, a few revolutions, opposite to normal rotation and each cylinder measured 120 lbs. not ideal, but engine not broke in since rebuild, and I think I can lap the valves a bit better.

do you think the above test tells me the compression release was the issue for my low compression ?

there is no compression release mechanism.......I looked, but I presume its the cam lobe shape. also, I saw a statement from Briggs that "most of their engines have some type of compression reduction" and another stating, "the low compression at start up due to open intake valve until just before tdc, became a non issue as engine speed picked up as the valve speed overcame the short open time".

I would love to find an official Briggs description of the compression release operation of my specific engine. even my interactive comprehensive briggs manual says nothing.
 
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Old 04-21-14, 09:58 AM
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I think you are probably okay since you're getting 120 lbs on an engine that isn't broken in and is possibly dry and probably turning pretty slow since it is connected to a drill.
 
  #5  
Old 04-21-14, 10:04 AM
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I am not an expert on L-head motors and can't even say if it has a compression release on it (probably does). I believe this B&S spec says that you should have some valve clearance, even on the L-heads. With the piston at TDC, the intake should be 0.004" to 0.006" and the exhaust should be 0.007" to 0.009".

Briggs and Stratton Valve Clearances

This guy seems to have the same, loss of compression issue that you have however he is adjusting the valves on a Tucumseh L-head.

https://www.youtube.com/watch?v=yQiE50YAPuA
 
  #6  
Old 04-21-14, 11:07 AM
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Cheese: I think your right, I will make sure the valves are lapped well, check all clearances and put it back.

OEagle: I didn't know this engine had a compression release, but it seems by reverse engineering it must.

I checked out that video, yes, same process as my twin.

My manual clearly says to check clearances 1/4" past TDC. At TDC, the intake is closed (by my patented blow air by mouth through intake opening, ), but .004 feeler would not enter tappet/valve end space.

they claim the after TDC measurement is to eliminate the compression release effect of the cam lobe.

thanks to you both for your feedback!

Ed
 
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Old 04-21-14, 03:26 PM
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As I recall, it doesn't have a mechanical compression release, it is part of the grind on the cam.
 
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Old 04-21-14, 03:45 PM
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you recall correctly. I looked at cam last year....no mechanized item.
 
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