Compression & Decompression

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  #1  
Old 10-03-02, 09:41 PM
dave4.3
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Tight compression stroke on B&S 4hp

I took apart a Briggs 4hp lawn mower engine and cleaned everything and put it back together. It turn over smoothly with out the spark plug in. Put the plug in though and it gets very hard to turn over. The compression stroke is very hard and it kicks back.

The timing marks on the crank and cam are correct. The only thing I can think of is I used form a gasket on the sump pan. There is supposed to be some free play in the crank. It is so tight it broke the vertical pull starter.

I did get it running by using a drill to turn the flywheel nut.

Any ideas?

Thanks
 
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  #2  
Old 10-04-02, 12:14 AM
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Hello Dave4.3!

If it turns over very easily without the plug, then the form a gasket is probably not the problem. I bet your valve lash is too much, or...is there any chance you are trying to start it without the blade attatched? If so, that is your problem.

Let us know what you find!
 
  #3  
Old 10-04-02, 01:13 AM
Fisher
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Or if you had the flywheel off, you may not have retightened
the flywheel nut enough and partially sheared the key.
Fish
 
  #4  
Old 10-04-02, 05:18 AM
mikejmerritt
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While your into this you do need a few thousandths of end play on that crank.....Mike
 
  #5  
Old 10-04-02, 08:10 AM
dave4.3
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Would mixing up the lifters cause this? I took the sump pan off last night and there was a lot of fine metal in the sump.

The flywheel was torqued to 50ft-lbs with the key installed.

I was trying to start the engine with out the blade attached.

Thanks. This is driving me nuts!
 
  #6  
Old 10-04-02, 05:38 PM
dave4.3
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Well I bought a proper sump gasket today. It didn't help. The last inch of travel is very tight on the compression stroke. I know it compressing but this is over kill. It broke the starter.

There was metal fuzz in the bottom of the sump. I did get it running. It ran for 15 mins or so. I shut it down and then started it up with an electric drill. It still tight.

Any ideas? It also backfires through the carb.
 
  #7  
Old 10-04-02, 09:04 PM
dave4.3
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Question Compression (PSI) for small engines

I checked the B&S manual and they only talk about leak down tests. What should a health small engine have for compression?

I checked 3 lawn mower engines and they are around 45psi +-5. One is brand new 6hp B&S with 5 hrs max on it.
 
  #8  
Old 10-05-02, 03:31 AM
J A Boggan
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On most of the engines I've checked, it runs around 90 to 100 PSI. You should also take in cosideration, some mowers have compression release devices, usually on the camshaft, designed to make the pull starting easier. It will generally lift the exhaust a small amount, then when it starts, centrifugal force will return the valve to it's normal position. Tecumseh uses this a lot on their 'easy - spin' engines.
 
  #9  
Old 10-05-02, 04:32 AM
J A Boggan
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Like Mike said you need to definitely need to 'feel' a few thousands endplay. Usually, you can get the sump gaskets in a variety of thicknesses. You can even double up the gaskets to get the proper thickness.
 
  #10  
Old 10-06-02, 01:03 AM
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Have you reinstalled the blade? The engine, unless equipped with a non-compliance flywheel made of cast-iron, depends on the weight of the blade to get it cranked. It will break the pull cord and maybe hurt you if you try to crank it without the blade.
 
  #11  
Old 10-06-02, 01:06 AM
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It seems to be kind of difficult to get an accurate reading on most small engines. If it has enough to blow the compression past your finger when pressed over the hole, then it should be enough. (this done with someone giving the rope a good pull).
 
  #12  
Old 10-06-02, 11:39 AM
dave4.3
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Nope. Nothing is attached. Just the engine bolted down on the deck. Aluminum flywheel.

I have no idea why but it bust a drill yesterday tring to start it.
 
  #13  
Old 10-06-02, 12:43 PM
Fisher
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Again "Listen to Cheese". On pushmowers, they have aluminum
flywheels and need the added weight of a steel blade otherwise,you have the symptoms you describe.
Fish
 
  #14  
Old 10-06-02, 01:11 PM
dave4.3
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Thanks I will have to try that! I will also buy some new glasses! Thanks again guys!
 
  #15  
Old 10-06-02, 02:42 PM
dave4.3
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I just tried with the blade on..... It made no difference.
 
  #16  
Old 10-06-02, 08:43 PM
Fisher
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Did you recheck the flywheel key?
Fish
 
  #17  
Old 10-06-02, 11:55 PM
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I agree with Fish, check the flywheel key...and make sure your armature air-gap is set correctly and not right up against the flywheel.

What was the engine doing to prompt you to take it apart? Did you remove the piston and rod?
 
  #18  
Old 10-07-02, 12:04 AM
dave4.3
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I did recheck the flywheel key and it is there and happy.

I decided to pull the engine apart for my own "enjoyment".

www.picturetrail.com/rangerdave

I took a few pictures of the engine in pieces and building up. It totally took everything apart.

I have a 3.5hp B&S in pieces now.
 
  #19  
Old 10-07-02, 09:17 PM
mikejmerritt
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Shot in the Dark....

Piston in reversed (X on the underside pin area toward flywheel), rod reversed, rod cap on backward or any combination could cause this anomaly. Its been so long since I was into a 3.5 (current labor and parts prohibit such fun...:-(.. ) but I think if the rod was in wrong it will hit the block but the cap could still be on backward. If the rod cap has been installed backward there is no need in attempting to right it once the bolts have been torqued, it won't stay together long. When installed properly at the factory the crank, rod and piston will break in and find their center and the variable is the all important end play in the crank......Mike
 
  #20  
Old 10-07-02, 10:58 PM
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Mike...I always wondered what difference it would make if the piston was installed on the rod the wrong way. I have never done it to my knowledge. I do know that the rod will hit the crankcase if installed the wrong way.

Dave: The rod cap should be on the camshaft side of the crank journal.
 
  #21  
Old 10-08-02, 06:12 AM
mikejmerritt
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Well, if you pulled a brand new engine down and reversed the piston there probably wouldn't be any problems. I assume the factory puts an X on them for tear down/reinstall purposes although none condone rebuilding. Funny they don't recommend rebuilding/internal work to any extent but they all sell over/under sized parts. Once an engine has been run a while it will break in and find its center and reversal of the piston could cause a very tight or ill fitting situation that may allow the engine to run but wear will be very premature.....Mike
 
  #22  
Old 10-08-02, 07:26 PM
dave4.3
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Thumbs up 3.5hp Briggs runs again!

Well I ripped apart a 3.5hp Classic. Cleaned everything, honed the cylinder, new gaskets, lapped valves, new plug, etc. It runs like a dream!

I have no idea why the 4hp Briggs I posted earlier is giving me grief. I rip it apart and try again.
 
  #23  
Old 10-08-02, 07:30 PM
dave4.3
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Guys,
I checked the rod cap and it is on correctly. The piston is on the correct side and I checked my 3.5hp to verify which is the side cheese said.

Are the lifters different lengths? The larger spring goes on the exhaust, correct?

Also what about decompression? Maybe that isn't working thus I feeling the full compression.

Thanks again guys!
 
  #24  
Old 10-08-02, 07:31 PM
dave4.3
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Decompression

How does the decompression work on these small engines?

Moderators Note:

Since these two questions where closely related, I merged the two into one topic.

If you still have questions on these engines or similar questions on 4 cycle engines, kindly use the "REPLY" button and add the additional questions or replies. Doing so helps everyone reading the topic follow along more easily.

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Last edited by Sharp Advice; 10-08-02 at 08:46 PM.
  #25  
Old 10-09-02, 01:58 AM
Fisher
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If you mixed up the lifters, then you may need to recheck your
valve clearances.
You did go back with the old head gasket or a new one, didn't
you? I saw where you used form a gasket at first. Using that
for a head gasket would raise your compression ratio and give
you similar problems. Just a guess.
Fish
 
  #26  
Old 10-09-02, 12:51 PM
dave4.3
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I used the original head gasket Fisher. I used form a gasket on the sump, intake, and breather. I've since replaced the sump gasket with a proper one.
 
  #27  
Old 10-09-02, 01:25 PM
mikejmerritt
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A Couple of More Answers...

It would be almost impossible for the decompression function to be disabled if the valve lash has been set correctly because decompression is a function of the cam. The back side of the lobe on the exhuast has a bump so to speak that raises the exhaust valve just a moment as the piston comes up on the compression stroke.
The lifters are identical but as Fisher points out reversing them would cause the need for setting as would a general teardown.

Kart engine tip while on the subject: with a benchtop sander like a 1 inch belt sander remove about .060 from the back side of both cam lobes (point opposite the high point of the lobe), polish a bit and reinstall and set lash to .006 exaust, .010 intake, when it settles in it will be spec .007 & .011. Also, use the long valve springs like the the exhaust on both valves. Rob another long spring from a junk mower. A bit tough on the arm (hey nothings free) but a fantastic power booster. Also remove the carb baffle on 5HP Briggs horizontal and that dude will hit like a Harley, particularly impressive at idle. Sorry to ramble on this modification but dave4.3 seems like the kind of guy who would have try this on a 5HP Briggs horizontal.

In case dave4.3 missed it above the long valve spring is for the exhaust in a stock situation......Mike
 
  #28  
Old 10-09-02, 04:42 PM
dave4.3
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Mike,

I noticed on the 3.5hp Classic made in 1991 that both springs were the same size and being short in size. The 4hp had one short and one long. I'm going to double check I didn't put the long spring in the wrong spot.

Also hows the best way to measure lash?
 
  #29  
Old 10-09-02, 11:11 PM
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To check the lash, bring the engine to top dead center, remove the PCV valve/valve cover. Using feeler gagues, measure the amount of space between the lifter and valve stem. If the space is over specs, then this will cause excess compression when starting.
 
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