B&S 18 HP No Compression

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  #1  
Old 08-17-04, 02:16 AM
kaw550
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B&S 18 HP No Compression

I have an B&S 18hp twin II that has no compression.

Well according to both my compression gauges there is no compression. The engine is about 5 years old. It went after about 5 minutes. I beleive it was a connecting rod or piston. The parts were taken out of the engine it was replacing.

Since then it never ran. Repairs have been attempted several times. I have just obtained the engine/mower, so some of the details on what have been done are a little fuzzy.

When I found there was no compression in either cylinder I took off the heads. I found that there were no head gaskets. THose were replaced but still no compression. The cylinder walls look perfect. The valves look good.

What should I check? Could the valves not be seating? I am stumped.
It has spark but wont fire. It will occasionally pop from one side.
 
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  #2  
Old 08-17-04, 02:47 PM
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You didn't mention and I don't want to assume, but were the pistons moving up and down the cylinder? After installing the head gaskets, did you torque the head bolts down properly? Answer these ?'s and we'll move on.
 
  #3  
Old 08-17-04, 04:24 PM
Azis
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I had in mind the same ? as puey when I first read this post. Simple check for compresion of any sort, pull a spark plug and hold a finger or thumb over the hole while cranking the engine. You should feel some sort of pressure and suction. If so this should give a good indication that the pistons and valves are moving correctly and point else where.
 
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Old 08-17-04, 05:49 PM
kaw550
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Yes

THe pistons were moving with the heads off.

I torqued the heads down to about 14 there was nothing so I tried 20.

I did put my finger over the plug hole and there was some compression.
 
  #5  
Old 08-17-04, 07:53 PM
Azis
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Well since the engine has been disassembled to your knowledge, and you have spark and some compresion, and you mentioned it pops now and then... Do you have spark on both high tension leads? If so the flywheel shear key may be suspect. If it is sheared wrong or missing.
 
  #6  
Old 08-18-04, 12:02 AM
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The flywheel key is a definite possibility. It also sounds like someone completely inexperienced put the thing back together (no head gaskets), and it's probably a good chance they didn't get the camshaft installed correctly and may be out of time. Watch the valve movement as the engine turns to see if it is timed correctly if the flywheel key doesn't pan out.
 
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Old 08-18-04, 02:03 PM
kaw550
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IT does have spark on both sided.

How do I determine if the vlaves are working properly. WHen should they open and close? Open when the pistion extends?


They do alternate between intake and exhaust on both sides.
 
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Old 08-18-04, 02:04 PM
kaw550
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Can I check the timing with a my timing light?
 
  #9  
Old 08-19-04, 02:20 AM
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No, a timing light won't really do anything much for checking valve timing. As you rotate the engine clockwise by hand, the intake valve will open as the piston begins to move down (intake stroke). When the piston reaches it's farthest position down, the intake valve should be closed and the piston will begin to travel toward the head (compression stroke) with no valve movement. The piston will travel all the way to the top most position and go back down with no valve movement (power stroke). At the bottom, as or just before the piston starts to go back toward the head again, the exhaust valve should open (exhaust stroke). Then the cycle starts over again. In between the exhaust stroke, and the intake stroke, when the piston is all the way at the end of the cylinder (top dead center), both valves should be closed or almost completely closed. You should be able to rock the engine back and forth at top dead center and see the 2 valves rock open and closed. If not, the valve timing is off.
 
  #10  
Old 08-19-04, 06:22 PM
kaw550
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almost

The valves seemed to work almost exactly as you described with one exception the intake valve did not close all the way until the piston was anout 1/4 of the way back up(or headting toward TDC).

Now when one pistion is in between the exhaust and intake stroke where should the other pistion please.

Is it even possible for the valves and pistions to be off with one camshaft?
 
  #11  
Old 08-20-04, 04:08 PM
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Each piston will be in the same positon in the bore as the other, since the crankpins are 180 degrees apart on the crankshaft and since the cylinders are 180 degrees apart as well. The problem you have is that the intake valve appears not to be timed correct. Which means the exhaust valve timing would be off as well. Sounds like a disassembly is in order here to correct the crank and cam timing marks. My guess is that the person(s) who repaired this engine mistakenly thought the crankshaft cam gear keyway was the timing mark, when actually there is a dot on the cam gear just to the left of the keyway. If you happen to have an engine with a PTO side ball bearing, post back as this timing procedure will be different.
 
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Old 08-20-04, 06:53 PM
kaw550
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on a scale of 1 to 10

Ok how hard of a job is it. Sounds really tough.
I am somewhat experienced with small engines rebuilds but they were all 2 stroke snowmobile or jet ski's.

How tough is a lawnmower engine?
Are manuals available?

Is there any chance it will run!!!!!


I am confsed to how it could be assembeled wrong. and still rotate.
 
  #13  
Old 08-21-04, 01:38 PM
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It can rotate in any timing configuration. How far does the intake valve hold open when it chould be closed? All the way open? It's not very hard...I think you'll be fine doing this if you've been into other engines, even if they were 2 stroke.
 
  #14  
Old 08-23-04, 03:43 AM
kaw550
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I got the engine out. I removed the bolt holding the upper and lower sections of the block but could not split it. I will work on that tonight.

I did remove a panel from the back of the block that allowed me to look inside. I assume the adjust ments I have to make will be with the small gears inside.

What should I be looking for?
 
  #15  
Old 08-24-04, 12:07 AM
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You'll want to be sure the timing marks on the crank gear and cam gear line up properly.
 
  #16  
Old 08-24-04, 02:27 PM
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So, do you know yet if there is a ball bearing on the PTO side of the crankshaft? As far as manuals to help you out...yes, go to your local Briggs shop and ask for manual part number 271172. This covers all opposed twin cylinder Briggs engines and has a section detailing the timing procedure very well. Good luck and let us know of your progress.
 
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