Valve Timing Briggs 15.5 HP model 28N707


Old 11-01-07, 04:53 PM
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Join Date: Nov 2007
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Cool Valve Timing Briggs 15.5 HP model 28N707

Just swapped out a block on my Briggs and Stratton lawn mower engine. All parts were from running engines with the same model number (one was a type 0173 and the other a type 0160) From on-line sources I verified that the cam and crank part numbers matched.
I got the block from an Ebay member with a good reputation. He said I would only have to set the timing since he inspected the block and did not put the cam in correctly. So I opened it and matched the two valve timing marks on the crank and cam. Everything else was clean.
Tried everything and it will not run. Tried several valve setting and cleaned carbs. Checked for restricted fuel lines. Installed new flywheel shear pin. Even hooked the engine harnesses to a running Craftsman mower to make sure I had not screwed up the wiring. It does get spark and does fire up but as soon as you let go of the starter, it quits.

Question 1: Before I attack the rings or something worst, I would like to know if it is possible to set the timing gear 180 degrees out of phase? I know this is possible on a car engine but I have limited experiance on a small engine of this type.
Question 2: Is it possible to verify the installed cam/gear by markings?
(Compression was 95 when I checked it. Will recheck it tomorrow...seems it should be a lot higher.)
Jim R.
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Old 11-02-07, 02:12 AM
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If you are able to see the timing mark on the crankshaft gear and you lined such up with the camshaft timing mark correctly then it is not possible to have the valve timing off 180 degrees. I'm unsure what you mean in your second question, please clarify. Ninety-five pounds compression is OK, ideally you'd like to see it a bit higher but 95 is well useable. Did you notice the spring loaded mechanism on the camshaft? This is the MCR (mechanical compression release) mechanism and its purpose is to relieve compression pressure on startup by lifting the exhaust valve slightly on the compression stroke at starting RPM. This mechanism will give you a false sense of compression if it's doing its job properly. That's why I say that 95 is OK.
Old 11-03-07, 07:14 AM
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Did not see an MCR though I saw it in the book. I just assumed mine did not have one.
One thing I did not check was the key way of the timing gear. But the timing of the valves seems to be about right when you rotate the crank.
Maybe I set the mag wrong leading to a weak spark. I am open to suggestions. Can check that easily.
I did put some Marvel Mystery Oil in the crank, about 4-6 oz. Could that over dilute the oil?

Trying to avoid an unnecessary teardown!
Old 11-04-07, 05:21 AM
Join Date: Nov 2007
Posts: 128
It sounds like to me, that in the process of parts
swapping, some minor electrical differences in the donor
machines might be your problem. If it will start when
cranking, then die, as you stated, then that seems to confirm
You also mentioned swapping harnesses and such. No two
tractors will have a harness that will interchange, unless
they are identical, and also with the keyswitch, and so on.
Also it is a safe bet that your engine has a fuel shutoff
solenoid, which has to have voltage when in the crank and
run position of the keyswitch, and none when the key is off.

More over, the terminal that is on your ignition coil/magneto must never have voltage on it, or you will destroy your expensive ignition coil/magneto., so swapping
can be dangerous. When the key is on the off position, that
should put the wire/terminal to ground on the block/frame,
that is how the mower shuts off, the ignition is grounded.
With the key on the start/crank position, and on the run
it should not go to ground, or anything. Any voltage applied
will ruin your ignition

Old 11-04-07, 08:23 AM
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Location: Upstate New York
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What do you have the valve clearances set at? What did you set the armature (ignition coil) clearance at? Have you had the cylinder head off and possibly not replaced the head gasket and, therefore, a possible leaking head gasket?
Old 11-05-07, 05:08 PM
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Join Date: Nov 2007
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Last I checked, the charging circuit was putting out and I also had a spark. I will check the mag circuit again but it does fire the plug during crank. I have no idea if the spark quits when I disconnect from the starter! That would explain the whole thing if it did. I held the governor once or twice to make sure it was not cutting the engine off.

I swapped head gaskets to the one I was using on a running engine. It looked ver good when installed. (I also had to use my old OHV since this block did not have one). If I have to tear it down, I was going to recheck it.

I only used my tractor once to confirm symptoms. Currently I am using a 12v wire to the fuel solenoid and 12v positive to the starter (negative to engine grounging point). I use a spare 12v motorcycle battery. The solenoid seems to work (clicks) and I have fuel in the bowl.
Air gap is set between 010 and 012. I just reset it again but it was OK. I set valves to spec but after reading some threads here, I adjusted to 003-005 intake and 008-010 exhaust. (Spec for the 28N707 (OHV) series is 003/005 and 005/007 I think)

So far, every major part number, cam, crank, etc between the two blocks (types 0173 and type 0160) are the same. However there is a troublesome shift in some part numbers after a specified serial number on the newer block (0160)...that worries me but it mainly affects rings and the piston. I do not have the serial number of the newer block, only the model and type numbers.

Any more suggestions before I tear it apart. (ugh!)
Old 11-05-07, 05:25 PM
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Something I forgot to mention. There is no muffler on this engine right now so I can see the exhaust flame. There seems to a lot of flame coming out when its firing during cranking. Don't really know whats normal. Makes me thing I have the wrong cam gear or something! That would explain the low normal compression too. Wonder if this is why the previous owner said I would have to reset the timing!
Old 11-05-07, 07:57 PM
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Those head gaskets aren't meant to be re-useable. I don't think that's causing it not to run though. It does sort of sound like the valve timing could be off a tooth or two. Are you sure you lined it up correctly? Are you sure the valves aren't set too tight? Are you keeping voltage applied to the fuel solenoid the entire time...along with keeping the engine grounded electrically?

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