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Discussion Starter #1
I recently bought a used Murray that has a 17 hp twin (42B707), now I have worked with smaller Briggs motors and have a sense of what's right or wrong with them but this is the first twin Briggs motor that I have had so I'm a little shy on knowing anything about them.

After cleaning it up a little I was able to get the engine running but it has a terrible vibration and what I would call a knocking/rattle. It smooths out some at higher rpms but it's still a rough and loud running motor at any speed.

To add to this, the top of the engine has a some play in it, in turning the flywheel by hand, at times the flywheel will make a slight contact with the coil which you can feel the resistance and can see clean metal from it a slight making contact.

When the engine runs, there's no smoke, changed the oil (little thick) and the plugs (one plug looked normal, the other not too bad but more black with little buildup).

Now in reading around here I understand the L twins run with little more vibration and noise, also saw several people recommending Seafoam to clean out valves depoists which I plan to try.

What do you think? this mower is a 1996 model with a 46" deck which needs new spindles and the hydrostat drive works good, is my engine normal or should I tear it down and rebuild it? If it helps I will try to get a video with sound up online in the next 24 hours to let you judge for yourself.

My first thought was a rebuild and in fact replacing the drive shaft bearings but in getting a tear down parts lists, it doesn't have ball bearing and most likely may need a new drive shaft. Right?
 

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Jersey Mechanic
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The first thing you need to do is check compression in both cylinders and confirm that a rod is not broken. IF that is good, remove the blower housing and adjust the magneto so the flywheel is not hitting it, that may be your knock. While you are in there, attempt to shake or move the flywheel from side to side, it is possible that the block is worn out allowing the crank to "walk".

Report back
Dan
 

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My guess is you are running on one cylinder. Check by removing one plug wire at a time and noting the results.

Walt Conner
 

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Discussion Starter #4
Re: Noisy and vibrating Briggs 16 Twin II

Checked the compression, it's showing just a little under 90 PSI on both cylinders, don't have a spark tester to check for spark but I will see what I can rig up to check that.

Didn't get a chance to adjust the magneto coil but from just moving the top left to right I can see that there is some play in it which I'm still assuming is coming from the worn crank shaft or bearing but from all I can tell the motor doesn't have replaceable bearing?

I was postponed in working with this motor as I found and bought an identical mower with same sized motor over the week what needed only carb work and between the two mowers, got one up and running. The other motor was a much quieter and had no vibration so I suspect there's something more serious with the this motor.

Also, it's 16hp motor, not a 17 as I first indicated.
 

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10K and Climbin!
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Is that one of those OHV engines? If so you may have rocker arm troubles which could cause your problems.. Also if you feel the flywheel rubbing the main bearing could have a problem.. use a dial indicator to measure free play.. Just a few thoughts.. :goodl:
 

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Discussion Starter #6
I don't think it's an OHV, shroud label states 16 HP Twin II, Briggs model is 42B707. I'm rather sure the valves are in the front of the cylinder.

I'm wondering since the flywheel is making contact to the coil that there isn't some damage to coil and I'm missing a spark to one cylinder. While it's making contact, it's not major contact, just enought which the metal is shiny on one side, would like to think that I might be able to get by with a minor repair rather that a major tear down as I have two of these now, I'm wanting to sell the extra mower and free up garage space.
 

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Discussion Starter #8
Flywheel key is good, I pulled the flywheel off last weekend which was a bear to get off as as I broke my bolt puller trying then picked up a another one at Harbor Freight which I broke and after returning it and getting another, I finally managed to get it it off.

Ohhhh... just looked up the price for the crank shaft online, it's $200 plus so I'm not going to be replacing it anytime soon; this "project" is sounding like a patch and sell project..... I hate to tear it down and find out it need something like new crank.
 

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What could have happened is that the engine was run low on oil, breaking one rod (that is your vibration). The problem you have with the coil is probably the result of the upper main bearing being worn out. The prognosis for this engine is not good.
 

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Jersey Mechanic
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What could have happened is that the engine was run low on oil, breaking one rod (that is your vibration). The problem you have with the coil is probably the result of the upper main bearing being worn out. The prognosis for this engine is not good.
Read post #4, I have never seen a cylinder with a broken rod register "a little under 90 psi", most likely the upper "bearing surface" which is machined into the block itself is worn out, thus letting the crank move, which is in turn allowing the flywheel hit the magneto. The crank and flywheel not turning true are causing your vibration.

Dan
 

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The engine has a replaceable top main bushing (bearing) but to install the new one properly takes some special tools that are very rare now days and a machine shop is going to charge an arm and leg to do it, "line bore" the new bushing.

IF what you are talking about is a pleasant "chuckle" while idling and goes away when throttle is picked up, don't worry about it as long as the crankshaft seal isn't leaking, wet around it. This is generally caused by idling too slow OR low oil or both.

Walt Conner
 

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I had excessive vibration on a Yardmachine with Briggs 17hp twin engine and a 46" deck; after much troubleshooting it turned out to be the engine mounts were loose!
 

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Re: Noisy and vibrating Briggs 16 Twin II

Just a quick tip to rule out a broken rod, pull the spark plugs on both cylinders, and rotate the engine by hand while holding a pencil or wooden dowel in each plug hole. This can let you know if the pistons are moving up & down like they should. You should also probe the piston tops to make sure there aren't any holes in them. (though I think you'd be seeing a lot of smoke if this was the case).

I had a Murray (B&S 20hp) that was making the same noise; the noise finally went away when the 2nd rod broke; Very quiet machine now, you can just barely hear it rust! :crybaby:
 

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I'm not sure, but I think that the plugs are over the pistons. All the flathead Briggs I've worked on were that way (plugs are over the pistons).

If the plugs were over the valves, that would take up more combustion chamber space (to allow for clearance for the plugs) possibly resulting in lower compression, require more fuel, and add to the potential of burnt valves or damaged spark plugs if the wrong length of plug was installed (of course that could happen in either case).

Another method would be to remove the head bolts, and pull the heads, and physically look in the cylinders. You'll probably end up there anyway, if you are thinking of keeping/rebuilding the engine. It sounds like you have solved the initial problem (replaced the noisy engine) and are up & running again; but it would be nice to have a spare on hand if the replacement lets go.

Good Luck
 

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Discussion Starter #17
Engine mounts are solid, the vibration is bad at idle but it's the clunking metalic sound that worries me which doesn't improve too much with the engine RPM's except to get worse and make a louder sound as the speed increases.

I'm in the middle of finishing up another Murray just like this one with deck spindle repairs along with another 38" Murray ride which needs a little TLC so it may be a couple weeks before I can start a tear down on this motor, I'll post more info when I get back into this one.

Anyone interested in 46" 16HP hydrostat mower that needs a "little" engine work? :) Wife is slightly bent out of shape about the three mowers in "her" garage parking spot....
 
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