B&S 16.5 hp 31C707 UNGODLY loud under load, any ideas

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Old 05-01-11, 03:56 PM
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B&S 16.5 hp 31C707 UNGODLY loud under load, any ideas

Ok, So the story starts, I pick up a like new garage kept lawn tractor from CL dirt cheap, owner tells me hes only used it a couple years and cant get it started but it ran in the fall. I notice the tank is bone dry. So take it home and sure enough, main jet clogged, clean it up real good add a battery & perform all maintenance ie oil, air, plug ect. New gas with carb cleaner additive and it fires right up runs good but the muffler is loose, another shortened story, new exhaust gasket and all is well. But still loud under load.

I've read many post about this engine and decide to check valve clearances, pull the rocker cover and the vales are dead on , actually a little tight, intake .003 exhaust .005, thinking theyre a little tight I attempt to get them to the middle .004 & .006 respectively. And yes I followed directions in the B&S service manual 276781-8/09 section 1 page 21

Thinking thereís a cam problem, while the cover is off I spin the crank(by hand) and observe the valves in operation, I was very happy all was well, valve springs compress about 85-90% int & exh 80-85%. This appeared normal, smooth without any strange noises from the crankcase.

So, my problem is this, the mower runs fine until its placed under a load (blades engaged) then the engine is UNGODLY LOUD and sounds like a Harley with drag pipes @ WOT. I wear ear protection (good pair not cheapies) and still my ears hurt after mowing.

So I check the muffler no apparent defects that I can find although the interior is not visible

starting = good, no apparent problems
Power = good
Power under load = good
Idle = good
WOT = some surging as if the gov is kicking in with a little afterfire popping until placed under a load

Engine specs = 31C707-0175E1-031014Ze
Mower = Murray (cheapie) 425000x8 / 16.5HP 42" grass eater

PLEASE any ideas on why so load or is this normal for these engines :GI2:

Thanks
 
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Old 05-01-11, 04:33 PM
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Normally you don't ave to wear ear cups for lawnmowers, but it's always a good idea to protect your hearing. Aside from backfiring an engine will produce the highest decibels at full load = the most the combustion chamber will hold in terms of fuel/air to handle the load. A muffler designed for that engine in good shape will normally suppress that noise level.

You can alter the exhaust system either by changing the muffler or possibly redirecting the sound away from you as you mow. Maybe the muffler you have isn't the correct one or it's past its service life.
 
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Old 05-01-11, 06:56 PM
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Check your engine mounting bolts. Sounds like they may be just a hair loose and it will make a terrible racket when you load the engine.
 
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Old 05-01-11, 07:45 PM
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I had one with loose mounting bolts once that shook during off load enough to make it run lean. Tightened bolts and it was ok.
 
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Old 05-02-11, 06:53 AM
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thanks for your replies and suggestions, 38racing and cheese I checked the mounting bolts and all seems well. marbobj, I may consider changing the Murray muffler to a stock B&S later on but its in good shape for now.

Ya know, sometimes you answer your own questions, I work 10pm to 630am, and I've been mulling over this just about all night and came to a simple conclusion. My engine is fine and my problem is carb related, its after firing aka backfiring but not a full backfire more like an engine without a muffler.

To help anyone interested I posted a very poor cell phone video on youtube at the following link. My camcorder battery was dead its charging now and ill re-shoot the video if necessary.

YouTube - 16.5 hp 31C707 B&S engine afterfiring

Its poping, afterfiring, and surging in the video when the governor kicks in then it gets louder but you donít feel the surge while mowing the engine rpms are constant and noise level loudest

If you have the time take a peek and tell me what u think. Regards and thanks
 
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Old 05-02-11, 09:19 AM
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Just a link to a higher quality video showing the promblem im having with this mower any suggestions would be welcome

YouTube - Murray B&S 31C707
 
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Old 05-02-11, 06:57 PM
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Your carb is partially restricted still. It needs more cleaning. Use automotive carb cleaner in a spray can and clean out every jet and passage you can find.
 
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Old 05-03-11, 07:20 AM
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Thanks for the help cheese, Do you think a carb overhaul is warranted or just another cleaning. When I initially disassembled the carb to inspect and clean it the carb nozzle and main jet (that little brass looking screw like part) was clogged solid. After clearing with a single wire clipped off of a wire brush I hit every part pretty good with carb cleaner and then inspected it again but saw no obvious clogs. Any clues on the most likely culprit.

I put a bottle of GUMOUT carb and choke cleaner in 5 fresh gallons of gas and ran through 1 1/2 tanks(1.4gal) about 2 gallons so far. I was hoping that would knock out any varnish or partial clogs I may have missed, guess not. Is there a product that maybe you prefer or think would give better results like seafoam for the areas I cant see or access?

Not a bad looking mower for only $125, With a new battery, blades, plug, oil ect, Iím still into it for less than 2 Franklins.

How was the video? First time ever posting a video on youtube.
 
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Old 05-04-11, 12:10 AM
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The video was great... good job.

Go to a welding supply company and pick up a set of torch tip cleaners. They are about $3. Take the carb apart and push the largest cleaner that will fit through every hole and jet. Look into the throat of the carb where the throttle valve closes, and you will see tiny holes where the valve hits the carb body. Push the tiny cleaner through all of these as well. If you have access to compressed air, blow through all the passages well and see if it will run right then. Sometimes you have to clean one several times before it does right. Had one today that did this to me. I think I cleaned it 3 times.
 
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