A little help


Old 06-15-06, 06:15 AM
Thread Starter
Join Date: Oct 2005
Posts: 6
A little help

I have an older Craftsman 36" cut riding mower with a replacement Briggs & Stratton engine on it. (I will get the model numbers of each if needed). The problem that I am having, is that the engine will start, but it runs like it has a huge lift roller cam in it, as it idles like: WAP WAP WAP WAP. Once the blades are engaged it smoothes out, but man, it is annoying at idle. I have tried to clean the carb, and make adjustments, to no avail (SP). The motor is missing a muffler, but could the lack of any back pressure cause this ?

Any help is greatly appreciated.

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Old 06-15-06, 08:59 AM
Join Date: Jun 2006
Location: St. Louis
Posts: 2,365
Sounds like the engine is too lean when the engine is not under some load, open the adjustment screw 1/4 - 1/2 turn(with the engine running if possible) and see if it makes adifference, if not go the other way. It is never good to run without a muffler, if for no other reason than the noise will drive you nuts.
Old 06-15-06, 09:16 AM
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Join Date: Apr 2004
Location: Pennsylvania
Posts: 592
Air cooled engines shouldn't be idled for long periods of time, They depend on the fan to cool them and lower RPMs mean less air to cool. Having no muffler is probably contributing to the noise you hear.
Old 06-15-06, 09:29 AM
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Join Date: Oct 2005
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Yeah, I am going to try and get a muffler for it ASAP. The engine noise isnt the problem, its just the real choppy idle. I have tried to adjust the carb, in and out, and it has not made any difference. I have not officially rebuilt the carb, but have taken it apart to spray it out with carb cleaner, and an air compressor.

Old 06-15-06, 04:10 PM
Join Date: May 2006
Location: Iowa!!!!!
Posts: 3,728

The muffler on a four stroke plays a role in the tuning of the engine although I would say its more in a two stroke.

If after replacing the muffler the problem persists, I would think in terms of valve leakage which would be determined with a compression test. You can rent a tester at auto parts stores. If you're in good shape it should test over 90 psi.

Test, if low add oil to the cylinder, spin the engine to distribute the oil and blow out excess, retest. If low it's probably valves, but could be head gasket.
Valve leakage would contribute to the idle you're experiencing.

I would also check the point gap as it would affect the timing and could affect the idle as well.

Hope this helps
Old 06-16-06, 04:24 AM
Thread Starter
Join Date: Oct 2005
Posts: 6
I have pulled the carb off, and have torn it down. I am going to rebuild the carb, and get a muffler this weekend to see if it makes any difference. It also needs an air filter pretty bad. I will update with what happens.

Thanks all for the replies.

BTW, the engine is actually a Tecumseh OHV 12.5 hp, and not a Briggs.
Old 06-17-06, 01:51 AM
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Check the engine mounting bolts for tightness. Might sound odd, but it's funny how badly an engine will seem like it's idling when the engine bolts are loose. The engine twists and hits the bolts against the sides of the holes in the frame with every revolution, making a hard hitting sound and kind of shakes the mower.

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