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Compression lock - cold start


bobj

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This is a 14 hp Briggs that was rebuilt last year by a Simplicity guy that has been selling and servicing these machines since they were new. Machine is a 3314 1970 or 1971 Soverign. It runs awesome, low, high, and transitional speeds are right on. It idles at 300-400 rpm till the cows come home. The engine was really messed up prior to the recent rebuild, a botched previous rebuild attempt. Crank was junk, cylinder had to be sleeved. If I pull it through once cold it spins and starts the way it should. If I don't pull it through it locks repeatedly on compression. Once warmed up it always restarts the way it should. I don't feel this is an ignition timing problem because it did it initially during the first start after the rebuild. At that time I gave it 10w-30 oil, pulled the plug and spun it. Reinstalled the plug and spun it again, without fuel, and experienced the first lock up much to my suprise. That lock up was not as pronounced as when fuel is added. I put a compression gauge on it figuring I would get a very high initial reading, but that isn't the case either. It always reads about 80 pounds and it doesn't lock up using the compression gauge. I never have put a compression gauge on it after warm up, maybe I should. Attached is a pic of my engine. The numbers check out as the correct engine this machine came with. I have read that cam timing is such that compression relief exists during cranking speed, maybe the cam is worn. Maybe more so with the exhaust valve which might work against it's original design. Has anyone run into this before?


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Check the clearance in the exhaust valve. Since the B/S uses a bump on the exhaust cam as a compression release, if there is too much valve clearance then the bump will not open the valve and release the compression. I have a 10 Hp with no bump on the cam. I filed it off to give more low end torque when the engine was lugging down for tractor pulling. It turns easier when cold than when warmed up, so I don't know why yours is the opposite.
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Rod, I think the compresion release on a Briggs is built into the intake valve. But the results can be the same. I'd check the tappet clearance on both valves, just to make sure. With it being a Starter/Generator I commonly see the belt slip if its not tight enough. If the belt is tight and not slipping I'd say your S/G is either getting weak or your battery or connections are weak, not supplying good amperae to the S/G. Try jumping the tractor the next time it's cold. If it spins it over well with jumper cables then the battery is likely loosing charge from sitting andis bad.
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The starter/generator pulley could be worn to the point of the belt not contacting the sides of the pulley. I have heard of this happening quite a bit from people on here in the past.
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I had a similar problem on a Kohler recently and in this case it simply was not enough battery for the engine. It is the easiest thing to check before going into timing and valve clearance.
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Battery is new, 400-500 cold crank, belt is new Simplicity, pulley is great never slipped, starter recently gone over and checked by electric motor rebuilder. When I pull it through it's very difficult, second time is a breeze.
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Then I would pull the plug out next time you cold start it. See if it has this hard spot then. I'd be on the look out for liquid in the cyl.
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Does this initial cranking produce a mist or smoke? I have had some with ring probs suck oil into the cyl when it is shut down, leaving it for the next start up.
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No liquid, no steam, no smoke. Has been spun w/o plug. Engine just rebuilt, less than 4 hours run time total. Spark plug looks like it could be resold as new.
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I don't know, that's why I posted this. Looking on the web Simplicity filed for at least two patents over the years to bleed compression during start up. First patent late '60s, second patent mid '70s. Early patent had a bump on the cam, second patent had a much different mechanism. Trouble is the same part number is listed for both cams.
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For a very fresh engine, sometimes the rings can be a little 'sticky' on the first couple cycles. What oil is used? Perhaps a different oil could shed some light on that.
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