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compression!!!

B

bdunegan

Member
I have a 21hp B&S ohv single cylinder with problems. It starts and runs great, but you have to hand spin the flywheel past compression first. I have tried a new heavier battery, set the valves according to specs, and checked flywheel key to make sure it wasnt sheared and throwing timing off. I even thought starter may be dragging, and put another good starter on it. nothing helps. you can turn the key to start it, and it will spin to compression, but wont spin past without hand turning it. just hoping it is not internal problems. anyone have any suggestions!
 
Try adjusting the valves again. If the exhaust valve isn"t adjusted properly, the compression relief doesn"t work properly. If it"s within spec"s, the compression relief has likely broken on the camshaft. You"ll have to split the cases and check it. Some of the Briggs engines have plastic camshafts and don"t last too long

Hope this helps.

Joel
 
just went out and re-set valves just in case they may have been off, and no luck. I can hand spin it backwards about a turn or so and then hit the key, and it will spin past compression and start right up, but if it is close to compression at first, it will not turn past. this is on a 2011 craftsman mower that looks it is brand new. it cant have more than 20 or so hours on it. Surely the acr on the cam is not bad already. is it possible for them to stick?
 
I tried that one too. i went directly to starter using heavy cables from another runing mower. still no luck but thanks for the input. this one has me flubbered!
 
First if you're not doing it is valves must be set with piston 1/4" into downward travel after TDC of compression stroke otherwise valve tappets won't be clear of cam lobes & acr cam & you can get false settings.

If you are setting valves as above & still have problem,remove plug & crank eng 3-5 sec,reinstall plug,crank eng,if eng now cranks over then most probable cause of problem is float valve in carb leaking flooding eng & hydrolocking piston on compression stroke,if carb not the problem other possible cause of problem,acr or cam bad.
 
I set valves 1/4 down on compression stroke, and also took plug out and cranked. put in plug and same thing again. I know fuel solenoid is working and shutting off fuel. dont notice any signs of flooding. Not to sound like a dummy, but walk me through normal procedure for tdc just to make sure my mind hasnt slipped. i am going to check valves again tomorrow just to make sure something didnt slip. only other thing i can think of would be the acr. thanks for your response, i appreciate it.
 
If you're rotating eng to top tdc of compression stroke(both valves closed)then dropping piston 1/4" & setting clearances,then you're doing same correctly,I'm also assumeing whatever manual/spec sheet you're looking at calls for .003"-.005" on intake & .005"-.007" on exh,then you're setting valves correctly,a lot of guys like to set both for .004" on both to make for a longer time between adjustments,you might try that.

If carb not leaking other possible cause's of problem,if cam gear plastic,gear may be worn or bearings for cam worn & cam jumped it's time,acr cam worn or one of the pushrods may be bent.
 
I have a friend that works for a company that does warranty and repairs for Sears. He says this is a fairly common thing on the large single cylinder B/S like yours. He said some of them don"t get 10 hours before this happens. It is the acr on the cam.
 
just re-set valves for third time, set both at.04 1/4 inch down after tdc. checked both push rods and neither one bent, put it back together, and a lot better now. every once in a while it will still hang and not crank past compression, but most of the time it will start right up. if it hangs, i just roll it backwards a turn or two then hit it and it will start. I am guessing valves still not set just perfect but i can live with that. at least i can get it started now pretty easy. thanks for all the responses, it really helped a lot!
 
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