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Garden Tractors Discussion Forum

cam trouble?

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old bones

07-30-2003 19:46:15




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have a briggs 2 cyl(opposed pistons) 18hp on my grain auger. was running great (as it has for many years), but it sounded a little different(sorta like running out of gas), slowed down to an idle and quit (no smoke). could start it up but no power. checked shear key on flywheel -ok. took carb off and apart -clean as a whistle. when cranking, was getting air back up through carb throat AND through "blow hole" in top of carb body. did compression check - one cyl was 20lbs, other was 30lbs (extremely low). took heads off and cleaned off carbon (very little). no carbon under valves. cyl walls look good -no scores or scratches. turned it over by hand and watched valves open & close. noticed that intake valves don't completely close until piston is about 3/8" from tdc. i'm not an expert on small engines, but i would assume that the intake valve should be closed when the piston starts the compression stroke. also, the exhaust valve is still closing when the intake valve is opening. exhaust valves seems to be opening at beginning of exhaust stroke. my question to someone with small engine background- do i have cam trouble? or am i overlooking something? thanks in advance.

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Jake

07-31-2003 11:13:14




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 Re: cam trouble? in reply to old bones, 07-30-2003 19:46:15  
I have not worked on any of the 2-cylinder models but most if not all the single cylinder models have some sort of built in compression release that holds a valve partially open during cranking. Yours may have that. Some engines have a mechanical flyweight and mechanism on the cam shaft to produce this. Some have a small extra bump on the cam shaft to do this. On the single cylinder engines you need to spin them backwards to get a true feel for compression. Book states to sharpy spin flywheel backwards and it should rebound off of compression. However this may not work on your two-cylinder. I don't know if I've helped you much. Get a good manual or find someone who knows more.

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Mike

07-31-2003 07:26:06




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 Re: cam trouble? in reply to old bones, 07-30-2003 19:46:15  
Your engine may have a compression release built into the cam get a manual on your model eng. to find out.



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shannon

07-30-2003 21:44:22




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 Re: cam trouble? in reply to old bones, 07-30-2003 19:46:15  
check valve seats to make sure they did'nt sink down in block.. this will alter compression and cause it to lose power..had this happen to a 16 briggs(notorius)..



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OLD BONES

07-31-2003 15:18:28




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 Re: Re: cam trouble? in reply to shannon, 07-30-2003 21:44:22  
valve seats are not sunk in.



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Gene Davis

07-30-2003 20:38:37




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 Re: cam trouble? in reply to old bones, 07-30-2003 19:46:15  
Did you check for valve to lifter clearance? It sounds like there may not be any valve clearance , this will affect the valve opening/closing timing.



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old bones

07-31-2003 15:23:26




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 Re: Re: cam trouble? in reply to Gene Davis, 07-30-2003 20:38:37  
as nears as i can tell, the lifters are solid. i don't understand how they could "grow", especially that much. i would think that they would wear down, instead, resulting in the valves not opening enough. if i'm wrong in my thinking, correct me. any other ideas? thanks.



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Rick

07-31-2003 16:04:30




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 Re: Re: Re: cam trouble? in reply to old bones, 07-31-2003 15:23:26  
The valves will wear down and result in too little clearance on the solid lifter models. The steps are to remove the heads and anything that is in the way, remove the plates that cover the valve stems and push rods then the valves. You then grind or file the valve stem to get the right clearances.



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old bones

07-31-2003 19:40:45




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 Re: Re: Re: Re: cam trouble? in reply to Rick, 07-31-2003 16:04:30  
i don't think the lack of valve clearance is my problem. i'm getting compression back through the carb, actually blowing gasoline out the "blow hole" in the top of the carb body, as well as back through the venturi.



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Wally G.

08-11-2003 05:03:46




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 Re: Re: Re: Re: Re: cam trouble? in reply to old bones, 07-31-2003 19:40:45  
You better redo the valves. There is no compression release on the Briggs and the valves do wear into the seat and will eventually hit the tappet and lift off the seat. I usually give an extra .003 to .004 extra clearance.



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