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2019/04/08 15:02:58

westjw

Briggs & Stratton 20hp OHV Intek problem

Hello. We have a Craftsman 42" riding mower with the B&S 20hp OHV Intek single cylinder (model #31P977 type 0635 E1). This mower is approx 15 yrs old but hoping to get a little more usage. However I am scratching my head and wondering what I could have missed. After it's off season sleep added fresh fuel and it started right away and moved 50 feet to garage to prep for new season. Did notice in that short span it ran at full throttle and would not slow but did not give much thought.

Gave carb side of engine a good clean with carb cleaner and that where problems began. Tried to start again and only result was a very slow rpm that would run 15-20 seconds. I checked fuel flow before and after the fuel cutoff valve and fuel filter there was no clogs. Remembered in past couple seasons at low idle it would have some surging but continued on to check spark.

Cleaned the surface rust off flywheel magnet (only one-correct?) and made sure the magneto had proper gap at flywheel. Did the old school test of viewing plug while turning motor over and there was a visible spark. Put plug back in and attempted restart but there was no improvement.

At this point decided it was time for new parts. I have replaced the carb, magneto and spark plug looking for the culprit. After all this best it will do is maybe a one minute run at the very low rpm and that is not an immediate start.

I am a novice with small engines and at my wits end. Wondering at this point if there might be some interior part failure and thinking perhaps might be time to perform a compression check. Blown head gasket? Any advice would be greatly appreciated.

Thank you

9 comments Leave a comment

Mikel1

When you said it ran in full throttle, were you in full throttle just couldn't throttle down?
If you spray some carb cleaner in carb do the rpms pick up? Also when's the last time you checked the valve clearances?

2019/04/08 17:28:05

westjw

Mikel1
When you said it ran in full throttle, were you in full throttle just couldn't throttle down?
If you spray some carb cleaner in carb do the rpms pick up? Also when's the last time you checked the valve clearances?

Hello Mikel. Referring to the full throttle this was at initial start....after a short turnover with choke, throttle was moved to full to start. Hoping for some idle run time after moving discovered it would not throttle down. With the old carb did try spraying with cleaner while running and no change. After installing the new carb did not try any spray while running.

Valve clearance has not been considered. Valve cover has never been removed so no checks performed. Please forgive if this is a dumb question.....what would cause these clearances to change?

2019/04/08 18:05:57

AVB

Normal wear. Briggs recommends adjusting the valves every 50 hrs of use. Some engine go longer and others less. If it has never been adjusted in 15 yrs. I would not be surprised that you find a dropped push rod.

2019/04/08 18:20:44

westjw

Thanks AVB. Very good suggestion. I've wondered if issue could be internal and this would be good thing to check. From what I have found the clearance for both valves is .005/.007. So I would think set the clearance to .006? How can I tell which is intake vs exhaust and is there one that should be done first?

2019/04/08 20:50:14

AVB

Briggs has the setting as follows:

Intake Valve.003-.005 in (.08-.13 mm)
Exhaust Valve.005-.007 in (.13-.18 mm)

Briggs has the procedure as to bring the piston to TDC compression and then a 1/4" pass. THis makes sure that the ACR is not affecting the adjustments. Then you adjust both valves. Personally I still do it the old way by doing each valve separately; just how I have done valves for 50 yrs and it is hard to change now. To which valve to adjust first doesn't matter. On the 310000 vertical the intake is the one closest to the oil pan (crankcase access cover). It is also the valve that has the ACR bump on. Being an OHV I always adjust to tighter clearances IE .003 IN and .005 EX as I know these will open up over time but setting in middle will work fine too.

Just remember to cycle the engine a few times and recheck before sealing up the valve cover. Also remember if using RTV make sure both contact surfaces are oil free and remain oil free for at least 24 hrs so the RTV will properly cure.

2019/04/09 07:54:38

westjw

Thank you so very much for these instructions. Removed the valve cover today and parts fell out,discovered the alum push rod broke aargh. After $130 in other parts my issue probably going to be less than ten dollars to correct. Will return to these instructions upon reinstall.

2019/04/09 13:52:50

AVB

That would the intake push rod (692003). Also make sure you find both of the valve stem caps ((buttons) 691843). If I remember correctly this engine does use two. Some only has one on the exhaust.

2019/04/09 14:38:13

westjw

Hello AVB. Thank for all your advice and per your instructions have been successful. Installed the new push rod and set the valve clearances on the tighter side since it is a 15 year old motor. That along with the new parts I probably did not need it runs like a top. Hoping to get a few more years out of this lawn tractor before we are able to upgrade.

Just a side note that scared me at first. When I first started it was without the shroud and air cleaner and was super smooth. In a few minutes began an erratic surging watching the governor bounce back and forth along with belching smoke like a skeeter fogger. Installed the shroud and air filter and that situation went away.

Thanks again for all your help and the B&S lives to mow another day.

John
Wellborn, Fla

2019/04/09 18:37:29

Mikel1

Yes,Yes thank u so much AVB! Back to PF

2019/04/09 21:15:48

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Briggs & Stratton 20hp OHV Intek problem

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