Switching to synthetic oil in B & S air cooled engine and engine problems

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Randy Dockins
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I'm considering switching to a good synthetic oil for my 21 hp Briggs & Stratton engine. I have always used 30 wt. B & S brand oil in the engine and keep it changed about every 50 hours and new oil filter every 100 hours. My main purpose for thinking about going with synthetic is to hopefully reduce engine temps. From what I've read on the subject synthetic will let the engine run a little cooler and provide better lubrication. I am having some problems with the Boxxer 421 not wanting to run right after it gets hot and is either put into idle or shut down and restarted. It will back fire and run rough when trying to throttle up. Also, I can tell it's running rich on the fuel mixture. I'm going to try and adjust the carburetor and see if it helps with that. I had a compression test done at Jon Don and it was a low (40 psi I think). I was told there was nothing I could do to correct the problem which doesn't make any sense to me at all.

Can I switch from using regular oil to synthetic? Does anyone have any thoughts on why the engine runs rough and back fires when it's hot when you put it in idle and then throttle it back up? I'm seriously considering shelling out about 1500. for a new engine if I can't get this problem licked soon. I've already had the carburetor rebuilt and had the exhaust heat exhanger removed and cleaned. Every time I think I have this problem licked it rears it's ugly head again.
 

Gnu

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two words.. "royal purple" full synthetic.

hmm the engines issue might be timing. check the flywheel woodruff key. take the cover off the cooling screen if your has that. and see if its lined up right. there is a square bore on the shaft. and on the fan/stator assembly. sometimes they break and heat expansion will cause it to shift along with the timing. its a long shot but sometimes thats what it is.
a rebuild kit shouldnt be much for that engine. you can do those in-frame. it might be about $250 for new pistons, rings and gasket kit. and it should be sleeved those are cheap too. not easy to remove the old sleeves though
 

Ara Klujian

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They should have been able to tell you if it was the rings or valves causing low compression. Without seeing it, I can only throw darts. Lots of things can be causing your problem. Blown head gaskets, blown intake gasket etc...
As far as oil is concerned I dont think switching to synthetics will help your problem. But I do think you should start changing your filter everytime you change your oil.
 
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i have a b & S 31hp. it only has about 350 hrs on it. if i shut it down after it has ran for a while and restart (usually the next job) it acts like its flooding out. it will usually only idle, if i throttle it up right away it runs very rough and it sounds like its running on 1 cyl. and then usually dies. i can't figure it out either so my remedy has been to just let it idle for a few minutes while i do other stuff then it seems fine.
btw i only use synthetic oil and i have an oil cooler on it so i don't think that will fix your problem (sorry). i knew i should have bought a kohler (since i had trouble before with the last briggs) but the briggs went on sale and i couldn't resist (wonder why they went on sale).

check your valve clearances, if its backfiring and your comp. is low, maybe they are not set right.

also be careful adjusting (leaning out) your carb. it may seem better at first but may burn something after running 2hrs full power on a hot day.

good luck.
 

Ara Klujian

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Everyone is certainly entittled to their own opinion but I have to say Briggs & Stratton makes quality engines. Usually when there are issues throttling up its important to look into the intake gasket. Just spray ether onto the the neck of the carb and see if rpms rise.