adavid
04.06.2021 15:02:08
It may just be a lazy piston ring due to being left standing but you never know. [links]
See full version: Compression in a 2 stroke outboard
It may just be a lazy piston ring due to being left standing but you never know. [links]
After finally putting a fresh petrol mix in it on Saturday I managed to get it to fire for nearly 10 seconds on one cylinder! Then it died and won't play any more. Despite me using my electric drill on the flywheel nut to turn it over. more
Compression testing a two stroke tells you very little about the state of the engine. They dont 'suck' the mixture (at least not much) like a 4 stroke: the suction part takes place in the crankcase as the piston rises; as it comes down again it compresses it and forces it in to the cylinder when the ports open. So although cylinder compression is important, more so is the crankcase compression. Worn rings will upset the crankcase compression and suction, but far more commonly the crankcase seals leak, reducing the crankcase suction / compression cycle. If the crankcase compression is low, it will not blow through the combustion chamber enough to overcome the pressure of the exhaust gasses, so they will not purge properly, again preventing the engine starting or running properly. It is very difficult to measure crankcase compression, but very often it is the main cause of poor starting/running
The compression is less important than the variation between the cylinders.
If the variation is more than 15psi between the two then you need to investigate. more
What should the compression be on a small 2 stroke outboard? more
In most two strokes 120 + is good, tho there are some weird motors that run less.
6.5 - 7.5 to one is pretty typical swept compression (only counting above the exhaust port).
I'm at work now, but could post formulas and measurement techniques tonight.
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compression seems to be.
hello Rudy, in a two cycle, cylinder pressure is not a big deal, the crankcase pressure is what runs the engine and unless the case has a leaking gasket or hole in it, it should not be reduced much from new, and if 90 is not fouling plugs, then use it. in abos third edition outboard motor service manual copyright 1965, mcculloch lists 75 psi for the 3.6 hp, Clinton lists 60 psi for the 5hp. mercury lists " not more than 15 psi between cylinders " for 6hp to 100 hp. john more
Thanks John you are a Prince. Your information is a Godsend and of course I always look for parameters and this not exception. The plugs are clean and dry. Do you know any books that might tell me how bad, bad is, as in what are the high and lows that are acceptable. Maybe I just spent too much time in Government Defense work. Thanks for your help, Rudy here
. too much of a variable on these small 2-stroke engines, Rudy. I'm no expert on the subject, but I have worked on many 2-stroke engines; from small model glow types to the marine outboards. I don't recall ever seeing compression specs. Just as John said, if the crankcase is sealed, rings are good, fuel & spark are available, she'll run! I had a problem last year with our 5hp o/b where the rings got stuck in the piston grooves over the winter (forgot to add oil before storage!). Blasted thing wouldn't even pop with ether! The problem was obvious since it was TOO easy to pull! After digging the rings out of the grooves and replacing them with new (didn't NEED new ones until I broke the old ones!), she started up on the 2nd pull! To repeat again what John has said, don't get too excited about these things. If it starts and runs well, just go sailing! Jim Maroldo s/v: Lil' Wass Ett