[Public-List] atomic 4 compression test
brooks.glenn at comcast.net
Sat Dec 1 12:41:45 PST 2012
Black soot on the plugs is typically an indication that the fuel air mixture is to rich, commonly caused by a dirty air filter that reduces air intact for a given quantity of fuel burned, hence the carbon build up on the plugs due to incomplete combustion.
Or, also common, a carb that is out of adjustment. You can adjust the fuel air mixture screw on the carb and observe noticeable changes in the way the engine runs. Turn the screw all the way in until the engine runs rough... 2 to 5 second lag in engine performance... Then back it out until you get smooth running condition. Easy fix IF, the carb itself isn't messed up. however, old carbs build up 'varnish' deposits on the interior fuel flow surfaces of the carb body, and actually reduce fuel and air flows in the carb. This causes dirty black soot build up on plugs and produces constantly bad running engines. You can try soaking and flushing out the carb and rebuilding, but in my experience,,, from rebuilding dozens if not hundreds of small engine carbs, more than say 60% of the time it's better to buy a new carb and throw the old one away, as it's often impossible to flush the buildup out of the labyrinth little maze of journals that are cast into the carb body.
BTW, sometimes carbon deposits build up on the cylinder heads of old engines that have been run for a long time with fuel air problems and cause the same affect as bad fuel air mixture from carbs, e.g. Running rough, loss of power, etc. an easy fix is to take the head off and clean the cylinder heads with solvent and green scouring pad to remove the carbon deposit, then reassemble and voila, find that the engine runs like new!
Good luck on chasing down the problems with your engine. Try the simple stuff first before rebuilding...
Sent from my iPad
On Dec 1, 2012, at 10:36 AM, "John Boor" <mahseer at kos.net> wrote:
> 100-110 is quiet normal as long as they are even on each cylinder.
> John boor
>> We are currently rebuilding an old Atomic 4 and trying to determine a
>> baseline for compression. We were testing the engine when it was cold,
>> using a battery, and got results in the 100-110 range. Adding oil
>> increased compression to 115-125ish. The plugs are fouling very quickly in
>> regular useage (black soot, not oily) and the engine is running rough at
>> times with significant vibration. It has been suggested that it needs a
>> ring and/or valve job. But the compression seems to be in the specified
>> Any thoughts would be much appreciated.
>> Mark Bisset
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