Re: Proper techniques for doing a compression test
We probably shouldn't have thread-jacked shadetree's post, I appologize. But I guess we should keep your information and questions here, or it might get confusing.
Your number four sounds low, 10% low is a typical value you are a bit more than 20% low compared to your highest value. If you compare to your middle value, at least the numbers sound better. There has been discussion of this in the past.
Did you say you check the compression while on muffs? I check the compression with just cranking speed, all the spark plugs out and the wires grounded. I used to use WOT during the check, but someone pointed out to me that was only necessary on a four stroke. Someone else said that WOT will compensate for a uneven opening of the throttle plates, but if you are synched correctly that won't be a problem.
If your low compression was caused by a sticking ring, a second decarb and a recheck of compression is in order. Perhaps you've caught it before you rocked the ring and failed it. If the compression stays low, I would remove the port cylinder head and have a look-see.
Check for water leaks at the exhaust cover between the cylinders while you are running on muffs, just for inspection purposes.
If one cylinder is running lean or coking up, you may be hot in that cylinder, and it may be time to rebuild your cabs. On my 90 each carb is essentially two carbs with a common float bowl. A plugged orifice could cause lean running in one cylinder. You may also be off on your link and synch, so you may want to check that as well. If you do have low compression on one cylinder, you need to find out why.
good luck,
fp
