Quote originally posted by DougMN
I bought a 1997 classic, the carbs had just been cleaned. With 285 miles on it. It was never as fast as my sons 1996 classic. Keep getting slower, a compression test and a look in the exhaust port showed burnt pistons. Honed the cylinders and put in SPI pistons and rings. Ran good, 85 mph but over the next 60 miles it started to slow down and started hard. Compression test 98 lbs.in both cylinders. Are these SPIs known as good or bad. What other rings will work on SPI pistons. What type of rings are SPI? Thanks.
So, lets try to break this down....Your engine performance all of a sudden gets mediocre. An exhaust port and compression test show a drop in compression and burnt pistons. You hone the cylinders and install new SPI pistons and rings. Now you have how much compression after the rebuild? (THIS IS A MISSING LINK...) But you have renewed performance after the rebuild, so we'll figure you have 125 to 140Lbs. of compression after the rebuild just for the sake of conversation. And then after 60 miles the performance drops, after another compression check now you only have 98 lbs. compression (In Both Cylinders).
So, what would make BOTH cylinders lose compression at the same time? Since we have no report on spark plug condition ie. color or if there was debris on the plugs. And a second exhaust port check has not been described in detail. Just a note on exhaust port checks, unless you have a lighted scope to look in through the exhaust port to the intake side or the evidence of failure is on the ex. port side. they're pretty much so inconclusive. Pistons don't always burn on the exhaust side. So, again we go back to the only physical evidence we have given to us, the loss of compression in BOTH cylinders at the same time, and the sled still runs,but is hard to start. The cylinders were honed, were they honed straight?. or was too much material taken off when done?...we don't know. But the chances that both cylinders were equally egg shaped or over honed so that the compression levels were the same in both cylinders ?...LOL...even Pro's aren't that good.
And, if they were that bad when the cylinders were reinstalled, the sled would have never shown a performance increase then a drop off.it would have been horrible to start. And cylinder wear would not happen that fast as to only get 60 miles on it before a noticable drop in performance of the engine.
So, what parts were replaced that could wear or fail that fast and POSSIBLY show an equal drop in compression in both cylinders at the same time? It is something that has not been looked at or been accused as a culprit to this problem.
I would almost bet that if you have an equal pressure drop in both cylinders without any other conflicting evidence here I would have to say it's either a blown head gasket or base gasket at this point. and If you retorque the head and base bolts incorrectly either done out of sequence or improper poundage they're gonna blow. and they will cause a drop in compression in both cylinders, and make the engine hard to start not only because of the drop in pressure, but in most cases it will let water into the cylinder and lower the flamability of the fuel. other evedence of this wich was left out is water droplets on the spark plugs and or back firing when trying to start,another note is that backfiring is also caused by a severe lean condition as well.
I have run SPI's for a long time and never had problems....
Hope this helps either fix or rule out some of your problems....
