I brought my 88 560sl to a mechanic because it was misfiring. He told me that the plugs were fouled and asked if it was burning oil and I said it was and that I was planning on having the stem seals replaced in the spring. He then took a compression test and said that 2 of the cylinders on the left side of the engine were low about 78psi and that I should probably pull the engine and do the rings as well. Do bad valves affect engine compression? He said the whole job including timing chain would cost about $4000usd here in Montreal. Any opinions?