Re: 135 pistons
Rob, I've used muratic acid to remove the piston aluminum which has transferred to the cast iron cyl walls. Used sparingly, it can be applied with a brush or eyedropper. It will dissolve the aluminum, leaving the cast iron intact. When the alum is dissolved, then clean the cyl area quickly after it's dissolved. No problem-works fine. Does look like a detonation problem. If it were a fuel/oiling issue, likely the rings would have overheated, then broke/cracked, then the ring pcs would have pitted/dented the piston crowns and head faces. That did not happen. Just got too hot from too much timing. These engines normally run rich at idle, so likely the WOT timing was too advanced. Couple of additional sidebar comments that may have been contributing factors to your failure: 1. The 135 cyl heads were the tightest (highest compression) of all the V4 crossflow engines ever made. So everything has to be just perfect ie: The 76 service manual says the 135's were made to run on 86 octane, either leaded or lead-free. The factory never said they will run well on today's 87 gas/ethanol crap. So, with today's fuels, run this engine on a steady diet of 92 ethyl only. (unless you retard the timing.) 2. The timing on this engine needs to be set to factory spec, but only when running at 4300-4600 rpm's. Can't set WOT timing at idle rpm's. (it is a valid method-only if you know what you are doing.) The timing on this engine will creep 2+ degrees from idle to WOT, due to the characteristics of the ignition electronics. This would distort any max timing setting that a prior owner may have done, if he set the WOT timing to factory spec-when done at idle rpm's. 3. There is no base idle timing spec, other than to initialize timing when first started after an overhaul. Once running, set the idle rpm's (650-700 rpms) in forward gear when the boat is floating in the lake. It's the setting of the idle rpm's that determines the final/correct idle timing-not the service manual.
