A few inline facts from retired Merc service service school instructor<br /><br />I had a long talk with my old retired from Merc buddy last night. The one thing I intended to ask him that I forgot was exactly what year they stuck the power ported pistons in the 1150's, so I still don't know that for sure. A couple of things that he did tell me were that the 140's (later ones, not the 72 model) were the EXACT SAME ENGINES as the year earlier 150's. He said the only reason for this change was that it was the same year that Merc introduced the V6 150 and they didn't want both engines rated the same. He also said that all the inline 150's and 140's had the high dome pistons. I always suspected that the 140 HP rating reflected the lower pistons, but he said this was incorrect.<br /><br />He said the 140 was dropped in about 84 or 85 with the 115 being the top of the inlines as far as ratings go. He also said these 115's were slightly modified from the previous 140's for more bottom end which had always been a problem with 150's and 140's and they did have the power ported pistons. Also, this 115 DID reflect the change to propshaft rated horsepower for the first time on these particular engines. (He said there was no specific year when Merc rated at the propshaft on all engines, that it varied from engine to engine).<br /><br />Somebody asked the other day when the distributor got gone on the 6 cyl inlines. He said it was mid year in '79 and the ADI type engines were actually called 79 1/2 by Merc. (by then I was knee deep in V6's and didn't care, so I never owned one)<br /><br />Someone stated the other day that triggers would cause #3 to burn pistons. I'd never heard this before either so I asked him. He said that in all his years in the warranty department he'd never seen it, but some of Merc's tech reps mentioned it, so when he taught service school later he'd bring it up too. He said the Merc engineers explain it this way: There's always gravity, even in small amounts in the incoming intake charge. In that the inline feeds 2 cyls off each carb, the upturning charge in each pair mixes slightly leaner than the downturning one. That means that #'s 1, 3, & 5 will always be slightly leaner than #'s 2, 4, & 6. Back to the piston burning, the defective triggers (if they did exist) would start advancing the timing in all cyl's, but with 1, 3, & 5 being the leaner already they were the most at risk. Of this group, there may be slightly better airflow to #3 & 4, so this makes 3 arguably the leanest in the engine and the most at risk. He said that they taught in school to stick the .082 jets from the XS in the middle carb to give it a margin of safety. (I think that's the size he said, didn't write it down, but anyway the jet that's stock in the 1500XS) He also said that they recommended that the 150's/140's be pitched to turn about 6K at WOT with usual load and fuel, and not over 6250 with light load. He said pitchin' em down in the 5's greatly increases piston burn risks.<br /><br />As far as the inline 90's, he said they had very small ports and were very different from the other inlines. They were built as a low RPM high torque engine, and he wouldn't pitch one to turn over 5400 at WOT, not because of damage worries, but just because they won't make any power up there.
