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Corsair M25Y problem reaching max rpm


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Any clues for me?  I have a Black Hawk w/corsair 175 M25Y engine that has ~40 hrs on it. (It sat idle in a hanger for quite a few yrs before I bought it from a friend).It used to consistently reach max rpm of ~7800 on the ground & ~7400 airborne. It has recently started having problems getting above ~6200 before it starts sputtering, but it will get up to ~7400 after a few minutes airborne.

Some background: ~6 engine hrs ago it became hard to start and would sputter & die if I even tried to throttle up from idle, so I put in a new plug NGK BR9ES, overhauled the carb WB (found some litter in the screen). I have always used the all black fuel pump diaphragm but this time I followed advice from Alex Varv's web-page and used the tan/clear diaphragm... And everything seemed fine for a few flights (~2 engine hrs) but the sputtering came back. Then I discovered that he spark plug cap/boot was bad so I replaced that, and now the last 2 flights I am having problems reaching 7800rpm after warmup before launch. I am thinking I should install the old style black fuel pump diaphragm first and go from there.

Other notes: Under plug CHT has shown no more than ~380F max during any of this. I warm my engine up to ~200F before all run ups or launches.


Greg C. in Va.

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