'85 Mercruiser 454 Top RPM

[california91207]

I'm looking at an '85 Sea Ray 340 Express with twin Mercruiser 454s with 600 hours. On Sea Trial, engines seemed to perform very well at idle and cruising speeds, cruising at about 26 mph at 3000 rpm. They ran very smoothly, no visible exhaust at startup or while cruising, no signs of any type of leaks in the bilge. <br /><br />Only concern is that at full throttle, top RPM was only about 3400. I've heard conflicting reports that this is either o.k., or the engines should be topping out in the mid 4000 range and this could be a problem.<br /><br />Any input would be greatly appreciated.

 

[ODDD1]

Re: '85 Mercruiser 454 Top RPM

I would be concerned too, califonia....if run at wide open throttle the motors will destroy themselves from overwork.<br /> the rated rpm for these motors is on a red tag on the valve cover of each engine....I think it is 4200 to 4600 rpm.....lots of reasons, weak motors, dirty hull, overweight boat....irregardless, dont buy until you see those motors make rated rpm.

 

[california91207]

Re: '85 Mercruiser 454 Top RPM

Thanks for your quick response. Any additional input from members would be greatly appreciated.<br /><br />David

 

[california91207]

Re: '85 Mercruiser 454 Top RPM

Thanks for your input. Just visited the boat today, where I read the labels on the engines. Engines are 340's, not 350's, and according to the manufacturere, Maximum WOTRPM is 3800-4200. Took her out again and we ran aruond 3400 without any strain. She's also very quiet and smooth at low rpm, even immediately after starting, so compression's probably not an issue. Prop is 17 by 15 bronze, not sure if it is original or how/if it was cupped. <br /><br />Looking into tuning, carbueration etc. -- any additional input that you can provide would be greatly apprecaited.

 

[california91207]

Re: '85 Mercruiser 454 Top RPM

After being told by the mechanic that basically we needed a tune-up, we took posession of the Sea Ray about two weeks ago. He completed his tune-up on Monday, after the re-built carbs came back from the shop. He replaced everything with all Mercruiser parts, including electronic alternator. We took her out today, and she really soared! Specs on the engine plate called for WOTRPM of 3800-4200; we had her at 4100 and it looked like there was plenty of room to go. Looks like major culprit was the carbs -- secondaries kick in at around 2900 rpm, and before the rebuild they were not operating on either engine. Now when you hit just over 2900 rpm, it feels like afterburners kicking in. At 3000 we were doing about 26mph, at 3500 around 32 (with the trim tabs bringing the bow up), and we were getting into the upper 30's before I pulled back on the throttles.

 

[Fouled Plug]

Re: '85 Mercruiser 454 Top RPM

Glad to hear your mechanic got things in order. Let us know which gas card you have so we can all buy stock in the company! My uncle has a 42' with twin 454's he charters out of Juneau. I love to hear it at WOT, but watching the tanks empty makes me glad I don't pay the bill$!

 

[california91207]

Re: '85 Mercruiser 454 Top RPM

One of the first things that the mechanic suggested was to learn how to enjoy running at around 2900 rpm -- just before the secondaries cut in and she really starts supporting OPEC!