One more QUESTION.. 1978 Merc 1150

[johnson1]

One more question for you all..<br /><br />I have the 78 Merc 1150 on a 17 ft Fiberform Tri Hull. I am getting up to abour 4800 RPM's and tops out at about 30MPH. Does this sound about right.. Not alot of other weight in the boat.<br /><br />When I back a little off the throttle it seems to speed up a mile or two an hour. Could it be that he has the wrong mix ratio, or Prop. Thanks!

 

[angus63]

Re: One more QUESTION.. 1978 Merc 1150

sounds like cavitation. Cavitation plate even with keel? What pitch prop?

 

[jim dozier]

Re: One more QUESTION.. 1978 Merc 1150

I don't know the max rpm for your Merc but I'd bet its above 4800 rpm. Sounds to me like you need a prop with less pitch. At WOT you should be running at or near max rpm.

 

[Gold Bear]

Re: One more QUESTION.. 1978 Merc 1150

johnson1<br /><br />"When I back a little off the throttle it seems to speed up a mile or two an hour." That sounds like the Throttle Stop screw is not adjusted correctly. <br /><br />With the engine not running, move the throttle control lever to the WOT setting, then look at the butterflys in the carbs. The blades should be straight in line with the carb bore - to give you maximun air flow. If the butterfly plates are opened to far, then when you "back off it a bit" you are probably improving the alignment of the butterfly plates. If that is the case then the stop screw should be adjusted.<br /><br />Do you have a Service Manual? Perform and Link & Synch.<br /><br />The RPM's should be about 5,400. If all else is OK, then maybe you are over propped. What is the pitch of the current prop?<br /><br />Good Luck,<br /><br />Gold Bear

 

[Boatin Bob]

Re: One more QUESTION.. 1978 Merc 1150

RPM's sound low, my 76 1500 goes to 5400 at WOT, spinning a 13.5 x 19 prop, approx 50 mph (if the speedo is accurate) on a 16' Glastron CV16, a smaller pitch prop will bring your RPM's up but you typically lose a little at top end.