What is the problem? Can't get max RPM's

[fishez4alivin]

Evinrude 150 ficht 25" SS 14 1/4 x 15 prop new install, I had a local mechanic remove my merc and hang the Rude, bolt pattern was not the same, difference being the uppermost holes and lowest mounting holes were used. The cavitation plate is 2 1/2" below the keel of my SeaCraft 20 MA normally heavily loaded. I only acheive 4600 RPM's at WOT....Checked the plugs...they all looked the same, compression is constant across all 6 cylinders, triple checked my throttle cable to make sure im getting full throttle. Only getting 46 maybe 4700 RPM's at WOT. This was a used engine and I'm wondering if there is some kind of initialization with a laptop for engines that were disconnected from a power source, sort of like rebooting the computer. Im just wondering hate to have to bring it to a shop this time of year, all the dealers want a three week appointment, with a three week estimate of time in the shop....would the fact my engine is mounte d too low, create the kind of drag that makes my engine unable to attain the reccomended 54-5600 RPM's at WOT? Thanks for taking the time to answet my questions.

 

[rickdb1boat]

Re: What is the problem? Can't get max RPM's

I think you've answered your own question. The motor is mounted too low. You are going to have to get a jackplate and get that motor up about 3" for it to perform properly. Too much drag having the lower unit that low and a bad running attitude are causing your low RPM figure, which BTW, should be around 5800 RPM's...

 

[Dhadley]

Re: What is the problem? Can't get max RPM's

Mounted that low you also stand a chance of injesting water into the cylinders. When the AV plate is that low the motor can throw water (either a spray or a mist) straight up and can possibly get injested into the motor. If it were mine I would not run it until the situation was corrected.

 

[jim dozier]

Re: What is the problem? Can't get max RPM's

In addition to the above comments it is possible the prop will need to be changed also.

 

[Dhadley]

Re: What is the problem? Can't get max RPM's

If we're looking for another 1000 (at least) rpm's that would mean we'd have to go to about an 11" pitch. 20' Makos and Duskys and 196 Aquasports all run 19" props with that motor. Normally Sea Crafts do too. I'm sure we have other issues.<br /><br />With a nice diameter like 14 1/2" the motor can come up a tad more too. So now we're at least 3" low. Maybe 4" depending on the style and design of the prop.<br /><br />When the tech mounted the motor he should have hooked it to a computer to check it out. Were there any codes or issues that came up?

 

[LubeDude]

Re: What is the problem? Can't get max RPM's

Anyone that mounts a motor 3-4" too low from the start isnt a "TECH" in my opinion.

 

[Dhadley]

Re: What is the problem? Can't get max RPM's

Exactly.

 

[fishez4alivin]

Re: What is the problem? Can't get max RPM's

Ok thanks, I'm gonna lower the engine all the way, and remeasure, maybe its more like an 1 1/2, i'll get back on that. I had a 25" shaft Merc on there before, this is the a 25" rude, I did notice that the rude was mounted on the highest and lowest sets of bolt holes, as I told him that i wanted to use the existing holes. I didnt realize that it would set that engine too low, but he should have. I'll remeasure and get back. Btw I had a 14 1/2 x 19 aluminum prop but could only get 4200RPMS. Went to the 14 1/4 x 15 and figured i would gain the extra rpm to put me close, but i didnt know that the change from aluminum to stainless would lower the amount of change id see. Thanks for the information.

 

[Dhadley]

Re: What is the problem? Can't get max RPM's

If you went down 4" in pitch and only gained 4-500 rpm, something else is wrong. In addition to being too low.

 

[Hooty]

Re: What is the problem? Can't get max RPM's

Maybe I missed it but what size wuz the engine you replaced?<br /><br />c/6<br />Hooty

 

[fishez4alivin]

Re: What is the problem? Can't get max RPM's

i went down in pitch, but went from aluminum to SS, i was told that if i switched to the exact same sized prop but changed to SS, that i would lose 200-300 RPM's..I had a 1989 Merc 175, with over 4000 hours since its first powerhead replacement, and it was smoking, had a switchbox problem, and was ready to be laid to rest....the fuel was killin me, fuel out here at a gas station is 2.25 a gal. I was also wondering about the possibility that the timing is off, or something with the ECU is limiting only 75% of full throttle???? Thanks again for all the input, i've got to get that measurement on the A/V plate to the keel, i know its below, not sure how far...know its over an inch