5800 RPM'S but HOW???

[peterskeeter]

Hello all, I have a 1989 Evinrude 90 on a 16 ft. Skeeter SD-80. I keep reading how I should be turning 5800 RPM'S with this motor wide open with an average load but cannot get close to that number. A little info on the motor, it has a new CDI power pack, new coils, 120 psi in all 4 cylinders, the motor is hung at the highest mounting position possible, and runs great. I have tried a 13x17 SST prop, a 13x19 SST prop, and a Raker 14x1 8 prop and cannot get over 5000 RPMS with any of them. It appears also when the motor was rebuilt, the mechanic retarded the timing 2 degrees according to his mark on the flywheel.The boat does abot 50 mph wide open according to my GPS with the Raker prop at about 4950 RPMS with a light load. I am worried about lugging this motor to an early death. Is it time to experiment with a jack plate possibly??? I need some suggestions as I am stumped at the time. Dhadley has got me worried reading his posts!!! Thanks for any help as it is always greatly appreciated.

 

[jaymasta]

Re: 5800 RPM'S but HOW???

Did you notice and RPM difference in the 17" and 19" SST? Maybe you should try a 15" prop, a decent prop shop should let you borrow a prop to try it out to help solve your problem.

 

[surrender]

Re: 5800 RPM'S but HOW???

I haven't done the calculation yet but you may want to check your tach. With an 18 in prop at 5000 rpm can you get to 50 mph?
Just did the calculation and looks like with zero slip the fastest you could go with an 18 " prop at 5000 rpm is 42.61 mph. Someone check my math but if 50 mph is correct then the tach is way off.

 

[kenmyfam]

Re: 5800 RPM'S but HOW???

That outfit and a 13 X 17 prop should be spinning a lot faster than you are getting.
Check the tach before moving on.

 

[peterskeeter]

Re: 5800 RPM'S but HOW???

Tach is only about a year and half old but I've always wondered how accurate it was. I may go to harbor freight and try the optical tach they sell, has anybody found it to be reliable? Figured I could also ues it on my old 25 Rude also. By the way, I don't know if you could hear signs of an engine lugging, but at wide open, it doesn't sound like it is struggling. Just seems like a 15 inch prop is too small but I don't have a whole lot of experience when it comes to prop combinations.

 

[ezeke]

Re: 5800 RPM'S but HOW???

Surrender is correct in that the 18 pitch prop has a maximum theoretical speed of 43 MPH at 5,000 RPM with your motor.

Check that the tachometer is set on 6P.

 

[clanton]

Re: 5800 RPM'S but HOW???

Why retard the timing 2 degrees? I will check my charts, and post timing specs.

 

[peterskeeter]

Re: 5800 RPM'S but HOW???

I bought the boat from a friend who had the motor rebuilt before he sold it to me. I'm not sure why the mechanic retarded the timing 2 degrees, only guess is that he wanted to increase reliabilty/longevity of the motor slightly? Maybe he was accounting for the newer fuels of today? Would this change in timing cause much of a difference? When I get home today, I will double check the tach setting, but I'm almost positive its set to 6p.

 

[clanton]

Re: 5800 RPM'S but HOW???

Timing should be set to specs, no need to retard 2 degrees. Timing pointer alginment should be checked, before checking and setting timing. Forgot to get specs engine should have decal with timing spec. I think 28 degrees BTDC at 5000 rpms.

 

[Jacob645]

Re: 5800 RPM'S but HOW???

Would he have retarded the timing for a "break in" period?

 

[clanton]

Re: 5800 RPM'S but HOW???

Maby, have to ask the mechanic, what he was thinking, not recommended in service manual. Maby jet size increase. Then depends on the pistons he used OEM or after market. After market piston may have there own requirements.

 

[peterskeeter]

Re: 5800 RPM'S but HOW???

Let me say I greatly appreciate EVERYONE's help. I checked the tachometer setting after getting home for work and it is and has been set at the 6p mode. Thinking back to the old tach, I never remember the motor getting over 5000 rpms, ever. I just thought it was the nature of these old V4's until reading otherwise here...

 

[surrender]

Re: 5800 RPM'S but HOW???

The old V-4's will most definitly get above 5000 RPM. My 88 special goes to 5700 and I think it likes it around 5000 cruise. Probably a little high but boat rides good there and engine actually sounds about right there. Burning more fuel then if i kept it at 4200 cruise but I like to get where I'am going

 

[peterskeeter]

Re: 5800 RPM'S but HOW???

I cruise right now around 4200 rpm and max out at about 4950 rpm. That is according to tach at least...

 

[bob johnson]

Re: 5800 RPM'S but HOW???

your motors factory recomendation for WOT is 4500-5500...pretty generaic across most of the outboards close to that size, except a few 60 degree motors.. most of what I have heard is that those loopers like the high 5,000's

I have seen 5000-6000 rpms a few times...but I have also seen 4500-5500 for the same motor...

very confusing...but the cross flow motors all seemed to be 4500-5500

good luck

bob