6200 rpm's on a 135hp???? Isn't that too high?

[paperdesk]

My recently aquired 135hp 73 outboard (on a 73 rienell trihull) didn't have a functional tach. Recently I fixed it and was surprised to find that at WOT is 6200!<br /><br />Not only that, but I can barely push up to 35mph at that! Once I saw how high the rpm's were I was scared to have the motor running that hard, especially since my manual says 5500 is max rpm's.<br /><br />I have been running a bent up 13 1/4 x 17 aluminum prop. I just bought the same size stainless prop which seems to give me the same performance. I am assuming I need to go with a larger prop, or else make sure I don't let the rpm's get that high. Would you agree?<br /><br />My motor has low compression (110, 115, 107, 100 if I remember right). Does this mean I should try to baby the engine a little, and not rev it high?<br /><br />Hope I get some helpful feedback.<br /><br />Ted

 

[JB]

Re: 6200 rpm's on a 135hp???? Isn't that too high?

Hi, Ted.<br /><br />If your tach is correct, you are probably underpropped and need to go to a longer pitch.<br /><br />Rules of thumb say 200 rpm per inch, which would call for a 20 or 21" pitch to bring your WOT rpm down to 5300-5500rpm.<br /><br />Verify your tach calibration first. It may be set incorrectly.<br /><br />The 15% spread on your compression is at the limit for variance, but the 100psi minimum is probably okay.<br /><br />The engine could probably use a good decarb treatment.<br /><br />Good luck.

 

[paperdesk]

Re: 6200 rpm's on a 135hp???? Isn't that too high?

How could I calibrate my tach?<br /><br />My motor calls for a 13 1/4 x 17 aluminum prop, so maybe it is the tach? It does also allow for a 19" or more especially when using stainless though.<br /><br />I want to mostly pull tubes, waterskii etc with it, so I don't need a lot of top speed, but 35 just seems really low!

 

[paperdesk]

Re: 6200 rpm's on a 135hp???? Isn't that too high?

I did double check that the tach settings were correct for my motor (4 cyl etc).<br /><br />I just looked at the manual again, and it says the motor shipped with a 13 x 19 prop, but allows many other sizes including 13 1/4 x 17.<br />For stainless props it recommends:<br />13 3/4 x 15<br />13 3/8 x 17<br />13 x 19<br />12 3/4 x 21<br />12 3/4 x 23<br /><br />I'm confused! Does this mean I shouldn't use the 13 1/4 x 17 stainless? I read posts about people purposely increasing their rpm's. Is that healthy?

 

[G DANE]

Re: 6200 rpm's on a 135hp???? Isn't that too high?

Glad to hear you got it running Ted. <br /><br />If it runs at 6200 motor sounds healthy.<br /><br />Dhadley is the one with the big knowledge at this point, but I think propping for 6000 would be your best goal. A 19" or 20" Pitch prop would do that, and mayby add a little speed too, as steel reeds dont like much above 6200. Just dont run it at 6000 all the time. According to OMC propchart, a 17" pitch should give you up to 42MPH at 5500 Rpm. How do you measure speed. Boat speedo ? Notorious for wrong speeds - try to borrow a GPS.

 

[bubbakat]

Re: 6200 rpm's on a 135hp???? Isn't that too high?

I have 76 model 85 h/p which there isn't much difference in these two and I was over turnning to and went with a 19p and brought it down to 58 g's. Got 5 more mph out of her and comes on plane good.

 

[Dhadley]

Re: 6200 rpm's on a 135hp???? Isn't that too high?

Ted, you mentioned your tach settings were correct but you didnt tell us what that its set at. Make sure its an OMC tach. It should be on position 6. Not 4.

 

[paperdesk]

Re: 6200 rpm's on a 135hp???? Isn't that too high?

Cool. Thanks guys. I'll see if I can get a hold of a 19 or 20 pitch prop. I am basing my speed on the boat speedo, which seems to be accurate based on a previous GPS comparison, however, I do realize they aren't reliable.<br /><br />Ted

 

[DJ]

Re: 6200 rpm's on a 135hp???? Isn't that too high?

I do not believe the boat speedo. I suspect the tach MAY be inaccurate too. As I recall, Reinells are NOT light boats.<br /><br />As Dhadley said, make sure the tach is set at position 6, not four.