Maximum RPMs?

[krosemond]

1979 Johnson 150. What should the engine be able to turn full throttle? I replaced my prop with the OMC standard (I am not sure what the pitch is) from a dealer at the beach. It was turning 5300-5500 with the old after market prop I inherited, now it will not turn but 4500. But it planes and runs great at 3700. Is this an acceptable range? It is a little slow to get out of the hole with the new prop. I was also told to blow it out at max RPMs occassionally to clear the carbon. Is that right? Kind of scares me with a 23 year old engine.<br /><br />Ken<br /><br />P.S. To the guys who helped in the past month: Bad gas is gone, it is running without a knock or skip and idles, and I adjusted my throttle cable (after it pulled free and left me stuck in reverse)and she engages without so much as a thunk. Gear oil was clean. Thanks for your help!!!

 

[JB]

Re: Maximum RPMs?

Hello again, Ken. The "standard" prop you got from the dealer must be "standard" for a smaller and/or lighter boat than you have. Go back to a prop that will let her rev to 5500 at w.o.t.. Sounds like you had a good one and replaced it with too much pitch.<br />I would be suspicious of anyone who offers a prop as "standard" for an engine, without regard to the application. Decarbonizing additives are a lot safer and more effective than over-revving an engine or running for long periods at w.o.t.<br /><br />Happy boating.<br />JB

 

[Dhadley]

Re: Maximum RPMs?

Assuming the engine is running fine (running on all 6 -- throttles wide open, ect) get rid of that prop! The best way to kill a 2 cycle motor is to lug it. Even if you only run it at 3000 your combustion temps are too high. That causes carbon build up (coking) and very soon the top ring will break. Excessive combustion heat is the mortal enemy of any engine. <br /><br />There are only two things that will limit the potiental RPM's on your 150 cross flow. One is the rev limiter in the packs. Yours may not even have any. The other is the reeds. Cross flow reeds start to flutter at around 6200 to 6300 RPM. <br /><br />I know the cross flow is very old technology but it's a great motor. Prop it out at 5800 with an average load, feed it fresh 87 octane fuel with good oil, Champion plugs and it will live forever. A friend of mine has a 1978 150 that has out lived several boats.<br /><br />Good luck!

 

[krosemond]

Re: Maximum RPMs?

Thanks guys, I suspected such an answer. I knew that "pitch" was about the weight and design of the boat and that "standard" was sort of an oxymoron. I was just unclear on high high it should rev. It is a 20' Robalo<AMF>, center console skiff, about 3500 pounds wet.I replaced the prop after dinging the old one up some oyster shells trying to get back to the dock at an exceptionally low tide. I will have the old one reconditioned and put it back on. How many times can you recon a prop? Can I have the pitch on the new prop changed? I had a prop shop change the pitch on my brass wheel on my former diesel inboard, but can they change aluminum? <br /><br />One more question. When I get it to 5500 w.o.t., what is an acceptable cruising rpm. I am trying to get the hang of outboarding.<br /><br />Thanks.