7.5hp '55 evinrude FUEL MIX BOOGER! F-R?

[jbjennings]

Guys,
As you may have noticed on another thread, I have messed up the fuel mixture on my 'rude. It runs flawlessly but I have been using 24:1 instead of 16:1 as it should be. I have only run about 3 gallons through it at this mixture. It runs great but doesn't seem to have any more power than a 5.5hp. Could the extra friction by not having enough oil be causing this? the cowl says 7.5hp. The head has #203656 cast on it, and the exhaust cover has 203374 cast on it. This is a 7.5hp, right? It ran this way from the get-go, never had much power. One fellow posted he got 18mph on a light 12ft. boat with his '55 7.5hp and I can't get near that with mine and I have a fairly light 12 footer also.
Any ideas?
JBJ

 

[F_R]

Re: 7.5hp '55 evinrude FUEL MIX BOOGER! F-R?

Yep, that is the number for a 7.5 exhaust cover. I doubt you have significantly hurt the motor by running that 24:1. It didn't try to sieze did it? Are you sure you are getting full power from both cylinders? Try removing and grounding one plug wire and see how much power it has. Then do the other one. Should be about equal power loss with either.

 

[jbjennings]

Re: 7.5hp '55 evinrude FUEL MIX BOOGER! F-R?

Thanks F-R,
No, it never tried to seize, just not as fast as I'd hoped. It does have new coils but I can't remember if I put new condensors on it. Will try grounding plugs and see if anything sign. happens. Also replaced impeller, l.u. seals, so it shouldn't be overheating. I think I'll test it on my "new" 12 footer and see how fast it is on it.
Thanks again,
JBJ

 

[wbeaton]

Re: 7.5hp '55 evinrude FUEL MIX BOOGER! F-R?

I have a 1955 7.5 hp that won't plane my 12' deep and wide tin boat either. It pushed my old narrow 12' boat up to about 18 mph, but not the bigger one. Hull shape and style make a big difference.

 

[JB]

Re: 7.5hp '55 evinrude FUEL MIX BOOGER! F-R?

"I have been using 24:1 instead of 16:1 as it should be."

According to the 1964 Johnson Service Manual and to BRP you have been running the correct mixture.

You will do no harm by running 16:1, except for faster carbon buildup and risk of fouling plugs more often, but anyone claiming that 24:1 using TC-W3 oil is harmful is just plain wrong.

 

[wbeaton]

Re: 7.5hp '55 evinrude FUEL MIX BOOGER! F-R?

To be honest, I thought OMC had switched to 24:1 for ALL 1950's outboards, but I can't find anything in writing to support it. Either way, I've found its an unwinnable argument.

I run at 24:1 with synthetic oil. I know some run as little as 36:1, 40:1 and OMG 50:1. They will all run on leaner mixes, but the motors longivity is reduced. Obviously, I'm with JB on this one, but you can't go wrong with 16:1 either.

 

[R.Johnson]

Re: 7.5hp '55 evinrude FUEL MIX BOOGER! F-R?

I have the same engine, have always mixed 25-1 with tw-3 oil, never a problem.

 

[samo_ott]

Re: 7.5hp '55 evinrude FUEL MIX BOOGER! F-R?

I have the '50 7.5hp and ran it all year on 24:1 with fully synthetic oil... Other than the LU full of water (see other thread) it ran well and has noticablely more power than my 1961 5.5hp.

 

[jbjennings]

Re: 7.5hp '55 evinrude FUEL MIX BOOGER! F-R?

I'm thinking of putting the 7.5 on my new 12 footer and taking my GPS tomorrow morning duck hunting. Will see how it performs on that boat with some good 16:1 gas (well, close to 16:1 anyway). If I take it I'll post results. Only thing, I love my old '57 18 johnson so much I hate to not run it!
Thanks for your responses,
JBJ

 

[samo_ott]

Re: 7.5hp '55 evinrude FUEL MIX BOOGER! F-R?

quack... quack... I dunno, I'd MUCH rather be cruising with the 18... they rock!