Evinrude 5.5 - Open up intake manifold?

[GerryWm]

When I rebuilt the carburettor on my 1960 Evinrude 5.5, I noticed that the port on the intake manifold is .100" smaller in diameter than the carburettor outlet, with a cast-in step. I'm trying to get this motor to plane my 10' aluminum jon boat, and I'm tempted to open up the manifold port to match the carburettor. Does this step have some function in creating turbulence for better mixing, etc. or could it have been a marketing move to limit the engine's output to the nominal 5.5 hp? Has anyone else tried this?<br /><br />- Thanks, - Will Thur

 

[WillyBWright]

Re: Evinrude 5.5 - Open up intake manifold?

You'll need to lose 100lbs or hang 2 more of those 5.5s of the back to get a little boat to plane-off. On the other hand, we had a 20' jon boat that would plane-off with an old 5Hp Scott Atwater. That's because of 8x the surface area to ride on top of the water with. Trust me, you don't want to go planing speeds with a ten foot boat!

 

[jtexas]

Re: Evinrude 5.5 - Open up intake manifold?

agree with WBW...used to run a 6evinrude on various rented jonboats, wouldn't plane a 10, nor hardly a 12, but a 14ft planes easily even with 2 big guys & fishing gear.<br /><br />although that doesn't answer your question, sorry I can't help with that.<br /><br />good luck!

 

[steelespike]

Re: Evinrude 5.5 - Open up intake manifold?

If you get an extension handle so you can sit on the middle seat you probably can plane it.We used to do it with a flat bottom steel boat and a 5hp Sea King by Gale.<br />It planed easily with my 180 lbs in the middle seat.You'll need to be careful handling may be tricky with that flat bottom.Its true a bigger boat will plane easier because of the larger planing surface.And the longer length helps it to plane easier too.

 

[rodbolt]

Re: Evinrude 5.5 - Open up intake manifold?

i dunno I run either a 33,a 25 or a 35 on my 72" beam 14 jon. no problems planing, my 6HP fisherman just wont cut it. however cornering can be a bit tricky, not as tricky as getting back out of the marsh grass, but a bit tricky. if that old 5.5 runs well I would play with the weight distribution and just run it.

 

[Chinewalker]

Re: Evinrude 5.5 - Open up intake manifold?

I certainly would NOT try to squeeze more out of the 5.5 than 5.5 horses. If your motor is otherwise running well and healthy, then 5.5 is all you're going to get. The aluminum rods and bronze bearings simply won't take the stress of tweaking the motor for more power...<br /><br />As was noted - play around with the set-up a bit for weight placement, prop selection, etc.<br /><br />- Scott

 

[JB]

Re: Evinrude 5.5 - Open up intake manifold?

The critical dimensions in the intake stream are the carb venturi and the reed plate. Think of it as a chain, where each component is matched to the others.<br /><br />The diameter of the intake throat is sized to allow enough mixture velocity to keep the fuel suspended. If you enlarge it without also enlarging the carb venturi and the reed opening cross section there is risk of fuel puddling in there. . . leaning out the mixture that gets scavenged into the cylinder. You could be placing a weak link in the chain.<br /><br />Two strokes are even more sensitive to upset in the balance of portions of the flow path than are 4 strokes.<br /><br />Your engine is sister to the Johnny CD series, and they are among the most reliable outboards ever built.<br /><br />I recommend that you leave her be.

 

[JasonJ]

Re: Evinrude 5.5 - Open up intake manifold?

As JB stated, don't mess with it. You can't approach small 2 cycle engines with a car motor mentality, it is apples and oranges. Leave her be, and find a 7 or 9 horse motor, or work your weight distro. Good luck..