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1991 40 HP Evinrude - Carb Balancing

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  • 1991 40 HP Evinrude - Carb Balancing

    We are in the process of breaking in a requilt powerhead and, although the engine is performing well so far at low speeds, I am concerned that a lean fuel mixture on my upper carb may have contributerd to the failure of the original head.We used to have a VRO (the other thing we suspect might have caused the failure) but have bypassed it and are mixing the oil into the fuel (currently 25:1 for for 12 gal for the breakin). However, in some earlier forum responses I received, a number of you indicated that one carb running lean could have caused one of the cylinders to wear out or fail prematurely. There was also metal missing on the edge of the piston.If this is symptomatic of a lean mixture I assume this means that one carb is not carrying its weight and therfore causing excessive wear and incorrect ignition for the cylinder it supports.Anyway, how does one go about determining that the fuel/air proportions provided by two carburators are the same over the same RPM range?Hope you can help.


  • #2
    Re: 1991 40 HP Evinrude - Carb Balancing

    What motor is this and year

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    • #3
      Re: 1991 40 HP Evinrude - Carb Balancing

      Nevermind i see it on the post title.I sinc mine as follows.assure mix screws are set the same.Remove air box .Move throttle control to WOT.May need someone to turn prop while shifting.Assure that both carb flaps are equally and fully horizontal.If not, loosen respective lock screw on linkage and adj.Next,Move throttle control back to nuetral and insure both are closing fully without binding in carb throat.Here's what I do next.If you have a precission tool as an adj. hole guage move throttle to various open positions and guage between flap edge and carb throat.I usually move mine to 1/4,1/3,and 1/2 open and measure at all points and fine adj. as needed.This gets these little jewels within a couple of thousanths sinc at various throttle ranges.Also make sure your linkage roller is aligned on center with a factory scribe mark on throttle linkage cam.Hopefully there is a scribe mark there as it designates throttle position in relation to ignition timing.

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      • #4
        Re: 1991 40 HP Evinrude - Carb Balancing

        (VRO not likely the cause of only one cylinder)>be sure the floats are adjusted properly>be sure the throttle valves are in syncTo test for equal contribution at idle, run with one plug wire shorted to ground and check rpm. Then ground the other and check rpm. They should pull close to the same. If your a finatic, one could connect up 2 vacuum gauges and monitor each cylinders crankcase pressure at idle.Just a thought...At higher engine rpm, under load, check the exposed side of the cylinders to feel if one is hotter than the other
        Alias "schematic"

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        • #5
          Re: 1991 40 HP Evinrude - Carb Balancing

          ob and schematic, thanks for the replies.ob, I like the approach and, if nothing else, it sounds like I can do the whole process on shore. We might give it a shot this weekend if the weather clears up here in Mich.schematic, I'm with you that the VRO might not be the problem. The top cylinder was worse than the bottom and that is why I am wondering if fuel (therfore oil lubrication) might be a root cause. However, the bottom cylinder was not really that great either but still better thatn the top. On the other hand I'm not a marine mechanic that sees hundreds of problems and failures so I can't be sure what an average piston/cylinder should look like in a 10 yr old engine. So, to be on the safe side, we bypassed the VRO. We always wondered about how much longer it would last, especially with it being 10 yrs old, and it was always in the back of our mind when we were using the boat. As a result, we decided that at virtually no cost to convert we could put that issue to rest.I like some of your suggestions and we might try them once we get the boat out again.Thanks

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