Why would There be Different Size Main Jets?

[RBuch1974]

Hello and thanks for reading this,

I am rebuilding my two carbs on a 1996 90 HP Evinrude (E90TLCDC-pretty sure). After removing the main jets (and cleaning whole carb) I began re-installing the main jets only to notice that they were different sizes one is #56 & the other #59. Tore apart second carb to verify and, yup, same thing. The #56 was on the port (left) side standing behind motor (I assume on both carbs). Manufacturer specs the #56. I have two spare carbs I can steal parts from to re-install (2) #56's.

My question is, Should I put all #56's back or is there a condition that warrants the mismatched sizes? Seems odd to me. Old Powerhead has (2) 0.30 over cyl. and new one will to but not sure that it's the reason? http://forums.iboats.com/johnson-ev...-re-jet-carb-after-boring-30-over-643014.html

Thank you

 

[Daviet]

Re: Why would There be Different Size Main Jets?

The parts breakdown I looked at shows #56 in both sides of the carb. It does not even list a #59.
If it were me, I would install the parts that are listed for your model engine. Maybe someone will come along and offer another suggestion.

 

[racerone]

Re: Why would There be Different Size Main Jets?

Perhaps the rebuilder put bigger jets in to compensate for the wee increase in airflow.------Lots of different jets listed in a factory service manual that are not listed in the parts diagrams !

 

[Chinewalker]

Re: Why would There be Different Size Main Jets?

It's also possible that in practice, the cylinders fueled those jets were running leaner than the others. In theory, a multi-cylinder outboard motor is essentially made up of separate single cylnder motors, each sharing a common crankshaft. It is not uncommon for one or more cylinders to run a little differently from the others, due to quirks of the cooling system, crankcase flow, casting variations, harmonics, etc. Manufacturers try to reduce these variances as much as possible, but it is possible to have all items to spec and still have one plug running a little lighter than the others. I knew a guy who ran different heat plugs in all four cylinders of his racing motor in an attempt to get all four running the same color.

I'm not saying you shouldn't change back to the original jets, just offering a possible reason as to why they might have been changed...

 

[emdsapmgr]

Re: Why would There be Different Size Main Jets?

I'd put the factory spec jets back in the carbs, unless the engine has been bored to over .030. If both cyls on one side were bored to say, .040 over, it's possible that is the reason for the jet size difference. I would not expect the jets to be so much larger than the stock ones, however. Maybe one jet size.