Carb jetting on rebuild

I have a 1981 Johnson 140 HP that I've just had bored out .040" (cyls 1,3,4) and .064" (cyl 2). The block was already machined out .030 before I lost some rings, that's why the large sizes. Cylinder 2 had a piston dowel migration that scratched that one too far for .040, so my machinist took it to .064, and I'm using a Wiseco in that cylinder. All the others are cast.

I got it back together this weekend and I'm getting ready to start the break-in. But I have a question on the carb jets. The machinist recommended that I use some larger jets because of the bore size. I haven't pulled the jets yet (rebuilt the carbs last year, can't remember what the jet sizes were though). On BRP's site, it looks like the standard jets are 67C. Can anyone give me an idea of what size jets to replace them with? I imagine that the one .064 oversize cyl would need one size larger than the others. Or can I drill mine out a couple of thousandths? Any advice is greatly appreciated.

Re: Carb jetting on rebuild

you dont need to jet for larger bore size but if lean condition caused failure then i would jet up

Re: Carb jetting on rebuild

Thanks dehydrated. I don't believe a lean condition caused the failure. I had rebuilt the lower unit last season, and read that for the first 10 hours on the LU break in should be done below 2500 rpm. My lady and I cruised at or below that for at least that long on a day. I believe that just lugging around at that speed cause carbon build up and seized the rings / migrated the piston dowel(s). It's our first boat, and we didn't fog the engine for winter either. Plus, we stowed it with the engine up on the transom saver and some water probably stayed in the block, cracking it. Machinist did a fantastic job of TIG'ing the crack and decking the block. Lots of (pricey) lessons learned in our first year already!

Like I said, I've already done the carbs, and replaced the fuel pump too. The VRO was removed before I got the boat, so we just premix (going to do 25:1 for the break in). I just don't want to waste all the money put in the machining / parts / time etc. just to fry it again over a cheap set of jets.

Re: Carb jetting on rebuild

If I were you i'd just watch the spark plugs for a while. Bring your plug socket with you when you go out. Document the differences between the large, larger, and extra large holes. That way if you do indeed need to rejet then you'll have a better starting point for which and how much. Post 4 before pics, 4 during pics, and 4 after pics of the plugs (and which cyls) and I'm sure you'll get the right information.

Re: Carb jetting on rebuild

Great arch! Figure I'll give it a couple of hours of break in and see how they're doing. I'm pretty sure the carbs were supplying fine before...i think we just lugged it too much without any WOT to blow it out. In fact, before I tore into it, I did the Seafoam decarb thing and blew all kinds of black out the exhaust. It was running on three cyls (as a comp test showed number 2 at 20 psi or so) and wouldn't idle much, but that seafoam cleaned it out well, plus dripping all over the midsection too. Appreciate all the advice!....now time to go check the 5 day weather outlook....