Johnson 140 carb jets

[rdeyoe]

I have a 1981 Johnson 140 cross flow that has three pistons at .040" oversize, one at .064" oversize (lots more information here: http://forums.iboats.com/showthread.php?t=519282 ).

All the high speed jets are factory (67C jets). I'm considering changing for larger sizes, after reading many posts on this site (plus, I don't want to burn any more pistons...it's getting tedious). But, what size should I go to? And, since the factory parts manual doesn't list larger jets for this particular year, will earlier years fit the carbs? I see that some 1974 v4 carbs had 69C jets....will they work? The carbs are the 1 5/16 throat size on a J140TLCIM with cyls 1,3 and 4 at .040" oversize, and cyl 2 at .064" over.

 

[Dhadley]

Re: Johnson 140 carb jets

The jets are listed in an OE service manual but yes, the C series is for your carbs. Even at .044 or even .064 over the stock jets (for your carbs) should work well at 6000 rpm and less as long as the carbs are working as they should. If you keep burning / melting pistons something is wrong. Double firing ignition, wrong plugs, cut heads, center hub magnet slipped in the flywheel, etc....something. If you do change jets just make sure to not use jets that have been drilled or enlarged from their stock rating.

 

[rdeyoe]

Re: Johnson 140 carb jets

Thanks for the reply, DH. I know you to be one of the 'vets' here. I've seen so many posts about bigger bores needing larger jets. In a car, the jets don't have a "C" or other designation, just a size. But, between me and my girl, if you just look at this thing wrong, it'll burn a piston! (it's becoming a running gag now). I'm assuming the "C" is a series, or type designator, with 67 as the size, so anything with a "C" should go in fine? I really want to make sure that at long periods of WOT, the internals are getting oil. Cooling system is ok...don't think that more cooling would keep pistons alive. I want to make sure that when we run it hard at wide open, it's not going to burn another. Thinking of going with 68c on the .040, and 69c on the .064...does that sound right?

 

[Faztbullet]

Re: Johnson 140 carb jets

I myself stay with stock jets up to .020 oversize. At .040 I would jump to the 68C which I dont have listed in my chart on the wall and the .064 I would go with the 69C (part # 319776) as it will run a tad hotter due to thinner cylinder liner plus its a Siezeco...

 

[Dhadley]

Re: Johnson 140 carb jets

Yes, the C series are high speed jets for crossflow motors like yours. The D series are for loopers. Don't confuse the two, just stay with the C series. If you're really that convinced the problem is in the jetting then just run big jets in all 4. The worst that will happen is the motor will blubber but the root cause of that many pistons being damaged will most likely lie elsewhere.

 

[rdeyoe]

Re: Johnson 140 carb jets

As in my last piston replacement thread, there was trash in the high speed jet that burned one new piston. We bought the boat with (apparently) .030 OS in it already. The pins migrated out of two pistons and the rings destructed....that's why the latest set of pistons. It runs well at WOT, but I'm afraid that it may be more lean with the big pistons now. It may last just fine with the current jets, this is just more of a 'peace of mind' move.

Fazt: I believe the machinist gave the .064 bore a couple thousandths extra for the forged piston. I've been reading both good and bad on the Wisecos, but the general consensus seems to be to give them a hair more room....time will tell, i suppose. If that one does lock, the whole block's going in the trash anyway...

 

[Dhadley]

Re: Johnson 140 carb jets

The 31 series pistons (3173 in this case) are fine. As long as the machinist knows what the correct standard bore size is and bores accordingly the skirt to wall clearance will be fine. Pin migration is usually a sign of excessive combustion temps.

 

[rdeyoe]

Re: Johnson 140 carb jets

I was under the impression that pin migration was caused by low combustion temps, or high carbon build-up. I've ordered three 68C and one 69C jet. If I had high combustion temps, this should cool it, yes? ...or at least, there will be more oil hitting the moving parts. I can deal with blubbering, and these holes are so large now that I really can't see that happening anyway. Just trying to stay away from a lean burning condition. We did lug it around 2500 rpm for a full 10 hours breaking in the lower unit. That may have helped it...i didn't want to run new gears at WOT....I probably broke the rings doing that. The gears are good, but the engine suffered, I feel.

I've checked the cooling system thoroughly, I believe. Water deflectors are excellent, and in correct positions. Impeller has maybe 10 hours on it. Pee stream is strong enough to dig holes in the yard. Thermostats are new, and tested in a pot with an IR thermometer, opening around 140*. Cooling jackets have no scale or junk in them, block or heads. Screen on lower unit is intact. The water tube seals are in good shape and in place. Permatexed the mid section gasket around the water passages. Checking head temps at idle often, starboard is a bit warmer at 135, port runs about 125....but this is on muffs at idle. Will get more readings when we take her out. Don't know what else I can cover. Might be hot at WOT with the bored holes, hence the larger jets. Won't know until i get there...

 

[stop8]

Hi, I have broken down my carbs today and was wondering what the correct position is for the high speed jet. On three of the carbs it was dialled out from fully closed approx 10 and a half turns but on one of the carbs the high speed jet was fully dialled in.
Any ideas on what is the correct position?
Cheers

 

[emdsapmgr]

I'd be inclined to increase jet size slightly also, considering what you are going thru. I oversize jets on any piston more than .030 over. Try Fastbullet's recommendation. .068 C jet= 328555 .069 C jet= 319776 After you break the engine in, you can test for high rpm overfueling and dial them back if that's a problem. The high speed jets need to be screwed fully into the carbs, snuggly seated. Not sure just what you are describing regarding the position of the high speed jets you found when checking them. If they were found unscrewed 10 turns from fully seated (normal) position, those jets may have been so far back that they conflicted with the fuel flow to the pickup tubes. That's a possible reason for burning pistons.