Jet size for overbore

[fmjnax]

1984 Johnson 115 V4 (J115TLCRD). I've been rebuilding the engine piece by piece (yes, I know it's not economical). I originally thought I was only going to have to bore to 040 or 044, but it had to go to 064 to clean up properly (and I would have just trashed the block, but I can't find a good replacement to save my life). I was planning on changing ALL jets based on the 044, but now I'm not sure what I should jet to for the 064. Everything is stock to the 115 except for bypassing the VRO and using an '83 fuel pump.

 

[emdsapmgr]

Good question. The factory put .0585 jets as the stock high speed jets. For your .064 piston, why not try a .060 jet for that side of the carb- and test from there. 321449. If you break it in slowly, should be ok-checking the piston crowns every so often. At the point you want to do go to WOT, plan on it for just long enough to do a high rpm throttle chop and read then plugs, visually check the piston crown. Document how it runs, bogging , mid-range issues, idling, piston color, etc. I'd do this each time you do a WOT run till you see develop a track record on just what is going on inside the combustion chamber. At this point you'd still probably be on the break-in oil. May need to recheck when you go to straight 50-1.

 

[Faztbullet]

Make sure the cooling system(impeller,t-stat,poppets and springs) are new and break it in slowly as Wiseco pistons need several heat cycles to temper up. Also drop the max timing 2 degrees.

 

[gm280]

For my information and certainly not trying to hijack any thread. But if a carbonator is designed to allow the proper fuel air mixture through at any volume allowed by the physical carbonator throat size opening, why do you increase the jet size when boring out an engine? I mean the throat area is the same and therefore the fuel air mixture would stay the same. So why increase the fuel to a higher amount. I could easily understand if the throat area gets changed... IDK!

 

[fmjnax]

Thanks guys, and I'm curious as to the answer to gm's questions as well.

The machinist recentered (not sure what that means) and said there looks to be plenty of liner left and doesn't think I'll have any problems with it... but if it blows *knock on wood* again, I think I'm throwing the whole thing away and getting a newer engine (which I really should have done from the get-go, but in it too far now).

Fazt - Impeller was replaced last June and only saw water 4 times. T-stat (all parts in the kit) is new and stainless steel. Have new heads, new alarms, all new hoses, deflectors, all new carb jets, thoroughly cleaned carbs, rebuilt starter, new fuel pump. Just about everything is new (or was already recently replaced).

 

[racerone]

The bigger bore would pull just a little more air through the " carbonator "-----Same throat diameter so air moves through it faster , yes.

 

[fmjnax]

Do/should I change the idle and intermediate jets as well, or just go with the 60C high speed at first and monitor as suggested before stepping up those

 

[emdsapmgr]

My crossflows always seem to overfuel slightly at idle. If you note, the pistons, plugs and cyls seem black and wet after idling... I'd probably leave it alone unless you end with up with a lean sneeze at idle. You may need to have a look at the mid jet size. Esp if it seems to have a slight bog at midrange. Change the high speed, then do your lake testing and decide. Let's put it this way-the high speed jetting at WOT must be absolutely dead on-or it can cause lean problems. Concentrate on that aspect first.

 

[fmjnax]

Makes sense, thanks emdsapmgr. I had already planned on the 60C's with the 044, but I was also planning on 36's @ idle and 30's as intermediate. I'll just order the 60C's for now and see how it goes.

 

[Faztbullet]

For my information and certainly not trying to hijack any thread. But if a carbonator is designed to allow the proper fuel air mixture through at any volume allowed by the physical carbonator throat size opening, why do you increase the jet size when boring out an engine? I mean the throat area is the same and therefore the fuel air mixture would stay the same. So why increase the fuel to a higher amount.

The bigger jet changes the AFR(air fuel ratio)by richening the mixture. The extra fuel helps cool the piston crown from the increased cc's and lubes the cylinder. A perfect AFR is 14:7:1 which is 14:7 parts air to 1 part fuel, the larger jet would change this ratio to (hypothetically) 13:5:1