78 115 to 140

[jason915]

So I have a rebuilt 78 115 that runs great, but I have an opportunity of picking up a blown 78 140 power head complete with carbs and everything. How much better did the 140's run? I think everything would be basic bolt on transfer other than the bubble hump exhaust swap. What's exactly involved with that? Can I leave my power head on the motor or will I have to pull it off to modify it? I live in Colorado so some of the lakes I go to are 8K and higher so more power is always useful.

 

[jason915]

Re: 78 115 to 140

Ttt

 

[emdsapmgr]

Re: 78 115 to 140

Both engines use the intake filler blocks. The 140 uses larger carbs and has the tighter heads-bolt on changes which would easily swap to your 115. You may need to jet the 140 carbs down a bit. It's not easy to transfer the bubble back exhaust system, as the powerhead must come off and the back of the block must be decked once the exhaust filler blocks have been added. Depends on how much time and effort you want to invest.

 

[kenmyfam]

Re: 78 115 to 140

That's a whole lot of work to do even without the unexpected issues that will crop up during the process. Personally If I wanted a 140hp I would look into finding one that is in good running order and then sell my 115hp (or keep it as a back up)

 

[jason915]

Re: 78 115 to 140

Both engines use the intake filler blocks. The 140 uses larger carbs and has the tighter heads-bolt on changes which would easily swap to your 115. You may need to jet the 140 carbs down a bit. It's not easy to transfer the bubble back exhaust system, as the powerhead must come off and the back of the block must be decked once the exhaust filler blocks have been added. Depends on how much time and effort you want to invest.

Cool thanks for the reply, I may just swap carbs and cylinder heads then, and install some boysen reeds. Would the CR be too high for regular fuel?

 

[jason915]

Re: 78 115 to 140

That's a whole lot of work to do even without the unexpected issues that will crop up during the process. Personally If I wanted a 140hp I would look into finding one that is in good running order and then sell my 115hp (or keep it as a back up)

I agree, I don't want to remove the power head to modify it for the bubble back exhaust. But the carbs, reeds, and cylinder heads seem easy enough to do for a nice potential HP increase. 125-130 crank HP from 115 for little $ and time.

 

[iwombat]

Re: 78 115 to 140

You can put on the bubble back exhaust w/o the filler blocks, and therefore w/o having to deck the powerhead. There's some evidence that once you up the RPMs into the high 5ks it runs better w/o them anyway. I removed mine and saw no difference either way.

 

[jason915]

Re: 78 115 to 140

You can put on the bubble back exhaust w/o the filler blocks, and therefore w/o having to deck the powerhead. There's some evidence that once you up the RPMs into the high 5ks it runs better w/o them anyway. I removed mine and saw no difference either way.

are you saying that the 140 bubble back will bolt on the the 115 block without modifying the block?

 

[emdsapmgr]

Re: 78 115 to 140

I personally like the composite reeds. A nice addition to any engine. They are worth installing. The 140 heads are considered high compression and with today's poor fuel quality, I'd run premium fuel only after the head swap-or take a few degrees out of the full advance timing.