TOPIC: Bubble Butt

I've heard that the "bubble butt" cover on the back of the V4 Johnny-Rudes is a major power upgrade piece, and have a line on one I could buy. Can somebody please fill me in on weather any of those would fit my '73 115 Johnson, and what else would be required to boost the HP? I think the (bigger) carbs were mentioned, but maybe a main jet up-size would also work? Any thoughts/pros-cons/advise would be greatly appreciated.

Mark,

Not a direct swap for the tuned exhaust. The 135 had filler blocks attached to the exhaust valley. These directed the exhaust through the tuned ports of the bubble basck. The mounting boss for these blocks is not on your engine. Without the blocks, the bubble does nothing. Also, I believe your engine has all of the ignition and charging components attached to the rear exhaust plate. The bubble does not allow for this.

Increased jet size will only waste fuel & load up the engine. It will actuallyt makle less power. Different heads & reeds could give a little boost, but not much. If you had the engine torn apart, you could have someone try to modify the port timing. The problem is that the 115 was a high performance engine out of the box. Doing modifications are very expensive, net small increases of power, and will likely wreck the driveability of the boat. The engine was designed to have a broad power range for everyday use. If you change something, you may actually have it performing worse for your needs. Best performance comes from a motor in good shape, clean boat bottom that is straight, and a high quality performance propeller, correctly matched to the boat.


Bruce

I am going to be blunt here and say that the filler blocks will bolt into the 115 HP block ( must be machined in situe for proper gasket fit ) and adding the bubble does NOT interfere with any ignition components in any way.

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If you know , then you know what fits and what does not.

Many thanks to both of you gentlemen for your input. The 115 runs great as it is now, I guess the old "motor head" in me peeks out from time to time! The only time the quest for more power is a bad thing is when it screws up something that was running well before hand, and is not doing so after the change, IMO. I wouldn't want to chance that, may have to get another V4 to "play with". This motor has run like a top since you guys helped me with the major tune-up and carb/fp rebuild, she's a good one "as is".

If the motor runs great, leave it be... If you want to mess around with one for grins & giggles, get a blown motor and build it up. Then you're not on shore for the season if something goes wrong...

Well Mark - looks like I may have given some bunk advice. Sorry about that.

Fortunately, the ever humble Professor was able to set the record straight. 3 cheers for the Prof!

That's okay Bruce, don't worry about it. You're human and none of us are perfect. Possibly one of the greatest things about this wonderful "web home for Glassaholics" of ours is the ability to get input from several knowledgeable folks, and use the "cumulative total" of the info given.

I may go ahead and get the bubble butt, and save it for a project as Scott suggested. I'd like to build a backup anyway, and love the OMC V4 motors. Plus, if I can't get the Merc 1000 straightened out for the GT 150 project, a "hopped-up" 85-115 would be ideal. They seem to be fairly plentiful in my area, and I have access to a great machinist, so a little "massaging" is not an obstacle.

Thanks for the input guys.

Bolted the filler blocks into a 90 HP block a few years ago.

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The ports are smaller and needed to be opened up to the 115 / 140 HP size.Had to machine the exhaust area and machine the back area for gasket fit.

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This project is easy to do, but it is done on a work bench and not on the back of a boat !!

I understand completely that this wouldn't be a "bolt on" mod, Professor. More of an "upgrade during rebuild" kinda thing. As I said, I'll probably keep the 115 on my Starflite stock, and build another motor. How did the motor you modified turn out? Were you (and/or the owner) satisfied with the results? Did you replace carbs, jets, reed valves, etc..? It occurs to me someone like myself without your experience would be guessing at proper fuel ratio/delivery/etc. without a dyno to measure results. Maybe I should stick to stock rebuilds for now......

It was done to turn a 90 HP block into a 115 HP ( OMC turbo jet )

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no problems encountered.

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The newer ( propshaft 115 HP blocks ) had the filler blocks and " bubble " tuned exhaust.

The newer 115 crossflow (c.1985 & up) is essentially the older 140 crossflow. Same porting, carbs, bubble, etc.

Older 115 blocks have the same porting as the 140s of the same vintage. They just lack the exhaust tuner. Depending on the year, some of the 115s will even have the 140's 1-5/16" venturi carbs.

If you do decide to build one up, use one from the late 1970s or later, as it will have the heavier crankshaft, better power trim system, better ignition, and mid-range jetting in the carbs for smoother acceleration. As you get into the 1980s, you'll find that the 115, 110, and the GT/XP-100s use the same larger porting of the old 140s, and all would work for a crossflow build-up.

- Scott

79 and newer used the " big center main bearing " on the crankshaft.

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Note

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These engines all used the same crankshaft / rods / bearings / pistons for a 90 / 115 / 140 HP versions--Basic same block too.

This is exactly the kind of info I was looking for, thank you Gentlemen!

Look at these pictures.

All of the 100-125 engines that I've see have the same exhaust valley divider that runs from top to bottom of the block. This design goes all the way back to the fat 50 - the gasket outline is exactly the same - save one extra bolt hole on the left side. The 135 and 140 block have a shortened divider that stops 1/3 of the way down - to make passage of the exhaust outlet from the tuner. Looks to me that you would have to mill that section from the 115 block & weld the bottom water jacket passages closed to use the bubble. a lot of work.

The gasket is not the same back to the 50 HP !!!

professor wrote:

The gasket is not the same back to the 50 HP !!!

Well, actually the outline IS the same with the exception of the bolt boss on the bottom left.

Here is 50 block from my bone pile (1st pic) with a 140 bubble back gasket dropped on (2nd pic) as you can see the outline and the bolts holes do line up as I said they do. The 115 & 125 gasket would have the center water jacket poerion that is deleted at the lower 1/3 of the 140 gasket. I don't just sit around and dream this stuff up.

yaaaarrrrgggghhhhhhhh

What welding needs to be done ??

Below pic is a 115 block. There are 5 water passiges in the divider that run to the main water galley between the cylinder banks.

The red lines outline the area omitted on the 135 & 140 blocks. This would have to be milled away from the 115 block. The area contains 2 water passages (yellow arrows pointing at them) the openings for these passages would require welding to close them off

The next picture of the 140 block clearely shows where the casting was modified & the bottom 2 divider passages plugged. This would have taken just a slight modigication to the mold at the foundry.

On the newer say late 80's ----NON BUBBLE BUTT blocks

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I would say those bottom 2 water jackets do not go into the main water jacket down the center of the block.---No welding required to modify them , just some simple machining.