Carb swap

[Tkibby]

Will the carbs on a 1975 johnson 135 bubble back, fit on a 1976 johnson 85 flatback? And if so, am I right in saying it will then become a 135, and not a 85?

 

[flyingscott]

No those blocks are completely different.

 

[Tkibby]

Even though both carbs take the same rebuild kit? I understand blocks are different, but what about induction?

 

[racerone]

The 135 HP model was a bigger displacement motor !!----Different exhaust timing / tuning .-----This " bolt on a bigger carburetor " myth just does not want to go way !!

 

[jimmbo]

No, and it's getting worse with the 'Chip It' Mentality of the younger Generation.

 

[jimmbo]

Even though both carbs take the same rebuild kit? I understand blocks are different, but what about induction?

The carbs share the same Fuel Bowl, and mounting Flange. There were at least 3 different sizes of the carb Throat and/or Venturi. Some Rebuild Kits cover several models of Carbs.
Induction? Sometimes there are differences in the size of the Intake Runners.

Starting 1978, the 85hp was increased to the same Displacement as the 100, 115, and 140, that lasted until 81, when the 85 and 100 were dropped and a 90 was introduced. A person can spend a lot of Money and Time and convert the 90 into a 115 or 140, and the 115 into a 140. Parts need to be changed, Blocks Ported. In the case of turning the 90 or 115 into a 140, the Bubble/Turtle Back and all it parts need to be added, that will require Machining of the Block too

 

[Scott Danforth]

bolting big carbs on a small motor will cut your HP in half......

because the motor will run like crap.

you would be better off adding a bunch of stickers and a JC whitney rear wing.

 

[F_R]

If a bigger carb would make it a go-faster, imagine the stares you would get as you go whizzing by with your 3hp with a 40hp carb!! Yeah, I know, somebody will try it.

 

[Tkibby]

I thought this was a "help" forum, not just a place for smart ass keyboard warriors to make their egos feel better after a long day with their thumb up their a$$. And the bigger carb idea came straight from this forum, so if you dont want it perpetuated, remove it.

 

[tomhath]

Ask yourself a few simple questions:

1) Is the displacement of the two engines the same?
2) Is the compression of the two engines the same?
3) Is the flow of air/fuel into the cylinders and the flow of exhaust out the same?

If the answer to all is "Yes" then changing the carburetor will make a difference; otherwise the effect is unknown without doing a significant engineering analysis of what will actually happen. The answers you were given came from people with extensive knowledge of these engines.

 

[racerone]

Another poster slamming those with knowledge way beyond putting a wrench on their first motor !!----Are we not helping you by saying it will not work ???

 

[jimmbo]

Okay, a more detailed response
That 85 has small Intake and exhaust Ports in the block, so right there its ability to Breathe is restricted. So putting bigger Carbs on it is just going to result in lower Air Velocities in the carb at all throttle Plate angles, which means weaker Signal at the Main Nozzle, thereby delaying when it starts flowing fuel, Idle and Off Idle calibrations will also be off.
You will end up with an Engine that MIGHT run a few RPMs faster, but will have Stumbles and Hesitations, It will become a Chore to Drive and not much Fun