2001 Yamaha 50 to 70 carb conversion *done*

[will941s]

Alright folks, I'm not a newbie to working on outboards; and have repaired, serviced, rebuilt, modified MANY. My newest challenge was modding a Yamaha 50. I installed a shaved head, ported intake exhaust passages, installed carbon reads, modified the exhaust tuner. Motor is on a 16' havoc and was running about 36 mph before, now running 41 @ 5,800 RPM's. Now to the carbs. Type it in google and there's a forum or two about how 70 carbs "bolt right on".....BS!!! Plain and simple. This post is the solution to that. Your buddy telling you to eBay a set and you'll have a 100 mph Jon with a bolt on mod?...no. Here's how it works. 70 carbs throats are 1/16" bigger then the 50. In turn makes the carb a total of 1/8th " taller all together to accommodate that larger throat, they look the same, but trust me..they're taller. So, here's what you do. Every bit of extra metal on the top of carb 3..needs to be shaved off. Number 2...you need to take a 16th off the top and bottom. And #1 take a 16th off the float bowl. THEN they'll bolt on to your intake plate. The butterfly arms off your 40 or 50 carbs will need to switched to the 70 carbs. Why? The cam contacts the roller on the 40/50 carbs a lot later then the 70. They won't open unless you do this. Next, where the idle screw is on the 70 carbs...there's a brass plug there, drill out the center and pop the plugs out. That way you can access the idle screw, you'll need it. Swap the .150 jets in the 70 carbs with 145's. Port the intake plate to match the new larger carb openings. The pulse hole on the intake plate behind #3 carb, plug it. After all that's done and you get them to fit, back off the idle screws 1 1/2 turn, make it RICH. Why? If your running carbon reeds and it sneezes..your buying new reeds. So make it rich and lean it up as need be to achieve a idle. It's best idle is going to be rough now. Hole shot, mid and top end will blow you away. This is a 50 plus MPH now, cranking right at 6K. So far there are no bad side effects, this one is more trial and error. Purpose of this post, if you want to do this; this is what you've got to do. No Matter what some of the other forums say. You need the compression from the shaved head, the ported ports, the carbon reeds and larger tuner. Otherwise your going to rich running motor that's going to run like crap. It's a fun project, let the replies roll in!

 

[tommarvin]

Will,you the man, is this a way to modify a motor for racing? This is way over my head, I had no idea you could do this.

 

[will941s]

This guy uses it for duck hunting and fishing. I guess one could say just to buy a 70, but there's no fun in that. 50 is lighter, and freaks people out when you pass their 70's. If you were building a true race motor, with bigger Pistons, lightened flywheel, no rev limiter, low water lower unit, chopper prop...no telling what you could get out of the carbs. Out of everything I did to the motor the carbs really put it over the top. Took a lot of measuring, headaches, and elbow grease but worth it if you want to take on a cool project

 

[99yam40]

If the motor produces more HP with all of these modifications you should be able to run a higher pitch prop or a larger one.
Did you install a different prop or have it modified?
Kind of a waste if it cannot run a prop for more HP

 

[will941s]

Did have to change the prop. Was running a 14p turbo stainless, Ended up with a 15p regular black steel Yamaha prop, seemed to hold it down better and performed better all around.

 

[Hoosier238]

I assume these are square bowls you had used and had the fitment issues with? I have an 02 yam 40 that I have 6h3 70 square bowls on there way and should be here Feb.10th. They do not include linkages but correct me if I'm wrong I beleive my 40 carb linkages will fit up on the 70 square bowls with new style tiller. Heads will be shaved similar to hydrotec style, 50 tuner, lightened flywheel, advance timing, delete rev limiter.