40 yamaha tuner box questions

[bassstrat]

Just got a 1993 40 Yamaha that has been souped up and was wondering what if any of these might need to be reversed in order to prolong the motors life. Here are the mods:70 carbs(70 jets),70 exhaust tube,tuner box green wire has been clipped(what does this accomplish?)flywheel has been ground/shaved to allow more RPM`s?,and a hole was drilled just above water intake to allow exhaust to escape quicker?and the oiler has been unhooked and gas was being mixed at a 50:1 ratio. Thanks for your suggestions,and recommendations!!!Bassstrat

 

[mphelle8vld]

Re: 40 yamaha tuner box questions

First look at my response to your earlier post, 40 with 70 carbs. Your motor has a displacement of 42.6cid with ports sized to match, the 70hp has 51.8cid, also with ports sized to match. Yamaha designed your motor (42.6cid) with a max hp of 50hp, it was also offered (detuned) as a 40hp by using smaller carbs and exhaust tube. If you keep your current setup, find a way to monitor the head temperature or at a minimum keep a close eye on your sparkplugs to determine if the motor is running too hot.

 

[bassstrat]

Re: 40 yamaha tuner box questions

OK,I just found the earlier post,so,you`re saying that even though this motor has the 70 exhaust tube (along w/70 carbs)that the motor might be running to hot? What would the plugs look like if they`re running to hot?Finally,could I reinstall the original 40 carbs and just leave the 70 exhaust tube in the motor? Thanks, Bassstrat

 

[mphelle8vld]

Re: 40 yamaha tuner box questions

I've owned 3 of these motors and still have two, I converted two of the 40hp to 50hp with carbs and tube and in each case increased the rpm by 500rpm and gained about 5mph gps. That being said, I have no idea how this powerhead would react to such large carbs and exhaust. If the motor has been properly ported it might have a peakier powerband but not sure about driveability and reliability. I brought up the heat issue because of the post regarding better fuel economy which I thought might be related to a lean condition. The spark plugs will have a blistered white appearance and possible metal deposits if it's running very lean.
Let me know how this motor is performing in it's current state, does it start easily and run smoothly? I'd also be curious if the 70hp exhaust tube bolted directly in place of the 40hp.

 

[bassstrat]

Re: 40 yamaha tuner box questions

Not sure how the exhaust tube bolted on but will ask previous owner. The motor takes about 8-10 pulls to start cold, then about 1-2 pulls when warmed up.The motor seems to run pretty smooth,but occasionally wants to bog or choke off when I first give it the gas,If I rev the handle a couple of times it will usually jump right up!As I said I want the best milage,while retaining as much speed as possible without hurting the motor(a lot to ask for I guess!)What do think would happen if I just changed back to the 40 carbs and left the 70 exhaust tube on? Also I`m not sure what you mean by porting. What should I ask the owner about this? Would this have been another modification? Thanks for your help! Bassstrat

 

[mphelle8vld]

Re: 40 yamaha tuner box questions

Porting is using a grinder or files to change the size and shape of the intake and exhaust ports, if it's done right, it allows better flow into the cylinders which is what your motor would need to take advantage of the large carbs and tube.
Leaving the 70hp exhaust tube in place is a tough call, it's sized to allow enough flow at high rpm but also provides some back pressure to keep the incoming charge from following the exhaust gases out of the cylinder.
Putting the 40hp carbs back on will result in less performance but starting and driveability should improve. ( but see note above ) Let me know if you need my old 40hp exhaust tube.
If you're ok with it's current performance and you don't think it's running hot, just go fishing.

 

[bassstrat]

Re: 40 yamaha tuner box questions

Thanks!I do have the old 40 exhaust tube,but since I`ve talked to you,I found a local tech that suggested I would be better off turning my 40 carbs into 50`s(he has a milling machine and has done it before).My question to you is it worth the trouble of doing this to go from a 40 to a 50? You said that you had converted two motors this way,right? Is the gas mileage tradeoff worth getting 5 more mph from a 40 to a 50? The guy wants to do this work for me in exchange for the 70 carbs and exhaust tube,so I guess he`d be milling my carbs to 50`s and putting the 40 exhaust tube back on I guess? Would he need to do some porting to make all of this worthwhile? Thanks for suggestions! Bassstrat

 

[mphelle8vld]

Re: 40 yamaha tuner box questions

You do not need to do any porting to convert the 40hp to a 50hp, you only have to replace the carbs and exhaust tube to a 50.
I'm not sure simply boring a larger hole in the venturi area of the carbs is how Yamaha increased the 40 carbs to 50's, I'll take a look at mine tonight and compare them to see if there's any slope or curve in the venturi.
You will need an exhaust tube from a 50 to make this change.

I'm curious about your mileage concerns, I know gas has been high lately but anytime I'm motoring to the next fishing hole, it's the last thing on my mind.

 

[bassstrat]

Re: 40 yamaha tuner box questions

Well ,I plan on making runs of say 45min to 1hr(one way) on occasion as I will be using this boat in some bass tournaments where I run a long distance just to get to my area,and then I need the small boat(14'3) to get into small hidden lakes and then get back for weigh in!So,not having to carry as much extra gas(in this small boat,with limited space) plays into the gas milage scenario for me! So,having said that what`s the difference in the gas milage of a 40 hp compared to a converted 50hp?Is the burnt fuel worth the 5mph more speed I`d gain? If there is a good bit more gas being burned,then I might would rather change it all back to a stock 40.See where I`m coming from? Thanks!!! Bassstrat

 

[mphelle8vld]

Re: 40 yamaha tuner box questions

Well, we are now making the transition from the technical aspects of the Yamaha 40-50 to the practical aspects of catching some fish.

As you say in your #5 post, you're asking alot from this fine motor. Set your priorities. If you want a reliable, easy starting, fuel efficient, "slow" motor, clean the old carbs and replace the tube and in a couple hours you have nice 40.

If you want a little more speed, keep an eye out for a blown 50 on ebay to get the proper carbs and tube for this powerhead, I have no experience with gas mileage on these motors, if you run a 40 at full throttle, you could run the 50 at 3/4 and perhaps get the same mileage, but at least you would have options on speed if the clocks ticking on a tournament. I don't think you'd be happy having your tech enlarge the 40 carbs, even if all went well, you would still be looking at new jets, emulsion tubes, metal debris, and you would still need a 50 tube.

Ah, the 40/70. For all I know, (unlikely) you could be getting 55-60hp, reliability and great mileage, find a way to quantify what you have and then switch back to the 40 and compare. Make a run from point A to B at a speed determined by gps and measure fuel consumed. Repeat for 40hp. Make an informed decision.

Just a couple of load suggestions, if you use a 12gal tank, switch to two 6gal to balance the load, put 5 1/2 in one and start with that, when it runs out you know you can still make it back for weigh-in. If you have beer on board switch to Jack, it's lighter, and last but not least, if you're fishing with a partner make sure it's a 110lb blonde. Good luck on the water!