2004 yamaha 2-stroke 40hp will not reach top speed

[chaddyt150]

I have a 2004 yamaha 2-stroke 40hp oil injected outboard motor and it will not reach top speed. It idles fine and runs good at a low speed but when you hammer down the throttle it stutters a little bit and then will not reach top speed. Every once in a while when the throttle is all the way open it will open up all the way but once you throttle down it will just repeat what it always does. That's how I know it is not reaching it's maximum potential. I have an rpm gauge and when it reads about 4,700 that is when it's maxed out. It will usually run about 3,800 when it's doing bad. Just cleaned out all 3 carbs to see if that was the case. It is not. One other odd thing is the bulb will go soft but even when I pump it up it doesn't matter. Also just replaced spark plugs. That didn't help either. Water is also not in the gas. I have ran numerous tanks of gas and it has always done it. I have narrowed it down to maybe needing a carburetor adjustment or a fuel pump. Any help or suggestions would be greatly appreciated.

 

[Ruggybuggy]

Re: 2004 yamaha 2-stroke 40hp will not reach top speed

I kind of have the same problem. Same motor and year. I pretty sure mine is running a tab rich. Every once in a while when I can get the RPM's higher (wind and current in my favour) it will take off and pick up another 5mph (approx). Do you know what jet size your using? Can anyone else comment on the main jet size of a 40hp 2 stroke?

 

[99yam40]

Re: 2004 yamaha 2-stroke 40hp will not reach top speed

My C40TLRX has #115 s on the top and bottom, but #118 on the center one. Just like the parts diagrams show.
I had to replace them all as I found a previous owner had drilled them out trying to get more HP I guess. I had the same problem as it would drop a cylinder at WOT because it was running too rich

 

[Ruggybuggy]

Re: 2004 Yamaha 2-stroke 40hp will not reach top speed

Re: 2004 Yamaha 2-stroke 40hp will not reach top speed

I did find your old post about this. I also purchased this motor used. Did you actually figure out what hole size the 115 and 118 jets are? Do you know why they used the 118 in the middle? Ever since I bought this motor it has always seemed to use more fuel then it should and not very impressed with the high RPM performance.

 

[99yam40]

Re: 2004 Yamaha 2-stroke 40hp will not reach top speed

Re: 2004 Yamaha 2-stroke 40hp will not reach top speed

No I never did find out what the #s meant.
All I know is what I put in that thread, a small drill bit shank real close to 1.18 MM in diameter would slip into all 3 old jets and would not go into any of the new ones ,the best I can remember.

I also had unburned fuel/oil ( black goo)drip out of the exhaust while sitting in the drive way sometimes before I changed the jets and I used a lot more fuel back then also, but I was trying to find my problem and would run up around 5500 quite a bit more than I do now. I try to keep the RPMs down around 4200-4500 and I get good mileage now.

I never did find out why the different size jets for different cylinders, but these are not the only ones that are set up that way. Engineers must have done it for a reason though.

Also, something to look at would be the enricher circuit to make sure the needle is extending properly as it is heated up so that it will not be putting in extra fuel as it does for a cold motor. Check that diaphragm also to make sure it does not have a pin hole or tear in it, along with the fuel pump.

Hopefully you have the red lever in the normal position as it should be.

Chaddy150 your 40 hp motor should get to 5500RPM WOT unless you have motor or set up problems or is over propped.
Check out this site and see what they were getting on a set up like yours.http://www.yamaha-motor.com/outboard/products/perfbull.aspx

Too much fuel or not enough will not let the cylinders fire properly so check the fuel pump and pull and clean the carbs if the pump is not a problem.
adjustments on a carbs will not effect WOT RPMs as far as I know.
Compression, spark, timing ,and fuel/air mix are the things that can effect proper power of a motor, so measure what you can and then work on those things you can not measure

 

[Ruggybuggy]

Re: 2004 Yamaha 2-stroke 40hp will not reach top speed

Re: 2004 Yamaha 2-stroke 40hp will not reach top speed

I did pull the carbs and the jets look like they haven't been tampered with. This is a motor with very few hours (under 100). I did check the enrichment and it seems to be operating properly. The diaphragm held vacuum and the solenoid did extend when voltage was put to it. As a test I could put the red valve in the closed position to eliminate the possibility the enrichment system is causing a problem. Am I mistaken, was it you that made a timing adjustment from 7 to 14 degrees and found it helped? Sorry for steeling the thread but maybe this will help chaddyt150.

 

[99yam40]

Re: 2004 Yamaha 2-stroke 40hp will not reach top speed

Re: 2004 Yamaha 2-stroke 40hp will not reach top speed

I had to change the CDI to get the idle timing back to where it is suppose to be as it was at 14 but needed to be at 7ATDC. Testing things is the easiest way to find a problem.

I did not see a problem with my jets either, that is why it took close to a year to find my problem and every one said just to look at the #s on them

 

[Ruggybuggy]

Re: 2004 yamaha 2-stroke 40hp will not reach top speed

I'll put a timing light on it and order new jets. My motor was mounted on a large 17' boat with a 11 pitch prop when I bought it. Maybe someone thought they could get a little more from it by drilling the jets.

I found out why Yamaha used the larger 118 jet in the middle cylinder. The middle cylinder runs warmer then the two outside cylinders so they dump more fuel into it to keep it cool.

 

[99yam40]

Re: 2004 yamaha 2-stroke 40hp will not reach top speed

Interesting, Thanks I would not have guessed that

 

[chaddyt150]

Re: 2004 yamaha 2-stroke 40hp will not reach top speed

I only saw two jets on all three of the carbs. Are there 3 jets on all three carbs?!

 

[jeffersondodson561]

Re: 2004 yamaha 2-stroke 40hp will not reach top speed

Have you verified the fuel pump does not have a torn diaphram?

 

[chaddyt150]

Re: 2004 yamaha 2-stroke 40hp will not reach top speed

Yep. It pumps fuel fine. Just checked the fuel pump out. What is the CDI? Do yall think it could be one of the coils gone bad?

 

[99yam40]

Re: 2004 yamaha 2-stroke 40hp will not reach top speed

He said to check for leaking diaphragm, it will still pump fuel, but will enrich #3 too much if it is leaking.
You can test in puts and out puts of the CDI to see if you have a ignition problem, but you will need a peak reading voltmeter or a DVA adapter and a regular meter to do so.
Also sounds like you need to pick up a service manual for your motor

 

[chaddyt150]

Re: 2004 yamaha 2-stroke 40hp will not reach top speed

Does this motor have an O2 sensor (oxygen)? If so do you know where it is located?