Yamaha 2003 F25MSHB Carburetor Questions...

[templar33]

Hello, I do not have a service manual for my outboard, but I just ordered it and should have it in a few days...

For 2 years since I had the carburetor rebuilt professionally it has run flawlessly, but it is now having problems and running rough and sluggish. When I took the cowling off I noticed that part #18, 65W-14373-00-00 COVER, PLUNGER CAP has broken into pieces and some fuel is leaking out of it now. When I push down on the pieces while running it seems to help make a seal and it begins to run right again. I have this part on order.

***FYI, I always run only non-ethanol fuel I purchase from a local marina, and I treat the fuel with stabilizer and I keep the boat in a garage at all times when not in use, and I run it on a regular basis at least once ever 2 weeks through winter too. So I am wondering why this rubber boot/seal would last only 2 years and if this is normal for it to fail every 2 years??? I see it comes standard with a carburetor repair kit.

But what I would like to ask any of you who might know, I was told by a Mercury outboard technician that I could increase the power of this outboard, my 2003 F25MSHB 4-stroke, by increasing the size of the jets. So I purchased a brand new in the box carburetor for the 2003 F40MSHB for a good price on ebay last year and have it sitting on the shelf keeping it in mind for when a carburetor problem popped up.

To increase power would swapping internal jets be the way to go?

Or, should I just repair the 25hp carburetor and leave it as is?

Not sure if this is important, but the F25MSHB carburetor is model 65W-14901-10-00, and the F40MSHB carburetor is model 67C-14901-13-00

Also, I realize that if I were to increase the power of this outboard with larger jets or, the 40hp carburetor, that I would more than likely have to change the pitch of my propeller up from the 11 pitch I am using now. I have the original 13 pitch prop that came with the outboard, and I also have a brand new 12 pitch prop in the box ready to go. I am familiar with playing around with the prop pitch to achieve optimum running rpm's using a tachometer and following Yamaha specs on this issue.

Any thoughts or comments greatly appreciated! Thanks in advance.

 

[99yam40]

I have read that the size of the jets in the 25 are larger than the 40,
and some install the 25 jets in the 40 to get it to run better due to the 40 comes with small jets that plug easily

I would fix the 25 carbs

 

[templar33]

Thank you for that suggestion. When I had both carburetors apart today side by side I compared the jets and found you are absolutely correct which sort of surprised me as I would have thought and hoped that the 40hp jets would have been larger. I was hoping the reverse of what you said above would have been the case, but it was not. So I did as you suggested and left the 25hp carb as is and only replaced the deteriorated rubber boot. I did carefully check to make sure the jets were clean and clear of any debris or blockage and inside the carb- which was perfectly clean. All is well.

It took me about 2 hours to remove the carb, disassemble, clean, repair, and rebuild it and re-install it. I was not in a hurry and just took my time with it. Once done she started right up and seemed to run great. Tomorrow morning I will take it out on the river to really give her a run and she how she does.

I was amazed at how easy it was to do this job. I had been dreading it, but it was not as bad as I thought it would be. And it saved me some money doing it myself too. My mechanics are near $100 per hour for labor, so this job could have cost me a couple hundred easy. As it was, I did this repair for free since I had the other 40hp carb to part out. Heck, just that little rubber boot is $20.00 to my door- $13 for the part and $7 in shipping. So it was nice to already have a brand new part ready to go for free.

I would still like to have an answer as to whether or not I can increase horse power by increasing fuel flow by enlarging the jets slightly? Is this possible? And if it is possible, to what diameter should the jets be drilled out to?

 

[99yam40]

How many carbs and cylinders does the 25 have and how many on the 40?

My guess is the 25 has less cylinders and carbs maybe.

My C40 2 stroke has 3 cylinders and when I bought it 3rd hand,
it took me about a year to figure out one of the previous owners had drilled out the main jets in all 3 carbs
causing too rich of a fuel to air mixture ,which caused a high speed miss.

You cannot increase fuel volume without increasing the air volume and expect the right mixture in the cylinders to burn properly

 

[templar33]

You are right again! I just checked and the 40 has 3 cylinders while the 25 has only 2 cylinders with a single carb. I will leave it alone and not try and modify it any. She runs like a bat out of hell and I'll leave it as is. Thanks again! Good advice.