Converting a 1999 Force 40hp to 50hp

[GoFastr]

I've done a little digging via google and seen a few posts on various sites about folks wanting to convert their 40hp Force motor to 50hp by simply changing out the carb for one with a larger set of jets setup for the 50 version. Unfortunately they were old posts and had no follow up as to whether or not they succeeded. Does anyone here have experience or know of someone who did this successfully by changing only the carb?

It's my understanding the power head is the same for both the 40 and 50hp models of 1999. There's even a sticker on the powerhead of mine that indicates Force 40 - 50hp.

If this is the case, it would be silly not to purchase another carb spec'd for a 50 and slap it on there and get a cheap 10 extra ponies.

I have a '99 Force/Mercury 40hp.

[DRYHEAT]

I'm not sure what your goal is, but I don't think you will notice any gain, just more fuel consumption.

[Doctordeere]

It's not the size of the jets that makes the difference, it's the size of the venturi bore. And there's also probably a difference in the programmed ignition advance curve within the ECM.

But, unfortunately, unless you can buy a donor 50 REAL cheap... like with a roasted powerhead but good carbs and ECM, it's probably cheaper to sell your 40 and buy a good used 50.

[GoFastr]

Yeah i get all that, but I read a post ( i think it was on iboats forums) where a guy did it and gained a couple clicks on the speedo and a few more revs by doing this. He was given a hard time cause he was asking if there were any other stock mods that could be bought and done, but posters kept coming back and saying he weas wasting his time using auto, cycles, atv's etc. experience as reasons why not. He had to repeat himself by saying he had already done the carb thing and had in fact gained performance etc. "Apparently" everthing between the 1999 40hp and the 50hp is identical. But there are multiple different main jets based on different altitudes of operation with .096 being sea level.

Soooo, I ordred the $7 main jet change and will give it a shot. If nothing is gained or noticed with baseline info, then oh well, nothing gained if nothing tried. I can put it out of my head and look at maybe an engine swap down the road.

[LocoCoco]

Wouldn't jetting bigger just make it run richer if the engine isn't provided any more air? In a perfect world, there'd be a restrictor plate on your 40 that would just have to be removed.

I do know that on some motors it's easy to convert them, but on others it's just not worth it. For reasons I can't remember, converting my 1990 Evinrude 9.9 to a 15 was not worth it. On older models the exhausts were different between the two. A subtle difference that makes a big difference. The biggest challenge was a problem with the ignition timing. Swapping the 9.9/15 carbs would make the converted 9.9 bog down at full throttle. Then again, one year Evinrude made an "XP10" or something like that. At the time, there was a demand for more 9.9s than Evinrude had in stock, and they had a surplus of 15s. So they just added restrictor plates to a bunch of 15s and sold them as 9.9s. Switching them back to 15s is as easy as removing them.

Something else to look at is what I discovered with my Mariner/Mercury (made by Tohatsu) 2.5hp. After some research, it turned out that the 2.5hp and bigger 3.3hp were nearly identical motors and carbs. The only difference was the 2.5 had a taller slide in the round-slide carb, so "full throttle" came sooner since the slide would hit the top of the carb sooner. I successfully converted it to a 3.3 by simply removing the slide, cutting it shorter with a hacksaw, then reinstalling it. Then the throttle lever just had to have the plastic slot extended a bit. Now the carb is able to fully open and run it's true power. Got the fastest 2.5 on the lake!

Anyway, my point is it may or may not be possible. But on an oldschool-technology Force I'd bet that it'd be a simple task.


LC.

[Doctordeere]

Wouldn't jetting bigger just make it run richer if the engine isn't provided any more air?

Yep. All you'll accomplish by putting a bigger jet in the otherwise unmodified carb is boogering up the air/fuel ratio.

[GoFastr]

All good points and perspective, and makes logical sense. When considering the carb tho, does the fact this thing is not gonna suck as much air or whatever, due to it's relatively small size and HP as compared to larger HP engines or V8 auto motors? Just a passing thought or possibility.

I'm gonna pull the carb anyway just to clean it up real good including inside of the bowl, and since it's apart I'll slap in the new jet when it arrives. Do a couple test runs, examine the throttle lever action though it appears to open wide, meaning plate is a flat 90 degree angle to the carb opening. If it runs like crap, I'll just ensure the original is clean and put it back in and pocket the knowledge to share with others. BUT based on part numbers all being the same, etc. etc. without being able to verify any carb physical differences, it is an interesting experiment.

As an fyi, I found an interesting website on the topic of jetting carbs:
http://www.jetsrus.com/FAQs/FAQ_jet_kit.htm

Also here's a poster from 2004 who saw a slight increase:
http://forums.iboats.com/showthread.php?t=126615

And another really good discussion from 2008. Would like to know how this turned out for the guy as the topic just dropped off:
http://forums.iboats.com/showthread.php?t=248886