want to switch 1988 90hp carb.'s to 120hp's

what is the bore size for evinrude 90hp carb. and what is the next size up? i've got a pair of 1 5/16th carb.'s and want to install a size larger, what would it be.
thanks

Re: want to switch 1988 90hp carb.'s to 120hp's

why? is the motor and hull set up properly. engine height correct, weight distribution???

Re: want to switch 1988 90hp carb.'s to 120hp's

Not sure the carbs are compatible. Your 90 hp engine is a crossflow, when the 120 came out it was a looper. These are different engines and the carbs will not exhange. If the carbs look identical, your extra set of carbs may have come off some other higher hp crossflow. You can swap crossflow carbs with ones with larger throats. Larger-throated carbs tend to have been on powerheads which may have higher porting, intake filler blocks, bubbleback exhaust and high compression heads. If you don't do/add these other performance items to your 90, you may find the swapped larger carbs will run, but may overfuel the engine at high rpms- causing poor perfromance. You may need to spend time rejetting the new carbs to get them running properly. I put 1 5/16" (135) carbs on a stock 85 crossflow years ago. After all rejetting was done, I picked up just over 200 rpms and a few top-end mph.

Re: want to switch 1988 90hp carb.'s to 120hp's

Yes..i always had thought the only difference between varying HP models in the same block was just the carbs...but I have learned a lot in this forum. Emds is correct about the differences, so trying to get a few more HP out of a motor is really not worth it. Just sell your 90 and go buy a 115...should be able to do that cost neutral or close to it.

Re: want to switch 1988 90hp carb.'s to 120hp's

The 120 looper carbs won't fit on a stock crossflow.

Keep in mind that if you over-carb a motor (more fuel delivery than the port area will support efficiently and/or the cumbustion chamber/ignition can handle efficiently) you'll probably end up with a motor that accelerates sluggish and burns more fuel.

Yes, we run 4 two barrel carbs on a V4 and 6 two barrels on a V6 but nothing about those motors are stock.

Re: want to switch 1988 90hp carb.'s to 120hp's

The boat and motor were bought new in '88. its a 17ft ranger fisherman 680. It'll do 46mph empty with me.
I had talked with a few guys who say that for their motors like a 9.9 and switched carb.'s to a 15hp. simply bolt on. I was told that the same goes for a 25hp motor and switch to 35hp carb.'s. The only difference for some of these motors is the diameter of the bore in the carb.
I had talked the boat mech. here and asked him about this and he said if i can find a size larger bore on the carb. that i can make the switch no problem. I have the 1 3/16th carb.'s now on the boat and a pair of used 1 5/16th in a box. I'd hate spending the time to switch these out and find out it wont work. thanks for the replies...so i'm more unsure than before.
One more thing, this boat hasnt seen water in over 7yrs. When i first rebuilt it it would do 46mph and after about 10trips out it will barely make 44mph. any idea as to why i've lost some power? wonder if the spark plug wires or plugs? I did switch the wires when i rebuilt this this past summer.

Re: want to switch 1988 90hp carb.'s to 120hp's

SparkieBoat said:

Yes..i always had thought the only difference between varying HP models in the same block was just the carbs...but I have learned a lot in this forum. Emds is correct about the differences, so trying to get a few more HP out of a motor is really not worth it. Just sell your 90 and go buy a 115...should be able to do that cost neutral or close to it.

Click to expand...

the boat and motor were free and i've put about 1500.00 into it. I'd rather not spend money on another 2stroke if i dont have to. It just needs some cosmetics to be back to show room quality.
I didnt know that the transom was solid, no wood from what I can see.

Re: want to switch 1988 90hp carb.'s to 120hp's

Getting more HP out of a 2-stroke outboard is rarely just a matter of slapping some different carbs on it. Listen to DHadley - if there is anyone around here that knows a thing or two about "tweaking" outboards, it would be him.

That said, it sounds like to me, that you have a few other issues to get straightened out, before spending a bunch of money trying to get your boat to go 48 mph, instead of 46 mph. I know that sounds a little judgemental, but understand that we see these threads constantly and most of them begin with a variation on, "my buddy told me I can turn an "x" hp model into a "y" hp model, just by putting bigger carbs on it. Trust me when I tell you that 98% of these "buddies" have never heard the term "volumetric efficiency" and have no idea what it takes to improve it.



???

Re: want to switch 1988 90hp carb.'s to 120hp's

I'll take your words for it..thanks very much. I've gotten a lot of help from you guys in the past, thanks for sharing.

Re: want to switch 1988 90hp carb.'s to 120hp's

there is not any documented proof, that putting a 15 carb on a 9.9 will make the 9.9 a 15hp.

Re: want to switch 1988 90hp carb.'s to 120hp's

if you sell your 90 for around $1k and buy a 115 for around 1K it is cost neutral..except for your time to remove and reinstall motors, and time searching for a 115 and selling your 90.

Re: want to switch 1988 90hp carb.'s to 120hp's

Switching crossflow two barrel carbs from one model to another is easy (you origainaly asked about putting looper carbs on a crossflow). Swapping cf carbs will work - as in the motor will still crank up and run, no problem. But it will in most cases mess up the efficiency. OMC was really pretty good about matching carb size to port area and port timing. There are cases where a simple carb change will result in more power but it's rare with OMC motors. In those cases the blocks are identical, including port area and port timing, but the motors are rated at different rpms. Even in those cases putting a larger carb on usually means the motor accelerates slower but may gain a few rpm on top. All this has to do with air velocity thru the carbs and intake.

However (there's always a "however" - gotta love it), there is a lot of untapped power in the stock intake on a crossflow.

Re: want to switch 1988 90hp carb.'s to 120hp's

tashasdaddy said:

there is not any documented proof, that putting a 15 carb on a 9.9 will make the 9.9 a 15hp.

Click to expand...

No, but from all the reasearch myself and others have done, doing so will give the 9.9 identical parts specifications (and likely the same performance) to a 1974-78 OMC 15hp model. Later years had other differences, for example a tuned exhaust for the 15. Whether the early 15 at the time was making a genunine 15 crankshaft-rated horsepower is another question. My 1978 Johnson 15 was faster than all 9.5/9.8/9.9/10 hp engines I encountered from various manufacturers, but most of the time the difference was only a couple of mph. I'm sure the propshaft-rated 15 of 1981 and newer was a little faster.

Re: want to switch 1988 90hp carb.'s to 120hp's

Dhadly, could you recommend the easiest and cheapest way to get more HP out of a crossflow?? like aftermarket reeds, and things that will not cost a fortune, I would also want any changes made to not effect in a big way the life expectancy of the motor. Anyone whos may know can feel free to input. I mean there are certain things you can do that work well together.
I mostly know about car motors, we use to get headers, a high rise intake, a big 4 barrel carb, and a bigger camshaft, high performance ignition and plug wires, then change the rear gears to 3.55 or 4.11, but if you wanted the best two things to do that would help the most the cheapest, do headers with dual exhaust, and a high rise intake, this could be done for under $300 back in the 80s. and could get as much as %25 increase on HP.
add a bigger carb, with a camshaft, high pro ignition and see another %25 but that was gonna cost another $500-$700