carbon build up.

How much power loss can carbon buildup really cause me? I have my boat running good enough to get by as the lake I fish most has a 40mph speed limit. But 40 seems to be my glass ceiling. With a 13X19 prop my 74' 135 turns 5000 rpm wot at 40 mph on th gps. With a 13 3/4X15 it runs 40 at 5600rpm. Would bad carbon build up cause enough power loss to bog it down that much? It has less power than the 90 horce that was on the boat when I bought it.

Re: carbon build up.

not sure of the recommened rpms

but a decarb with seafoam would not hurt.

Re: carbon build up.

Carbon gets behind the rings and causes them to wear in an irregular manner, often catching on the ports and destroying the cylinders and pistons. This is especially common where the engines are lugged by being over-propped. Carbon is a great insulator, causing irregular temperature patterns whereever it accumulates. Carbon in the firing chambers may stay red hot, causing misfires, pre-ignition and sometimes a run-away.

We try to run the engine at a level where the engine is not being lugged but does not allow the engine to run at an RPM much higher than designed. In this way the engine does it's job without strain.

Because of the design of the pistons on the 1974 engine, which have a very high placement of the rings, these engines are particularly at risk from carbon build-up.

The specified full throttle operating range for your engine is 4500 to 5500 RPM, found in the specifications sheet of your owner's manual and factory service manual. The target for the setup should be to prop the motor so that it slightly exceeds the range at WOT with your normal load.

De-carbon treatment on an annual basis is recommended for all two stroke outboard motors. http://www.evinrude.com/Content/Pdf/en-US/PressRelease/maintenance_schedule.pdf

Re: carbon build up.

Last thing I want to do is discourage you from doing the decarb, but that motor might be capable of making more power than you're getting from it.

We need to know what boat that motor's on, and how high it's mounted on the transom (height of the antiventilation plate in relation to the keel, with the motor in neutral trim). Also whether you have power trim and are using it properly.

I don't know your gear ratio, but assuming 1.7:1, a 19-inch prop at 5000 RPM is 53 MPH, so 40 MPH means 24% slip. Seems pretty high to me.

A 15-inch prop at 5600 RPM is 47 MPH, so at 40 MPH, slip is about 14%. Still seems high, but it's nearly twice as efficient as the other prop.

Are your props in good condition?

Re: carbon build up.

The propeller gear rato is 13:25 on this motor or 1.923 on most of the canned calculators.

So, a 19 pitch prop has a maximum theoretical speed of 47MPH at 5000 RPM on this motor.

Re: carbon build up.

Im told that this motor should put out the same power as a modern 115. If thats the case then 47 would be what Im shooting for. As for prop condition, the 13X19 was brand new. The 13 3/4x15 was about as bad as I would expect a prop to get before failure. I just put on a brand new hustler 13 3/4x15 and I will run it tomorow. I am ordering a 13 1/4x17 to see if that gets me where I need to be. I am just hopeing that I will be able to get the revs I need for it after doing the de-carb tomorrow. I just didnt know if there would be a noticeable difference.

Re: carbon build up.

The lake I fish most of my tournaments at has a speed limit of 40 anyway so Im not looking for speed just to go fast. Im just trying to make sure I dont have something wrong that could be terminal. In fact, with the 15p prop this thing launches and gets to top speed so fast that I am easily beating the guys with the 250s. Just want to make sure this thing will last me for the rest of the season.

Re: carbon build up.

47 mph is how fast you would be going, at 5000 RPM with a 19 pitch prop, if you were moving though something solid instead of water. But some of your thrust gets wasted pushing water around instead of just pushing the boat forward, so you're only going 40. 40 mph is 14% less than 47. If you had the perfect hull, perfectly balanced, you might expect that to be something like 8% or 6%, which would be 43 or 44 mph.

To get to 47 mph with a 19" prop and 7% slip, you'd have to make 5400 RPM.

Re: carbon build up.

I understand slip, what you're missing is that 5000 rpm is way to low. It should be spinning 5800.