Bought this motor, had some problems.

Replaced the cooling fan blade, head gaskets, carb rebuild, new fuel line, fuel filter, put synthetic oil in it, new oil filter, and cleaned it up a little.

Heres the problem. Took it out by myself in my 1648 welded flatbottom, couldnt get much speed out of it. Seems to run smoothly, sounds good, looks good, doesnt miss or knock, doesnt lose power when going through shallows or over stumps. Checked the throttle line and everything looks tight on it. Seems to be fully revved up, but it wont go very fast at all. I know these motors arent speed demons, but it should go much faster than this. I was getting maybe 5-6 mph out of it.

The prop on it seems to have an 8 in diameter, no clue about the pitch. Research shows people like 9x6 props on this motor for 2-3 people in it. Would this extra inch make that much difference? Anybody got any info on where to start? What could be the problem?

What are your rpm's in and out of water? If you don't have a tach, get one.

Ok, what do you think about the prop? Seems alot of people post this same question with this motor, but never really say what the problem was.

I do believe a 9x6 is what you want. That could definitely be the issue if yours is only 8". Without knowing your rpm's, it's hard to say what the issue is though. If you are only turning 2000 rpm's, doesn't matter what size prop you have, it's not gonna push very hard. If you are turning over 4000 and only getting 5-6 mph, the prop is probably the first thing to look at.

You need a 9x6 prop

How much improvement should the prop show? Should the RPMs in and out of water both be at full throttle?

Does the motor sound like it is getting to high RPM's like a riding lawn mower?

Yes, anything more that 1/2" of wear on a prop is very noticeable

I think it is revving all the way up, I will check the RPMs tomorrow and let yall know

Ok out of the water it goes up to about 4500-4800. In the water it goes steady at 2800. Picked up a little speed that wast there last time.

turn it down to a maximum of 4300 OOW it should be runnning ~4000 in the water.

So will the change of the prop change my rpm level as well? Does there seem to be something else wrong?

A worn prop will raise the RPM's in the water due to less surface area with a load on it.

You my friend have a problem!
Wether it be a fuel, spark, throttle linkage, or compression problem.... It's hard to say without some more details.

What other details do you need? Where would be the first place to check out?