1999 Johnson 40hp prop

Hey guys,
I bought my boat 2 years ago and I put an engine on it that needed some work done on it.
On the engine i needed to replace all the seals in the lower unit plus the drive shaft.
This engine used to be on a 20ft pontoon boat (pontoon boat now has a new engine)
So i took this and put it onto this new boat i got and i also took the rpm gage with the engine. The rpm gage says that it is running at 6500-7000 rpm's at WOT. Barly moving the shifter forward makes the rpm gage jump up to around 3000 rpms's.
I was wondering if this is most likely a faulty rpm gage or is the pitch of my prop makeing it run at such high rpm's?
There was no problem when the engine was on the pontoon boat...

most likely the tach is at wrong setting

ok thanks mainliner!
i dont wanna run my engine at such high rpm's and do other damage to it.

What boat do you have and what prop?

it is a 1967 Grew 14foot and prop i am not sure i would have to look next time i am at the cottage...!
but would changeing the boats from pontoon to a runabout effect the rpms that much?

You should have a rev limiter to keep it from running that tight.Probably a bad tach or wrong setting. Taking that moter off a 20ft pontoon,it probably had a 11p or 12p on it.According to how heavy you load your boat,you may need about a 19p or so.

I would put a tach on it that i knew was good so you could put the prop that it needs on it.Wrong prop is hard on it.

changeing to a 19 inch prop what would that do? would it give more speed? or what

It just depends on how heavy your boat is.its very important to have a tach that is true to get the correct prop.If the prop is to small then yes but most importantly it will bring your rpm's down and keep from damaging your engine.

Each increase in prop pitch will lower your rpm's about 200.Each decrease in pitch will raise rpm's about 200.