compression omc 49 cu

[Stone Runner]

I have a 1977 70hp and a 1977 75hp stinger, checked compression and the 70 hp was about 90 lb on all cylinders and the 75hp was about 115 lbs on all cylinders. I realize different gauges can give different readings. That is a difference of 25 lbs between motors, is that normal or is the 70 tired? or should I say more tired than the 75 (my wife would hate that gramar).

The 70 is on a jet pump so I was thinking of swapping the 75 on the jet pump while I rebuild the 70 and make it a 75.

To build up the 70 I have gleened over the search function and so far come to this conclusion. This is not a race motor only a fishing motor. Shave the head 100 thousands, chamfer the exhaust port to 45% and from a Fast Fed post add a mill of timing to the transfer over port port (I have a copy of the post just not with me).
To add a mill of timing is that on the top of the port to advance the opening of the port?
Set of reeds.
Is there a significant advantage to bore 0.030 over? It doesn't seem like a a lot to me compared to a 3" bore.
One more question, I have a Seloc manual, is better to have the proper factory manual, is it the "Factory Service Manual?

Any input would be greatly appreciated :)

[powerabout]

75 might have the hi comp head which was an option in those days
70 to 75 - you will need the bigger carbs

[Stone Runner]

The carbs are the same on the two motors, since it isn't a race stinger it seems to have the stock carbs. It is adding a mill of timing to the boost port for low end power.
What does square tuna mean?

[Lil' Blue Rude]

If your planing on rebuildin g the 49 cu for the jet pump don't waste your time they're a dog because they don't have any low end grunt to them, the 56 cu in motors are so much better with a jet pump because they have alot more low end torque to them. You can change over the powerheads pretty easy. the only other thing you need to change will be the exhaust adapter that the powerhead bolts to and you will probably need to swap the 49 crank over to the 56 block unless you get a 86-87 mdel 56cu, I tink they still used the 4 spline crancks. Then you can make your sqaure tuner like fred talks about because the exaust runner is sqaure on the 56 cu motors.

If your going to stick with the 49 cu the best bit of advice I can give you is dont' port it like a prop motor! Prop motor are like sprint cars because they dont' need alot of low end but they usaully have alot of top end a jet pump is like a pulling truck they need alot of low end grunt but they don't have alot of top end. Jet pumps aren't deazined to turn much over 6000rpm because they will being the impeller will begin to slip.
Have any questions feel free to ask

[Stone Runner]

The 49 cu came with the jet and the boat, is a 4 spline. I'll look for a 56 cu power head to rebuild. What years would the 49 parts be compatable?