Re: Performance upgrades on 1982 Johnson 115. Any Ideas out there?
your top speed is way too low, its miles too slow for that boat and that engine.
its a 115hp, ok so its rated at crank ,but that would still be a 100hp today so should be quicker on that boat.
you should easily be 48mph plus with your set up at moment.
But the engine is a bit too low for high top speed, it needs to be up a good deal, the higher the better as long as the
water pressure is still good.
exactly what prop is this your using ? make and material ?
to have a 21 with 5500rpm with 2-1 ratio means your getting tons of slip or prop is not correct size as stated.
you should be getting 5500 rpms minimum when full throttle or the engine is not getting full power, but you might have bad tacho and not getting the 5500 you say it is revving at.
you should be able to swing a 133/4 x23 prop as well no problem, unless like said at the top of this topic the boat is very wet in the foam.
All boats with foam are great UNTILL water starts to get in, then they are rubbish and take on the water but the foam don't release it,
even over time it never fully dries out.
take the boat to a local weighbridge and see what sort of numbers come up, its easy to work out weight of trailer and engine so then see what the boat weighs.
you might find the foam needs to come out and be re-newed, I just took 500 pounds of weight out of my 13ft boat by dumping the old water logged foam.
found the boat sits 3 inch higher at standstill too.
Now the fin on the engine will not make the slightest bit of difference when up on plane, the fin wont even be touching the water if
the engine is in right place on transom.
putting the cav plate level with bottom is fine on a slow cruiser,on a speed boat hull that is wrong place for it.
the cav plate realy don't do much at all when the boat is on the plane, and should be at the very worst scenario slightly touching
the water at full speed.
cav plate level with hull is old school and wrong.
this is why jack plates do come into play.
hydraulic is obviously better, but you can get a manually adjustable plate to do a good job.
if its just the top speed yopur after then a manual adjustable jack plate will be perfect as you can adjust it by a simple spanner and get it idel for top speed.
but going up with the motor will give slower pull away from idle to plane !!!!!!
do a decarb and get the compression up a bit, give the carbs a real good clean and check the timing.
if everything engine wise is good then the boat itself has a problem !!!! very wet or untrue hull bottom.
also make sure the bottom is nice and clean with no algae, ive lost 5mph before now on a speedboat due to dirty algae growth on hull bottom.
post a pic of the transom with engine etc
try to get a pic of the gearbox when running at full throttle, youl need someone to lean over and take a pic, then you can see if the cav plate is above or below the water when full throttle.
if its touching the water its too low and will be killing your speed big time.
to get the most speed from the engine the centre line of propshaft needs to be 1-2 inches below the very bottom of the hull at the very centre of transom, then youl need a chopper type prop and will see high speed numbers.
the cav plate would be around 5 inches higher than bottom of the boat
with the 21 pitch you should be getting 49mph with 10% slip at 5500rpm.
post a pic of the back end so we can see what the engine to hull looks like
thanks again for the help.
