- The Response - It looks like some fine work
there - nice pictures too
[Sorry I can't put up all the
pictures I get - Rick]. I appreciate the
update. Case matching is always a good idea. Its benefits
may be marginal but every little bit helps and it
certainly doesn't hurt. Over the winter I rebuilt my
GP760 motor too. I decided not to part with the $600+ for
an after market exhaust pipe so I concentrated on making
the motor more efficient. When I pulled off the head it
was easy to see how the factory achieved the staggered
compression, the whole combustion chamber is recessed a
bit on the rear cylinders head. The static compression of
that cylinder would read about 5 to 7 lb. less than the
front one. I CCed the head on the motor with the pistons
installed to establish the uncorrected compression ratio
(UCCR). It turns out the engine is shipped with an UCCR
of 12.28 :1 (up front). That's pretty good for running
low octane pump gas (86 whenever possible). The tub head
combustion chamber design and over cooling help to
keep
- everything in check. The squish area of the head was
so thick I think it was virtually useless. When measured,
the squish was determined as being 2.46 mm thick on the
front cylinder and 2.84 mm on the rear cylinder - ugh. I
decided to base all design changes off the front cylinder
because I still wanted to retain the staggered
compression - this would allow the rear cylinder to still
run lower compression. I ran the head data through the
two stroke software to help me design a better head. I
knew that if I could make the squish band useful I could
gain additional performance from the motor. I still
wanted to be able to run pump gas (89 octane would have
to be acceptable since 92 is not available at all
marina's). I found out that if I milled 1.1 mm from the
surface of the head I would be able to achieve a maximum
squish velocity (MSV) of 30 m/s on the front cylinder (a
good number for a heavy watercraft). Though this is a
vast improvement over the stock set up for the front
cylinder, the rear cylinders squish thickness (about .40
mm thicker) would only get a marginal improvement (to
about 22 m/s). But milling 1.1 mm from the head would
remove some of the squish area from the front cylinder
while it evens up the squish thickness difference a bit
between the two cylinders. Some of it would have to be
recut into it - but very little work would be needed. I
would much rather have a properly designed squish band
working for me than not. I would settle for opening up
the dome of the front cylinder to make better use of this
tub head design. After the material was milled I recut
the squish area as needed. I touched up the rear cylinder
to make it parallel to the piston crown and removed some
material from the dome of the front cylinder since it was
still a little bit too small. I ended up with squish
thicknesses of 1.52 mm up front and 1.78 mm out back.
This calculated out to a MSV of 29 m/s up front and 25
m/s for the rear. These are much better figures than the
16 m/s and 11 m/s it came with from Yamaha. The UCCR
would end up being 13.65:1. That's a little over 10%
higher than when I started - the numbers looked
good.
-
- After running the porting specs through some two
stroke software it became very clear that the transfer
port area was quite a bit too small. Without raising them
I was able to increase the time/area by a significant
amount. I changed the port roof angle from its "choked"
angle to a design which has more area as well - much more
flat top port roofs. This basic cylinder block has its
ports cast the same as they were when it was a 701 cc
motor - maybe even from when it was a 650 cc package. I
also changed the vertical angle of the main and secondary
port walls so they will release their charge a little
more directly across the piston to help peak power. The
match between the liner and the aluminum was terrible - I
spent a great deal of time making it right. I also
increased the exhaust port time/area by widening the
port. I choose to make the port oval instead of giving it
an unusual shape like we talked about this year. The 760
engine uses 84 mm pistons and its exhaust port has the
same area as the 701 which uses 81 mm pistons. The
additional bore size lets making the oval port shape much
easier. It would have been even easier if I was going to
raise the port as well but I didn't want to decrease the
duration of the power stroke. I was able to achieve a
good shape and time/area by widening the port to 64% of
the bore. Though it weighed heavily on my mind (the
thought of cutting the aluminum too thin) it turns out
that there is plenty of material for this modification. I
was able to increase the exhaust pipe entry port area
too, though its shape is not as oval as the exhaust port
or the aluminum immediately adjacent to it.
-
- While working on the exhaust I discovered a 5 to 6
inch long crack in the aluminum casting of the inner wall
in the rear most section of the pipe. I welded it to plug
the water leak. I'm glad I decarboned the pipe or I may
mot have seen it. When I measured the bores of the barrel
I was surprised to find that they were still extremely
round and measured exactly 84 mm - they needed only a
fresh hone. The time meter showed 108 hours of use in the
2 years since I bought it - and much of that time has
been spent towing multiple tubes with kids on them (I
know I'm not supposed to do that). At the time I ordered
replacement parts for this project I was unable to get
the Pro-X cast pistons I wanted. I could not justify
paying the amount of money Yamaha wanted for OEM
replacement piston sets - literally 2 1/2 times more than
after market, so I bought a set of Wiseco forged pistons
for it. I did not get the XPS series (coated) pistons
because I heard they had problems with shrinkage -
collapsing of the thrust surfaces. The new Wiseco pistons
came with .003" piston to wall clearance cut into them -
what I call a perfect fit. The pistons I took out had
.007" clearance. Additionally the new pistons are over 3
ounces lighter than the chunky Yamaha pieces.
-
- Since the carburetors are of the down draft type and
the intake makes a 90 degree bend into the crankcase, I
added the Boyesen reed cage stuffer to the intake side.
The stuffer has different sized passages (the horizontal
divider is placed very low) to compensate for the mixture
which wants to follow the contour of the intake manifold.
It should prevent the mixture from slowing down there as
well because the intake manifold opens up very large to
allow it to feed the reed cage. I also topped of the
carbs with a Pro-Tec K&N flame arrestor and velocity
stacks which was left over from a Wave Raider project - I
had to buy the larger adapter plates for use on the 44 mm
carburetors.
-
- In addition to the engine modifications I replaced
the worn out stock impeller with the Solas I (eye) design
- it is very close to the stock part as far as I could
tell, and added a Rule 500 gph bilge pump to help keep
the inside of the boat dry. If you haven't seen what
cavitation can do to an impeller take a look at the photo
I put up - click
here.
-
- The instructions that came with the Wiseco pistons
said to add 2 ounces of oil per gallon of gas for the
first tankful when used in oil injected motors. I thought
that was a lot of oil to add to the already oil rich
something-in-the-neighborhood-of 32:1 mixture it gets so
I added only 16 ounces of oil to the 13 point something
gallon fuel tank. I didn't like the thought of leaning
out the fuel to air ratio by adding that much oil to the
gas either. I left the low speed mixture screws alone but
I opened the high speed mixture screws 1/2 turn. I did
this to compensate for the additional exhaust area and
head modifications. When it was done I started it up for
a minute then let it cool off completely. I then
performed a compression test using a Snap-On compression
gauge. Up front the reading was 175 psi - the rear was
showing 170 psi. This was an increase of 15 psi in both
cylinders.
-
- Last week I was able to take the craft out for its
"run in." Immediately I noticed a deeper throatier sound.
The throttle response seemed much more crisp than it was
before. I planned to drive around the lake at different
operating speeds until the first tank of gas was gone.
I'm not one to baby a machine during any time in its life
so I pretty much broke it in the way I drive it, with the
exception of holding it at WOT - I would wait a bit for
that. It was clear right away that the Wave Runner had a
different personality - it would pop out of the water
like a cork on steroids. That low octane sluggish feeling
was gone.
-
- My kids and my nephews and nieces were bugging me to
take them tubing and I wasn't even through 5 gallons of
gas yet. I told them it would have to wait a bit. I took
my daughter Lauren for a ride - actually she drove while
I observed. We pulled into an area with some tall grass
and tree tops sticking out of the water to take a look at
the fish that were in the water. Every few minutes we
would start the boat to move to another spot, then we
would shut it off and drift in so we wouldn't scare the
fish. When we were going to leave the area I clicked the
starter but the engine wouldn't start. I figured it was a
fouled plug since I had done so many starts without
really running the engine - perhaps it had become oil
fouled. I jumped in the water and swam with the boat to
push it to the shore as Lauren poked fun at me. I yanked
the plugs out as she started building a sand castle to
establish a home base. The spark plugs were the stainless
steel Champions that my brother gave me last summer when
we were at lake Havasu - they only had about an hour of
riding time on them. Somehow I had forgotten to pack a
couple of spares. It was kind of hot out so I wiped them
off with a piece of paper and rested them on the hood -
maybe they would dry off. While waiting Lauren started
asking me the "what if's." What if we couldn't get off
the island... What if we didn't have anything to eat...
What if we had to stay there all day... What if we had to
stay there all night!?!
-
- It really wasn't an island, it was the shore of the
lake. Though we were about 7 or 8 miles from camp we
could walk there if we had to and I could see the marina
from where I was standing. Lauren decided that she could
remove and bend a nail from the nearby fence, attach it
to a piece of string that she found and tie it all to a
stick so she could catch us some fish for dinner. She
would rub 2 sticks together to start a fire. She would
spell out S.O.S. by using rocks on the beach so aircraft
could see us and come to our rescue. I was asked what
S.O.S. stood for - I really don't remember! I told her it
meant "rescue us and we're not kidding!" I'll have to
look that up (save our souls?!?). I wondered about the
stick rubbing thing to start the fire and decided that if
I could get any kind of spark from one of those Champions
I could start a GREAT fire - we had gasoline! She had me
thinking like a kid - the world's so big to kids. I
looked over that fence she was going to take the nail out
of - it was holding back cows! I never did like fresh
water fish that much - I asked her how she felt about
having a hamburger instead. She laughed and said "Oh
yeah!" It seemed funny to me that there was no traffic on
the lake right then. I remembered it was a Wednesday -
not many people out during the middle of the week. It's
not often we leave for a trip on a Tuesday, my clock was
off. I put the plugs back in as I tried to sell her on
the idea that the boat would now start. When it still
wouldn't I knew I would have to replace them. Lauren
started making more detailed plans and was clearly
thinking about digging in for the long(er) haul.
-
- After a few minutes I successfully flagged down a
boat - the second one that drove by. I decided that I
wasn't convincing enough when the first boat went by (it
didn't stop) so I over compensated the hand and arm
gestures to make sure the second boat stopped. Three guys
in a blue boat pulled up. I told them I would like to
give them money to go to the marina to buy me a set of
plugs. The driver said that would be fine. I gave them
one of the bad plugs so they could match it up and a ten
dollar bill. I had a tow rope but I thought I could do 2
things at once this way. I could get new plugs and get to
spend a few more minutes on the "island" with my
daughter. We started using words like shipwrecked and
marooned. I decided to name my GP760 the S.S. Minnow. We
started singing the theme from Gilligans Island. Lauren
asked me if I thought the guys we gave the money to would
ever come back. I told her that since they stopped to
help us there was a good chance they weren't going to run
off with the $10 I gave them. I told her they would come
back. If I wanted to walk about thirty feet I could have
looked to see if their boat was at the marina. I wasn't
worried, I figured sooner or later my brother would
cruise by riding his XL1200 anyway. His son Jason would
probably be with him on a Wave Raider. We had lots of
daylight left.
-
- It seemed like it took longer than it should have to
drive the 1/2 mile to the dock, run in and grab a couple
of plugs and come back. Lauren asked me again if those
guys looked like they could be trusted. Before I could
answer, I saw them coming closer. One of the passengers
in the boat handed me the new plugs and the change (about
30 cents). I thanked them for the assistance as offered
to buy them some beers if they cared to stick around long
enough for me to screw in the plugs and get started. The
driver smiled and suddenly looked real thirsty. It
occurred to me that he could probably put away some
serious beer. The Wave Runner took a few seconds of full
choke to start. I thought that was a little curious
because it was about 85 degrees outside - it never takes
choke on warm days. I only had $22 dollars left on me and
I (now) wanted to get a spare set of plugs too, but I
could make another trip for them if the beer was really
expensive at the marina. It was expensive - the 2 plugs
were $9 something (what a rip) and the beer took just
about the rest of the money I had. I didn't really care
because we were going home! It turns out that out
rescuers were our neighbors and were parked about 4 or 5
campers more inland than we were.
-
- I later found out that the plugs were not fouled
after all. The reason the motor would not start after we
were done looking at the fish was because it needed
choke. I did not think it needed it (actually I thought
it was oil or perhaps fuel fouled) so I never pulled the
knob. I later turned out the low speed mixture screws
(1/6 of a turn) and my starting problems went away. I
should have considered that I would be running a bit
leaner at low speed too and richened up that circuit
before I started as well. When I had the spark plugs out
(on the island) with the sun fully shining on the lathe
cut piston crowns I could see the burn pattern real well.
Things looked good inside the motor - I was getting
happy.
-
- I poured some more 89 octane gas into the tank and
headed out for another ride. I had run about 1 full tank
through the motor so I was thinking it would be a good
time to see what it had on top. With the water as smooth
as I've seen it on that lake the speedometer (actually a
ballpark guesser to the tune of several miles per hour
I'm told) showed 62 MPH. That's 2 MPH higher than I've
ever seen it register - 60 was my previous best and I
only saw it one time. Most of the time it used to read 55
or 56 as a top speed on smooth water and about 49 or 50
in rougher water. The natural, part throttle cruise speed
the stock motor liked was about 45. At that speed the
motor would turn about 5500 RPM. Maximum RPM is limited
at about 6700 according to the tach. It now became clear
that not only was the motor able to take off with much
more authority it actually had some speed left when the
tach showed 6000 RPM. It used to be useless to turn it
higher than that because the speed would not increase.
Another thing that became clear was that its natural part
throttle cruise speed was now up to about 52 - quite a
difference. I never saw 62 again last week but I never
had flat water like that again either. I saw a lot of
58's and 59's and several 60's. It would be difficult to
say that the top speed was not improved - because I'm
sure it is up a couple mph's. When I entered the stock
engines parameters including port time area and output
figures into the 2 stroke software and compared it to the
new port time areas using the same BMEP, the data shows
power to now be 102 HP @ 6300 an increase of 12 HP at the
same RPM.
-
- I spent some time trying to figure it all out but I
think it would be safe to say from my experience that it
indeed is possible to increase low speed power and
midrange and peak speed as well when modifying (at least)
this model of personal watercraft. When towing the tubes
full of kids the boat planes them (and me) instantly.
There's a little thing that kids like to do to see if
you're paying attention while cruising in the no wake
zone or just before you get to it - they submarine the
tubes. Doing this exposes the inner circle of the covered
tube to the water and the center section starts to fill
up with water, creating a huge drag on the tow vehicle.
When they do it, it usually stops my forward progress -
it's like a couple of parachutes have been opened, or
like a couple of anchors have been dropped. When I'm not
paying attention it can catch me off guard in such a way
that I almost tip over. Real funny to the kids. When I
see them doing it sometimes I'll gas it real quick to see
if I can knock them off their tube - catch them off
balance. Unfortunately there is so much drag against the
water that even when I pulled the throttle all the way in
it would prevent much forward movement. Kind of like a
stall in an airplane - the motors running but there's no
progress. I am happy to say that this situation has been
rectified. I know some kids who can attest to this by
telling of how much water they drank and how long they
coughed after I pulled the trigger last week when they
were doing their little antics. This boat now has enough
power to keep pulling the submarined tubes under water
until the tube passengers have to let go because of fear
of being deep under water - heh. I doubt this boat will
win any drag races against 1100's or 1200's but that
wasn't my intention with this project anyway - this was
what I call routine maintenance. It occurred to me that
my gas mileage seemed to be better as well - though I've
got no hard data about this. I had no trouble burning 40
gallons of fuel in the 5 days on the water. With
watercraft fuel mileage is a funny thing. The power
delivery is so inefficient (talk about slippage) it's a
wonder they do as well as they do. I'm sure my wallet
will let me know.
-
- Rick
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