The Thread Spread

-Water Testing-

Flat Water = Fast Facts
The Question - Is there any more to this Yamaha watercraft related thread?

The Response - Yes!

I enjoyed reading your letter about the GP760 project and your adventures at the lake that day. It sound as if you and your daughter had a great time. Since I emailed you last, I have put the motor back in the trued Raider 1100 hull. That took quite a bit of time with a 4" by 4" block of wood and some sheet rock paper. I now have about 6 hours of riding time on new motor. I did much like you when it comes breaking it in. I did not baby it nor did I hold it WOT. I also mixed the oil (Klotz synthetic) to about 20:1.

My first ride was with some friends who own 98 SeaDoo XP's which have factory pipes and K&N flame arrestors. We decided to ride to the sandbar (a popular place for boats and jet skis) which is about 20 miles or the opposite end of the lake. The water was extremely rough as you might expect for a weekend. Anyway, on the return trip we decided to pick up the pace a little - which to these guys means full throttle. I was able to run side by side with just a little over 1/3 throttle and pull away at 1/2 throttle.
The next weekend and several tanks of gas later, I rode with another friend who has a 98 GP1200. We did exactly the same ride as the weekend before only this time the weather was terrible and cold. The wind was blowing about 20 MPH and had the lake transformed into an ocean, but not a boat in site. I was getting soaked and a little cold, therefor I decided to make the ride time a little shorter. I let the engine run at it's natural cruise speed or just when the main jets open (about 1/4 throttle). When we got to the main channel there were 3 foot tall long rolling swells. Much different form the usual boat chop. I had the timing down perfect, hit the gas in the bottom of the swell and let of when the boat goes air born. By the time I reached the sandbar I was exhausted. My friend was about a mile back. He later told me that he was trying to keep his ski at about 45 mph. I figure the natural cruise speed is about 51 to 54 mph.
Later that day we went back in a long cove and did some drag racing. This is the first time I have tried this with the Group K modified Solas J impeller. As soon as we both open up the throttle, I can jump about 2 boat lengths in the first 75 feet or so. From that point on it is a steady pull away. I have also noticed, now there is enough power to cavitate the impeller for about 1/2 sec. as soon as you hit the gas. It never did that before. I figure the top speed is around 63.5 to 64 mph.
The next weekend I was feeling pretty good about all my work which I had done over the winter. I decided to mount the tiny tach and go get some peak rpm numbers. Last year I was getting a peak rpm of about 6730 with an air temp of about 70 deg F. The air temp on this day was about 92 deg F with high humidity. I did numerous runs on glassy smooth water. I am using Ocean Pro flame arrestors with about 2" tall elements. They have been taper bored while bolted on to the carbs therefor they match up very well. With the flame arrestors on I was only able to reach 6680 to 6690. It did not seem to make any difference where I adjusted the high speed screw, I began thinking that there was enough signal to get the high speed screws to respond. I then taped off about half of the flame arrestor with duct tape in order to increase the vacuum in the inlet tract. The RPM then dropped to 6650 with still no effect from the high speed screws. Completely frustrated, I decided to take the elements completely off. The RPM then jumped to 6710 although adjusting the high speed screws still made little difference. Last year I was running 135 mains 2.0 needles with 16 lb. of pop-off and 90 pilots. This year I have been using 145 mains 2.0 needles with 16 lb. of pop-off and 90 pilots. There seems to be no hesitation or sputtering when the throttle is cracked open from a long 1/2 throttle run. I think I am in the ball park when it come to the jetting though maybe a little rich on the mains.
I feel much like you did with the GP760 project. I know that I have increased the bottom end and midrange but I am not so sure about the top. My goal was to get the engine to turn 6850 to 6900. I am not sure what to do from here. I would like to try some 44's (carburetors) with K&N filters. Maybe that's the trick but I would hate to spend the cash only to see a 30 or 40 rpm difference. Group K says that their Big Bore 1100 with Type 2 Porting turned several hundred more with the 44's but used lots of gas.
HUUUMMMM for whatever that's worth lots of contradictions in their articles. Rick, let me know what you think.
It sounds like you're close to where you want to be but I agree you, the engine could be turning a bit higher. Do you know the point where the rev limiter shuts things down on that engine? I don't suppose Group K use some ignition mods to achieve those revs do they? It may be you were losing the revs due to the air quality only and that testing another day with better air/water would get them back. Air temperature, water temperature, altitude, RAD (relative air density) and the condition of the water affect your final outcome. Testing on a perfect day will be tougher to achieve, but I'll bet your numbers will improve. What were your final numbers for the compression, compression ratio and port timing anyway?
I'd consider the 44 mm carburetors a waste of money to get the extra revs you're looking for - and I doubt the addition of them alone will get you the almost 2000 rpm's you're looking for. It could be that you're just a bit over propped. Since you are experiencing a cavitation problem from take off (not a good thing) you may want to look into that as a major source of your peak water speed problems as well.
One of the nice things things about the 1100 triple is that it can achieve high on water speeds with what seems like little effort. I am amazed by the power produced by that engine - especially the midrange hit. It's all midrange - even when revving out it does so with a sort of lazy attitude. And that big engine in the light, flat Raider hull makes it even more noticeable. The Raider may be the best high speed hull ever made for a runabout watercraft. On glassy water there are few, if any boats that can match the high speed of the modified big engined Raider.
I have also had the opportunity to try out the higher displacement version (1200) of that engine, the power delivery is nothing less than extraordinary - even when pushing the additional weight associated with the XL1200. I am also impressed by the on water manners of the big XL. In rough water it inspires confidence when the Raider would have its driver begging for smoother water - and maybe some Pepto-Bismol. I have wondered if I'm the only person who has literally blown lunch while riding a Raider in heavy chop too soon after eating. Try to remember that the day the engineers had the first test Raider hull on the water there was no chop - so it made its way into production...<jk>.


Thanks for your suggestions. I just got back from the lake and may have arrived upon a few conclusions. First I think you are probably right about the 44 mm carburetors.
I have a good friend who owns a high performance jet ski shop close to my house. He has an 1100 Raider that was ported by Riva Yamaha. At the time the porting was done Riva did not offer any type of recreational porting, only full blown race porting. This ski has 44 mm Boswell carbs. In case your not familiar with Boswell, they were very popular a couple of years ago and were very expensive. It also has a Riva head with the compression set at 180 (static). I had a chance to measure the Riva ported cylinders before this engine was put back together. The results are listed below.
My Motor

Exhaust port (from cylinder deck - mm)

35.5 mm
35.5 mm

Exhaust duration (degrees)

191 degrees
191 degrees

Transfers (from cylinder deck - mm)

52 mm
53.5 mm

Transfer port duration (degrees)


Static compression



Boswell 44 mm venturi
Stock taper bore 38 mm's
After warming both skis up, we ran side by side across 4" chop. My boat was about 1 mph faster on the top end. Next we went into a cove and started from a dead stop. After about 4 seconds I was about 75 feet in front. Oh well, so much for the bigger carbs. We played around for about 20 minutes when a guy with one of those brand new Kawasaki Ultra 150's came up and started talking. Did not get a chance to run the guy from a dead stop however we did run wide open across some smooth water. This is the part that made all of my work over the winter seem worth it all. My Raider was about 1 to 2 mph faster.
As far as getting the rpm's I'm looking for, Group K said they could repitch my Solas J one more time. All I have to pay is the cost for shipping. They said that "every boat is a little different as far as getting the right pitch".
I also agree with you about the weather thing. I have noticed 40 to 100 rpm's more on a cool day with low humidity. By the way the stock rev limiter is set at 7050 and Group K does not modify the stock CDI unit. If I use the stock impeller, I am bumping the limiter on smooth water. I have a feeling that I am very close to reaching the limit of this motor especially without the help of a different pipe.
Also you're right about the 1200 motor - it has an unbelievable amount of mid range pull. Sure would like to try that motor in one of those 320 lb. Purple Hull Raiders. Wow talk about a screaming machine.
When I wrote that last letter I was thinking you were 2000 rpm's, not 200 rpm's away from your target. Once I reread the part about the rpm's I realized you were much closer to where you want to be than I was thinking. I think the bigger carbs may well get you the extra revs (200) your looking for but the cost associated with buying them is enormous and probably not worth it.
I'm very surprised that your friends Riva Raider isn't faster. With the higher compression and bigger carbs it makes me think it should be. Is there a difference in the way the two boats are propped? The compression that boat is running is on the high side for a 1100 cc triple. He may be losing some upper revs do to too much compression - I wonder what rpm his motor pulls at its peak.
Thanks for including the specs of his engine - I'll add it to my database of specs for ported motors. Having the prop repitched again will probably allow you to hit your target. Now that Klemm knows where your revs are, what you did to your engine and what the result was he should be able to set the blades just right.
I used to like to race Bill's Raider against other boats which were supposed to be faster than it. It's 700 cc Riva mill is a rocket! I like the revs it turned and the sound that came from the engine compartment. He and I raced (I was on my stock GP) at lake Havasu last summer and the smaller engined Raider easily pulled away from me - did I say EASILY with enough emphasis ?!? Unfortunately since he bought a XL1200 last fall and a new (dare I say it) Sea-Doo GSX Limited (I think that's the model) [nope - wrong model, see below - Rick] this spring both of his Raiders either have been or are about to be sold. The best thing about the big Yamaha triples is their ability to cruise at high speeds with little effort from the engine. The gas tank seems to need refueling more often though - for some reason ;)
Since the last time emailed you, I have gotten about 50 or 60 extra RPM out of the motor. I am now at the same rpm as I was last summer when the air temp and water temp were much cooler. That's good because when the temp does start to drop I will be about 50 to 80 rpm's more or some where around 6800.
I gained the extra RPM by doing a couple different things. First I have two water bypass fitting coming out of the side of the boat. Each hole was supposed to be 6 mm according to Protec's dual cooling line specs. One of the holes was only 4 mm so I drilled it out to 6 mm. This should have dried the pipe out some at peak RPM. Next I raised the front of the ride plate 5 mm. My hopes were to raise the bow up some, thus reducing the wetted contact area at top speed. I knew that 5 mm would probably be excessive, but I wanted to see what kind of effect, if any, it would have. The boat seemed very fast on top end but was very hard to stop from bouncing. I then took 2 mm away from the front which made it 3 mm. That seemed to be the happy medium. A little bounce, but still a noticeable difference difference in bow height.
I have also modified the stock ride plate - it is cut off flush with the back of the hull, and the six large mounting holes have been filled with epoxy and sanded smooth. I then drilled out the mounting holes only large enough to accommodate a stainless Allen screw.(about 7 mm dia.) I am not sure if this actually helped or not but I figured it couldn't hurt. I did not mention, but the ride plated was modified earlier in the winter, so it was not responsible for the extra revs.




It has been sometime since I wrote last. I think at that point I was getting ready to Radar my 1100 and give you the results. We did this about 2 months ago when the outside temperature was around 95 F deg. It radared at 64.8 to 65.2 with a couple of peaks at around 66. The max rpm at that time was 6690 with the Solas J prop. Now that the weather has turned much cooler it is turning somewhere around 6750 to 6790. I am not sure how fast that is but I would say, it should be a solid 65.5 mph.
My friend is working on a GP 1200 and has put a set of Buckshot 44's carbs on a motor that he has just finished porting. I'll let you know how that turns out. Anyway we are getting ready to remove my modified 38 mm carbs and put on a 44 mm intake manifold using his stock 44 mm GP 1200 carbs. The only thing that has been done to them is removal of the choke plates. We are not exactly sure what to expect, but we are hoping it will turn about 150 to 200 more RPM on top end. If it does it should make this boat run about 67 or 68 at about 6950 rpm. I will probably jet the 44 mm carbs a little richer than a stock GP 1200, since it is very cool outside and the fact that my motor has been ported.
P.S. - I'll let you know what happens.


I've got so much to write and no time to write it! I buried in work because I started taking in more outside engine builds. In addition to that we're heavily involved in youth soccer. One of these days I'll write about my trip to New Melones Lake with my GP760 - there were a few surprises.
It sounds like you're right on target with what you wanted to accomplish. That's fast - and it seems even faster on a Raider hull! I think you'll notice the carbs will "hit hard" in the midrange and offer some revs on top. The decrease in fuel mileage will be noticed.
This summer, like the past few I've been fortunate enough to be able to spend a week aboard a house boat on some of California's greatest watercraft hot spots. This year we spent the time aboard the big floating home on the water of New Melones (ma-lone-eez) lake - it has about 100 miles of shoreline and is a wonderful place to spend a watercraft vacation. One of the nice things about the house boat we were on this year was that it was equipped with a refueling station at the rear of the boat. Gassing up was a piece of cake because we wouldn't have to drive a long distance (and burn considerable fuel) just to replenish the thirsty fuel tanks. This boat was also equipped with an electric watercraft ramp. At the end of the day when riding was done or when performing service we could drive our machines onto the ramp then use the winch to raise the machines completely out of the water - classy.
Bill's new XL1200 was going with him as would his even newer Sea-Doo XP Limited. Both of these machines have been fully "run in" at our local lake (San Antonio) so there would be no babying any of them during this trip. I was bringing my freshened up GP760.
Before going on this trip I searched around a little bit to discover some information about the boats that would be with us - I wanted to know the power to weight ratio of each model so I could compare them. I found out that my GP760 had a ratio of 5.22 lb. per 1 hp in stock form (470 lb. and 90 hp). In its newly modified form the ratio changed to 4.60 lb. per 1 hp (470 lb. and 102 hp - calculated using two stroke design software) - a decent improvement. The XL1200 has a ratio of 4.51 lb. per hp (609 lb. and 135 hp) and the XP Limited has a ratio of 4.32 lb. per hp (550 lb. and 130 hp). The numbers indicate the XP Limited has an advantage over the other two machines in this category. I knew what to expect from my GP since I had already established its power delivery has been improved after the modifications I performed to it earlier in the year. I wished Bill still had his Riva Yamaha Wave Raider 700 so I could use it to make a benchmark comparison against. It was gone and its replacement (the XL 1200) was no slouch in the speed or handling department. I knew very little about the XP Limited - but after looking at the numbers it wouldn't be slow.
During one of our "site seeing" tours of the lake the three of us (Jason aboard the XL1200, Bill on the XP Limited and me on my GP) found a large inlet where the water was relatively smooth. Smooth enough to do some preliminary (ahem) testing. There was about 4 to 5 inches of chop in this large cove (more like a branch of the lake). It was there that the personality of these three machines was discovered and directly compared for the first time.To start things off Jason and I ran against each other. You can imagine my surprise when I raced against that 1200 and faired very well - beating it in fact by a bit. During repeated attempts I could consistently pull the large triple by 3 to 4 boat lengths out of the hole. Remember, the power to weight of these two machines was now very close (4.60:1 for my modified GP and 4.51:1 for his XL). Through the middle of the power range and even on the top end the XL1200 was not able to recover from its starting line deficit. After a short time Bill joined in on the post-adolescent fun (for he and I anyway). Armed with a new XP Limited he had plenty of machine between him and the liquid surface. The three of us lined up to race. Though most of my attention was being spent on my driving experience (I didn't want to screw up) I gave a small peripheral glance and noticed both of the other machines were playing catch up against my torquey little GP. When my 760 was topped out and running just below the rev limiter (it could go no faster) Bill went by the two of us and began steadily pulling away. The race between the 1200 and 760 was too close to call, but it was clear that Bill's ride was faster on top - by at least a few mph. After several more runs which turned in the same results we found ourselves at the end of our smooth/clear water and onto the more mainstream and much choppier (about 8 to 10 inches of chop) water.
On that kind of water the outcome was definitely different. Even though I could still pull ahead of both of the other boats from a standing start, the XL1200 was able to pull ahead of my GP760 through the middle of the speed range and keep it up through the top speed and slowly keep pulling away. The Sea-Doo also showed a better personality in the cut up chop by pulling away with even more ease and enthusiasm - it could easily walk away from either of us on top. There was no way my GP could make up the ground (water) it lost through the rough stuff because the choppy top had it sucking air instead of throwing water. It became clear that the other boats with their larger engines and bigger pumps could recover from cavitation much more quickly. They could regain acceleration sooner if the pumps lost their prime in that kind of rough water condition.
During our week long stay the time came when we found a cove with water that was very, very smooth. We had to do some more testing (ahem). Once again we found ourselves acting like little kids with new toys. The area was smaller for that test (ahem) but the water was very smooth and flat. Jason and I started the racing. Time after time I could get a great holeshot on him and keep the gained distance through the entire speed range - he could not catch up. In fact I was very slowly pulling away from him. The 1200 just could not win against the 760 on that kind of water. Even when racing for the longest distance the area would allow the outcome was the same. When Bill entered his XP into the mix it become even more fun and interesting. Once again the 760 easily pulled the other boats from a standing start - by about 3 to 4 boat lengths. It would lose all the advantage to the Sea-Doo once its smaller engine was out of RPM's, power and/or impeller. The 951 cc twin of the XP could put its power to the water very well - especially up on top. It let that boat show its style by outdistancing the competition in (almost) humiliating fashion. It must have a 4 to 6 mph advantage on top.
Late one afternoon after towing the kids around on their tubes and wake boards I looked across the water in the cove where we were parked and noticed an unfamiliar sight. Glass. The water was as flat as water can get. Barely a ripple in sight. It's shiny surface showed a near perfect reflection of the surrounding land. I climbed aboard my GP and headed out to make some speed runs. Within a couple of minutes the on board speedometer registered the highest reading it has ever showed me. I briefly saw 64 MPH before it settled back to a solid and unwavering 63 mph. I was able to duplicate that speed reading on several attempts. I do not have a radar gun so comparisons come in handy. In an issue of PWI magazine I noticed a test of the 1997 GP760 with and without a Riva Yamaha RPM Engine kit (September 1999 issue). Their baseline testing showed the GP760 to peak at an average of 50.57 MPH @ 6800 rpm (stock - without the RPM kit). After installation of the kit its peak speed increased to an average of 53.98 MPH @ 7100 rpm (an increase of 300 rpm ). I checked the Riva web site and noted the price of $1222 for that engine kit. PWI magazine also lists the stock 1998 XL1200 as being able to achieve a speed of 54.47 mph at its peak according to their radar gun and the top speed of the stock Sea-Doo XP Limited (1998 test) averaged 62.63 mph.

I did the math. It seems possible (if not certain) that the modifications I performed may have produced more favorable results than the $1200+ Riva bolt ons. Judging by the numbers, I may have gained about 1 MPH over the Riva equipped GP Wave Runner. The Riva Yamaha RPM Engine kit consists of a new tuned exhaust pipe, a Pro Series billet head, carburetor adapters, flame arrestors and a jetting kit. I find it interesting that the fuel octane requirement for their engine kit is 93 and the static compression was 175 psi. I'm running virtually the same compression (175 up front and 170 in back) and 89 octane is the minimum octane requirement according to my engine building software resources. I suppose the pricey tuned pipe could account for the increased octane requirement.

The results were in and I could live with them. It became clear that the overall power was increased (and perhaps the top speed as well as evidenced by going as fast as the XL1200) after performing the engine modifications to my GP760. In stock form I doubt its 5.22:1 power to weight (weight to power) ratio would have been able to stay ahead of or even with the 1131 cc mill of the XL1200. After riding my GP760 this summer it is clear that the octane I am using is fine and the modifications I chose to perform were very inexpensive and extremely productive. I'm glad I had the opportunity to race against such premium competition while vacationing. It felt good to know that under certain circumstances the little GP could definitely strut its stuff.


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