Kawi 750 engine info

[Vumad]

Please post information you have on Kawi 750 engines here.

Specifically I am looking for information having to do with crank sizes and porting and etc.

Tom and I were discussing information on different 750 engines. The 750 engine was made for several years. There is a big pin and small pin version. There are different boats the engine was in (750 STS, 750 XI SS, 750 SX, 750 SXI, 750 SXI PRO). The topic of porting was brought up. Does the small pin engine from my sit down have the same porting (and expected perfomance) as the same year stand up? or are they ported differenty for different types of performance?

For example, I just found the following information about 750 porting. While it is not specific, it does address porting changes in the 750 engine. It does confirm that the change from small to big pin is not the only change to the stock form of the 750 engine.

Sure, the 750SX and SXi watercraft use the same 743cc Kawasaki engine. But that base engine is also in several models, including the STS, the 750ZXi, and the SS model runabouts. These engines are the same, but different.

To get more horsepower, the SXi uses twin 40mm carbs, connected to a revised intake manifold that uses a balance tube to equalize the carbs' efforts for more low end response. For better top speed, the cylinder porting has been changed, without raising the compression. And to handle all this new power, most engine parts were beefed up for durability.

As i understand it, all 750 engines '94 and older were small pin, which means they have a lighter crank and better bottom end performance. engines '95 and newer are big pin and generate more horsepower, but have a heavier crank to get it, so less bottom end than a small pin. but... how does the porting in our engines change from engine to engine? how does this affect our expected performance? I want bottom end!

Please post specific differences in the engines from year to year and model to model as you find it.

The changes to carbs and exhausts is not terribly relivant, because most of us change those out, but it is certainly useful information, and welcomed. I don't know if any of the 750 engines used different flywheels or electronics that would change performance.

This isn't really the proper place to talk about 650 electronics. The discussion is about changes to the 750 engine based on year and model.

PS. The porting changes mentioned are the same as when the cranks were changed. It is possible that the porting is identical from all models in all small pins and all models in all big pins. I'd like to know for sure... Please post what you know.

 

[DangerBoy]

Wow, where to begin
I"ll start with this.
First off, big pin / small pin has little to do with bottom end punch , the port timing , ignition timming and compression ratio is mostly responsible for that

2nd, same year different 750 cc models. had different port timing. be it exhaust port heights of 38mm 40 mm and 41 mm ect

I will try and look for my notes from years gone by (at least 10 years), and post more tomorrow

you didnt post the year of your stand up vs sit down but chances are the are different in at least the exhaust port height.

for bottom end we use cylinders with the lower (ie 40-41mm) exhaust ports

.

 

[Vumad]

Specifically, my engine is out of a 1994 750 XI SS, but I intended this thread to be very generalized to cover all 750's. I don't intend to take my engine apart any time soon, but some people might, and there could be a lot of knowledge to be shared.

 

[DangerBoy]

Ok going from a foggy memory:scratchchin: that motor has a lower exhaust port of 40 or 41 mm and will make better bottom end than say the 95 sxi stand up there was also a xir in that era with yet different port timming
Im sure I have my notes, but not here at home
Db

 

[tom21]

how come you don't want to know about 650 electrics? is because you don't have one? or because mine can be advanced with a simple screw adjustment, and my flywheel is lighter than yours? :nana:

nice thread btw! either way you will still need the same mods I would guess. and if when you tear into the motor you can address the port timing to get more bottom end if you did need it. but at least its a usefull thread. wish pete and the others could condense down all their knowledge and put it in a nice format. that right there is a lot of reading on two sites!!!!!

 

[Vumad]

how come you don't want to know about 650 electrics? is because you don't have one? or because mine can be advanced with a simple screw adjustment, and my flywheel is lighter than yours? :nana:

Because noone purchasing a stock 750 motor has to wonder which version of the 650 electronics are in it. 650 electronics is a modification that isn't terribly relivant to this thread. I'm after 750 stock specification, if we went into modified, it'd get crazy with total loss and lightened flywheel and other information that would detract from the goals of this thread.

However, I do get that you are teasing me. You're right, you 650 electronics have more potential than my 750 electronics, but at the end of the day.. my ski runs... :lmao:

 

[DangerBoy]

Ok I looked up your specific motor 94 xi and found out how bad my notes really are .. but
exhaust port height of 41mm (good for bottom end) but it had the retarded timming curve (CDI) and questionable stator your cdi curve if stock is
initial 13 deg
16 @ about 3000
13 @ about 6000
the 95 on Cdi is
13 initial
21 at about 3000
19 at about 6000

that is why some 750s ran great with the timing advance plate and others didnt
hope this helps

 

[Vumad]

Wow, great info. Where is that info from? Where can I find more information like that? I would love to see a complete list of all of this information so everyone can benefit from it.

Information like that on the 650 electronics would be awsome too, but should probably go in it's own thread, maybe?

Thanks!

 

[mikey23]

If you buy a MSD enhanser there is a paper in the box with total timing curve of your engine with MSD and with out msd

maby someone have one to post it here

greetings from the netherlands

 

[Crab]

This is why we like 650 electrics, easy to advance timing and you can use super light flywheels. A Dales CDI will have no retard feature, only has what ever timing you set it at. Back to porting, my small pin is raised about 2.5mm on the exhaust and widened, I run no base gasket and an SXI 28cc head for 170psi. The motor has a massive hit around 4200 rpm, most likely from a combination of porting and the superjet limited chamber. It pulls hard to 6950rpm with stock cdi, I have been meaning to try my 86 cdi but really no need since its doing 50mph now. And yes when you pull the hammer, its a rocket.

 

[dbrutherford]

Best info on the differences of the 750's is found here:

http://www.groupk.com/k750.htm

http://www.groupk.com/k750sxi.htm

http://www.groupk.com/k650.htm

http://www.groupk.com/k800.htm

http://www.groupk.com/k849.htm

As I have read all those articles I kept in mind that Group K will not sell you a ticking time bomb. They are on the conservative side of things.

Also I am not sold on the whole small pin better han big pin I frequently read on here. I am personally going to go for an 800 SXR engine for my X2. Why well #1 you can buy a crank from Hot Rods which saves a few hundred over Kawasaki's cost. Also it is already a big bire compared to a stock 750 engine. I believe their actual stock displacement is around 735 cc's? Plus if I use 800 electrics I can use an aftermarket ignition like MSD or Advent. Deep down I want ot run an MSD Total Loss ignition. A small pin does have one advantage... cheaper initial cost compared to a big pin newer engine.

 

[Vumad]

This entire discussion is fueled by costs. of course I'd rather run a 800 engine, but I'm not going to get one for $100 like my 750. I'm sure everyone is in the same boat.

 

[dbrutherford]

Yep, I am in the same boat $$$ wise. I had looked at buying 800 cases from Kawie $500ish, a Hot Rods crank$500ish, Pistons $300ish, ect... Not worth it for me!

So at this point, I am thinking just buy a whole used SXR then part out what I don't need.

No matter what 750engine you have, it is definetly a better power plant than a 650. Atleast there are more parts out there still manufactured.

 

[w-nut]

the cheapest way i found to build a 800 was to run a 800 cylinder on small pin cases and crank . that is the set up that i have here at the moment and it had great power

Yep, I am in the same boat $$$ wise. I had looked at buying 800 cases from Kawie $500ish, a Hot Rods crank$500ish, Pistons $300ish, ect... Not worth it for me!

So at this point, I am thinking just buy a whole used SXR then part out what I don't need.

No matter what 750engine you have, it is definetly a better power plant than a 650. Atleast there are more parts out there still manufactured.

 

[Vumad]

The "BIG PIN" 750 series - The 1995 and later 750 motors differ greatly from all the previous 750s in several important ways. 1) The wrist pin and big end of the connecting rods have been enlarged for better durability (hence the term "big pin"). This means that the pistons of the early and late engines are not interchangeable. 2) The inlet tracts, reed cages, and carburetors have been downsized slightly to increase inlet tract air speed. This increased airspeed results in much better overall acceleration. 3) The cylinder has much higher exhaust ports than any previous 750s. These larger ports work for these 750s because of the improved low end power offered by the smaller inlet tracts. While some of these parts are interchangeable with earlier 750 models, we don't recommend it. The SXi cylinder will yield poor results on an Xi and vise versa. The low exhaust port/large inlet tract, and high exhaust port/small inlet tract seem to be matched designs that don't like being mixed.

Reads to me like the big pin engines are better for bottom end. They have a heavier crank but better porting. The technology makes up for weight and size.

 

[Vumad]

~

EXHAUST SYSTEMS - While there are many aftermarket pipes available for the 750's, we consider only a few of them are a clear cut improvement over the stock pipe. The two best choices for the SX model are the "Factory Pipe Products" LIMITED pipe, and the "Coffman's Rocket series" pipe (both can use the stock waterbox). For the SS, ST, and Xi models, we recommend the Coffman rocket pipe with waterbox, or the Factory Pipe Products "half pipe". For the SX, SXi, and XiR we recommend only the Factory Pipe products pipe.

We recommend that a stock waterbox silencer be used on all the Sleeper and Hammer 92 kits. Contrary to rumor, the stock 750 waterboxes yield excellent overall power levels in all pump gas applications. Unlike most after market waterboxes, these stock waterboxes can offer quiet sound levels along with strong power when water is diverted away (water diverters are a part of every Group K 750 engine kit) The SX and SXi Hammer 105 should be fitted with a Jetcraft Eng. resonator style muffler. This muffler permits the engine to pull easily into the higher rpm ranges while maintaining tolerable increases in sound level.

 

[Vumad]

I wonder if the Mikuni carbs were arround at the time this was tested. Is the Mikuni better because it is easier to tune, or because it creates more power? I don't think group K cares which is easier to tune, as much as which creates the most power.

Edit: it does mention the mikuni carb, so Group K does believe the kehien performs better than the mikuni.

NLET SYSTEMS SINGLE CARB - It's our position that the inlet system that can provide the best inlet tract air speed and fuel mixing, at all throttle positions, is the system that will provide the best overall acceleration and horsepower. While the stock oem Kawasaki dual carb kits can work very well, our testing showed us that a recreational 750 with a well prepared single carb could easily generate as much speed, and acceleration, than most jet pumps can ever deliver. The few aftermarket duals that do offer better performance, do so by consuming "allot" more gasoline. Where riding range is a big consideration, a single carb intake is by far the best choice.

In the SLEEPER kits, the stock 40mm Keihin carb (it's actually 37.2mm) is bored to a true 40mm. (Note: The 38 mm carbs on the big pin motors are actually 33mm. We make them a true 38.) This throat boring is an extremely cost effective way to increase intake ability without compromising easy starting, smooth idling, easy adjustment or fuel consumption. For customers who want to install a larger single carb, the 42 Keihin carb, used on most single carb Group K Hammer 92 kits, offers better overall fuel mixing and throttle response than any large throat single carb we have tested. This 42mm carb bolts directly onto the stock 750 intake manifold and provides throttle response and acceleration that few dual carb setups will match. The single 42mm Keihin carb has no troublesome linkage, and it features easy "two screw" mixture tuning. Your stock flame arrestor matches perfectly to the opening of the 42, however the intake manifold must be bored slightly to accommodate the larger 42mm carb throat diameter. NOTE : Boyesen reed stuffers are recommended for all pre 1995 Group K 750 Kawasaki engine kits.

DUAL CARB INLETS - The stock Kawasaki dual Keihins perform as good or better than any aftermarket duals we have seen. Group K offers the same throat boring mod for these as the singles. These bored 40mm(and 38mm) Keihins can easily meet the fuel demands of the most heavily modified 750 racing engines. Our tests showed that perfectly tuned dual 44 or 46 Mikunis offered absolutely no advantage over the bored stockers.

 

[Vumad]

Change the electronics I guess

IGNITION - The stock rev limiter on all early 750's is set at about 7200 rpm. The stock "big pin" 750 ignition's limiter comes in at about 8000 rpm. We see no need to rev pump gas 750's beyond these rpms, nor have we seen a power increase by changing the ignition firing "curve". Given this, we recommend that the stock cdi box be retained for all pump gas 750's. For customers utilizing the stock ignition flywheel, Group K offers a flywheel lightening modification. While we only lighten it by 1.2 lb, the change is very noticeable because the majority of that weight comes off the outer perimeter of the flywheel where it hurts the inertia the most.

105 octane customers may be interested in a total loss type system. The aftermarket is full of affordable (?) total loss ignition systems that have no limiter. Because of the constant battery maintenance and questionable reliability of these units, we do not recommend them for any recreational or amateur level racing application.

For 105 octane customers who want the absolute best in performance and reliability, we only recommend the Kawasaki 650 flywheel and stator plate used in conjunction with the Dale's Jetsports "bad attitude" brain box/coil. This conversion offers full charging capabilities along with a much lighter flywheel, increased voltage, and no rev limiter. The increase in acceleration and peak waterspeed is big ... very big. The 750 cases do require machining in order to accept the 650 coil plate. The crank cases must be completely empty for this machining process. However for the ultimate high output motor, this trouble-free ignition is easily worth the hassle of a complete engine teardown.

 

[dbrutherford]

What you have to keep in mind about Group K is they sell machine work. They don't sell a lot of parts. But Harry K was keen to sell you a ground out intake manifold and a Keihin. I think though if you follow his recommendations, you will have a more powerful than stock ski that is still pretty reliable.

 

[Vumad]

What you have to keep in mind about Group K is they sell machine work. They don't sell a lot of parts. But Harry K was keen to sell you a ground out intake manifold and a Keihin. I think though if you follow his recommendations, you will have a more powerful than stock ski that is still pretty reliable.

So, are the mikuni's better or do people just like their ease of tunability and cheaper parts price? It's like the debate of the 750 pump conversion. being able to change props easier isn't a selling point for me. I'll do a 750 conversion when someone can back the performance.

I guess where I'm going with this is the carb sizes. I've been put under the impression that dual 44's or dual 46's mikunis is the way to go on the 750. Is the power return really much better? Or is having 15 pounds less fuel in the boat better (because your larger carb is just wasting gas)? Group K seems to suggest dual carbs are a waste. So what's the truth? Or is everything I've been told based on someone saying "that seems powerful" and no actual testing.

My 750 came with dual 40 keihin's. I only took them off because I couldn't tune them intime for daytona. I'm not sure if I should tune the stock or stick with the aftermarket when I go to my B-pipe.