Custom/Hybrid Melmack's very unique 300sx build

Melmack · Feb 16, 2011

OK, I've looked into everyone's take on how to build the best 300sx. I've studied the BOB, WFO's project, and many others. I recently purchased a bone stock, nearly mint 300sx. My first impression when riding it was "what a turd!" Of coarse I'm pushing 190+ pounds. So I didn't expect it to be quick. But, once it got on plane, this was by far the funnest feeling hull I have ever ridden. I have had just about every OEM ski at some point and a couple aftermarket hulls so I know my skis. This thing is right at the top.

I managed to barrel roll it several times off a boat wake but it couldn't pull me out. Of coarse, this ski has no power at all. To top it off, it has a lousy 440 pump designed for high rpm.

What the BOB guys did was great, what WFO did was pretty cool too. What I wanted was something a little different. I wanted a ski that can do a little of everything from surf to flat water to jumping wakes and hood tricks. What I didn't want is to drop a big heavy engine into this thing making it no different than an FX-1 or a 550. This hull has tons of potential but some severe limitations as well.

First there is the issue of power. I have read a lot about the 300 motor and wondered what kind of power it can actually produce. The 300 produces 30hp. Compared to a 440 that has 27hp and a 550 read valve makes 41hp. My old 550sx actually produced a ton of power with only bolt on mods and was a lot of fun to ride. Not to mention that ski was almost 35lbs heavier. Also, the 550 hull has much less lift than the wide and super light 300sx has at only 210lbs.

The porting and compression are nothing special so there seems to be room for gains there. The Carb is also very small and the old round body type.
My goal is to get 45-50 horsepower. If this takes porting, high compression and high octane fuel, so be it. This thing sips gas so I have no problem paying a little extra for some 100 octane avgas. So far I found a really nice Kerker pipe, some Boysen reeds and have a 38mm square body carb laying around.

The next issue is the pump. I can make hp all day but with that 440 pump it will never pull me out of the water like I need it too. I tried the 550 swap but unfortunately, the 300sx has a specific drive shaft and mid shaft housing that doesn't swap like the old 550-440 swap.

So, It was time to think out of the box. The last thing I wanted to take on was another FX-1 style 144 conversion. I did that 2 years ago and while it turned out great, it was a nightmare. But that's just where I got a bright idea. I basically had a worthless but mint condition FX-1 pump laying around from the previous conversion. With a little mock up, I found it was quite simple to bolt up the FX-1 pump and shoe to the existing pump cavity. There would be some obstacles but I had tons of parts laying around and was completely confident I could make it happen.

So, I have completed much of the project to this point. In the process I have had to fab up some parts, destroy others, and do a lot of measuring. The drive line is complete. What's left now is the motor.

I will be posting all my picks up soon. I took pics all along the way. I will be working on the motor this week and hopefully have it water tested by this weekend.

SuziQ · Feb 16, 2011

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ontGrowAutofit/> </w:Compatibility> <w:BrowserLevel>MicrosoftInternetExplorer4</w:BrowserLevel> </w:WordDocument> </xml><![endif]--><style>  </style> Glad you are doing this, and extremely interested in everything I read, less the hood tricks notion. I have desired to test a modified 300 for a long time now, and the thought is really the only thing that keeps me from going ahead and moving on a FX-1. I hope you post some ride video when completed. Please elaborate on this statement, “It is the funnest feeling hull I have ever ridden”. Is it nimble/squirrely like the FX-1? That is what I like about them.

Melmack · Feb 16, 2011

Actually, it's like a tiny, super light SN super jet. Its extremely stable, it carves relatively well, and it rides pretty flat. It also flicks around with ease if you want it too like an FX-1. I had no problem with it porpoising and it didn't seem to want to buck me like a 550 or my FX-1s. That was the one thing I hated about the FX. One thing is that that bucking usually happens at around 45mph on those other hulls. This ski will likely never reach 45mph. If it could go that fast, It doesn't feel as if it would.

I'm going to have to get every inch of power out of this thing using timing and low pitch. That will severely limit speed. But it will be fun. I will have it at LSFR for sure. I will get some footage once we water test it and tune it this weekend. I had to have a special prop made so I have no idea if the pitch is even close. I have the bigger carb, the pipe and the reeds but the head is still stock and I'm waiting to port it. I'm not sure what to expect this first ride. But I can't wait anyway. I'll post up afterwords with the results. Should be riding this weekend.

SuziQ · Feb 16, 2011

unctuationKerning/> <w:ValidateAgainstSchemas/> <w:SaveIfXMLInvalid>false</w:SaveIfXMLInvalid> <w:IgnoreMixedContent>false</w:IgnoreMixedContent> <w:AlwaysShowPlaceholderText>false</w:AlwaysShowPlaceholderText> <w:Compatibility> <w:BreakWrappedTables/> <w:SnapToGridInCell/> <w:WrapTextWithPunct/> <w:UseAsianBreakRules/> <w

ontGrowAutofit/> </w:Compatibility> <w:BrowserLevel>MicrosoftInternetExplorer4</w:BrowserLevel> </w:WordDocument> </xml><![endif]--><style>  </style> Thanks for your thoughts. I guess it is not what I am personally looking for then, but it does seem like a very interesting project indeed. I wouldn’t think you could expect to get everything exactly how you want it first try, but maybe you will get lucky (wouldn’t that be nice, lol)!! I look forward to your video, and hope you have a really fun time!

Melmack · Feb 16, 2011

Dony get me wrong, the ski is extreemley nimble like the FX-1. Its just less likely to throw you off suddenly for no reason.

Liquid Graffix · Feb 16, 2011

Subscribed.

SuziQ · Feb 16, 2011

unctuationKerning/> <w:ValidateAgainstSchemas/> <w:SaveIfXMLInvalid>false</w:SaveIfXMLInvalid> <w:IgnoreMixedContent>false</w:IgnoreMixedContent> <w:AlwaysShowPlaceholderText>false</w:AlwaysShowPlaceholderText> <w:Compatibility> <w:BreakWrappedTables/> <w:SnapToGridInCell/> <w:WrapTextWithPunct/> <w:UseAsianBreakRules/> <w

ontGrowAutofit/> </w:Compatibility> <w:BrowserLevel>MicrosoftInternetExplorer4</w:BrowserLevel> </w:WordDocument> </xml><![endif]--><style>  </style> Thanks! It would be nice if I could make it to the meet, but I can not. I weigh less than half of what you mentioned, so all skis ride quite differently with that variable alone of course. What is your estimate of the ski weight upon completion, compared to the average weight of the FX-1 with the known standard mods please? If not too much trouble, please post a link to the video within this thread, or shoot it to me in a pm.

Melmack · Feb 16, 2011

I imagine it will weigh about the same. Stock it is 220lbs. The pump is about the same, the modification I had to do to the mid shaft is slightly heavier. So I guess it will weigh about 222 when complete plus gas. The 300sx was the smallest ski ever made. Hopefully with a little power, it will be one of the best.

Here's the kicker that I haven't mentioned yet. I am working on something really big. Not big in size but what I have is going to change Stand ups and possibly PWCs forever. Since the wise old EPA has officialy killed off the 2 stroke, I have come up with an EPA friendly, high power to weight solution. I have shared this idea with very few. It has been tried before and worked but never caught on. This time, I think I have the perfect application for this unusual motor type. All I can say for now is that it is a four stroke, it weighs much less than a 701 and currently makes 50hp with potential for more, lots more. This hull is the perfect vehicle to try this in. Being so light, this ski with 50hp should absolutely rip. It won't have BOB like properties but it is a ski that could be manufactured in the USA and still be profitable. Not that I'm out to make millions but to stay in business I have to make a little something.

Anyway, I am working with an engine manufacturer. They are beginning to produce several engines for other applications. Once they get me my prototype, I plan on bolting it up in this ski for a trial run. If all goes well, I will be looking for a hull builder and a few assemblers to help make this happen. Cross your fingers.

chadetroit · Feb 16, 2011

Sounds interesting, keep us all posted. Always great to hear individual people innovating to move the sport forward, Good Luck!

special fx · Feb 16, 2011

Cool project, with a very different direction. I believe the FX pump and 440/550 pump are awful similar though? FX=123mm, 440 = 122mm. If I remember (way back in the day) one of the issues with working the 300 engine was vibration. Most of the JS300 hulls I played with had extensive cracking in the motor mount area webing in the hull due to the vibes.

McDog · Feb 16, 2011

Sounds cool.

Melmack · Feb 16, 2011

special fx said:
Cool project, with a very different direction. I believe the FX pump and 440/550 pump are awful similar though? FX=123mm, 440 = 122mm. If I remember (way back in the day) one of the issues with working the 300 engine was vibration. Most of the JS300 hulls I played with had extensive cracking in the motor mount area webing in the hull due to the vibes.

Thanks for your input and your interest guys!

Vibration could be an issue. That's why I'm not lightening the flywheel. Keep in mind, this is only a prototype. So, if the hull cracks, we'll just make the production hull stronger in that area. Plus, the motor we are planing 0n utilizing will have very little vibration if any.

As far as the pump goes, the size of the 440 and FX-1 pumps are almost identical. However, the hub size on the 440 is much smaller than the hub on the FX-1 pump and designed for a much higher rpm. It was designed for speed. The key here is that The fx-1 pump bolted up easily and I know what it can do. Also, it is small enough that the tiny motor should be able to push the prop. A 144 pump would be too big for this low hp motor.

So, the FX-1 pump can handle up to around 80hp before cavitation is a major issue. The 440 pump would handle that hp but would have terrible bottom end.

I'm not planning on achieving 80hp with this 300 engine but, once you see the engine we are working on, you will understand our need for a pump that can handle a wide range of HP. Ideally, the production ski would be fitted wit ha FX-1 style pump but have the ability to be converted to a larger pump for serious back flip performance. It would come with 50 and 80hp configurations with the ability to add another 30+hp relatively easy but would require a larger pump.

My experience with the FX-1 was that it hooked up fine as long as it was relatively stock. It was very responsive and could do everything I told it to do. That ski weighed 260lbs though. This ski is 40lbs lighter. Imagine a 300sx with FX-1 power but stock weight and a pump that produces better bottom end response.

Melmack · Feb 16, 2011

Spoke with my engine manufacturer today. The smaller engine is closer to being a reality. There is a possibility of a 40lb motor with 60hp. That's basically a single carb 701 that weighs 50lbs less. The larger engine may be a while.

As for the actual ski, I got the engine and pipe all cleaned up and back in the hull today. The ski is ready to go as soon as I pick up a carb kit. Also, I noticed one of the motor mounts is damaged. I will be looking around for one as these are impossible to find aftermarket. If anyone has a good 300sx motor mount, please let me know.

WFO Speedracer · Feb 16, 2011

I have a good engine mount, pay the shipping and its yours. On the 300 engine here is what you want to do, pull the head, keep the stock gasket as you will need it for a pattern. Now buy a stock 650sx head gasket. laying the stock gasket on top of the 650SX gasket you will see several cooling holes that need to be enlarged, mark the profile of the gasket onto the 650SX gasket with a magic marker then cut the gasket out with a pair of heavy duty shop scissors. Install the new gasket in the correct orientation and torque the head down. Congrats you have just increased the compression, now lets move to the carb, the carb you want is the stock carb off of a WR500 91 or 92 model, the jetting is dead on the money and it is a direct bolt on.Top it off with a K&N adaptor and a fly or Tau Ceti flame arrestor.One more item to be on the lookout for is an intake manifold that has the boost bottle attachment on it, I found one on Ebay after a little bit of searching.

Melmack · Feb 16, 2011

Thanks for the input WFO. Mine already has the boost bottle. What the heck dose that do? I was going to block it off. Maybe I shouldn't? You told me about the gasket before. I may do that before I have Group K modify the porting and head. They actually redesign the head dome for $40. Not a bad deal. Group K also recommended the 34mm carb off the 500. The problem is that it's only slightly larger than the stock carb. Many motors with 650-700cc have dual 38 or even larger. With the big Kerker pipe I have I want to try to get all the power I can. I took a carb off a blown 1100 yamaha. Its a 38 and jetted for a 366cc cylinder. Jetting should be close but we'll see. If that fails, I'll do the 500 carb.

I'm going to hit you up for that mount. Sending PM shortly. Thank you.

Liquid Graffix · Feb 16, 2011

So are you running a group K 300 motor?

Melmack · Feb 16, 2011

Not yet. Right now it's all bolt ons. 38mm carb, Pipe and dual stage reeds. We'll see how much that helps this weekend. After LSFR, I'll be sending the head and cylinder yo Group K to see what they can do.

Soon as thease photos upload, I'll post some picks of what I've done to make this ski hook up better.

Melmack · Feb 16, 2011

Here are the three most critical parts of this conversion. Thanks to the aftermarket kawi style coupler, I can now add virtually any motor I like with the right adapter plate.

The prop is a 15/18ss prop from a VXR pro. I sent the prop to Impros who then cut it down to the FX-1 diameter and balanced it. This is much lower than the pitch of any FX-1 prop available.

The hub is slightly smaller than the FX stator so I had to modify it a bit. The 144 pumps have this same taper but a slightly smaller hub. If this prop dose not work out well, I'm planning on buying a FX-1 prop and having it re pitched and set back as close to the veins as possible.

The mid shaft housing is simply the guts of a torn Yamaha mid shaft glued into an aluminum tube and then welded to angle aluminum. The key here was the shimming process. I installed a gimble fitting on top for sevicing the seals but decided to put a set screw in its place for good measure. I can always screw in the fitting again, Pump some grease, then replace the screw.

The one missing photo is of the pump shoe. The shoe is a modified FX-1 shoe. I ground it out to fit in the cavity. I'll try to find that pick soon.

WFO Speedracer · Feb 16, 2011

Group k and I believe Advanced Techniques are about the only one I know about that ever hopped up 300 engines. I am not sure the latter is still in business.

Melmack · Feb 16, 2011

I've never heard of advanced but I have spoken with Group K and am comfortable with their work. Hopefully they remember what to do. I'm not sure that they've had many 300s to mod. LOL

Custom/Hybrid Melmack's very unique 300sx build

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