Building a C Mod motor

[Hydro doc]

We have participated in the C mod class for a number of years. I thought we should create a form on your thoughts on how to build a motor and what boats seem to work. Some have used a 102 to success while most use 302. The biggest out ward appearing difference between a stock Yamato is the exhaust system allowed in the Mod class. The stock system works well in the lower speeds, but mod motors increases measuability as the motor turns more RPM. The mod lower unit has a better water pick up which allows the higher positioning of the gear foot. Most units are modified by adding a larger sag and a more streamline front. Most units have a pick up hole in the lower part of the redesigned nose. Without a mod lower unit the mod motor can not reach its potential higher rpm.
A typical exhaust system has a 1/2 or 3/4 plate that directs the two cylinder into either a cast manifold or steel pipes that brings the flow into one pipe. That is the two most popular, however the first mod motor we pruchased had two In Depend manifold and two tuned pipes. While it has potential the two pipes are very cumbersome and we replaced it with one steel pipe system. The Yamoto motor has a very narrow cast block at the exhaust head. I have tried using a high heat silicone only to attach the manifold but iwater leaked into the exhaust. The motor will run but it will not run to its potential. I use silicone on both sides of a stock gasket. When you buy a manifold, it comes
With a plug that closes the bottom of the lower cyclinder. Fill any area that the plug does not fill with a high heat silicone. I recently used one called "Right Stuff". Motors seem to run better if you grind the exhaust passage ways into a smooth unit. Do not grind on the casting as this is not usually allowed. I will stop here and hope we can get others to express their experiences.

[DeanFHobart]

We have participated in the C mod class for a number of years. I thought we should create a form on your thoughts on how to build a motor and what boats seem to work. Some have used a 102 to success while most use 302. The biggest out ward appearing difference between a stock Yamato is the exhaust system allowed in the Mod class. The stock system works well in the lower speeds, but increases measuability as the motor turns more RPM. The mod lower unit has a better water pick up which allows the higher positioning of the gear foot. Most units are modified by adding a larger sag and a more streamline front. Most units have a pick up hole in the lower part of the redesigned nose. Without a mod lower unit the mod motor can not reach its potential higher rpm.
A typical exhaust system has a 1/2 or 3/4 plate that directs the two cylinder into either a cast manifold or steel pipes that brings the flow into one pipe. That is the two most popular, however the first mod motor we pruchased had two In Depend manifold and two tuned pipes. While it has potential the two pipes are very cumbersome and we replaced it with one steel pipe system. The Yamoto motor has a very narrow cast block at the exhaust head. I have tried using a high heat silicone only to attach the manifold but iwater leaked into the exhaust. The motor will run but it will not run to its potential. I use silicone on both sides of a stock gasket. When you buy a manifold, it comes
With a plug that closes the bottom of the lower cyclinder. Fill any area that the plug does not fill with a high heat silicone. I recently used one called "Right Stuff". Motors seem to run better if you grind the exhaust passage ways into a smooth unit. Do not grind on the casting as this is not usually allowed. I will stop here and hope we can get others to express their experiences.

And a "Short Tower Mid-Section" of course..... Just long enough for the Yamato Swivel and Clamps.

Other than that, I think you've covered it.

[Hydro doc]

Thanks Dean but not so fast.....and speaking of fast Dean has built a really fast C Mod that won the APBA national championship. In NBRA, in years past it has been legal to run any grear ratio in the unit. This year, it was you could only one tooth off units, like the popular Yamato stock foot. Starting with a used Yamato stock, you can have a legal mod foot for about $900. A komato new foot(lower unit) cost over$1,400. The kamoto unit is smaller in the R radius but not legal in APBA. You can use external water pick ups on your boat and run a line direct to the motor. This allows you to raise the motor even higher. Only draw back would be, a small piece of debris can clog the line and you will fry your motor.

[DeanFHobart]

Thanks Dean but not so fast.....and speaking of fast Dean has built a really fast C Mod that won the APBA national championship. In NBRA, in years past it has been legal to run any grear ratio in the unit. This year, it was you could only one tooth off units, like the popular Yamato stock foot. Starting with a used Yamato stock, you can have a legal mod foot for about $900. A komato new foot(lower unit) cost over$1,400. The kamoto unit is smaller in the R radius but not legal in APBA. You can use external water pick ups on your boat and run a line direct to the motor. This allows you to raise the motor even higher. Only draw back would be, a small piece of debris can clog the line and you will fry your motor.

Thanks Hydro Doc,

Our motor was built by Steve Benson. Our gear case is a 302 with a welded nose cone and shaped... built by Tom Cronk. It sounds to me that the kamato would be faster..... I'm assuming that it is one tooth off.

Best Regards,

Dean.............................................. .................................................. ...

[mercguy]

Who is this Steve Benson guy? Lol

[Hydro doc]

Not many people are willing to talk about improving the performance of Yamato motor. Since I am not building our motors, I am not a good person to speak on this issue. Some of our motors have not had the head reduced and others have. It's hard to say what works without changing the head at the test session. So far our fastest motor has a stock head. So go figure. We also have two heads that have 18mm plugs. That is going to have to change because there is no 18 mm plugs available. There does seem something to the idea that older motors seem to turn higher rpm and thus rods are enlarged to hurry that process along. We have not gain speed when this was done to our motors. Boring motors has improved performance, mainly IF the bore job is done correctly. This lowers the life of the block because the wear will prevent fitting the pistons to the correct clearance. So don't bore unless you have to. There is allot of debate about the best clearance, so if you are doing this for the first time ask an expienced person for his advice. In the old days before CNBC machines we improve the motor by bring the motor to specs. This is only valuable if your motor was built wrong. The Yamatos are right on and I see only slowing the motor down by changing the port timing. There is speed in more places.

[Jack Stotts]

The new 321 Yamato has only 18mm plugs and does not have 14mm heads available. Having said that, the 18mm plugs must be readily available in Japan. Jack

[bh/]

500ccMH APBA 102/302 only;

7. Top fill and/or “floatless” carburetors are permitted, provided that the carburetor body is not altered in any way (i.e. filing, drilling, sanding, or polishing),
9. It is permissible to use repaired or “after-market” crank pins and bearings so long as the stroke and other specifications are maintained.
10. It is permissible to mill the cylinder head. Motors must meet the cc specifications. No combustion chamber shape or configuration changes except for volume are permitted. It is allowable to machine the squish band on both Models 102 and 302 cylinder heads (18 mm and 14 mm provided the new dimension H equals 2.597 – 2.627 and H1 equals 13 degrees min to 17 degree max angle are adhered to. A detectible edge must surround the squish band.
15. Aftermarket ignition coils for the Yamato 102 may be mounted in any location external of the OEM location. Addition of coil mounting brackets or coil mounting plates must not require internal modifications to the powerhead.

-the head mods, and ignition are most significant. Could one go as far as a CD ignition on the 102 replacing the mag?
-port mods are not permitted, but they have relented some on edge chamfering .
-if running a 4 blade prop at 1/2" + above the bottom why add material to the case
if it is not in the water, save for some profiling to mod specs?
-a deep strong straight skeg is good .
-also , why are so many/most adverse to the 16mm OEM prop shaft?
$$$ wasted IMHO

[DeanFHobart]

Mike Ward who is the Yamato dealer in England must have the 18mm Spark Plugs.

http://www.yamatoracing.co.uk/home/4536916004

His e-mail: mike@yamatoracing.co.uk

Also, Ric Montoya in Seattle should have them: http://yamatoracing.com/

Phone: 206-283-4773

His e-mail: ricstr4@gmail.com

Both are easy to deal with.

[Hydro doc]

The C Mod class has allot of up-side; it's not a beginners class but it goes fast enough to be exciting. No one I know is bored driving a small boat in the mid-70 range. The class has a bulletproof motor when align properly with it other parts. You can run it for several years without repairing . Some drivers will take compression test after each race and tract the compression. When you see a drop off then it may need new rings. When taking a compression test you can continue to pull until there is no longer any improvement. This is usual five hard pulls. Most Yamato motors will show 120lb. This is a low compression motor so this an average. I use this as a gage as to the progress of the motor, with other indicators. Speed is important but only if the weather and site are the same. Altitude, hardness of the water, air pressure, wind velocity and direction and temperature of the water and air are important factors in the speed of the boat. The richness or lean of the fuel mixture should be adjust to improve the motor's performance. Fuel is another factor in the engine's performance. We are all looking for that little something that move us just a little faster or allot faster. We are fortunate to be able to buy 91 octane non alcohol gasoline two blocks from our shop. We use 8 oz of evinrude oil per gallon of gasoline. Some like to use more.

[Hydro doc]

I wanted to write about boats and heights of the motor too. Hydroplanes used in the C Mod class or 500 mod class range between 10'6" and 11'6" long and width of 36" to 37". The boat weight should allow you to be within 25lbs of the minimum weight allowed. My son is 155lbs, so he can use a heavier boat. The amount of lift built in the boat should be around 2" to 2 1/4" on a 37" bottom boat. Lift can be measured by using a straight 12' two-four on the bottom. The amount of distance at the front is the lift when the 2x4 is held on the bottom. If your heavier than the average driver, 210+ then you may build more lift in the bottom.
How high can you go with the motor? In the last 15 years, in stock racing, APBA have put limits on motor heights. In Mod racing there are no limits to how high you can raise the motor. With the introduction of the four blade propeller, you can raise the motor higher and still go straight. Deeper traps have also help keep the boat to go straight. As you raise the motor, you have less sgag or lower unit in the water. As the boat gets more air born, the boat will break loose and want to move the back of the boat to the left or right. It's possible for the boat to turn so severe that you loose control and the boat will turn over. This will happen while you are going in a straight line and is very dangers if you are in close competition. If you experience this, lower the motor till you have complete control.

[Hydro doc]

I had hoped others would write their experience in the C Mod class. Certainly there are many others that have more experience in this class. One subject that I have not mention is propellers. One truth or myth is the propeller must be fit the pipe. That when you find a prop that turns the RPM that is compatible with the volume (back pressure)in the pipe, that you have the best production of the motor. This is hard to measure because it can not set up on a dynamo. What we find is we try to just find the best pipe and the prop that fits the race course. Maybe, we have not found that perfect combination of pipe and prop. One problem that we have encountered is that some props will get a hydro on plane, but will lack the pitch to get a runabout on plane. Presently, we use the same prop on both types of boats. Interesting, the prop was made over 10 years ago, so I'm sure newer props could be better. I hope I have encouraged some one to try this class as it will be seen by one thousand people. What's Up Doc? Race Team.

[dpracer95]

Hello, I'm trying to somewhat build a C Mod set up. I was invited by the guys from Ecuador to come down and race in their national event. They basically run an Austin copy boat "A" boat with a piped out 302. Just curious what mods would be most cost effective to take down there to mount on a motor they are allowing me to run. I believe the race is around 63 miles and is basically a drag race.
I personally have never run a mod rig, but I'm extremely eager to get to testing before I go down! Any help or advice would be much appreciated! Thanks

[rumleyfips]

There's a thread here " A look inside Ecuador boat racing " It says the mods are a pipe and a Mercury 25 carb. Most use a megaphone but there is a picture of a chamber at Facebook " Ecuadorboatracing".

I tried running my Y80 on a 25 Bezoat in 1979 or 80. It just lifted the left side up 5 inches before I could get the throttle fully open. I needed a bigger boat or bigger something.

[rumleyfips]

Looking again I have to think that's real boat racing. A beer company issues special label beer for boat racers !

[johnsonm50]

I saw vids of the racing in Ecuador, they are on here somewhere I believe. It's real, they race in tuff conditions too. As a new driver to C-mod I'm glad this post is going. I never found C to bore but a mod is exciting. It isn't easy to get used to the speed, the boat, going on pipe & counting all at the same time but fun to try. I have been working on some issues & getting a little closer each heat. I hope to race more & learn to drive better next season.