Hi guys,

I race a stock Zen 26 in a 42" cat at our local club. Been using this motor for about 2 years and this is my second hull I'm using it in. Class rules state that the motor must be stock, as supplied by Zenoah, with either a 644 or 257 carb. Got the latter fitted to my motor. Further the rules state that any tune pipe can be used, as long as header diameter are no bigger than a 7/8" and the diffuser and baffle cones angle stays constant for the full length of them...no multi staged diffusers or baffles. The pipe can have a band or belly length if you so wish.

My boat performs very very well and if I keep it the right side up it finishes in the top 4 places every race. But I do have some weird issues with the motor.

From day one it seems that the motor runs very hot. Sometimes after a race you can smell the motor...you know that smell you get when a motor is very very hot. The motor also don't really react to the needle settings unless you adjust them a huge amount. Then the motor will not perform and so back to the 'old' settings. Checking the plug it will always show that the motor is running rich. Setting the needles will not alter the plug color much...not that you can get it the right color anyway. Then, when a race start and you throw the boat on the water, the boat will be a rocket for about half a lap. After that the performance will drop a tad bit and so it will stay for the remainder of the race. It's not alot, but it's enough to notice on the water. With that, the motor also sounds that it's struggeling to get to the top rpm. Almost like it's lazy in a sense. Like it's struggeling to get past that last 1000 or so rpm before hitting max.

We've checked everything regarding this issues and found nothing wrong...from cooling circuit to carb to drive line...everything!

Which brings me to my question...can my tune pipe be causing this. When we finished the first boat we needed a pipe to test the boat. This pipe was lying in the garage and we fitted it. This pipe were made locally by someone which use to race. Boat performed well and we never really bother to try any other pipe. I read up on tune pipes and it seems that my stinger are very small in diameter compared to other pipes? The stinger ID is 10.6mm. It also runs all the way to the belly of the pipe which measures to a length of 216mm for the stinger. Of that, 71mm are sticking out of the baffle cone with a silincer around it. The stinger have 4 x 6mm holes in that 71mm piece inside the silincer. The belly of the pipe measures to 55mm. The diffuser cone length are 175mm and the baffle cone length 145mm...no band on this pipe. At the moment my total header length, from face of piston to the diffuser, are 166mm.

I really hope that someone can help me here with this issues, or the strange behaviour, of my motor.

A friend had a similar issue with his stock mono. It ended up being that the foam tube floatation was too close to the carb. We cut away a large amount of foam around the carb and the problem was gone.

Possible that with a hatch on there is not enough venting for the eng to cool and have the carb in clean air? When the air heats up in the hull the eng runs hot and will drop off in power. Do you have pics of the setup? That would be a good start to help you out.

The engine is over heating and that is the reason for the drop in performance, First check to see if the barb fittings on the cooling cap are straight through and not stepped down, since Zenoah has used two different types of barb fittings 1- straight though and the other that was stepped down.
Internal stingers that go inside to the center of the band of the pipe will choke the engine and cause excessive back pressure, which will make the engine run hotter and at the elevated temp the engine will not perform at higher rpms. The internal stinger should only be 4" from the cone.
Try a different pipe and your performance will improve.

By doing the math ( converting to inches ), your pipe looks to be good. You could try it without the silencer to see if it's developing too much back pressure. Most 257 carbs like 2 1/4 to 2 1/2 on the low needle, and 7/8 to 1 on the high ( given good air quality ). Check the carb diaphram for wrinkling. The stock intake isolators are infamous for warping and causing slight air leaks with increased temps. You may have to flat sand it. Make sure your water "IN" is low, and water "OUT" is high, to eliminate air pockets inside the cap.

And a few pics would be helpful.

I had the same problem on a mono
It turned out to be the pick up in the rudder
was a double pick up and 1 of the 2 had a little bit of grass in the tube

Some pics for those who've asked.

Some more pics.

Thanks guys for the replies. I really appreciate it.

I'll check the barb fittings tonight and also the rudder pic ups...maybe, just maybe!

I'll see if I can find a pipe just to check the if it makes a differance. Come to think about it I've never tried any other pipe on my boats other than this one I have. Also...I've had this issue since my first motor and I'm now on my third motor. Also had it in differant hulls with same issues. Only stuff that stayed the same between the three motors and two hulls were the tune pipe and rudder with dual pick up.

I would get rid of the in-line fuel filter. Make sure you filter your fuel VERY WELL before it goes in the bag. And check the internal fuel screen in the carb. They can get slightly plugged up and restrict high flow. It could be that pipe needs a good clean out.