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Poly is good because it can be cut with a hot wire. Poly is bad because gasoline will melt it.

A free flowing exhaust has some value with the stock pipe only. A good pipe needs back pressure to work so they have to factor in the muffler. A larger stinger is used with the stock box while a smaller stinger is required with a free flowing box. The end result is about the same performance...

The concorde will need some load removed to be ideal. Otherwise they work very well.

Its a two cylinder two-stroke, they all sound like poop. Quieter the better.

Why do you want to "mod" the muffler???

Get a limited pipe and a low pitch prop, it will make suprisingly good power.

About 90% of performance (power) lies in the cylinder porting, pipe, and pump load. If any one of these are not correct then power (thrust) will be down no matter what else is done. There is simply no way to make good power with a stock (yamaha) pipe, or a stock 61X cylinder for that matter. A...

Now you know why almost all skis are in such bad condition. Annual maintenance is an absolute must or else your ski WILL turn into a POS very quickly. Every year remove and dump the tank. Check for chafing lines, hoses, and waterbox pads. Pull the front cover look for water. Slide the engine...

I always had a problem with the Westcoast gasket being too thick to get enough pressure against the domes. I found a Wiseco gasket that is slightly thinner. The holes need to be cut to clear the domes and water passages need to be matched. This cured all sealing problems.

The 650sx is not a popular ski here. Their engine/pump is modern and your 91 had the best engines. Plenty of parts around but make sure the crank is good before its modified.

Those ribs are for pole bracket support. End you reinforcement at the ribs, nothing is gained by removing them. Its best to add material to each side of the ribs if more strength is desired.

Why does everyone assume that a "specially modified" or bigger carb is always better than a standard SBN 44 or 46.

It is no longer economical to port/bore a 650 cylinder with so many used 701 cylinders available.

Just check the torque of the head and exhaust manifold bolts. The factory gaskets seal better than any but can crush slightly over time.

We have built in dynos with our pumps. All that matters is how much pressure is produced and the nozzle area. I built a thrust meter that raised more questions than answers. The only numbers that anyone is interested in is the size of their engine and carbs.

There is rev limit mod instructions on this site, replace the 91 ohm with a 75 ohm resister and call it good. Next get the lowest pitch prop available. Get a Solas concorde or yb-sc-s and have some load taken out, Impros already knows what to do. If you choose a large hub like a hooker make...

You need to simply need to lower your pump load and raise the rev limit. This can be done with a hammer, soldering iron, and a resistor. Thats all it took for me to turn an over ported/over propped dog into an exceptionally strong running ski.

You may be at the point where that big engine wont rev high enough for a 144 pump. Adding pitch works great when the ski is moving but just boils water when slow. Remove that TBM and install a stock concorde as a baseline. You will need to spin that over 7000 at low speeds in order to rule out...

There is a little more to porting than just matching the liner. Dont do it unless you are willing to spend a lot of time learning. A bad port job can be worse than stock.

Dont port unless you already have a good pipe and correct prop.