lightswitch power

[jetski9010]

I just built up a superjet 650 motor for a friend. I ported the motor for him. I widen the exhaust to about 66%-67% and rised it a little to match the port a little better. It wasnt rised any more then 1mm-2mm. All transfers where matched. Cases were also ported and compression is 165 psi. Timing is advanced 2 degrees. I am running a protec pipe on it that is for a 701. It had stock 44 sbn carb on it at first but is now running a blackjack. I am having a bad lightswitch pull of power right before half throttle. I thought I had put a good arc in the porting roof to avoid having this lightswitch feel.
I am wondering if its my porting that did this or could it be the 701 pipe setup on this motor?

 

[egbrig]

Two stroke engines are amazing. You can do so much wrong and they will still run. You don't have any real info on the porting written down so it would be very difficult to accurately determin if or how far off ports are. If you want to know wether the ports are poportional you'ld have to measure the pta's ect. From the photos's the exhaust looks over sized for the stock (although matched) transfer area. This means that it takes rpm to let the pipe work. The exhaust port shape looks ok but does it work with the transfer area. Its cool that you want to get into porting but there is alot to understand, and your lucky bcause its out there. This is why people who do this professionally charge what they do. There's a lot to learn and a lot to screw up to where you get it right. A sport port is worth about what the template costs but real performance costs. Don't give up, this is how you learn. There still could be pipe issues but the Pro-Tec pipe ain't that bad.

 

[jetski9010]

I didnt even think to measure the transfer windows. I was thinking it was just like a 701 where you can port the exhaust this size and it will run fine. Are the transfers area in the 650 motors much smaller then the 701's?

 

[Polish jet pilot]

Also, yo umention that you raisd the window by 1-2mm . That is a HUGE amount and makes a lot of difference in top end power (more top end) but torque can be nasty (lightswitch)... my 02.

Andrew

 

[kevbo]

That is the way all good running skis are. Probably the easiest way to soften up the power is by simply using two base gaskets. This will make the throttle response more lazy but will smooth out the pipe. The carb will need to be richened considerably.

 

[jetski9010]

That is the way all good running skis are. Probably the easiest way to soften up the power is by simply using two base gaskets. This will make the throttle response more lazy but will smooth out the pipe. The carb will need to be richened considerably.

I dont need to make the throttle response more lazy down low it hits really good down low but then hits even harder in the mid which is where I am getting this pull from. Its not bad if you just feathering the throttle setting up wake and stuff but if you try to go on a nice long ride right at the mid throttle where you are trying to hold it there it keeps hitting this lightswitch I am talking about like its got powervalves. I am thinking it might be what Egbrig is talking about and i dont have enough transfer surface area. I should have looked into that more.

 

[Sospikey]

The pipe you have; is it a dry pipe? Then you could dump more water into the stinger to tone down the hit. I know this worked on the type 4 I used to run. Obviously it would affect the entire range. It could be that the hit you get is when the pipe is "on" as in it's range.

 

[jetski9010]

No its not a dry pipe. I did try to mess around with the water in the stinger but still didnt change anything with this problem. Here is the pipe I am running.

 

[egbrig]

jetski9010 you are way ahead of most here. If you have decided to go find and read the info available out there, vs. just post for peoples opinion on this forum then you are on your way. Every real engine builder has done the same thing, the difference between a keyboard expert and someone who knows is, the keyboard experts can only type or barely worked on a real engine and the real builders will show you on the water. So do a search, there is are some good links on this board and read. Randy @ WATCON has some good material.

 

[jetski9010]

thanks for the info. I was searching on here but cant find any stock 650 cylinders port maps. I have a spare set of 650 cylinders back at home in the garage so I am going to check those transfers tonight. Now that you pointed me in the right directions I went back to the 2 stroke tuner handbook and was looking into what my transfer timing should be.

 

[kevbo]

The Yamaha 650 has very low transfers, just raise the whole cylinder and cut the head to regain compression. This will smooth out the pipe. You are not going to get rid of the pipe surge without hurting performance somewhere else. I tested several SJs on my thrust meter, all with pipes. There was an instant 150-250 pound jump in thrust when the pipe started working. The stock exhaust has no tuning effect and is very linear, just less power.

 

[TB]

the watcon 2 stroke manual is by far a great thing to have on the shelf..goes into great detail on every aspect of these motor's.