a friend of mine who just bought his 2nd banshee decided to take his stocker apart after it was about 2months old and do a port job that matches another friends professionally done porting. Needless to say even though it looks right he has no clue as to how to set the carbs or what jets are right. also to compund the problem he slapped on some fmf pipes. his friend who had the work done professionally runs 290 main jets, so he bought these. Needless to say he has problems. It will rev up just sitting, but if you try and ride he never reaches the powerband before the right cylinder goes weak and stops firing. So my question is does any one thing this is just a Jetting issue?, or is it a overly large port job?, Or my thought is some kind of compression leak on the right?, or is it just scrap metal now?

 

Are the carbs in sync? If they aren't then that could be the problem. Are the carb tops on tight, double check this as stock banshee carb tops are notorious for coming loose.

Have you run a compression test on both cylinders? Acceptable limits are 125 and up (I've seen banshee's all the way to 195), with the cylinders being no more than 10 pounds apart from each other, with less than 5 pounds being desireable.

It could also be a bad gasket, did he buy all new gaskets to reassemble the motor? Are the rings in good shape? Hopefully if he had the motor apart he replaced pistons and rings, and checked the piston to side wall clearance. I don't remember the gap for sure, but I don't think it's supposed to be less than 3 thousands, or over 9. <~ not sure though.

Those are the things I would start checking.. if it was my bike.

ONE THING YOU NEED TO BE AWARE OF!!! If you have one carb sticking open, or running up, while the other isn't, you're asking for a burnt piston on the dry side. I fried my last motor like this, it wasn't pretty. Think about it, if one cylinder is running hard, and the other isn't getting gas, and it's oil in the gas, you're going to fry the motor, count on it. Been there!!

Again, all this is just my $.02

Brad

 

Two millimeters difference in port heigths will create a paperwieght cylinder. I'll address the worst case scenario here.

2 mm is roughly 80 thousandths of an inch, to many people that's not much. If there is a mismatch between the transfer ports, i.e. one side is higher than the other then one side's transfers will open first, this will dump the pressure built in the bottom end(the same pressure the forces the mix up thru the transfers), and that one side will "over-power" the other set of transfers.

In the few miilliseconds that it takes for a complete revolution at RPM, if one transfer opens first, the other side basiclly doesn't have a chance to charge the cylinder bore from it's side. It's out of balance, and will run very poorly at RPM.

The angle that the transfer port opens out into to the cylinder bore has equal importance. There can be "short-circuiting" of the transfer ports, i.e. mix is drawn out the exh port into the pipe by the suction pulse of the expansion chamber. Thereby, getting blown out the exh pipe if the return pressure pulse isn't able to stuff it back into the cylinder before the piston closes of the exh. port.

If the exh. port was raised too high in relation to the transfer ports, the motor will be extremely "pipey". While the power will be there, it will be extremely narrow(the "power-band") and higher in the RPM range.

If the space between the rear transfers and the intake opening are opened too far, the ring end gap has no where to ride. A ring will pop out into the intake or transfer and break off.

If the intake port was "hogged" out too far then you risk having a cylinder fail. As in, break, often at higher RPM's it comes unglued. Catastophic failure.

Now, with the SOME of the worst case, "Do-it-yourself" port job possibilities out of the way.

Check the simple stuff.

Compression.
Good fuel, plugs, spark, air filter, reeds...

Good luck

 

I kind of figure his cylinder will have to be scrapped, what a goof ball, his train of though is got to be that of a bowl of jello. he did it because his friend had his sent off to Sandtrax in Tulsa OK and now his runs awesome, so they figured they could just duplicate the porting that Sandtrax did. I think that the porting is just not matched and one is right and the other is off, throwing the timing of everything off. but ill give him the news. I alread suggested he order a new one, but he thinks it must be jetting. i guess we'll see.

 

He should really send his cylinder to a professional. They will definately be able to tell him what is wrong. Also, they may be able to repair a botched porting job.