Reverse exhaust sound

[banks619]

I have an 81 650 with bs34's and uni pod filters. It has a stock exhaust chopped right past the pegs and when I let off the throttle I get this reverse exhaust almost a raspberry sounding noise. I have tried stock main jet all the way to a 155 and stock pilot and up one on the pilot. I've adjusted the air fuel screw from all the way in to almost all the way out. Cam chain was adjusted and so were the valves and I'm still having this noise. I bought 1.5" baffles but they don't fit in the exhaust. Anyone have any other ideas I'm at a loss?

 

[nj1639]

27" Commando or Dunstall repro mufflers. Can't beat the sound or performance IMHO.

 

[banks619]

I was hoping to get it solved without adding mufflers. Thanks for the info.

 

[fredintoon]

Hi banks,
the short pipe length has as much to do with it as the lack of mufflers.
I you don't want mufflers try grafting on some lengths of straight tube.

 

[banks619]

Ok. I am wanting some longer pipes anyway. Appreciate it.

 

[XSLeo]

Short pipes may look good maybe even sound ok, but as far as streetable performance they are about the worst thing to use, almost impossible to tune for.
Adding longer pipes or a muffler helps, not because of back pressure but exhaust reversion. When exhaust gases flow down the pipes they are not a smoothe flow, it's in pulses. As these pilses flow down the pipe when they reach the end of the pipe some of the gases get reversed ans flow back up the pipe, If the length of pipe is right this reverse pulse reachs the exhust valve while it's open. This pushes some oif the exhaust gases back into the cylinder. This perevents the incoming fuel charge from filling the cylinder. Thus poluuting the mix. Leads to poor burn, loss of power. Add ing a longer tube or muffler helps time this reversion so it reaches the exhaust valve when closed. Thus prevents all that stuff I mentioned from happening.
Leo

 

[banks619]

Thanks XSLeo! Very informative. Now to find some pipes.