Motoscrubs.com

Hello,
I have recently got my bike (250N/C) up and running and whilst the muffler which is a megaphone stye with a conti core, supplied by Phil@ road race is loud, I was just wondering if exhaust pooping on de acceleration is normal, I am still points ignition 6 volt with a SSI 27mm dellorto, Is it a tuning thing or just what to expect from a non restricted muffler...
Regards
JasonB

Smarter people than me will chime in, but I don't think it's normal though it may not be detrimental either.

I would guess there is unburned fuel in the exhuast that is igniting in the muffler.

For some reason though I think having an avenue for air to mix with the exhaust gasses from the head to the header pipe to the muffler exacerbates the problem. Make sure all exhaust pipe connections are tight.

It usually indicates a lean condition, try enriching your idle mixture screw by a 1/4-1/2 a turn. Can also be from an exhaust leak, check to see if the header pipe nut it tight.

Bill

I always understood that popping from the carb meant lean, popping from the exhaust meant rich. I stand to be corrected, however.

To Jim and kmev,
It can indeed also be from a rich mixture, for different reasons obviously, just in my experience it's typically a lean condition.
If it's kind of sputtering/popping on hard deceleration then usually lean, if it gets a louder pop or two as the motor is almost wound down then more likely rich.
If the richer mixture screw adjustment makes it worse then go the opposite way and see if it improves, checking the plug color may also help decide.

Bill

Thanks for your input guys. My exhaust pop is loud on de accelartion, I have checked for and exhausts leaks but all seems good.
I have been told its not going to do any harm just annoying.
Regards
JasonB

By: JasonB...
" I was just wondering if exhaust pooping on de acceleration is normal,
Is it a tuning thing or just what to expect from a non restricted muffler... "

____ It's not to be expected as normal.
I assume that that which you refer to is at it's worst when fully shutting-down the throttle after topping-out 1st-gear (with clutch left engaged) !? _ If so, that kind of fairly rapid crackle & popping like type of backfiring from out of the exhaust-pipe, is due to the carb's idle-circuit being set too lean !
__ So adjust the air/fuel-mix adj.screw incrementally just until any signs of the crackling is totally relieved, under the conditions I've stated. _ That should then provide a near ideal idle-mixture, (so then just reset the slide-stop adj.screw to obtain best idle).


DUCATIly,
-Bob

Perfect timing. thanks

My 450 was running near perfect a few weeks ago save the rarest pop on trailing throttle. I had planned on lowering the needle but didn't find it offensive enough to take it all apart. Well this past weekend I found it hard starting requiring repeated tickling to keep it running til warm and terrible popping from the pipe on closing the throttle. The head pipe nut is snug and no puffing felt there or at the muffler connection. So It seemed to me that somehow the idle circuit had plugged. no adjustment of the idle speed screw would improve it. What do you think? Will pull the carb when I get the time. Oh yeah, I've got an Amal 932

" My 450 was running near perfect a few weeks ago save the rarest pop on trailing throttle.
Well this past weekend I found it hard starting requiring repeated tickling to keep it running til warm and terrible popping from the pipe on closing the throttle.
So It seemed to me that somehow the idle circuit had plugged. no adjustment of the idle speed screw would improve it.
I've got an Amal 932 "

____ I've noticed a good number of times with Amal-carbs that after setting the 'mixture-screw', (not to be confused with the idle speed-setting screw), a couple-hundred miles later, the (closed throttle) exhaust-popping would become slightly noticeable, and by about 300-miles would then become intensified & quite annoying...
It seems that the mix.screw gets somewhat slowly twisted away from it's set position and always towards a more lean position. _ So thus requiring repeated resetting (back to a normal-running setting).
But if no resetting of the 'MIXTURE-screw' provides any improvement, then the cause is not a common one, and it's then somewhat more likely that the idle-circuit fuel-passage has become blocked with something.


Hopeful-Cheers,
-Bob

Well after work today I adjusted the idle mixture screw at 1 1/2 turns out went for a ride and found that the popping has subsided somewhat, still there if a accelerate hard say in 3rd geard and back off, but not as bad, so I am thinking I am in the right direction, Thanks to Motoscrub memebers.
Many thanks and if I could shre some nice Australian red wine with you I would (-:

Regards
JasonB