Exhaust Popping

[wcorey]

Mike,

You say adjustment doesn't improve the popping but don't indicate if moving the idle mixture screw changes anything else (is the idle circuit functioning?). Like if it's turned too far in either direction idle goes up/down and the motor quits at some point? I you have to use the tickler to keep it idling, sounds like an obstructed idle circuit.
Bob also has a good point on the the drifting adjuster screw, though as you've tried adjusting it then that's not likely the cause. Unlike most other carbs that have a spring to help retain the adjuster screw position, the amal concentric's only have the sealing 'o' ring to provide tension and they can get loose after a time. And ethanol may be eating them...

The idle circuit is pretty basic on these, an air passage comes in from the front under the intake throat and goes straight (perpendicular) to the adj screw, a fuel passage starts up the back of the float bowl and meets the air passage and adj screw pointy end. Then there's a plugged off chamber with two tiny holes just behind the main needle tube coming up on ether side of the back of the slide with a passage coming sideways off of that straight out to (in line with) the adj screw.

Easiest thing to try is blowing air through the adj screw hole (screw removed), as there's a good chance since it's a recent problem on a running motor that its a minor loose type obstruction and not profound clogging. if that doesn't work then the carb needs to come off. Next thing is to poke something (I pull a bristle from a wire brush) into the air passage as well as those two little holes in the the floor of the intake throat, then make sure the float bowl passage isn't gummed up.
And if you can follow all that you get a prize, lol....

Also, the needle position doesn't (shouldn't) effect anything at hard deceleration (closed throttle) and even if it did, lowering it would likely only exacerbate the problem.

Bill

[Bevel bob]

I was getting some popping when making gearchanges --3rd to 4th at half throttle in the upper rev range , dropped the needle to the top notch and its gone,,i'm thinking its time to try a new needle as the needle jet measures exactly 2.6 mm ie a 260.I also suspect a little air is getting in at the silencer clamp.

[MotoMike]

Thanks for the input Gents, Bill Bob and BBob and others. I was using the wrong term. the pilot or air mixture screw is the one that did not effect the the popping. I did ride it once like this but with no idle circuit it is a tough ride. Pulling the carb and checking and cleaning is really not a problem. I had planned on lowering the main jet needle a slot anyway, now I have the impetus to do just that. If I blow the obstruction clear, my mind would see it floating around in the bowl waiting to attack me again at the next least opportune time. I rebuilt the carb before this season and don't believe the mixture screw is drifting as the o ring is still holding it snug and it seems about where I left it. Of course I could be underestimating the power of the 450 as it seems to shake everything else loose.

I'll report back.

Mike

[DewCatTea-Bob]

" the pilot or air mixture screw is the one that did not effect the the popping. "

____ Keeping in mind that the suspected "popping" is the type that's most WORST when-ever the throttle is completely shut-down while left engaged in 1st-gear (at higher speeds)...
I think Bill was understandably wondering if the turning of the pilot/mix.adjust-screw had any noticeable effect, on ANYthing, what-so-ever.
__ I understand that today's carb.cleaners are so weak as to be next to useless, otherwise I would've suggested power-squirting a dose in-through the screw's hole (before trying anything else).


Hopeful-Cheers,
-Bob

[MotoMike]

I see in my other posts, I misstated my intention as it pertained to the main jet needle. My 450 had been popping occasionally on trailing throttle and I thought raising the main jet slide needle (lowering the needle clip) might address that issue.

Well I pulled the old Amal today and as has been the case in many of my previous motorcycle carb dissasembly and inspections, I did not find anything that had certainly caused my idle circuit's apparent absence. I did find a film in the bowl and some residue in the pilot circuit passageways, but the jet itself was open. cleaned it with carb cleaner, ran some orangewood peg stock (clock repair expendables to clean pivot holes in clock plates) through them and repeated til the residue was no longer coming out on the pegwood. more carb cleaner and air hose drying. once over for the rest of the carb and reassembled. Did raise the main jet needle on notch while there. started and idled on the second kick after one wet tickle. after adjustment I can't make it pop. went out for a ride and it is running as good as it has since I've owned it. The trailing throttle popping is as far as I can tell gone. will put in a new plug and check it for color when I get time. It is still a blast every time I ride it, and it is fun to see all the "what the heck is that" looks from the harley hordes that flock to my town.

[JasonB]

G'day fellow Motorscrubs gents,
just thought I would say thankyou for your suggestion, I have found be screwing in the idle mixture screw the popping has gone, I also wired up a Pazon ignition booster which seems to make the bike idle allot better.

Regards
JasonB

[Bevel bob]

Plus one on the booster, works for me too.