Popping sound when power off on throttle?

[craigcjc]

I all, my first post so I hope I am doing this correctly. I have an old Honda 750 Shadow 86 and when I decelerate I get a popping sound. I have searched for clues on this and tried adjusting the mixture which helped a bit... any ideas on how to fix this?

[pete376403]

Air leaking into the exhaust - loose exhaust clamps, bad gasket at the exhaust port, for starters.

some bikes (eg my 08 KLR650) have a vacuum controlled valve that opens on overrun, bleeding air into the exhaust port to burn any fuel rather than letting it go into the atmosphere - used to pop pretty bad on overrun before t was removed.

[MarkH]

I all, my first post so I hope I am doing this correctly. I have an old Honda 750 Shadow 86 and when I decelerate I get a popping sound. I have searched for clues on this and tried adjusting the mixture which helped a bit... any ideas on how to fix this?

I don't think you should adjust the mixture for closed throttle deceleration.

In the DR650 thread in another forum someone that knows a shit load more than I do posted this:

People get confused because they misinterpret the cause. Decel popping is caused by a lean condition. Not 'too lean' jettting. You can have jetting set up way too rich and still have decel popping. The condition is created when the throttle is closed at an RPM significantly above idle. The motor pulls more air through the carburetor while the bike is decelerating but the idle circuit is only designed to feed in enough fuel for an idling motor. The extra air makes a lean mixture which misfires. You can't set up the jetting for closed throttle deceleration - at least not without totally screwing up the idle mixture.

There are a few things that can be done to minimize the popping. Run the idle mixture slightly richer than ideal, Raise the idle speed slight higher than the stock spec, learn to wind the throttle closed instead of 'chopping' it suddenly.

I'm guessing this applies to most carburettor bikes and the amount you hear the popping probably depends a lot on the muffler.

[bogan]

+1 to what MarkH said. My bike has a wee valve built into the carbs which adds a bit more fuel on over-run; though it doesn't sound like (pun intended) yours does. It's normal, pretty harmless, and easy to avoid by just not chopping the throttle, or doing hard engine braking. Or just fuck with the idle speed/mix a tad.

[AllanB]

I'd put money on it that you are hearing either the popping caused by the clean air system which injects air into the exhaust or a exhaust leak between header pipe and muffler or similar. The air system will be a more subtle popping the other a potential racket.

My new Ducati crackles and pops on deceleration - apparently very normal for this model (and many other Ducatis).

[kiwi-on-wheels]

Differebt kettle of fish with the late model top end ducatis, they inject fuel across the valves without igniting it (titanium valves) to cool the piston crown and the valves, which promptly explodes in the exhauat pipe.

My cbr250 gets a small pop and crackle as you roll off the gas at high rpms

[AllanB]

Different kettle of fish with the late model top end ducatis, they inject fuel across the valves without igniting it (titanium valves) to cool the piston crown and the valves, which promptly explodes in the exhauat pipe.

Cheers - best answer for Ducati I've read - 90% response - it's a Ducati - they all do that. I like to know why.

[nzspokes]

If this is a new thing to the bike and it want doing it before then its either a lean condition caused by a block circuit in the carb or the valves need adjusting/checking. I wold think the bike is to old to have a Pair system which injects air into the exhaust to burn unspent fuel.

[AllanB]

If this is a new thing to the bike and it want doing it before then its either a lean condition caused by a block circuit in the carb or the valves need adjusting/checking. I wold think the bike is to old to have a Pair system which injects air into the exhaust to burn unspent fuel.

They do - I did a google on it. That unit will give a consistent low volume popping on deceleration. Volume will be relative to the mufflers on the bike. My old Kawasaki had one.

[spanner spinner]

Take the pair valve cover off and inspect the valve, it is a reed valve set up and can get a carbon build up holding it open which will cause backfiring. The carb needs checking as well as a lean mixture can cause the same problem if a jet has blocked up.

[craigcjc]

Thanks all for the heads up, I look into all the suggestions, I bought the bike in a pretty run down state, stripped it completely and rebuilt it although I did not do the motor.
The exhaust set up is odd and looks like it has been modified somewhat, see pic attached, never seen a bolt on system usually a clamp, also I think the baffles have been messed around with. What bike shop it Auckland still knows how these old school carby bikes?

[craigcjc]

Air leaking into the exhaust - loose exhaust clamps, bad gasket at the exhaust port, for starters.

some bikes (eg my 08 KLR650) have a vacuum controlled valve that opens on overrun, bleeding air into the exhaust port to burn any fuel rather than letting it go into the atmosphere - used to pop pretty bad on overrun before t was removed.

Changed Gaskets and used gasket sealer on connections, pretty sure air not getting in through manifold or exhaust, even though exhaust looks a dodgy setup.

[craigcjc]

If this is a new thing to the bike and it want doing it before then its either a lean condition caused by a block circuit in the carb or the valves need adjusting/checking. I wold think the bike is to old to have a Pair system which injects air into the exhaust to burn unspent fuel.

Thanks nzspokes, will check valves, guess I can just remove tank to get at valves or do I need to take engine out again?

[craigcjc]

Take the pair valve cover off and inspect the valve, it is a reed valve set up and can get a carbon build up holding it open which will cause backfiring. The carb needs checking as well as a lean mixture can cause the same problem if a jet has blocked up.

Where do I find the pair valve cover?

[craigcjc]

Air leaking into the exhaust - loose exhaust clamps, bad gasket at the exhaust port, for starters.

some bikes (eg my 08 KLR650) have a vacuum controlled valve that opens on overrun, bleeding air into the exhaust port to burn any fuel rather than letting it go into the atmosphere - used to pop pretty bad on overrun before t was removed.

Is that the Vacuum Chamber that is on the carburettor?