Bonneville America Forums Forums Technical Forums Exhaust why they pop

There have been a lot of posts concerning modified or aftermarket pipes snapping and popping during deceleration. Here is why it happens and why you shouldn't worry so much about it.
When decelerating, especially at high engine speeds, there is a considerable vacuum below the throttle plates. This pulls more fuel than normal through the idle jets and, at the same time, there is very little air entering the engine. The closed throttle position and the engine speed signals tell the igniter that the engine is under very little load and the max advance curve should be used.
The end result is that the spark occurs well before the maximum point of compression. Since there is very little air and not a lot of compression pressure, there aren't many oxygen atoms close enough to the gas to ignite. Very little, if any of the fuel actually gets burned in the engine. By the time the piston nears the bottom of what should have been a power stroke, there is nearly as much of a vacuum as near the end of the intake stroke. When the exhaust valve opens and you have relatively wide open pipes, the vacuum pulls air up the pipes from outside. This rush of air fills the vacuum, lots of oxygen is close to the unburned fuel and the heat of the pipes and exhaust valve lights it off.

So, what is to be done? There are a few choices.

1. Richen the idle mixture so that there is so much gas in the cylinders that you are sure to have some close enough to the oxygen atoms to ignite. The down side of this is that the plugs may foul during normal idle and, since the idle ports are used to richen the mixture when running hard, the mixture will be too rich and you will lose some high speed performance.

2. Tun the length of the exhaust system so that the sonic pulses keep the air from reaching the hot end of the pipes until after the raw fuel is gone. The downside is that this will only work over a limited range of engine speeds and you could end up with very long pipes.

3. put flap valves on the ends of the pipes to block the intake of air. Of course, this ads to maintenance and may look a little odd unless you are building a tractor theme bike.

4. Just ignore it knowing that it doesn't hurt anything and makes your bike sound like a dragster shutting down at the end of a run.

Is this popping on deceleration particular to Triumphs as some bike don’t seem to suffer from this?

Having recently installed tridents the popping was more evident than it was with TOR’s fitted.
With the Tridents installed the popping was much louder and happened more often...it even popped during gear changes. ..I assume this is because the Tridents are far less restrictive.
With the AI now removed the popping had decreased substantially.....the popping during gear changes has gone but the popping is still there when decelerating especially when the revs have been high and then are shut down.. For example reducing speed from 60MPH to 30MPH.
One query I have though is the issue of re-jetting.
If you use larger jets which increases the flow of fuel to the cylinders does this mean that the popping is likely to subside even further or will it increase?

I’m a newbie mechanic so this post shines a little more light on why things like this happen.

Thanks

John

Generally popping on deceleration means you are lean. However in rich situations when you leave it lugging on an extended downshift it can load up with gas and do little explosions on you.

AI makes it lean, if it got a bit better when you took AI off then you just need to increase fuel. That may mean re-jetting, yes. I don't pop much at all with 45 pilots and 130 mains about a turn out on the air fuel mixture.

There is a jetting chart in the tech vault I think.

Excellent Greybeard.....I used to love the popping on decel after a nice throttleup.......

Just plain cool. Always wondered why some complain about it so much.

Very thorough writeup on a topic that seems to pop up (pun intended) a lot. Much of what you said went right over my head but then, that's nothing new. I get the gist of it for sure. Thanks for all that.

+1 Greybeard. Thanks for the write up on this. There have been some posts, me included It's nice to know what's going on in the engine and pipes and have a good explanation of it. Thanks!!

I'm with others-I like the popping and rumbles. Sounds cool.

Nearly any engine will pop on deceleration with open pipes. Pre 1949 Ford products did it a lot because of the ignition advance system used. There was a time when some carbs (Holly I think, maybe Solex) had a vacuum controlled idle shut off valve to stop this. Most fuel injected engines read the manifold pressure (vacuum) and adjust the mixture so that popping is rare.
Removing the AI helps some by reducing the amount of fresh air in the exhaust system.
Larger pilot jets can reduce popping, but you might easily end up with an over rich mixture below 3500 RPM, the range where the engine runs primarily on the transition ports before the slides open significantly. On the other hand, adjusting the pilot jet size can eliminate mid range flatspots.
The higher the engine speed when the throttles close, the more vacuum/less oxygen in the cylinders so, the more popping.

Thanks for the info:

My EFI Speedmaster has started popping. I have had the bike for about 8 months. Personally i dont mind it, but was a bit concerned that it may be caausing some damage. If not, i'll leave it as is!

Check your header to muffler conection.Mine poped like crazy on decel, it would pop so loud my local PD would reach for there guns when I slowed down...thats bad... I took it apart,added high temp sealent to the joint and re tightened.Poping greatly reduced,but not gone.

Really great info and write ups in this thread. Thanks one and all. But in spite of all the 'why', 'how' and 'because' information I'm telling you (as I spoke of in another thread) by disconnecting the TPS, the popping is gone... totally!

Disconnecting the TPS alters the ignition timing. On detecting the lack of throttle position info, the igniter goes to a different timing curve that fires later than normal.
The extreme opposite of this condition could be found on old pre-1949 Fords. Those had a centrifugal advance that was modulated by a vacuum/spring controlled friction brake. Coasting with the throttle closed caused enough vacuum that the ignition went to full advance and the exhaust would snap and crackle like a bowl of Rice Crispies.

One consequence to richening your idle circuit too much to reduce popping is that you get a backfire on startup. So it becomes a balancing act between reducing the popping and reducing the backfiring.

Good write up.

Don't forget to seal the crossover pipe also and the slip ons where they join at the headers.

If it bothers you, just disconnect your TPS.

I had some popping on decel with my Specialty Spares long cannons. This season, I dialed it out a little by enrichening about 1/4 turn on each carb.
Previously, I would occasionally get a pretty loud backfire at start-up. That seems to have been eliminated by enrichening, which doesn't make sense.
Now, a new thing has popped up. If I am riding, and crest a hill and let off the throttle slightly, than kick back in, I get a loud "whoomph" thru the bike. This seems to be coming from the left cylinder. Is there a way to dail that out?

If it really bugs you just leave the choke out. I forgot to push it in once and wondered where the popping sound was until I realized I'd forgotten to push the knob back. (on a long days highway riding, I went over 100 miles with the choke knob out)

The advantage to our bikes is that it is not really a choke so it didn't stall the bike once it warmed up. It's just like having REALLY rich idle jets. You'll burn a little more gas but otherwise it didn't seem to affect performance except it idles really fast and you don't decelerate much at idle.