Triumphrat

Jsp500 · 08-25-2007

I was told you can reverse the connection to the reed valve under the gas tank that the stock AI tubes connect to. No more air flow...no more backfire with aftermarket pipes on deceleration. (it's rejetted and tuned but still backfires)
The emmision stuff is still next to the spark plug so it looks intact when in fact it's disabled.

You guys heard about this im sure.....anyone done it ?

**** edit **** My bike doesn't "backfire" it "pops" on decel

Ski_rat · 08-26-2007

this is interesting i dont know if it would work or how but I am tired of the backfires

Jsp500 · 08-26-2007

It was told to me by a Triumph Mech. I'm going to do the mod next weekend and see how it works out.

The theory is that your "rolling" the intake and exhaust tubes by installing them in reverse order.

Leeroy · 08-26-2007

This is copied from a Honda forum and the Pair Valve discussed doesn't pertain here, but the rest is good info on backfires.

Burn Baby Burn

The last thing I want to address today is the subject of deceleration backfire, or “popping”. This topic generates a lot of concern from inexperienced riders, or even from experienced guys who just hate the noise, so lets take a look at what causes it. But first things first, lets define the issue:

Deceleration Backfire is caused by fuel burning in the exhaust manifold or header.

No ifs ands or buts, that’s what causes it. But the bigger question is how does gas get there in the first place, and that’s a bit more complicated. Generally, there are a variety of ways it gets there, and a variety of things that can make the backfiring worse. But there’s a kicker, and something you should understand before we go any farther:

A motor in perfect tune will exhibit deceleration backfiring.

Therefore, just because your motor is banging it up, doesn’t mean there’s anything wrong. And consequently:

Getting rid of the noise means de-tuning your motor.

Yup. If you’ve jut got to eliminate that popping, you’ll have to accept the fact that your motor is going to be forced to run rich to do it, and that isn’t necessarily a good thing. So lets talk about what causes the problem.

Ok, so you’re riding along at some given rpm, and suddenly you decide to decelerate, and you reduce the amount of throttle. This causes an “overrun” – that is, the motors rpm is turning faster than the fuel provided can support, so the motor begins to spool down. This causes a couple of things to happen.

First, when you close the throttle, you are also closing the throttle plates. This reduces the air and fuel flowing into the motor, and increases the vacuum (lowers the pressure). This results in less air and fuel in the cylinder during the power stroke, which in turn results in a lower pressure in the combustion chamber. Remember I said earlier, that the A/F mix burns faster in proportion to the pressure applied? Well, when we reduce pressure this way, the mix burns slower. This results in two things happening.

1. The lower burning fuel generates less heat, and the cooling effect of the non-burning fuel tends to “quench” the flame front, or slow it down even further. Because the mix is burning much slower, the exhaust valve can open before all the fuel is consumed, and the unburnt fuel is ejected into the exhaust.
2. The engine designers, in order to promote smoother idling and better combustion, retard the spark when the throttle is shut, and this results in the mix being lit later.

So, now we end up with unburnt fuel in the exhaust, and burning fuel being ejected into the exhaust, and bang! Backfire. In addition, Honda has added a device called a “programmed air injection valve” (Pair Valve) that actually injects some fresh air into the exhaust to help this process along – since fully burning the fuel results in cleaner exhaust. So the backfiring is not only a normal part of the engines operation, it’s also intentionally amplified by Honda! Of course, normally, that massive bazooka pipe Honda hangs on your bike hides most of the noise, but it’s there, even when you can’t hear it.

So the bottom line, is: That backfiring is perfectly normal and expected. If you’ve just got get rid of it, that’s up to you. You’re entitled to set your motor up the way you want, and your goals are your goals. But don’t refer to it as “fixing” the popping. Rather, the correct way to think of it is “de-tuning a bit to get rid of the popping”.

There are a few ways you can do this.

First, use the stock pipe. It will hide the sound, by absorbing it into mass, and masking it with the larger baffle space. Second, you can add more fuel during deceleration. This has the effect of raising the chamber pressure slightly, which burns a little more before the exhaust valve opens. Lastly, you can remove the Pair valve, which reduces the amount of available oxygen in the pipe to burn the unburnt fuel.

TCL · 08-26-2007

Great lesson, Thanks Leeroy

Leeroy · 08-26-2007

Here's the whole read if you're interested.

http://www.vtxoa.com/forums/viewtopic.php?t=28918

Sheepdog · 08-26-2007

Good treatise, Leeroy, but it only addresses a portion of the problem. There is "backfiring" during decel and then there is "popping" on decel. One takes place in the exhaust and the other takes place in the combustion chamber. Unburned hydrocarbons within the exhaust ignite within the exhaust system when they meet oxygen, causing "popping" on overrun. A "backfire" will actually reverse pressure through the intake ports after a poorly timed ignition in the combustion chamber. Backfires are normally caused by bad things, like improper timing or an extremely lean condition.

Since our bikes are extremely lean as delivered from the factory and because the venturi is almost completly shut when the throttle is closed, the bike turns to the idle circuit to take over carburetion duties. Thus, the majority of the backfiring that is experienced due to an excessively lean condition on overrun may be corrected by properly adjusting the idle mixture located on the front/bottom of your Bonneville's carbs.

"Popping" is caused by the presence of oxygen in the exhaust system such as when the bike is first started or when your AI is active. This may be reduced by deactivating the AI or using a muffler with small inner spaces (This is why aftermarket mufflers with more open or absent insides often induce popping).