Co trim and backfiring

After removing and cleaning the throttle body assembly, I've noticed that the bike does a lot of backfiring when reducing throttle (mostly randomly). I did balance the throttle body both at idle and at 4k rpm.
So my assumption is: it's now getting not enough air to combust all the fuel. So I thought, since everything was balanced properly, rather than messing with the airbleed screws I'd mess with the CO trim, setting it to a negative number reduces the fuel in the mixture? Is this a sound way of thinking?

I set it to -10 and the backfiring seems a little reduced but that could be imaginary. If CO trim can be used in this way, what kind of number should I be looking at to see a noticeable effect? Or just trial and error?

Are you running a closed or open loop map? If closed loop playing with the CO will have zero effect.

Also when is this backfiring occurring? At what RPM? Are you running an open pipe of some sort?

Pete Roper wrote:

Are you running a closed or open loop map? If closed loop playing with the CO will have zero effect.

Also when is this backfiring occurring? At what RPM? Are you running an open pipe of some sort?

No O2 sensor and beetle map so closed! Good to know that CO-trim will do nothing in that case!

It mostly happens between 2500 and 4000 rpm, especially when engine breaking for a roundabout, traffic light. Its doesn't always do it either, and its more noticeable when the weather is hot.

No open pipe, everything is stock.

I replaced the rubbers of the throttle body, the old ones had cracks in them on the outside. Maybe it was sucking in more air before and now with the new rubbers not anymore? Should I allow a little more air in through the air bleeds? Atm 1 on the left side is closed and the right side is very slightly open.

No, the map is open loop if it's one of Mark's. (Open loop =no lambda input.) so CO trim will be influential. Thing is he usually turns the fuel off at idle throttle position down to about 2,700 so there should be no backfiring above that as, if the throttle is fully closed, there will be no fuel being delivered.

Obviously I can't talk about your specific map but that is usually the way. Check that the throttle is actually fully closed and you aren't feathering it subconsciously.

TPS reset maybe?

Check for exhaust leaks. Headers and pretzel.

Pete Roper wrote:

No, the map is open loop if it's one of Mark's. (Open loop =no lambda input.) so CO trim will be influential. Thing is he usually turns the fuel off at idle throttle position down to about 2,700 so there should be no backfiring above that as, if the throttle is fully closed, there will be no fuel being delivered.

Obviously I can't talk about your specific map but that is usually the way. Check that the throttle is actually fully closed and you aren't feathering it subconsciously.

I did some tests being more conscious about when it backfires. I found it mostly backfires when I reduce throttle (but still keep it somewhat open) and then at around 3500-3200 RPM it does a series of backfires. It does so both in the higher gears at highway speeds and the lower gears at city speeds.

beetle wrote:

Check for exhaust leaks. Headers and pretzel.

None that I can detect. Also with the lambda probe off can such leaks even cause backfires?

If there is no fuel being delivered then there is nothing to combust, therefore no backfiring.

As I tried to explain the fuel is turned off at crank speeds greater than 2,700 if the throttle position is below 5.2* or whatever it simply can't backfire as there is no fuel injected to be burnt, either in the combustion chamber or the exhaust.

If though the throttle is being held open, even very slightly, sub consciously by the rider or, if the throttle spindle bushes and butterflies are worn in such a way that when the throttle is closed above that point the air pressure on the back of the butterflies prevents them closing fully so the TPS reading remains higher than it theoretically should then some fuel will be being delivered and some backfiring at the correct harmonic points will occur.

Remember, one of the strange things with the stock Marelli closed loop maps is that when the throttle is closed, regardless of the crank speed, the ecu continues to deliver the idle fuel pulse. When the engine is spinning faster than the target idle of 1,250 +/-50, (On the 8V.) the higher above that point it is spinning the harder it is pumping, (All an engine is is a self propelling pump.). The harder it pumps the greater the differential between the pressure in the inlet manifold and the atmospheric side of the throttle butterfly. To put it very simply, the harder it 'Sucks', (Although this is a bad description as there is no such thing as 'Suck').

This means that the faster it spins the more air is 'Sucked' past the butterfly which in turn means that the mixture in the cylinder becomes leaner and at most points above, roughly, that 2,500-2,700 rpm mark it will be too lean to ignite in the combustion chamber and will simply be pumped through into the exhaust.

Because of the harmonics of the, (Mainly) exhaust though there will be points as the crank slows where combinations of unburnt moisture being pushed back into the chamber through the exhaust valves by the harmonic waves in the exhaust gas and the build up of unburnt residual charge left over from earlier pumping cycles will reach the point where the spark will ignite it in the cylinder. This, rather than happening every second revolution of the crank may occur every forth, or sixth, or eighth etc. Then, when the exhaust valve opens at the end of that cycle the still burning exhaust gasses exiting the valve will ignite all the previously un-spent charges from the previous cycles that are loitering in the exhaust. That is what causes the backfiring and it can only occur if fuel is being delivered.

With Mark's open loop maps, with no fuel being delivered above 2,700rpm the ecu has to be told that the TPS reading >5.2* to re-commence fuel supply. What is causing that to happen is what needs to be established but it has to have an actual physical cause. There is no 'Magical Thinking', powdered unicorn horn or faery dust involved, it's a simple switching operation and the laws of physics.

Find the cause, address it, and the problem will be resolved. Either that or simply learn to live with it. It will be exacerbated by running a pipe without a dB killer because of its effect on the pipe harmonics.

Pete Roper wrote:

If there is no fuel being delivered then there is nothing to combust, therefore no backfiring.

As I tried to explain the fuel is turned off at crank speeds greater than 2,700 if the throttle position is below 5.2* or whatever it simply can't backfire as there is no fuel injected to be burnt, either in the combustion chamber or the exhaust.

If though the throttle is being held open, even very slightly, sub consciously by the rider or, if the throttle spindle bushes and butterflies are worn in such a way that when the throttle is closed above that point the air pressure on the back of the butterflies prevents them closing fully so the TPS reading remains higher than it theoretically should then some fuel will be being delivered and some backfiring at the correct harmonic points will occur.

Remember, one of the strange things with the stock Marelli closed loop maps is that when the throttle is closed, regardless of the crank speed, the ecu continues to deliver the idle fuel pulse. When the engine is spinning faster than the target idle of 1,250 +/-50, (On the 8V.) the higher above that point it is spinning the harder it is pumping, (All an engine is is a self propelling pump.). The harder it pumps the greater the differential between the pressure in the inlet manifold and the atmospheric side of the throttle butterfly. To put it very simply, the harder it 'Sucks', (Although this is a bad description as there is no such thing as 'Suck').

This means that the faster it spins the more air is 'Sucked' past the butterfly which in turn means that the mixture in the cylinder becomes leaner and at most points above, roughly, that 2,500-2,700 rpm mark it will be too lean to ignite in the combustion chamber and will simply be pumped through into the exhaust.

Because of the harmonics of the, (Mainly) exhaust though there will be points as the crank slows where combinations of unburnt moisture being pushed back into the chamber through the exhaust valves by the harmonic waves in the exhaust gas and the build up of unburnt residual charge left over from earlier pumping cycles will reach the point where the spark will ignite it in the cylinder. This, rather than happening every second revolution of the crank may occur every forth, or sixth, or eighth etc. Then, when the exhaust valve opens at the end of that cycle the still burning exhaust gasses exiting the valve will ignite all the previously un-spent charges from the previous cycles that are loitering in the exhaust. That is what causes the backfiring and it can only occur if fuel is being delivered.

With Mark's open loop maps, with no fuel being delivered above 2,700rpm the ecu has to be told that the TPS reading >5.2* to re-commence fuel supply. What is causing that to happen is what needs to be established but it has to have an actual physical cause. There is no 'Magical Thinking', powdered unicorn horn or faery dust involved, it's a simple switching operation and the laws of physics.

Find the cause, address it, and the problem will be resolved. Either that or simply learn to live with it. It will be exacerbated by running a pipe without a dB killer because of its effect on the pipe harmonics.

Thank you for the comprehensive write down of how this works, I had not much of an idea so it's appreciated! I'll try to find the cause of the lean mixture and if I can't I'll live with it. To be honest I actually like the almost Harley-like occasional pom-pom-pom-pom, it's just that it's not supposed to do that

Make sure your inlet manifolds are tight and that all the jubilees are tight, I imagine any air geting in could cause a missfire.
My front left one was loose as were all the jubilees on a mates, neither of us had missfires though.