My theory on carb farts

[NZSpokes]

I've heard a few different opinions on what causes this, but your thoughts on this are new to me.

The odd thing is, why do some Firestorm/Superhawk owners experience this, while others owners don't seem to get this problem at all,

Chris.

This I cant answer but I do wonder if it has to do with atmospheric pressure and how worn carb components are. Air filter and how well it is sealed will play a big part.

Im happy to be proven wrong. I wonder if the bikes with the H wing have farts. my guess is they don't.

I personally think it may have something to do with atmospheric pressure as when my bike used to do it, it wasn't on a daily basis. I could go for days with it not happening once, then it could happen three times in one day

I know this is going against your theory, but I ended up leaning my bike off slightly, this seemed to help. Now whether that was due to more miles being covered (the bike had only done less than 5,000mls when I got it) or the freer flowing cans I fitted, I don't know,

Chris.

Thats not really against my theory as i dont think jetting has anything to do with it. More slide speed.

[VTRDark]

I did have some great links bookmarked regarding research into this but can't seem to find them.

Intake reversion or reversion in carbs is what you want to be looking up. I believe it is basically to do with the valve overlap period and scavenging. Scavenged gases blowing back out of the inlet valve instead of the exhaust which obviously fights against the clean incoming charge and farts.

So as one can guess there are many factors at play here, especially timing and possibly having a longer valve overlap period and a change in exhaust system that affects the pulse tuning. So I believe it to be more of a timing issue than fuelling though they are both related. What we call restrictors in the carb inlet rubbers of the later model I believe is some kind of anti-reversion plate.

There a little here. http://www.s262612653.websitehome.co.uk ... s/cams.htm

However the dynamics of the exhaust gases flowing out of the exhaust valve and their considerable momentum coupled with the momentum of the charge in the inlet port actually help to drag in mixture via the inlet valve. The incoming mixture also helps to push out the exhaust gases, this process is known as scavenging. Generally speaking, the longer this period of overlap, the less tractable the cam is at lower RPM and the hotter the cam is at the top end of the RPM range.

Cams with lots of overlap generally give good engine power at higher RPM, but give that irksome low RPM intractability known as 'off-cam' where the engine growls, spits and jumps, together with a distinct 'coming-on-cam' (very messy) at a particular RPM. This coming on cam feeling results from the harmonics of the exhaust flow reaching a critical point where the exhaust gases stop trying to exit via the inlet port (reversion), and do their proper job of exiting via the exhaust, and promoting scavenging. At low RPM when the engine is 'off-cam', the exhaust gases cause pulses in the inlet tract which lead to a phenomenon called 'stand-off' where inlet mixture is bounced out of the back of the carburettors and hangs in a mist around the inlet trumpets /filters. This contributes to the 'off-cam' feeling as the mixture then fluctuates between too rich and correct and is mixed with spent exhaust gases.

(:-})

[Bleh]

I did have some great links bookmarked regarding research into this but can't seem to find them.
Intake reversion or reversion in carbs is what you want to be looking up. I believe it is basically to do with the valve overlap period and scavenging....

Was going to banter on about something along these lines until I read this post. Suppose most of it has already been said.

But to put a spin on it, what is a carb fart? Well it's a fart almost like any other. Trapped gas finding the easiest route to escape. It's highly unlikely (imo) that atmospheric pressure has a part to play. After all, gas being collected and stored somewhere in between two warming cylinders will increase in pressure anyway, whether you're above 1 bar or below!

But think about it this way, you have two cylinders working in the four stroke system. Large cylinders that vary in total volume during each cycle due to the reciprocating mass (piston mainly in this instance) in motion. There's only a combustion every second approach to TDC (still unsure of the storms timing shamefully), for example spark at 13* BEFORE TDC. Before this ignition, the inlet valve opens just as the exhaust closes and draws fuel and air mix in, the fuel naturally ways more than the air and as we're talking carbs, not very well vaporised upon introduction.
These larger fuel particles can contribute to something significant (as a theory of the top of my head!);

Sink towards the piston faster and become unspent fuel until the exhaust/ next in-take stroke. At this point, any fuel left in the cylinder can become burnt off due to the temperature as well as altering the pressure in the cylinder upon in-take stroke and as the fuel has a greater mass, it's the air that has to be left out (linked to running rich/ lean in a way in that sense). Once the valve is closed, the system is now closed but there is still air in the system that needs to escape. For this air which has suffered friction by being sucked in to the cylinder near the speed of sound and then forced back out is now warm and by it's very nature travels upwards. This warm air then gets trapped until enough is gathered to increases enough pressure to push back on the carbs slide springs, up past the velocity stack and pops in your air box as a form of relief (a sudden increase in volume to fill).

Hope that makes some form of sense... As a theory!

I've found neither running rich or lean, standard cans or straight throughs makes no difference to the potential for carb fart. I've run lean but currently run rich. Yeah rich uses more fuel and requires more frequent plug changes but the bike feels nicer for it. Plus, I never ever suffer heating issues. What else I have found is that as it takes longer to reach mid-way on temp scale, I also suffer less puffs than I used to. This could be related but it may not... However, keeping the temperature cooler, especially the rear by running rich, there could be less chance to produce increases in cylinder pressure through unspent fuel.

I like theories

[mik_str]

My money is on cabbage-derived fuel.......

[sirch345]

I like your theory Bleh

But to put a spin on it, what is a carb fart? Well it's a fart almost like any other. Trapped gas finding the easiest route to escape. It's highly unlikely (imo) that atmospheric pressure has a part to play. After all, gas being collected and stored somewhere in between two warming cylinders will increase in pressure anyway, whether you're above 1 bar or below!

My thoughts on why I think it may have something to do with atmospheric pressure/conditions is because it only happens randomly, well that's what my bike did when I used to experience it. But then it could just be, that the last refill of petrol I put in was not quite what it should have been
Due to different heights in sea level we already know this will change the fueling, so then that brings us back to, is the bike set up too rich or too lean We could go around in circles here for ages
Even so, interesting to hear others thoughts,

Chris.

[Cadbury64]

Great thread fellas.

I can't imagine that Honda let the bike out with farty carbs, so I would suggest that there is wear and tear involved.

My theory (and I only thought about this today) is that it may be related to leaky float valves. My bike only farts under the same condition, a longish period of very casual deceleration with little or no throttle use and small blips on downshift, as when slowing from open road speeds behind cars coming to a junction. What I imagine happens is fuel slightly overflows from the float bowl, leading to an over-rich condition in the intake which then causes a misfire in the cylinder it enters.

I know from a fuel diaphragm problem that I also have at least one leaky float valve. But hey, its just a theory!