What is happening mechanically when an xs backfires through carbs?

[1980 Special Twin]

I know that backfiring through the carbs is generally a result of a lean condition. I've been troubleshooting mine for a year, which I think could be related to my needles because that's the point in the throttle where it backfires. Especially when cold. I'm running pipes and pods, pamco, just gapped my valves. Strong runner otherwise.
But rather than ask you all to be cyber sleuths, it would help me to resolve the issue if I could just understand what is happening when it backfires through the carbs. I think that backfiring in the exhaust is when unburned fuel ignites in the heat of the exhaust, right? But how does that happen before intake?

 

[Jan_P]

Not thought so much about it but I believe it is so that if the ignition is to advanced
It can fire with intake valve slightly open and kick into the carburetors
Read here they can come off the rubber
If I recall right others with pamco have had the problem

Shooting in the exhaust the opposite to late .
But the exhaust has another phenomenon if there is a leak It sucks in ( ejector ) oxygen and since there is rarely ever perfect burnt fuel in the exhaust the extra Oxygen make it burn again .. If no air leak the unburnt fuel exits at the end

So what to do then
It is the mixture / the ignition timing /spark perhaps more parameters

I fiddle with the timing and spark ( electrical ) almost never with the carburetors
If it was me and it is kicking into the carburetors I would try to retard later the ignition
Observing the sound change If I recall right you are into music and can hear if it sounds hard running
Throttle blipping and starting difficulties and if it loses power.
Setting it later makes it run quieter mechanically ( generally speaking )
If that not helps I would suspect the Pamco but others here knows about suitable jetting for your setup

 

[jetmechmarty]

Timing. Ignition is happening before the intake valve is closed. So, your timing is too far advanced or maybe the intake valve stuck open. Those are my best guesses. Do you have a timing light?

 

[1980 Special Twin]

Could it backfire through the carbs if it's too retarded? I feel like I'm just tinkering and fussing, maybe my timing light isn't great but the mark isn't dead-on on F in it blips around about a quarter inch and I can't tell what the best position is. Is it better to err in one direction or the other? Is it more advanced when it's closer or further from the T mark?

 

[jpdevol]

Here's a video of an SG @ idle (sorry I didn't rev it up there):

If I had given it some revs, the silver timing mark would have appeared to move clockwise toward the drain groove in the cover

 

[5twins]

It's more advanced when it's further away from the "T" mark. The idle timing spec is 15° BTDC. The full advance is 40° BTDC, much farther away from the "T" mark. I think your 1980 model just has one slash mark to the left of the "T" mark. That would be that 15° setting. Later models marked the idle timing with a "U". This showed a wider idle timing "range", probably like 13° to 17°. Normally when you checked it, you would find the factory setting right about in the middle (15°) .....

We've found that with today's fuels, these bikes actually run a bit better if you retard the timing a couple of degrees. The motor will be less prone to pinging and will run a little better/stronger at higher RPMs and speeds. Here's how I set my '83, over at the right side of the "U", so it's probably at around 13° .....

 

[1980 Special Twin]

Ok cool, yeah I found a sweet spot where it's more or less over the F (mind has an F and also the full advance marking) and now it doesn't backfire. It doesn't start as readily as it did when it was too advanced, but it runs better. I have a very cheap timing light, that was likely part of tie problem.

 

[jpdevol]

It doesn't start as readily as it did when it was too advanced,

Hmmm...it should, theoretically, start easier. Perhaps some fuel screw tweaking is in order now.

 

[1980 Special Twin]

It used to start on the first kick when stone cold, then third kick when warm. Now vice-versa, which is ok.

 

[Jan_P]

It used to start on the first kick when stone cold, then third kick when warm. Now vice-versa, which is ok.

Can take some time to find out the trick old HD -s have prime kicking before trying

I one year had a strange behaviour if I did not touch the throttle handle at all choke on
It started cold the first kick .. hesitantly but always. That went away after a while
to choke and throttle as the best method about half the times first kick cold.

 

[1980 Special Twin]

Thanks, i started hunting for TDC, holding the choke and it starts right up!