Timing tips with Strobe v Carb Setup Bike popping

[Tony Smith]

Bike pulls great, just backfires once I throttle off.

Replace the rubber unions between Carbs and cylinder head if they look even slightly cracked. If they are in good condition, tighten the clamps,

If that doesn't cure the popping, have a look at your exhaust system to see if air is getting in at the heads or the cross over pipe.

[jonathanrowley]

Bike pulls great, just backfires once I throttle off.

Replace the rubber unions between Carbs and cylinder head if they look even slightly cracked. If they are in good condition, tighten the clamps,

If that doesn't cure the popping, have a look at your exhaust system to see if air is getting in at the heads or the cross over pipe.

Why would air from the exhaust cause it?
How does that make for banging...

But I do think that could be it, the shorty pipes don't have a complete seal with the crossover for sure....

[montmil]

I also balanced at Idle the Carbs with the dials and set the Mixture by ear. Bike pulls great, just backfires once I throttle off.

Have you noticed the large, knurled screw on the bottom of the right-side perch?

It can be used as a sorta poor boy cruise control as the threaded end has a bit of nylon that contacts the throttle tube and holds it in place.

It's true purpose is to balance the carbs at around 3-4K rpm. This drill is done by tweaking the cable adjusters at the carbs, insuring balanced vacuum readings in the engine speed range normally seen. The cables, at rest, should also be free enough so that they may be lifted just a bit from the adj ferrules, insuring a carb butterfly is not being held open a wee bit.

[Tony Smith]

Bike pulls great, just backfires once I throttle off.

Replace the rubber unions between Carbs and cylinder head if they look even slightly cracked. If they are in good condition, tighten the clamps,

If that doesn't cure the popping, have a look at your exhaust system to see if air is getting in at the heads or the cross over pipe.

Why would air from the exhaust cause it?
How does that make for banging...

But I do think that could be it, the shorty pipes don't have a complete seal with the crossover for sure....

Ok, here goes.

When your exhaust valve opens it releases still burning fuel into the exhaust pipe, it also sends a supersonic shock wave down the pipe, and again when the valve shuts. The effect of this supersonic wave is that it creates a mild vacuum behind it (which helps to scavenge the combustion chamber) if there are significant air leaks around the header pipes or cross over pipes this will draw in fresh air, which if introduced tot eh still burning and somewhat rich mixture that results from closes throttle over-run - will explode - hence anything from a quiet pop and grumble to a full on backfire.

You should hear what happens when you have a burned exhaust valve!

[montmil]

Does your bike still have the horrid cold air injection system that Cali required and Motorrad supplied on ALL North American imports? Dump it. That is likely the cause of your "popping" on off throttle ops.

[jonathanrowley]

Does your bike still have the horrid cold air injection system that Cali required and Motorrad supplied on ALL North American imports? Dump it. That is likely the cause of your "popping" on off throttle ops.  

Howdy, my Bike is a European/UK Bike and running K&N style filters. Airbox removed

[jonathanrowley]

When your exhaust valve opens it releases still burning fuel into the exhaust pipe, it also sends a supersonic shock wave down the pipe, and again when the valve shuts. The effect of this supersonic wave is that it creates a mild vacuum behind it (which helps to scavenge the combustion chamber) if there are significant air leaks around the header pipes or cross over pipes this will draw in fresh air, which if introduced tot eh still burning and somewhat rich mixture that results from closes throttle over-run - will explode - hence anything from a quiet pop and grumble to a full on backfire.  

Hey Tony, thanks for taking the time to explain. Been out on the Bike today, in Dubais sunshine - pulled like a train but still banging away - sounds like a shot going off!

I think I'll run it into a place called Café Rider and let them set it all up - I've got limited facilities and it's damn hot here - circa 110F each day so not much I can do beyond the basic timing I've done.

I think I'll ask them to look at sealing the silencers properly where they join the original BMW pipes and cross tube. Sounds logical and great knowledge you have there!

[georgesgiralt]

Now you're talking ! KN filters, no muffler, bad jointing at the crossover make for a complete carb tune and adjusted jets.
The jets and needle of our Bings are set for the OEM air filter and exhaust.. not for the inexistent air filter or muffler... This explains this ...

[jonathanrowley]

Now you're talking ! KN filters, no muffler, bad jointing at the crossover make for a complete carb tune and adjusted jets.
The jets and needle of our Bings are set for the OEM air filter and exhaust.. not for the inexistent air filter or muffler... This explains this ...

Thanks, so needs a complete re-set as you say, I didn't know until posting, but yes very little back pressure. Wonder what Jets need to be?

Makes a great noise though! witness this short Vid I have just posted on YouTube. Witness an R65 Roar... Short Pipes are pretty much straight through, you should hear the first pop, just as it comes up the ramp.

Location is Media City with Dubai Marina in the background.

[media width=800]http://www.youtube.com/watch?v=SKV8V_hDgvI[/media]

[Justin B.]

To me it sounds pretty normal for your configuration...

[jonathanrowley]

To me it sounds pretty normal for your configuration...

Hey Justin, Thanks - so are you saying this is how it will probably be?
i.e. Go with the noise, everythings OK.. no harm is being done..

Here's an Air and Pipes close up