Dynajet In!

[Eddieb]

It will backfire with an aftermarket pipe.
The only way of reducing it is chainging the pilot/piolt air jets.
It's still there but not as much.

To bigger or smaller?

Dunno what carb access is like on a DR but changing the jets themselves once you have access is no big deal Dave.

[NordieBoy]

To bigger or smaller?

Dunno what carb access is like on a DR but changing the jets themselves once you have access is no big deal Dave.

2 bigger on the pilot and 1 bigger on the pilot air.

[Eddieb]

2 bigger on the pilot and 1 bigger on the pilot air.

So what would cause a standard bike with standard jets & settings to pop? This is my R100 PD.

When I got the PD the pilot mixture was set way rich, factory is 3/4 - 1 turns out of the mixture screw, it was 2.5 out one side and 2 5/8's the other.
It idled smoothly but had what I thought was a miss about about 2500 revs and I was investigating that when I found the pilot mixture setting. I guess it was so rich it was drowning at 2500rpm.

I've recently set the pilot air mixture back to 3/4 turn out and it now it pops at idle and very low speed but runs very smoothly as soon as you are off idle right through the range. The idle is smooth though, the motor isn't lumpy.

?

[NordieBoy]

So what would cause a standard bike with standard jets & settings to pop? This is my R100 PD.

Dunno about the R100 but the DR will pop even stock but as the muffler is so quiet you can't really hear it.

Only happens on closed throttle. Even a slight crack open and it's gone.

[warewolf]

So what would cause a standard bike with standard jets & settings to pop?

Emissions-induced lean out at idle. Fuel mixture leans off below that required to burn, fire goes out, raw fuel down exhaust, fuel mix richens slightly so it burns again, ignites the stuff in the exhaust, repeat... pop pop pop on the overrun!

Aftermarket exhausts tend to be a) louder and b) leaner (on std jetting), so they both cause more popping and let you hear it better.

Only happens on closed throttle. Even a slight crack open and it's gone.

Following on from the above, that's because then the fuel mix is now sufficient/consistent enough to keep the fire burning.

Which then means a slightly higher idle speed will likely reduce it. My 640A is supposed to idle at 1500rpm but it works A LOT better at 1600-1650rpm. It's enough to noticeably affect the low speed engine braking (-ve, or +ve as it encourages you to ride the bike not nanna it) stall resistance (+ve) and throttle-on response (+ve) but the emissions people won't be so happy, 1650 is 10% up on std. Some of the things done to satisfy the emissions people are not necessarily in the rider's best interests.