Deceleration Popping

Hi!

I've installed the CVP premium Kit, the CVP inlet and nozzle, and all I can say, that it is amazing, that a couple of modifications can make a huge difference! I'm really happy with the improvement!

I've got a sportster 95 XL1200 (no conversion), and have two minor issues. The first is that after a hard pull through full throttle I've got sometimes a popping when I release the throttle. I've read that in most cases this is due to an exhaust leak, therefore I've changed the mesh sealings and tightened the nuts with the specified torque, put a teaspoon of oil throught the sparkplug holes and fired the engine up to see from where the smoke is coming. I've also plugged in the end of the exhaust, an I couldn't find any leaks. I've backed out the mixture screw till the engine was running smooth and no backfiring occured through the carb. There is no hesitation through acceleration, the bike pulls very very hard. The other issue I'm having is, that when I'm over 100 mph there is sometimes a "fall back", it feels like as the gas would run out. When i was assembling the carb I've cleaned it through and checked the float level as well. The slide was drilled with the supplied drill. Can it be a vacum problem with the tank cap? Or the VOES hose? I'm using the 44 pilot jet, and the 180 main jet, as this seemed to run the engine better. Nevertheless I've tried the 175 jet as well, the popping still occurs, and the performance is not as good as with the 180. The mixture screw is 2-1/4 out from seat, which position was found with the idle drop method (I've used a digital tacho for accuracy). The manifold gasket is also not leaking, I used silicone grease and verified with an unlit propane torch. One more important thing to add: I got the bike with no modification(other than mentioned), except I have Screaming Eagle open end mufflers. I've also read that this could be normal (from the mikuni site):

Backfires in Exhaust Note:

It is normal for many high performance exhaust systems to moderately backfire or pop when the throttle is closed from mid-to-high rpm. In fact, one should expect a well-tuned high performance engine to "pop" and "crackle" when the throttle is closed at high rpm.

The popping is a result of the air/fuel mixture becoming very lean when the throttle is closed and the engine is rotating well above idle speed. It is also necessary that the exhaust system have rather open mufflers.Why This (normally) Happens:
1) When the throttle valve is in the idle position, fuel does not flow out of the main system (needle, needle jet, main jet). Fuel is only delivered to the engine by the pilot (idle) system.
2)The combined effect of the closed throttle and elevated engine rpm is to create a fairly strong vacuum in the intake manifold. This vacuum, in turn, causes a high air flow rate through the small gap formed by the throttle valve and carburetor throat.
3)Under these conditions the pilot (idle) system cannot deliver enough fuel to create a normal, combustible air/fuel ratio. The mixture becomes too lean to burn reliably in the combustion chamber. It gets sent into the exhaust system unburned and collects there.
4)When the odd firing of the lean mixture does occur, it is sent, still burning, into the exhaust system where it sometimes ignites the raw mixture that has collected ---- the exhaust then pops or backfires.
5)Completely stock Harleys do not do this until open-end mufflers, such as the popular Screamin' Eagle slip-ons, are installed. The exhaust must be both free-flowing and have an open exit for the popping to occur.

Thanks for helping!

cheers,
Tibor

Hi Ken,

thanks for your reply! I tried to adjust the mixture screw , but it didn't really help. The popping is still there. Next I'll insert the #45 pilot jet. My other problem is also still there, and I don't know where to look. When I ride with full throttle in fifth gear (only there) I get a sudden loss of power, the bike starts to stumble, and decelerates from 100 mph to about 65. When I release the throttle, it runs normal again. If I keep the throttle open it accelerates, looses power, accelerates again, looses power and it goes on and on. Really annoying, it feels like as the bike would suffer from a lack of fuel, or would be very lean. If I do open the throttle in 2, 3 or 4th gear, there is no problem at all. Really drives me nuts, that I can't fix these problems, please help!

Kind Regards,
Tibor

Hi Ken,

thank you so much for your suggestions. BTW I changed the pilot jet to #45 (main is #180) and the bike runs great. I still have the problem in the 5th gear though. I have changed the ignition coil to an ACCEL supercoil. Although it boosted the performance a little bit it didn't help the get rid of the issue. My second guess was also the fuel starvation, so I changed the petcock to a none vacuum operated one (pingel superflow) => didn't help. It really drives me nuts, because the bike drags till the top speed is reached, and then starts to loose power, regain power and then loose it again. The wiring should not be the source of the problem, because the bike is in a very good condition, no rust, no loose connections. Next I'll try the gas cap, but I'm afraid I have to wait till summer is here again. Thanks anyway for the great support Ken, I'll recommend you and your products to anyone I know and drives a Harley! Great source of knowledge!

Kind Regards,
Tibor

Hi,

I tried every possible pilot/main-jet combination with #42 #45 pilot jets and #175 #180 main jets. Didn't really help my problem, although I'm fairly confident, that the jets are now set to the correct size. The pipes are not drag pipes, but screaming eagle open end mufflers, like this one

as I said, the problem only exists in 5th gear near top speed (>100 mph). I can live with this problem, as I usually don'r ride my harley that hard, but it's just really crazy that I can't fix this problem.

Thanks for you help anyway!
cheer,
Tibor

Hi,

finally found the problem. It was the float level! After disassembling the carb again, I double checked the float height, and realized that it was way to low. After readjusting it all problems went away. Thanks for the support!

Kind Regards,
Tibor