OK, i have a basic understanding of why harleys do this, but id like to hear your explanations, Why when your on the gas, do these bikes reverberate fuel back out of the carb throat when your on the pump shot, just curious, i have a bike doing it particuarly bad in my shop, and cant seem to tune it out. chad.

 

My guess would be too much overlap, blowing back through the carb.. Got a big cam in it??

 

stock motor.

 

What kind of carb and exhausts and are you SURE someone else didn't slip a different cam in the engine? I agree with HGM that it sounds like an overlap issue. Went through the same process with reversion while dynoing mine with the larger VT EVL 5015 cam. It would even run differently on the dyno with the A/C off vs on and the reversion was visible.

We were finally able to tune the problem out with jet changes and by changing the advance curves on the Dyna 2000 ignition.

 

I had the same problem on my 67, it was solved when I switched from the Tillitson to an super e, I have heard different ideas to why this happens such as a lean condition to the type of cam to what pipes are on the bike. I would check the timing first.

 

Chad, this may be a little "out there" (but then, you all know me) Anyway, my CV (basically stock bike) did that until I replaced the stock accelerator pump nozzle (pedestal) and the check valve. I used the one out of a DynoJet kit which had a brass check valve (rod) instead of the aluminum stock piece and pointed the nozzle directly at the needle - contrary to the instructions of course

One more thing, maybe relevant - the later (well most stock) cams had a lot of lobe separation angle making for a much higher idle/low rpm vacuum and that tends to pull fuel past the stock check valve and allows them to constantly slober with the stock check valve parts. Usually not a problem with performance cams since idle vacuum is decreased, but it's enough to cause lower mpg and dribble gas all over the air cleaner at shutdown...

 

Part of the problem on the HD V-Twin is the short intake runner length. If I remember correctly the proper runner length should be around 17". Having short runner length allow the reversion pulses to force fuel and air back through the runners and out of the carb throat at certain RPM's. Combine that with the overly rich low/mid-range condition caused by most jet kits that exacerbate the phenomonom. This is also compounded by having an improperly tuned exhaust. Hope this helps !!!