Topic: The latest- runs ok, but POPPING under 1/2+ throttle. Opinions?

[BlackMax]

Okay, the saga that is my damn '78ss is coming into focus a little.  Here's where I am now:  I can fire it up easily (can use my hand on the kick start, when cold with no choke). It idles nice with a good 'lope' (cam).  The plugs look tan to dark tan.  On the kickstand, it starts, runs and revs very well.   When I drive it, as long as I don't get into the throttle, it's smooth and sounds good/solid, even above 35000 rpm.   However, when I get into the throttle, it stumbles (but I know the accelerator pumps are NOT working...so that's ok) then doesn't want to catch.  It sputters and catches and loses a cylinder or two, then they come back on for a second....etc.   However, what I'm REALLY noticing is the POPPING under throttle.  "POP....POP POP POP.....POP......POP....POP POP" while 1/2 throttle or more, especially above 3500.  

So, I'm guessing the idle circuit is fine to slightly rich.  But, the transition is going lean?  Again, I'm speculation.....and thinking that the main jet mixture is too lean to burn completely in the chamber, so it's igniting (popping) in the exhaust and also creating LOOOOOW power under throttle.  

Sound reasonable?  The main jets are 105s (don't look to be drilled out), and the bike has pods, a mild cam and 4-1 headers.   Of course, there could be other explanations as to why/how unburnt fuel is getting into the exhaust and popping under throttle.......maybe you have some thoughts to share?

....making progress!     As a review:

Head-            new with all new components
Cam-             new mild performance
Timing-          initial set to 5 BTDC (advance working)
Carbs-           checked and cleaned, floats adjusted, emulsion tubes clear
Accel pump-   NOT woking (none)
Coils-             tested to be fine
Points-           Filed, gaped, cleaned and working fine
Condensers-    unknown, but point spark seems normal/blue
Plugs-             good
Spark-             good whenever I check it (to the plugs)
Gas-                new
Petcock-           seems fine (fuel seems to be getting to carbs...no prob)
Compression-    140 in all
Intake-             pods
Choke-              open



 

[mlinder]

If it starts with your hand on the kickstart with no choke, it's too rich, especially for an accel pump set of carbs (they run a bit lean, using accel pump to richen when acceleration is needed). I had over-jetted the idle circuit on the kz650 and the bastard would pop all over place until that was rectified.

I can't remember, have you synched the carbs? And, really, I dunno if 105's are big enough for pods, but of course, it depends on the bike, the pods, and the altitude.

Your bike should work best at 5 grand to redline. Can you do a plug chop at WOT and tell us what it looks like? Plugs can go tan when rich on the idle circuit but lean on the main circuit. The big black deposits made at rich idle get cooked off in the lean top end, so they 'look' ok, but they aren't.