bucking at cruise... need a diff view

[BB73Challenger]

Soooooo... with the 500 It runs great on every plane except for cruise/keeping the speed steady.

I got the carb tuned pretty well and started doing a bit more driving, when I noticed some bucking or "surging" just while cruising.
Not wanting to get into a detonation scenario I start playing with timing curves... like crazy. My initial though was over advanced.
I have a MP unit with the adjustable advance, NOT using any vacuum, timing right now at 24 initial and 36 total - engines seems to idle & hard accelerate great with these settings.Even blending some race fuel to get me over an average of 95 octane, which shouldn't even be close to a problem.
Using MP's curve kit with all those dang springs, and right now I'm running a single stage advance using their brown/brown springs. Again came up with that combo after a ton of trials - I can get the dizzy out changed and back in time in minutes now..

So then I read that after going nuts leaning out the carb for idle quality that maybe I was getting a lean condition at transition. So, went back through and fattened it back up, with no improvement.

So, can this just be a characteristic of this engine coupled with the 4 speed, gears tire height ect..?

When I get things tuned into where I think they should be I get what I would consider a great looking plug, just turning tan, no fouling, no "specs" of material burned into them, no melted looking spots, very crisp off idle response, the engine pulls hard when RPMs climb, everything I could want. I can also get the motor to climb near 3 grand ( street cruising in 4th around 1500 to 1700 ) and smooth as silk. If over advancing at cruise it should get worse as RPM's climb right?

I'm sure I'm leaving some info out, but figured time to take a breath and maybe get some second opinions - thanks guys 

I can live with some give & take as far as driveability in trade off for the performance, BUT I don't want to cause any damage either!

[Cooter]

"If over advancing at cruise it should get worse as RPM's climb right?"
Not nessesarily. As you step down on the go pedal, the vacuum drops to the Vac. advance and the amount of advance should drop back to mech. and base setting combined if running Vac. adv.
SOME Vac. adv. units can be adjusted by using a SMALL Hex "Allen" wrench inside the nipple to decrease/increase the amount of Advance it gives. You might try this.

[Strawdawg]

He is not using vacuum.

So, what happens if you back the initial down to something like 16 degs and try the cruise?

[BB73Challenger]

Yep no vac advance at play here..
As far as dialing timing amounts I have been as low as 18 initial, and it idles just awful. You can feel the engine is sluggish with still no "cure". I have even been the other way and even locked out the distributor and ran at 32 all the time for a very short time.

The cruise RPM is my big sticking point despite changes 

I also have been "mapping" my RPM's versus Advance in 500 RPM increments. My current spring combo starts a little lazy, only adding only a couple degrees of advance at cruise and is all in by 2500. I've gone weeker, stronger, and 2 stage - the engine likes this combo from an acceleration standpoint as far as from idle, light pull and hard acceleration.

[wally426ci]

I was too lazy to read all the way through but could it be lean primary jets?

[Chryco Psycho]

I have seen this before , the advance springs could be on he light side , change one to a heavier spring & see if it changes the condition

[BB73Challenger]

I was too lazy to read all the way through but could it be lean primary jets?

Yep made sure I was not leaning out at cruise/transition -
will add 1 of the next heaviest springs & check it out.
The MP chart that came with the springs doesn't cover ALL of the different grades, but they do list springs and their rate of strength.