I just ran some tests on a mod I did to my ZDZ50NG.
This can apply to other engines.
The mod was to slightly increase venturi size with a slightly larger carb.
I was looking for possible more revs on tuned pipe/smaller prop setup.
I already had some background on performance with the supplied carb.
The new carb didn't add much at all.
In that the engine was not really smooth at full throttle - I wondered if it may be preigniting a bit -
So I started backing off the recommended 6-7mm setting.
The starting (idle) rpm dropped --and the full power rpm went up.
So it appeared that the increase in cylinder pressure was causing the problem- just as it did on my supercharged Olds-way back when - I had to retard the mag as I added more blower boost -or I got detonation and power loss.
Anyway -
I settled on 4mm advance ( I use a mm measure as its per the ZDZignition folder setup)
The engine responded very well.
I ran some repeated tests on the engine on 22x8 and 22x10 Mejzlic -stock props and a 32-1 Mobil1 MX2T fuel mix.
The tachs had fresh batteries and readings were steady and repeatable
the pipe used -a muffled sample I am testing - .
The tuning length is same as KS- I am set at 75-cm -to 70 cm
At 70 cm -the 22x8 ran easily over 7700 rpm.
on the 75cm setting - the 22x10 ran 7400 rpm.
Both smooth setups.
The apparant problem I was having was preignition and or detonation due to the higher cylinder pressure-from a tuned setup.
Maybe -- higher octane gas would have cured this - don't know .
This is the first time I have actually been able to see where increased cyl pressures were a bit much for stock timing.
If you are into tuning - you might try running the timing back a bit on full pipe setups- it may/maynot help.
Anybody else note the same thing on piped engines?