Had a 750 Mighty Demon on my SB..ran good with 18 initial timing slight stumble but no knocking or pinging.Changed to a 650 speed demon...no stumble but had to pull lots of timing out(10 initial) to keep from knocking and pinging...Went back to the 750 and put the timing back in...no more pinging.
Plugs looked the same from either carb...other than the obvious sings of detonation.
What makes 1 carb different than the other in the cause of detination?

Thanks
Rob

smaller venturie makes it burn a lil hotter and a lil leaner because of jetting and a stronger signal from the carb. The 750 with more gas being added cools the cylinder more during overlap periods. Kinda like the oval port/ square port debat on a BBC.

There's lots of things it could be. Perhaps the 650 was pulling in the distributor vacuum advance sooner/stronger than the 750? Maybe the power valve on the 750 was opening sooner than the one on the 650?

maybe the 650 was just plain lean? was it new out of the box?

My experience with Mr. Barry Grant , he is lost when he installs air bleeds in any of his carburetors . Go back to using Holley or AED your problems will be over.

Dave

Thanks for the ideas...I have used a 625 road demon on this motor too...no detonation problems.

I dont have the vacuum advance hooked up..so thats not it.

The carbs are straight out of the box as far as jetting just minor tweaks to get the idle mixture and such set.

I think it must be the secondary jetting cooling it off and thats why I didnt see the problem with the vacuum secondary carb since the open rate was different.

Thanks Again
Rob