Large rpm drop

nm9stheham said

Ruptured PV diaphragm from a backfire..?? Never had that happen myself, but supposedly a common issue for these PV's after a good backfire.

I don't understand needing 30* advance for this latest idle number, with this cam and this SCR/DCR, and the secondary slots closed off. Something sounds fishy in the timing advance measurements....??? Or just super rich with the PV problem....

Click to expand...

It seems like a combination of the excess fuel and the PVC not hooked up.

That carb has the backfire check valve, so if there is a damaged diaphram, its not from backfire.
I've had occassionally leaks into the PV cavity - its always something like the block not sealing or PV not sealing to the block.
The impressions on the block gasket look good, that's why I think its the PV gasket or somehting like that.