Timing Curve on 360 in an RV
but it’s actually fuel igniting in the exhaust.
no we're getting somewhere.
how is this possible?
I'm assuming this happens in Neutral at 3000rpm.
Perhaps the fuel-air charge did not finish burning in the chamber. This could be because there was not enough air in there to react with, so some unburned fuel left the chamber.
Or it could be too much fuel for the amount of oxygen.
Or it could because the fire was not lit early enough, so the fire just never had enough time to finish.
Pick one; I can't think of a fourth option.
But really it amounts to almost the same thing, the A/F charge is finishing the burn in the header/manifold; and it can only do this if there is oxygen already in there when the fuel gets there. How is that possible? Only one way I can think of; a faulty gasket, or poorly fitting gasket, at the head. A header, by design, will "suck" air in there, behind every slug of exhaust moving down the tube. And 3000 is the perfect time to do it, as the engine is approaching peak efficiency.
A log manifold does not do this as it is usually at a higher pressure than atmospheric.
Afterfires are sorta normal, but are usually contained in the header, so you don't notice them, Ima thinking that after you seal up the header-flange, your engine will want more no-load timing at 3000, and a change in AFR, probably leaner, but could go either way.
