the engine in that video is sitting a bit forward on the skid I made up. as stated in the original post, I have run the motor as far as tipped backwards 10 degrees, and it doesnt change the reversion.

every part comes into play dealing with this, cam, cam timing, intake(yes intake) heads, timing, and of course exhaust. seeing how your cam is getting close to the HP500 cams and they carb one was known to revert(mine did somewhat) I am not so sure what you are seeing is not normal...there is always scavanging in the exhaust, introduce water and there you go. Running thru the prop will make worse, I would put in boat run and check then, hard to tell without running complete

What is the ignition timing set to at idle?

ignition is currently at 14 BTDC with base timing at 800 RPM and a 22 degree advance kit. Dizzy is all in at 3200 RPM at 36 BTDC

quick post to get count up to 10

yet another

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OK, so I may have access to a bit of tools.... going to take the pile of exhaust parts and build this

the short H-pipe should help scavenge at low RPM, it will also equalize the pulses prior to where I am mixing water and exhaust (about 50" down stream.) and should eliminate any chance of reversion above about 3000 RPM, the cross over will become ineffectual however I only need it at low RPM's. an X-pipe would yield better high RPM scavenging, however I don't have room and reversion is at low RPM anyway.

the cross-over will be jacketed. just not sure I am going with a half-marmon flange or a hose





I did super-impose a 8.1/bravo for reference even though I have a carb'd VP 290 DP

Nice grafics....how are you going to cool the section of exhaust pipes exiting from the exhaust manifolds?