Ok here is the story: 577 motors with Big Brodie heads w/ 1050 carb Crane Hi-6m Box with MSD Dist. Crane Roller Cam#139661 with morel lifters. Timing set at 14 base and 34 total, all in at 3000rpm’s. Port motor runs strong with no problems what so ever turning a 28P B1 prop. Starboard motor ran great and the only problem was a crack around the head bolt boss close to cyl#6. Ran 15 hours this way… with the only problem was water leaking from the crack into bilge.
Removed head to have crack fixed and machine shop found 2 springs on #8 broken. Did valve job and replaced 2 broken springs. All other springs measured fine. Put head back on and new problem developed. Idles fine and runs good until put under load. At 3000 rpm’s backfire thru intake…OK I think valves too tight…readjust valves… gets worse when hot. Readjust again….same problem. Also motor will only make 4400-4500rpm WOT. Before both motors ran 5200RPM’s.
Ok here is what I have done: Switched Carbs, coil, Ing Box, Coil wire, and new Fuel Pumps. Fuel pressure is 9psi at idle and around 5psi WOT both motors. Oh I have adjusted the valves so many different ways I can do it in my sleep. Just did a compression test and all cyls are 170 to 175 on all cyls. On Port motor they are 175 to 185 Bought a leakdown tester and I don’t know if the readings are correct or not but it shows 30-35 % loss and all cyls on both motors, so I don’t think this is correct. I am going to try a different leak down tester.
So as you can see this all started when the 1 head was R&R. The Port motor still runs strong…Any help and advice..would be greatly apprenticed.

I would spend an hour or so with a dial indicator and check your cam lobes if you had some broken springs a lifter may have been "floating" and the impact may have flattened a lobe just an idea............. (you should be able to do this w/out pulling the cam)

I actually did do this yesterday..no flat lobe. I just hate to remove the head again if i don't have to.

Could a leaking intake gasket cause this problem?

I think i found the problem....I went back to square 1 and started checking everything. I decided to start with the cylinder #8 that had the broken valve springs. I found the lifter on #8 Intake collapsed. When checking with a dial indicator it open fine but there is no movement or spring action from inside the lifter... I think I finally find the Gremlin.

Anyone have a pair of Morel lifters they want to part with.

I had the same problem with the Comp Cam's top of the line hydraulic roller lifters....colapsed plunger/spring inside the lifter. I was fortunate enough that it didn't do any further damage to anything else in the engine. Replaced both engines with Morel lifters. A nice feature about the Morels is that they can be totally rebuilt for about $200.

What kind of valve springs are you using? Have you ever tried any of the Isky brand tool valve springs?

Well that did not fix my problem. Man I was so sure it would...next step... Would having the wrong seat pressure also cause b/f under load. I'm thinking the machine shop that repaired the head did not set the springs correctly.... am i off base...

Maybe you have a slightly bent intake valve due to the prior weak springs. Popping out of the intake makes this a good guess. You should be able to remove the intake valve spring without removing the head. Do that and spin the valve checking for a tight spot.

BT

Update...Update

Bingo...took head off, took it to a head shop and intake valve was bent...If weather holds up this week I will test this weekend..thanks for all the info.

Hope this thread helps someone in the future...

Happy For You Hope It Works:d