Back4More

Unfortunately I'm having the same issue...I go to get on plain and the Port motor backfires.
I don't want to hurt the motor or blower(10-71)trying to figure out the problem by anymore trial and error.
We have changed/new plugs, wires, cap, rotor, air filters, ignition box, fuel pump, exhaust system, new dist gear, checked timing, float levels and even took apart the carbs for a cleaning with new power valves.
A comp check (twice), with 120psi on all 8 cylinders.
The only other things we can think of is air maybe getting sucked into the fuel line...the mech gauge at the pump fluctuates from 7.5-6psi sometimes.
Or a valve train issue...broken/weak springs, bent rods or a bad lashing?
1/4 to 1/2 a turn should be within limits for a hyd. roller cam, no?
We are at our wits end on this one...time to pull the valve covers.

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Ttt

INDY27

I have not had the time to check into my problem, hopefully this weekend.

mo3d

Burnt or tuliped valve my guess too. Exact same thing happened to me in early summer. M1 w/ 5lbs boost. Been there and done that $$$$$
Time for a compression check. check #5 and # 7 first

INDY27

Did a compression check and #2 & #4 were about 125 lbs and all others were around 145 lbs, pulled the head and found that the intake and exhaust valves were leaking! Sent the head to have it checked, hopefully its just the valve seats. All the valves looked good. I should have it back in a few days.

Back4More

Glad to hear you found it...now its my turn to find my hic-up.

INDY27

They found a few valve seats leaking and had to deck the head a little, so I put it back on and got 175lbs compression on starboard, checked port side again and got 155 lbs. Ran the engine at the marina on the trailer under a load and still got the inconsistent popping through the intake, took it back to the shop, pulled the port valve cover to find one of the rocker arms had a lot of play so we relashed it and turned the engine over and saw that the rocker had less than half the travel of the others! So it looks like the lobe on the cam is gone... anybody got a suggestion on what cam to go back with? The one I have is the stock cam and this time I will use a roller cam and lifters, I want to be a little more aggessive but am worried about reversion since the silent choice is still on.

Back4More

need more info on your motor to make a cam guess...

INDY27

96 502 EFI with stock bottom end, heads have five angle valve job, large intake valves w/ incolnel exhaust vaves both are Ferrea, Crane gold roller rockers and either Comp or Isky springs... cant remember, Felpro 1047 gaskets, ARP head bolts, Stainless Marine exhaust, ProCharger M3sc stage II @ 5-6 lbs boost and ecu reprogrammed. I forgot to mention the cause of this whole problem was due to sucking up some sand and pebbles, I left from a beach area and within a mile or so the engine went into power reduction mode first and as soon as I looked down at the dash the buzzer went off and the temp was pegged, got towed in and the next morining took the hose of at the seawater pump and ProCharger's fuel cooler, flushed all the debris out and fired it up and everthing appeared to be ok so I took it to the marina ( about 5 miles) and had no issues on the ride over but the next time I used the boat is when all the problems started.

Back4More

Take a look at the ISKY #696275/284 Hyd. roller(228/238-.553/.578)...and have them grind it on a 114 lobe center.
http://www.iskycams.com/onlinecatalog.html

BTW....We might have found our problem being that the MSD caps we have are not of the vented type causing corosion and rust inside the dist.