5.7 Timing

I finally got my PMC 5.7 running. The timing was messed with by the P.O. so I've dialed it in by ear best I can. The specs I can find call for 4 deg. BTDC for the standard motor and 10 deg. BTDC for the Hi Po motor. Mind you, the whole thing is somewhat of a mutt, with a Mallory 609 ignition module in a Uniflite distributor. If I dial it to 4 deg BTDC at idle it runs pretty bad. more advance seems to run smoother, up until it starts badly. Where it starts best, it doesn't run smooth at idle. The other issue is, the gas is somewhat old in the tank - I'll be filling with fresh hi-test in the next couple days.

Any recommendations for timing this thing?

Ayuh,..... A SBC can run quite well at 8*btc,..... 'n addin' good gas to bad gas, makes more bad gas,......

I'll try 8 BTDC. Thanx for the recommendation. Unfortunately not much I can do about the old gas. Fortunately there's not much left in the tank. I'm hoping 90 octane and a can of Seafoam will improve things.

swampyankee said:

I'll try 8 BTDC. Thanx for the recommendation. Unfortunately not much I can do about the old gas. Fortunately there's not much left in the tank. I'm hoping 90 octane and a can of Seafoam will improve things.

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Ayuh,.... Pump it out, 'n start fresh,....

I rebuilt the carb and replaced the fuel filter/water separator and it ran better but was now backfiring thru the carb on acceleration, although at least it stayed running with idle set at 1000 RPM or so. I enriched the idle mix a bit and it idled better, adjusted idle down to 800 RPM or so and checked timing at 10 BTDC. We took it back out but it was still backfiring when pushed past 2200 RPM. It stalled in the fairway on the way back to the slip and we drifted to a nearby dock. The dock owner came down and had some experience with SBC's with Holley carbs and thought it could be bad gas, but I put my timing light on the coil wire and cranked it, and got what seemed like one signal per revolution, instead of the 4 or so per rev. It seems to be getting plenty off fuel as indicated by healthy spray from the accelerator pump. Since we were close, the marina owner came and towed us back to our slip. After an hour or so, it started up and idled fine, and I rechecked timing at 10 deg. BTDC.
I'm at wit's end at this point. The guy that helped at his dock suggested that I should put an HEI distributor in it, since the intermittent spark seemed to indicate a bad ignition (I've read that the Mallory 609 is less than reliable). Before I invest in a new distributor, I will replace the cap and rotor, and I was thinking of replacing the ballast resistor as well, since it it could lead to an electrical failure when it heats up. I will also check compression to see if a bad intake valve could be the cause of the backfire.
The engine wasn't backfiring before I rebuilt the carb and the only thing different was that I found 2 gaskets under the power valve and only put one back.

Anything else I should look at?

The engine wasn't backfiring before I rebuilt the carb

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Ayuh,..... Then you did something wrong there,.....

Backfires through the carb, is a classic symptom of a lean condition,.....

bondo said:

Ayuh,..... Then you did something wrong there,.....

Backfires through the carb, is a classic symptom of a lean condition,.....

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God suugestion tough lacking detail. I sprayed carb cleaner around carb while running to check for vacuum leaks and got nothing. I dismantled the primary side bowl and metering block, and double checked gaskets, ports, jets, etc. Blew out everything with carb cleaner. Finding nothing out of place, I swapped out the power valve with the old one which checked out ok. All back in place, started, warmed and revved engine and still got backfire. To eliminate secondary circuit, I held secondaries closed and revved. Still backfired. I installed a new distributor cap and retimed to 10 BTDC. Still got backfire.
Any other suggestions?

I've experienced a backfire through the carb when my distributor was not advancing the timing as I increased the RPMs. Are the weights in the distributor (assuming your distributor has them) able to move as they should? Have you checked the timing advance with a light at say 1500, 2000, 2500 RPM?

DJR said:

I've experienced a backfire through the carb when my distributor was not advancing the timing as I increased the RPMs. Are the weights in the distributor (assuming your distributor has them) able to move as they should? Have you checked the timing advance with a light at say 1500, 2000, 2500 RPM?

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As I was changing the dissy and rotor I checked for free movement and spring return and all seemed well.

I dismantled the primary side bowl and blew everything, and checked for correct and proper placement of parta, gaskets, etc. No change. While the motor was running I sprayed carb cleaner all over the carb body, base, and the 2 plugged vacuum ports. No vauum leaks detected. Revved the motor and got the backfire thru carb. I held the secondaries closed while revving to eliminate that circuit and still got backfire. I went thru sparkplug wiring and found 5 and 7 to be crossed. I corrected it and started and revved the engine, and did not get backfire. I shut it down and closed everything up and re-started, and got one backfire while blipping the throttle. UGGH!
The crossed plug wires could be my smoking gun, but I'm not sure whether they had been that way all along or I crossed them while changing the distributor cap, as careful as I was.
The backfire had previously been while trying to increase revs under load. Maybe wishful thinking, but backfire at the dock while blipping the throttle may be more of an accelerator pump adjustment?

I'd like to sea trial it, but I'm paranoid about getting stuck out and having to be towed in again...Maybe time to call a mobile tech
?