Anyone who can figure this out honestly deserves a medal. After months of working on this motor, I'm convinced that it is either possessed by demons or fundamentally built incorrectly.

Engine needs way too much initial timing advance (60+ degrees BTDC) in order to run semi decently. I realize that it shouldn't run or even start with this much advance, but it really does. At 12 degrees it backfires like crazy out of the exhaust (loud popping noise) and sometimes out of the carb. At 60-70 it idles okay but still backfires occasionally. No amount of timing seems to solve this issue completely.

Engine specs:

• 454 LS4 block, cast in 1975, very likely not original to the car (1974 C3)
• 91 octane pump gas
• Manual 4-speed
• Stock 8.25:1 compression ratio; flat-top pistons
• Aftermarket flat tappet cam, hydraulic lifters (unknown specs, seems mild based on idle vacuum reading); cam does not have a 4-7 swap
• Edelbrock Performer RPM rectangle port intake
• Edelbrock Performer RPM rectangle port heads; stock combustion chamber size (118cc)
• Brand new Quick Fuel 750 CFM double pumper style carburetor
• Brand new Holley mechanical fuel pump and regulator (set to 7 PSI, confirmed steady)
• New fuel lines and fuel filter
• All new: rotor, cap, plug wires, plugs, Pertronix Ignitor III ignition module, Pertronix 0.32 Ohm Flamethrower coil, distributor mechanical advance springs and weights
• New balancer with etched timing marks. TDC mark was confirmed multiple times with a piston stop tool, as well as with borescope. It also lines up with the stock timing tab.
• Headers, aftermarket exhaust system
• New vacuum advance; it is NOT plugged in while setting the timing
• New battery, Optima red top

Work that has been done:

• Compression test: 160 PSI consistent compression on all cylinders (+-5 PSI)
• Carburetor float levels set at half way up the sight glasses
• Vacuum at idle is around 17 inHg
• Confirmed the firing order multiple times 1-8-4-3-6-5-7-2 (5 and 7 are not swapped)
• Timing set is installed correctly, camshaft does not have advanced or retarded timing
• Confirmed timing light works (standard light, not dial-back), also connected timing light on all cylinders, all are getting spark
• Spark plug color appears to be consistent and good (tan, maybe a little too white) one plug appears to be oil fouled, but cylinder has good compression
• Valve adjustment: 0 lash with quarter turn of preload
• Confirmed Pertronix unit has good ground and full 12V while the car is running
• Spark plugs are gapped properly
• Spark plug wires have correct resistance
• No intake manifold leaks that we can detect and all vacuum accessories do not leak
• Mechanical advance works (brings total timing to 80 degrees)

I know that it is common to get the firing order wrong or to find TDC incorrectly. That is not the case here. I WISH it was the case. It has been checked more times through more independent methods than I care to describe. I know that "timing is everything" but any attempt to set it to the correct timing yields a motor that barely idles and backfires like crazy. When we took delivery of this car the timing tab had been removed so it was impossible to know what timing the PO was running. It ran even worse at that time.

Thanks for reading this very long post and for your help! I have reached the limit of my engine knowledge at this point and need to call in the experts on this forum. At this point, my next call will be to a priest.

 

I like the looks of everything there except two:

This was a "points & condenser" system or still is? (Pertronix)
Timing Tab was removed. H-m-m-m-m-m.

Which way are the advance weights installed. The old points & condenser weights are flipped over compared to HEI systems.
Obviously the number of degrees indicated are incorrect. You need to figure out which part is lying to you.

And just curious. Why did you choose such low compression ratio? Adding something later?

 

The original points and condenser was replaced by a Pertronix Ignitor I module by the PO. I figured that the module must have been defective to produce backfiring like this, so I replaced it with a Pertronix Ignitor III (a slightly more advanced unit), but the engine still ran just as badly. As for the timing tab, why it was not present initially is a question for the PO. To actually be able to time the engine properly I installed a new balancer with etched timing marks and an adjustable timing pointer which was fine turned to TDC with a piston stop tool.

The advance weights are installed in the stock orientation (this distributor appears to be original to the car). The reason for this is that the Pertronix system is meant to be a direct points/condenser replacement and the distributor otherwise remains stock.

As for the low compression ratio, this is the stock compression ratio for this engine (LS4). The engine could probably do with higher compression but the PO decided to install new heads with the same combustion chamber size as stock (118cc) and did not change the stock flat top pistons, hence retaining the 8.25:1 compression ratio. I figure that even with low compression, the engine should at least run passably but it clearly doesn't.

 

I hear ya. The 8.25 C.R. is pathetic unless one was adding a turbo later. I would think a C.R. of 9.5 - 9.7 would be more enjoyable, but its your car.

Just for clarification on the advance weight set. On HEI systems the weights somewhat appear to be like the shape of the state of Florida.
On the old points style, they would be flipped over, up side down FLA.

You got me, on the timing way off issue. Only thing that comes to mind is, when you verified the TDC with a piston stop that maybe it was on the Intake stroke?
Stranger things have happened to the best of us.

But I wonder about the new, after market, modified and questionable timing pointers accuracy?
Basically, something is off about 48 degrees.

 

45 degrees is an offset of ONE cylinder in the firing order. Another 15 degrees (or so) for 'normal' advance position would yield that 60+ degrees BTDC value you are quoting. I think you have the plug wires indexed ONE position off from where they really should be--and the 60 degree BTDC is compensating for that error.
With 'things' as they presently are, I bet the engine would run better at 65 degrees BTDC....

PM me on where to send the medal....

 

Correct me if I'm wrong but how you index the plug wires in the distributor shouldn't matter; if you clock them all over 1 position then your timing would be thrown off by 45 (90?) degrees, but you'd have to then rotate your distributor to get your timing back to where it should be (and get the engine running again probably). The timing light isn't really aware of the inner workings of the distributor, it simply flashes when the cylinder is sparked. Having the 1 and 2 cylinders swapped in the firing order would definitely result in the number one being 45 (90?) degrees ahead of where it should be, but I've checked the firing order multiple times and it looks correct. I wish it was something this simple but I've tried my best to find "dumb mistakes" and haven't come up with anything yet...

EDIT: the crankshaft makes two revolutions per camshaft revolution, so 720 degrees / 8 cylinders must give a spacing of 90 degrees between cylinders being lit. Unless my math is off that would mean that being off one position on the distributor would result in timing being off by 90 degrees rather than 45.

 

pics would help,
With engine off set the timing to 60* then pull the dist cap off and lets see where the rotor is as compared to the number 1 plug wire
the plug towers are 45* apart on the cap so if you missed the number 1 by one place you would see this issue.

Neal

 

I'll see if I can get some pictures.

I did this test earlier to confirm that what I was seeing on the timing light was real, and everything checked out. With the balancer at TDC the rotor was pointed about 25-30 degrees past the post for the number 1 wire (ie between the 1 and 8 posts), so it must have fired around 50-60 crankshaft degrees BTDC.

 

Have you confirmed that both valves are fully closed on the #1 TDC piston position?

 

Both valves are fully closed as the balancer approaches TDC on the compression stroke. This was confirmed with a compression tester and turning the engine over by hand. Clearly the cylinder is building pressure and is on the compression stroke as expected. I also checked this with the valve covers off, and the valves appear to be operating normally. The distributor is definitely not 180 out.

 

Are you sure your timing light isn't defective?

 

I've confirmed that the timing light is accurate. This was done by setting the crank to 0 TDC on the compression stroke, then checking the position of the rotor relative to the cap. It seems to line up exactly with what the timing light is reporting. I could get another timing light to confirm but I would imagine that if the timing light had some sort of defect or latency, it would show excessive retarded timing as opposed to advance. In order for it to show advanced timing, it would have to somehow flash BEFORE the spark happens.

 

So when you bring the motor to TDC, with your timing set at 60 BTDC, and you pull the cap off, the rotor is in between #1 and #8. I’m thinking that if moved all your spark plugs forward a post, this would effectively delay your timing.
I personally would set the motor at TDC, and pull the distributor and rotate the oil pump post with a long screw driver until I could stab the distributor where the rotor is pointing exactly at the post I want #1 spark plug to be. Then route the rest of the firing order and see if it will start with 0 degrees. Then just advance from there.
you mentioned that it doesn’t matter where you index plugs, but in my opinion it does. You can set a motor to TDC, and depending on how you orientate the #1 plug in relation to which post the rotor is pointing to, you are affecting the timing.
keep us posted I am curious what the answer is

 

I probably missed it but when at tdc is your rotor tip point at no. 1 cyl or is it positioned somewhere else and compensating with wire position on the cap?
Ive done that before...for one reason or another putting it where its supposed to be gave the right result
I dont see how it could even run on a true 60 deg somethings off

 

@randallsteel That's correct, moving all your plug wires to the next index WOULD change your timing. However, this would show up with the timing light. Maybe I'm misunderstanding something here, but what would the difference be between clocking all your plugs one index on the distributor cap and just simply rotating your distributor 45 degrees? As far as I can tell, these two actions would be functionally equivalent. The distributor cap has 8 arbitrary posts that do not have a preferred plug wire (ie there is no correct post to install your number one plug wire).

 

If carb is running super lean, it would take that amount of timing to fire it.

 

This is most logical. BUT i have had a slipped timing chain need similar advance to make it home. So the cam could be installed a tooth off

 

@wolfk This is one thing that I've considered. Running extremely lean would cause the mix to burn very slow. Is there any way to test for this? The carb is new and I've also tried another Edelbrock 750 cfm and got the same results. Maybe the jetting is way off?

 

@Matt Gruber This possibility was also considered. I took the whole front end of the motor off to get a look at the timing set and the sprockets and chain were installed correctly (0 marks line up). Cam also looked good and not worn. Not sure if the cam itself has some advance ground into it... even if this was the case I'm not sure how this would affect ignition timing since cam timing and ignition timing are fairly independent.

 

Nab
once i put a cam in my 72, i was sure it was correct, but i put on a degree wheel and it was off 1 tooth
glad i double checked.