@Matt Gruber Hmm that's interesting, it could be the case. I didn't install this cam so it's possible that it's bad. When you installed yours what advance did you have to run in order to get decent performance? My understanding was that cams only go in one way, so if the sprockets are aligned it must be right (see picture in my previous post).

 

What are the cam specs? Low compression ratio with high overlap cam likes LOTS of advance. Granted, probably not 60 degrees, but could be contributing to the issue.

 

Are you sure this is the correct alignment mark on the crank sprocket? I've only seen them up near the teeth but then again I've not looked at very many. Have you rotated the engine to see if there's another mark?

 

@wwiiavfan Unfortunately the PO did not provide the cam card and we don't really have any specs on the cam. The cam doesn't seem to be anything too ridiculous. It's a flat tappet and the engine pulls decent vacuum at idle, so probably something milder. I would have assumed that even with an insane cam you wouldn't need 60 degrees just to get an engine to idle properly and not pop out of the exhaust.

 

@67:72 According to the manufacturer of this timing set, this would be the correct mark. Depending on the keyway used, the correct symbol has to be selected. There's a triangle, square ad circle. The keyway selected is the round top one on the right, which is the "factory" setting (ie neutral cam timing). The appropriate symbol for that keyway is the O that you see in the picture. I didn't really see any other stampings (other than the square and the triangle) in the sprocket. I thought it was a problem initially because the stamping did not align with a particular tooth, but apparently this is normal on some timing sets.

 

xe262 comes advanced 4, that is what i used. THE reason for using a degree wheel is to check everything- parts get stamped wrong sometimes.
My base timing is +13 with 87 octane.
Have you checked cranking compression? Can i assume the vac. advance is disconnected when you test base timing?

 

@Matt Gruber The PO that installed this cam used the neutral (no advance) keyway so we just left it there. It's possible the timing set was stamped wrong, but I guess the only way to check would be to use a degree wheel as you say... might have to rip all the stuff off the front of the engine again. Cranking compression (with compression test using the starter) is 160 psi. Vacuum advance is not connected when finding the base timing.

 

I'd forget about tearing it apart if it drives normal. I'm lazy
BUT if it gets like 6-8 mpg, then i'd degree it.

 

@Matt Gruber Mileage is pretty bad, maybe around what you quoted. The main issue is the insane popping out of the exhaust, almost like it's timed too retarded or plugs are going off at the wrong time while the exhaust valve is open. It doesn't seem like there's any crossfiring between the plug wires and the plug wires are brand new, so it seems unlikely that that would happen.

 

What spark plugs are you using? Brand/part#.

 

@71 Green 454 Right now I'm running NGK BPR6ES because that's all they had at the store at the time. They are gapped to 35 thou. I have a set of AC Delco R42XLS that I might try though.

 

I agree you have checked most everything.

I first assumed you had a digital timing light and it was at fault, but then noticed you have a standard light. And you checked TDC it so that eliminates the balancer.
I assume that means you are using the timing marks on the new degreed balancer?

Cranking compression is ballpark about what I would expect for a 8:1 CR engine.

And you are correct the wire location on the distributor does not matter, they are 45 degrees apart, #1 will fire to the closest terminal. What is very interesting here is you seem to be MORE than 45 degrees off. But yes please stop it at tdc and check rotor cap to cap #1 terminal alignment. Which ever terminal you want to place #1 on is entirely up to you, and spark plug wire reach length. Confirm please.

Since you can check TDC accurately, and make a pointer, I assume you can degree in a cam with a degree wheel, that is what I would check next. The popping out the exhaust could be either very retarded timing or extremely lean.

But since degreeing the cam is a lot of work check 2 easier things first:

• What is the timing curve set up like? Idle, total, and at what rpm? How much does the vac can add? Does the vac can add it at idle? (ie: manifold vac or ported) Measure it and post.
• How is the carb AFR? Check and see if the idle mixture screws can drop idle rpm (50+) in BOTH directions, in and out. Confirm and post.
• Then set them for best vac reading.

Do you know the mfgr of the timing set?

It could also be that the duration is just too large for a 8:1 CR engine. This CR really needs short duration cams and careful LC and ILC settings to get decent cylinder pressure. It may be that it just has too much overlap for the CR. I have seen multiple engines that run very very poorly with that combo. And it is very difficult to figure out the cam while in the car. With a degree wheel and a dial indicator set to .050" valve lift you can get reasonably enough close tho.

The decently high 17 in idle vac seems to indicate a milder type cam, but it could be being artificially "propped up" by incorrect can timing.
Hopefully Jebby will chime in too. He is very good at tuning & combos. If not PM him.

 

@leigh1322 Yeah, the timing tab has been replaced with an adjustable pointer that we've aligned with the 0 on the balancer at TDC. All measurements are coming from the marks on the degreed balancer.

In terms of the distributor, while we are 45 degrees off at the crankshaft (since that is where the advance is being measured), from the distributor's perspective we are only about half a post off. In other words, at 60 degrees of timing advance, the rotor ends up being about 30 degrees past the number 1 post at TDC. I did this test already and confirmed that when the crank is at TDC, the rotor is roughly 20-30 degrees past the number 1 post (between 1 and 8) which agrees with the timing light reading.

It's looking like the front of the motor will have to come off again to get the cam degreed properly. The PO left no information about it so it could be anything. Regarding your points:

• Not much has really been tuned in terms of mechanical advance since it's really hard to curve a distributor when a motor is backfiring like crazy and possibly running dangerously at too much advance... what I do know is that the mechanical advance appears to be working and adds around 20 degrees of advance when the motor is revved (depending on what springs are used, it's all in at around 3000 RPM). During all timing tests the vacuum advance was left unplugged (and plugged off at the carb), but ordinarily it's on ported vacuum. It is working though as it was checked with a vacuum pump.
• The idle mixture screws don't really seem to help the problem, I'll have to check if they can affect the idle by 50 RPM

Timing set was manufactured by Cloyes.

Yeah this was another thought that crossed my mind. I'm concerned that whoever built this engine bought a big cam and high flowing heads/intake, which are not going to work will with the stock compression ratio. I'll have to see about degreeing the cam and seeing what's really in the car. I'll get back to you on your other points, thanks!

 

I guess you have older heads on that 1975 454 block since you aren't using tapered plugs.

 

20 degrees of mechanical advance all-in by 3000 rpm sounds good.
How much does the vac can add? Shoot for 10-12, more is not good. Add a limiter if necessary. And then please move it to a manifold vac port.
But neither of these sound like your issue.
Check carb mixture screws, then degree cam.

 

@71 Green 454 These are Edelbrock rectangle port aluminum heads that the PO installed (part number 6055). We also have the original cast iron heads and intake but the original heads appear to use a different plug size. Those heads are either original to the 1975 motor or to the original 454 that was once in this car, I don't really know at this point.

 

Maybe I'm missing something but you keep saying that the rotor is after the number 1 post in the dist, but that would retarded timing not advanced, if it were advanced it would 30* before the number 1 post at the dist.

Neal

 

@chevymans 77 Correct me if I'm wrong, but this is way I interpret it. When the balancer is at TDC, the number 1 cylinder is at max compression. If the timing is advanced, the cylinder would have already been lit and therefore the rotor should already be past the number 1 post. If the timing were retarded, you would still need more travel in the crank in order for the cylinder to fire, so the rotor would have be before the number 1 post.

 

you do realize that #6 is in firing pos?

 

Exactly. Moving all the terminal wires one notch, then having to make up the difference by turning the distributor base is non-productive.
The only reason people have to move the dizzy orientation is when the vacuum canister is clocked way out of proportion.

And Nab Yags, you don't have to tear apart the front timing cover. In my opinion, if that cam was truly one tooth off, it would act as a "No Start" issue. Yours not only starts but idles.
And 160 lbs of compression would not be likely be happening with timing chain set off a tooth.