Technical Distributor Rotor / Cap causing backfire?

Brother In Law

 

I did try the pump cam moving from #1 to #2 hole. That didn’t move much, and didn’t help anything, so I put it back.

I have t tried other cams. I know they exist.

PCV is over by #8. Having trouble imagining how PCV would suck anything out of the cylinder directly. #8 was only 135 PSI a couple years ago, so it’s probably just wear. Not too unexpected for a 1974 long block that I don’t think has ever been apart.

 

I haven’t changed wires yet, so that’s still a possibility.

 

If you're dropping to 4.5-5" hg @ light throttle opening I would think you'd want a PV that opens @ 3.5 . Its the seemingly low vacuum @ light throttle that bothers me ?, that " seems " more like a worn out motor / heavy vehicle scenario ?

 

You have 28 degrees of advance at idle? With no vacuum? How does it start? Does it buck the starter? I would think the idle timing should be in the neighborhood of 12-16 degrees with no vacuum advance. It looks like the mechanical advance is coming in too soon.

To me, the cruise ratio looks pretty good, if maybe just a little lean. WFO looks good at 12-13:1. Idle at 17:1 seems way to lean to me. How far out are your idle mixture screws? The engine runs on the idle circuit until maybe 2000 rpm, when it moves on to the main jets. If the idle circuit is lean, then you are probably running lean when the problem occurs. Idle flow is controlled by the idle feed restrictors in the metering blocks or plates. I don't know if your carb has changeable jets there, but you might try richening up those IFR's.
I have been playing with my AFR gauge for a couple of years now. I added a vacuum gauge right next to the AFR gauge. That helps understand what is going on. You can see when the PV is activating.
Something you need to remember is that with most installations there is only one O2 sensor, so you are only getting an AVERAGE of the four cylinders in one bank, half of the engine So that afr gauge is not necessarily the last word.

 

Cranks and starts fine. No bucking or kickback. The cam has enough overlap that this is where I ended up on the static advance. A more appropriate cam would probably work better with less static advance.

With the medium springs, I’m not getting a lot of mechanical advance. Going to the light springs for more got me in to detonation territory. Went back to medium.

Idle mixture was tuned for best vacuum at idle. I don’t know how many turns that is, exactly.

Currently have my vacuum gauge next to my AFR gauge. At 5”, the 6.5 PV is definitely in play here.

Good point on the O2. I’m reading the drivers side bank and assuming that the passenger side is the same. Not necessarily true. And it’s not per cylinder.

 

The bottom end is probably somewhat worn. A 74 long block that I don’t think has ever been apart. But the cam has enough overlap for not making a lot of vacuum.

Not especially heavy, a 37 Chevy sedan.

Lower opening PV is an idea. I was thinking about trying a 4.5 to see if that does anything helpful. Conventional wisdom is half of idle vacuum, which puts me at 7 (6.5), and that more fuel / rich is better than not enough / lean to keep from burning pistons.

 

You keep referring to the can , could you post the specs ?
28° @ idle is a bunch !

 

Crane Cams Fireball II 290H
Part #114802

Lift:
Intake @cam: 3027 @Valve: 454
Exhaust @cam: 3027 @Valve: 454
Rocker Arm Ratio: 1.5

Cam Timing @.001 Tappet Lift:
Intake Opens: 35 BTDC Closes: 75 ABDC
Exhaust Opens: 75 BBDC Closes: 35 ATDC
Advertised Duration: 290

Spring Requirements
Part #99848
Closed: 105 lbs. @ 1.700 (1 45/64)
Open: 265 lbs. @ 1.280

Cam Timing @.050 Tappet Lift:
Intake Opens: 3 BTDC Closes: 33 ABDC Max Lift: 105 ATDC Duration: 216
Exhaust Opens: 43 BBDC Closes: (7) ATDC Max Lift: 115 BTDC Duration: 216

Recommended RPM Range
Minimum: 1800
Maximum: 4400
Valve Float: 6500

 

Yes, 28* seems like a lot, but it’s where it seems happiest. Hashed out here https://www.jalopyjournal.com/forum/threads/sbc-non-stock-tune-up-does-this-seem-right.1147531/ a while ago.

 

I’ve been out of town on a road trip. Left northern Illinois last Friday. Two days to get to Atlanta via secondary roads, including a trip through Tail of the Dragon. Five days of Power Tour from Atlanta to Bristol. Then two days to get home via Cherohala Skyway and secondary roads.

Not a single backfire from the engine.

So I think I fixed it. I don’t understand the actual failure, but I changed one thing (power valve) and it got better. So it must be the PV.

I compared the opening of the old PV:



to the new PV:



The PV feeds gas to a pair of (drilled) restrictors, so I don’t know if this difference matters. Or if there is some other failure of the PV that I can’t see or measure.

 

I would think that length road trip would be an adequate shakedown.
Carry on!

 

Congrats David.........

 

This is a good read, and I remember the ignition timing thread. That thread helped convince a few of my friends that the engine will tell you want it wants for timing, and the specs it likes are the specs one should use.