Hey all,

First time poster so hope this is going in the right place. Currently working on tuning a stock, numbers matching ‘67 L71 and having trouble chasing down root cause for some exhaust pops occuring between 1500 and 1800 rpm under load.

Backstory: I was checking ignition timing and noted some crazy advanced numbers (near 30 BTDC) at idle. Found the mechanical advance to be hanging out slightly at idle. Cleaned up and lubricated the advance, restarted, and found initial timing to be closer to 18 BTDC without touching anything. Took it for a drive to see response and noted small exhaust popping under load between 1500 and 1800 rpm. No surging, pinging, lagging, or other negative issues were noted. Note that this condition was NOT present before I discovered the sticky advance weights.

I took data on the advance curve tonight and this is what I found. Vacuum was disconnected and plugged for this, and yes I did verify #1 TDC matches marks on the harmonic balancer.


RPM Advance
750 (idle) 18
1000 18
1500 22
1800 31
2000 38
2500 48
3000 48

Two things I’m seeing: 1) mechanical is coming in too early and 18 initial plus 30 mechanical overall seems crazy high. I will say the engine idle parameters are more stable at 18 BTDC than lower, but it’s not objectionable. My concern is the amount of mechanical advance.

Looking to see if I’m on the right track here or if I’m chasing a red herring. Thanks!

 

It looks like you have way too much mechanical advance so stiffer springs and/or lighter weights. It would be good to know if "under load" includes the secondary carburetors just starting to open. Mine would do that until I occasionally put the spurs to her.

 

Here’s some info on limiting your mechanical advance
https://www.racingjunk.com/news/perf...r-distributor/

 

Yes, 48 is very high at 3000 rpm. You should be looking at about 36° max at about 3000-3500. Once this is set, your vacuum advance will add more under steady load and could reach 46°-54°. If you're starting out at 48° the vacuum could top out near or above 60° which would likely be pinging pretty easily.

Take a reading after removing the weights and springs entirely to see what the base idle timing is. Your mechanical shouldn't be active below ~900-1000 rpm.

Send an email to Lars Grimsrud, asking for his write-ups on timing. <v8fastcars@msn.com> He hangs out most on the C3 forums but does pop down here on occasion. If you follow his step by step instructions you'll get a well running engine.

 

I am no expert, but your initial timing seems very high at 18 degrees BTC. GM specifications for a stock L71 are 5 degrees BTC @ 750 rpm. with mechanical advance starting at 900 rpm and 30 degrees @ 3800 rpm. Adding the mechanical advance to the initial advance gives you a GM total of 35 degrees for the L71. Of course GM is conservative and each engine can use more or less total advance but depends on various other factors not limited to mechanical variations, but also including fuel octane variations.
The RPM Advance GM RPM Advance 30 degrees total over 2900 rpm. Thus, ~ 1 degree per 100 rpm linear calculation.
750 (idle) 18 0 mechanical + 5 initial = 5
1000 18 mechanical starts at 900 + 5 initial = 5 so at 1000 rpm = 5+1 = 6 total
1500 22 popping estimate 900 rpm + 600 rpm = 5 initial + 6 = 11
1800 31 popping estimate 900 rpm + 900 rpm = 5 initial + 9 = 14
2000 38 estimate 900 rpm + 1100 rpm = 5 initial + 11 = 16
2500 48 estimate 900 rpm + 1600 rpm = 5 initial + 16 = 21
3000 48 estimate 900 rpm + 2100 rpm = 5 initial + 21 = 26
3800 ??? mechanical starts at 3800 + 5 initial = 29 so = 5+30 = 35 total allows for the 1 degree not accounted for in the linear calculations
Most articles I have read recommend 36 to 38 degrees total maximum mechanical advance, however, some engines may accept 40 degrees without detonation.
If you adjust your initial timing to 5 degrees (minus 13 degrees) you obtain the 35 degrees of the GM specifications. However, it seems most engines like a little more initial timing, maybe 5 degrees more than GM specification which must be accounted for at the top of the mechanical advance curve to prevent detonation. Lighter springs will provide more mechanical advance at the initial advance rpm and bring the total in at the lower rpm (~3000) you seem to be wanting to achieve. Your popping may be due to too much mechanical advance at 1500 to 1800 rpm. At 18 degrees BTC initial, you may also experience hard starting when the engine is hot.

 

Remove the cap, rotor, springs, and weights.
Use a mechanics mirror to see if the factory nylon advance stop bushing is missing or in really bad shape.

 

Is it possible you have a bad spark plug wire.

 

That would have no effect on the timing numbers he's getting and the length of the centrifugal advance curve.

I just sent all the timing papers, along with the distributor installation paper, to Jim. He'll get it sorted out...

Lars

 

Thanks for all the great input! Just pulled distributor and confirmed bushing slipped from position. It’s in good shape so I’m inclined to reseat it and see the results. Ordered a spring/bushing replacement kit just in case.

 

where is the vacuum advance come in? My L68 (400hp tripower) i have 5 degrees initial, 3000 rpm have 30 degrees. Starts fine but surges when driving constant speed. Ides fine, excellerates like a raped ape but trying to drive constant speed (any) it seems to be lopy .

 

Often there is a groove at the bottom of the pin for a circlip to snap in and hold the bushing in position.

 

If you disconnect and plug vacuum advance do you still have the surge?

 

A 67 L68 has a 12 degree vacuum canister. Vacuum advance begins at 7 in Hg and is full in at 12 in Hg. per the GM Heritage web site. If the canister is bad, you will have a vacuum leak and may not getting the advance needed at cruising rpm. Remove the hose at the carburetor and apply vacuum with a Mighty-vac or even just your mouth to see if the canister holds vacuum or leaks off
https://www.gm.com/content/dam/compa...t-Corvette.pdf
From https://www.gm.com/heritage/archive/...formation-kits

 

Where it comes in is in just about any idling and driving situation except for full-throttle acceleration when the vacuum signal drops below 7"hg (using the example above). Even at 60mph toolin' down the highway, the vacuum advance is engaged but when you stomp on the gas it'll drop out for a short time (low vac signal) and come back in when the load is reduced (high vac signal restored). Of course, some of this changes a bit if the advance is using a ported source rather than continuous source.

 

Thanks, I will try that tonight

 

If at steady highway speed it feels like a constant stuttering or light miss that is usually a sign of too much combined advance (initial, centrifugal & vacuum) causing a light kickback before TDC as the flame front is too far advanced before TDC. Installing the bushing and getting the combined initial and centrifugal advance back down to 35-36 degrees should eliminate that. If not you may have to limit the amount of vacuum advance by changing vacuum can or installing a limiter bracket.

 

Will test this too tonight, thanks for the suggestion.
I just got done checking the spark plugs (all were a little rich with about 400 miles on them) and replaced the spark plug wires (some of the boots had a tear in them). In the process I got a little frustrated and had to take a break because the car now had a miss after plug wire change. came back the next morning and found I had 2 wires crossed in driver's side. (Braided wires and that small distributor shielding I messed up.
Do I need the vacuum advance? a lot of people say no (even my engine builder) but I keep telling people this is not a race car, it needs to start and idle. as I posted it is at about 5 degrees with total advance at about 30 (with vacuum advance)

Since the motor was build it did the surging only when driving in town trying to 25mph and the RPMs below 1500 now it does it almost up to 2000
Here's another observation, I cannot physically get more than 5 degrees advance due to canister hitting intake manifold runner without pulling distributor and re clocking. it.

 

Your distributor was incorrectly installed if you cannot move it to more than 5 BTDC. Until your correct that there is nothing further you can do. A typical performance curve would have 12 initial and another 24 from distributor mechanical advance plus another 10 to 12 from vacuum. Vacuum is beneficial for a street driven car at idle and cruising.

 

That's what I was waiting to hear, It is a big deal to re clock (for me fat hands and fingers), (factory transistor ignition and the tach cable very hard to re-install ) but it has to be done so will have to set time to do this ...uggg

 

Email Lars Grimsrud for his paper on distributor install. He can be contacted at v8fastcars@msn.com