'63 327 340 hp misfire - finding the ghost!

[Duke W.]

Re: '63 327 340 hp misfire - finding the ghost!

The problem could be a vacuum leak, but you said you already checked.

It sounds like the centrifugal is starting at a very low RPM - maybe below where the engine will run stabley long enough to check and adjust. This is a common issue on SHP engines.

The best way to set timing is to rev the engine to a point beyond where advance stops and set it with a dial back light at 38-40 (VAC disconnected/line plugged), then road test for detonation; 36 is the low end of the 36-40 range recommended by GM Product Performance. Then try to determine where centrifugal starts by dropping revs to where the engine is about to stall and slowly bring revs up to the point were you see advance begin.

The only idle vacuum that is meaningful is in the normal idle configuration with the VAC connected and a vacuum gage teed into the VAC signal line. What you stated - 14" @ 650-700 does NOT correspond to any OE mechanical lifter cam, but it is in the ballpark of the L-79 cam.

The higher the overlap the more low speed advance an engine will tolerate, and one issue may be that you have too much at low speed, which could cause the symptoms you report. Your centrifugal may be too aggressive, and if the engine does idle at 700 @ 14" the B28 VAC is too aggressive. The 12" B26 would be more appropriate because its the least aggressive that meets the Two-Inch Rule.

The nominal setting on the AFB idle mixture screws is 1.5 turns out from the seat and with nearly any reasonable cam the final setting will likely be within +/- a quarter turn from 1.5 assuming the carburetor is propely configured and in good working order, and minor low speed/load surge can often be cured by an extra quarter turn out on the idle mixture screws.

Duke

[Patrick H.]

Re: '63 327 340 hp misfire - finding the ghost!

The total timing is within your range at 32* (w/vac connected).... 19 base + 13 from vac adv = 32 at idle, right? When I rev to 2500, its all in at 52, so I'm getting about 20 from centrifugal, right? This was 22 before I installed the distibutor bushing thinking I was getting too much advance at cruise, causing jerk.

No, you need to add the initial + centrifugal for total advance; ignore the vacuum advance for now. So, in your case it is 19 + 20 = 39, which is probably just a bit too high.
As Duke says, unplug the vacuum advance (plug line to carb with a golf tee), rev it up until the centrifugal is all in, and see where you are. I use a dial back timing light, set it at 36 degrees total (so that 36 is what I see at 4000 rpm) and call it good.

Use the dial back as well to see if you get 24 centrifugal or not, and also if it's all in by 2500 or so as Duke mentions.

I would not be surprised either if your springs are allowing advance at idle, thus the instability you see at 700 or so rpm.

Patrick

[William C.]

Re: '63 327 340 hp misfire - finding the ghost!

If I were to summarize the contents of this couple of days of discussions, you really need to take ALL the components of the fuel and ignition sustems, and have them checked/reset to the original specifications. Far too many "tweeks" have been mentioned in this discussion to ever know what the real problem is. Get it back to the factory settings, distributor and carb BOTH including the proper (not boy-racer) vacuum advance and see where you are. Also, have you actually checked the resistance of the plug wires, all eight plus the coil wire individually?

[Stuart F.]

Re: '63 327 340 hp misfire - finding the ghost!

The standard 63 L-76 engine from the factory by it's nature needs help. It was the first year for a single point vacuum advance distributor for an SHP engine. There is no doubt the engineers missed it on calibrating the vacuum advance. The PCV system design too is a disaster (I speak from the experience of having a crankcase explosion) which was addressed in a Service Release showing how to apply a flame arrestor screen in the air filter housing.

Now then, adding an incorrect camshaft to the mix and you got a problem, i.e. a 3461S AFB and a stock 63 distributor are not calibrated to work with a 30-30 cam. I agree, it is good advice to return as much as you can back to factory settings, but perhaps then go from there with reasonable corrections and improvements to these settings in order to live with the beast.

Stu Fox

[William C.]

Re: '63 327 340 hp misfire - finding the ghost!

I agree, this engine has too many "unknowns" and I see the camshaft as a huge one. The use of a '65 Vacuum can would be my next step to allow the vac advance to operate under lower vacuum conditions. Eventually, I forsee a camshaft replacement with a KNOWN Quantity duplicate of a GM cam, not one that a cam grinder or local speed merchant "says" is a replacement, even though the lash specs are totally different...

[Mel H.]

Re: '63 327 340 hp misfire - finding the ghost!

OK, heres few more things I did over past day or so;
1) Called engine shop in LA that did the rebuild. Says it wasnt likely a 30-30 and normally replaces with similar to stock (097 cam). Thinks cam was likley from Elgin?? (not familiar with them). He's gonna see if he can find the paperwork to confirm. Whatever, but doesnt sound like it was a 30-30. Duke's suspicions appear on target.

2) Inspected the breaker plate ground wire. Didnt look great, so replaced it and went for a drive. No change. Still jerks/misfires

3) Tested the carb plate when warm at 2000 rpms. When I closed the plates the rpms went down, so doesnt appear lean.

4) Turned in screws to seat. They both were 1.75 turns out. While idling, I attempted to turn in the screws. Began to struggle ~.5 turns left on either screw.

5) Did power time procedure w/vac unhooked and line plugged. One spring off, raised rpms to full cent adv and locked in at 36*. Idle was noted at 16* replaced one black spring with one silver spring, so now one black, one silver. Recorded timing curve as follows;
RPMS Timing "Hg Vac
500 16 10
1000 24 16
1500 30 18
2000 36 19
2500 38 19
3000 38 20

Replaced vac line. Idle timing 31* (I removed bushing from vac arm to allow a bit higher max timing and a bit more at idle for better low speed torque)
Total timing = 36 (16 base + 20 cent)
Max timing = 51 (16 base + 20 cent +15 vac)

Went for a drive. No change.

6) I did measure resistance when I intalled the new plug wires. New were mostly lower readings by ~5% or so, but no signifcant differences in any wires..., again, no change when I went for a drive after.

I'm down to trying new plugs (have Autolite 306s on order (=R45s)) and maybe coil. I'm now thinking this problem is electrical. Other reactions?

[Gene M.]

Re: '63 327 340 hp misfire - finding the ghost!

Mel,
I'm running Packard 440 solid strand wires and AC44 plugs, all non resistor. Also I have copper contact distributor cap with long rotor. I'm wondering if too much resistance is an issue for you. Point dwell is at 31 degrees.

A side from the plugs I changed everything else cause I was having the same surge issues you face. They were resistor wires and aluminum contact cap and short rotor. All the surges are gone now.

[Duke W.]

Re: '63 327 340 hp misfire - finding the ghost!

Leaness can usually be verified by using your foot on the throttle or hand on the carb linkage to bring revs up in 100 RPM increments from idle to 2000 and back down again. If it doesn't respond smoothly to this procedure it may be lean in the idle to main transition. Do this test with the existing centrifugal spring setup and the stiffest springs. Any difference in response may indicate too much low speed advance.

Did this problem start after you replaced the plug wires? What was the source? Something like bad teminal crimps could cause the problem.

RFI wires should have about 5K ohms per foot resistance. Remove the cap and rotor and carefully inspect for cracks and dress any corrosion on the terminals with a dental currette or Xacto knife. Also verify that the wires are indexed IAW the 1963 Corvette Shop Manual and the terminals firmly engage the cap sockets and spark plugs. Run the resistance check from the inside cap terminal to a spark plug center electrode, but you'll need a non-resistor plug. Wiggle the wires when testing and this should capture any terminal resistance issues.

It's still not clear to me where the centrifugal starts, but it looks like below 1000 and it's 8 degrees at 1000. That may be too aggressive, and could be a cause of the misfire because peak cylinder pressure is occuring too early and those cylinders want to run the crankshaft backwards.

Duke

[Edward B.]

Re: '63 327 340 hp misfire - finding the ghost!

I read Mel's original posting and immediately jumped to a conclusion. I then read all of the well reasoned responses and suggestions contained therein. For what it is worth, my initial diagnosis has not been changed: Notwithstanding the fact that they are new, replace the plug wires.

[Duke W.]

Re: '63 327 340 hp misfire - finding the ghost!

How do you make a "diagnosis" without a physical examination and running tests?

Think of how physicians arrive at a diagnosis - review medical history (What camshaft/plug wires are installed? Did the problem start after the new plug wires were installed? ) - physical examination (carefully examine all primary/secondary ignition system components including plug wire indexing) - order tests (check plug wire resistance and run lean check procedure with different centrifugal springs and w/wo the VAC signal line attached.)

These misfire issues can be caused by a number of problems that can originate in either the ignition or fuel systems, and the best approach is to run logical tests before you start throwing parts at the problem.

Another possibility is that the camshaft does not have a properly indexed distributor drive gear, so the initial timing could not be achieved without the VAC interfering with the manifold of coil bracket. The usuall "fix" is to move the plug wires one position, and now you've really got a kluge and sometimes a misfire.

That's why determining the camshaft is important. With an OE cam the dimple in the dist. gear should point the same direction as the rotor tip, but on some aftermarket camshafts it needs to be repositioned 180 degrees. This issue has been causing problems as long as small blocks have been around.

Duke

[Mel H.]

Re: '63 327 340 hp misfire - finding the ghost!

Duke.. problem been present since I bought the car 3 yrs ago. I installed new 'correct style' distributor cap and rotor early on. I dont know if was long or short rotor. How to determine?

New plug wires installed few weeks ago averaged 3.3 resistance/ft (range 3.1-3.5)....~5% lower than old wire resistance. Have double checked insertion of all wires (cap and plugs) and the arragnement/index per charts in maunual. Old wires were not arranged correctly so thought might be jumping/shorting to each other. Admittedly, new wires are off-the-self double silicone, but I just wanted to first see if old wires were the obvious problem. Gene's point on quality cap, rotor, wires is a good next step, but think I first want to eliminate plugs, too.

Centifugal begins ~700/750 rpms. Thats likley too quick so will try stiffer spring combo tomorrow. Raining today.

But an idea. I notice a timing mark bounce (maybe ~4* on timing mark plate), interestingly, at the very point of what I think is the 'misfire' (observed engine shake). The bounce seems to occur randomly every 2-5 seconds. Otherwise the bounce is ~1 or 2*, and smooths out at higher rpms, though still wiggles maybe 1* or so above 1200 rpms. During the bigger bounce, the timing mark moves 'up' the damper during the very quick, seemingly one spark misfire/stumble, (a retarding of spark, correct??) and both rpms and vacuum also seems to dip in concert with this misfire. Given this, what do you make of the distributor shaft end-play (see first post on data)? Would it equate to that much 'spark scatter' and cause this problem? And if so, would the scatter typically be that quick? Seemingly one spark misfire...maybe two in a row?? Just a thought.

[Duke W.]

Re: '63 327 340 hp misfire - finding the ghost!

Normal spark scatter with typical OE sloppy end play should be no more than a couple of degrees. If you've got an occasional jump higher, that could be an indication of a problem. Cross firing due to a cracked cap or wire leak could cause this symptom, but those parts are new...

Maybe it's time your 50-year old distributor got a blueprint overhaul. I started a thread on the subject based on work I did to a local chapter member's L-79 last December. We found some surprises. Maybe you will, too.

https://www.forums.ncrs.org/showthre...print-overhaul

Here's another war story about an intermittent misfire, which was happening on my '88 190E 2.6 about 15 years ago. At idle it felt like there was a misfire every few seconds and HC emissions on the last test were 1 PPM short of the limit.

I removed the entire plug wire harness and cap and ran resistance checks, and everything was okay. The Merc uses zero resistance wires, but has 2K ohm resistors built into the cap terminals and plug boots. So for a second test I removed the wires from the cap and tested the cap resistor while placing a bending load on the terminal which protrudes from the cap. On one terminal when I did this the circuit went open and then back to 2K when I removed the bending load. So I replaced the cap, which otherwise looked near new. The misfire went away and HC dropped to a low level on the next emission test.

Sometimes problems are not at all obvious and intermittents are the toughest to troubleshoot. Back in the day when tuneup shops has oscilloscopes these types of problem would usually show up on the scope wave form, and you could usually also tell which cylinder was misbehaving, which helped isolate the bad component.

Duke

[Mel H.]

Re: '63 327 340 hp misfire - finding the ghost!

Status Update: Progress!!

1) while studying the timing bounce, I noticed the timing light pickup was hanging pretty close to the #3 plug elbow/wire. So I moved it to #1 at the distributor. Sure enough!! The timing bounce went away. Not the engine shake..just the bounce, so maybe bad inductive pickup (or over sensitive)?? Also tried it on #6. Same...no more big bounce. Still a few degrees either way, so a spread of ~3-4* total. Is this still too much?

2) Installed heavier springs on dist. Now comes in ~900, all 2500. Went for drive.. No chanage. Still jerks.

3) Changed plugs to R45. Noticed some (1,2,6,8) were dirtier than othes. Ones I checked before just happened to be the cleaner ones. Lesson: dont assume all plugs are the same. Went for drive. BINGO! much improved! Feeling better! Still some jerk but I'd call it 80% gone.

4) When I bought plugs I also picked up new distributor cap and paid more copper posts. Its an UGLY grey, but installed it to see. Old one had correct markings, < 1 yr old, and looked great (black), but aluminum posts. Installed ugly grey one and went for drive. Again, noticable improvement!! Now maybe 90/95% of jerk is gone, and I can coast at 10mph without being 'all shook up'. Maybe rest is just carb problems like Stu described. One strange thing though, idle dropped about 200 rpms with new distributor. Cant explain why. So I just reset it to 700.

So feelin much better/encouraged! Clearly plugs most of it... aluminum cap, too. Will also try new points, coil, and may shim up that distributor.

Overall lessons learned.. #1) so much had been written about plug fouling, start there early!! Not last!! A lot of work to change, but fouled plugs have been root of multiple issues over the years. #2) Watch quality of electrical components. Made big difference in this case...and cant tell by simple looks.

Mel

[Mel H.]

Re: '63 327 340 hp misfire - finding the ghost!

Status Update: Progress!!

1) while studying the timing bounce, I noticed the timing light pickup was hanging pretty close to the #3 plug elbow/wire. So I moved it to #1 at the distributor. Sure enough!! The timing bounce went away. Not the engine shake..just the bounce, so maybe bad inductive pickup (or over sensitive)?? Also tried it on #6. Same...no more big bounce. Still a few degrees either way, so a spread of ~3-4* total. Is this still too much?

2) Installed heavier springs on dist. Now comes in ~900, all 2500. Went for drive.. No chanage. Still jerks.

3) Changed plugs to R45. Noticed some (1,2,6,8) were dirtier than othes. Ones I checked before just happened to be the cleaner ones. Lesson: dont assume all plugs are the same. Went for drive. BINGO! much improved! Feeling better! Still some jerk but I'd call it 80% gone.

4) When I bought plugs I also picked up new distributor cap and paid more copper posts. Its an UGLY grey, but installed it to see. Old one had correct markings, < 1 yr old, and looked great (black), but aluminum posts. Installed ugly grey one and went for drive. Again, noticable improvement!! Now maybe 90/95% of jerk is gone, and I can coast at 10mph without being 'all shook up'. Maybe rest is just carb problems like Stu described. One strange thing though, idle dropped about 200 rpms with new distributor. Cant explain why. So I just reset it to 700.

So feelin much better/encouraged! Clearly plugs most of it... aluminum cap, too. Will also try new points, coil, and may shim up that distributor.

Overall lessons learned.. #1) so much had been written about plug fouling, start there early!! Not last!! A lot of work to change, but fouled plugs have been root of multiple issues over the years. #2) Watch quality of electrical components. Made big difference in this case...and cant tell by simple looks.

Mel

[Timothy B.]

Re: '63 327 340 hp misfire - finding the ghost!

Mel,

If the idle dropped by 200 RPM's and the only thing you worked on was the distributor cap make sure the hose that connects to the vacuum advance is not leaking vacuum. Sometimes these rubber hoses can become oblonged at the ends and cause a poor seal.

A vacuum leak there can cause the V/A to hunt and also create a lean condition. Go for a ride with the vacuum advance unhooked and put a golf tee in the rubber line and see if taking out more timing helps some at cruise conditions. This test may give a indication where to go next, also what brand spark plug wires are on the engine.