'63 327 340 hp misfire - finding the ghost!

[Mel H.]

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

Good idea on driving check of vac.. will do

R43s were on car when I got it. They looked new, but several pointed out these are too cold so prone to more fouling.

[Timothy B.]

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

Mel,

One other thing I was thinking about with your lean engine condition. Are you using the hot air choke or is it blocked? If it's blocked the air that's allowed to pass through that circuit will not contribute to idle speed so the throttle blades will need to be opened further to make up for that loss of air.

What happens then is that more of the transfer slot is exposed at idle and less fuel emulsion pulled through the curb idle ports below the throttle blades. As a result, the emulsion screws are set leaner because engine vacuum is pulling harder on the transfer slot. Because of this, when you are at cruise speed there is not enough fuel through the transfer slot and curb idle port and it causes a lean condition. Proper throttle blade position at idle is important, keep in mind these carburetors are factory calibrated with these vacuum leaks, (hot air choke and PCV).

[Stuart F.]

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

Tim;

This sounds like some advice meant for me and my situation with the 3461S that we have talked about for a year or so.

First, I measured the vacuum draw on the fresh air hose when I had the standard hot air choke hooked up and found it to be insignificant (something around 1" hg or so). Second, why then is this not a problem with my 3721SB installed? (I have had both carbs off and on the bench up side down to check the transfer slots after each had been idled on the engine at 750 rpm. They were vertually the same as measured with numbered drills). I had found that the primary shaft bore was worn on the 3461S and thought that might contribite to such a condition, but I had new bushings installed and found no effect. Note: Most carb rebuilders will not do this as part of their re-conditioning unless they can detect a significant vacuum leak around the shaft. The shop I used tried to talk me out of doing it.


Don't mean to change the subject on this thread, but thought the information might be of some value.

Stu Fox

[Timothy B.]

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

Hi Stu,

I wish I could help more with your carburetor problem. There have been quite a few people over the years that have posted with the same surge issue with L-76 and the AFB, maybe there is a reason GM went with the Holley's in 1964 on the SHP engines.

The 3721 has a .037 IFR as opposed to the .035 on the 3461 and I don't know the air bleed sizes but they may be different. Other than those changes and the jet size I can't think of a difference in the carburetor bases.

I don't know if my post #32 helps some but it sure gives us something to think about.

[Mel H.]

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

Tim...rubber vac line recently replaced so good condition, but did test drive with vac line off/plugged with golf T...didn't notice any differences...but running so much better now there's hardly any jerk left. If anything, remaining issue seems more rpm stability...random drop of ~50 rpms downward at steady speed. Lasts only about a second. maybe once every 5-20 seconds. A vac leak somewhere? If so, wouldn't it be relatively steady?? Using hot air choke as original. seems to work well.


Anyway, I Tee'd into the vac line, ran hose thru dash, and a vac gauge in cabin so can see it while driving. Vac very steady above say 1500 rpms... but bounces at lower and at idle, with drops of 1-2" Hg coinciding with drop in rpms.


Still, this is livable for me as drivability is so much improved. And still planning to replace points, condenser, coil to eliminate those as possible contributors. And I misread your post on plug wires. Duralast brand 7mm silicone high heat. Cheap, but I installed to see if that would make any diff. It didn't.


Oddly, went for a drive last nite and idle rpms were back to 850, so reset to 700 where it was originally. Weird ...can't explain it.

Stu..I've also checked vac on this line. Mine also about 1" Hg. But I'm a novice on carbs so can't keep up with you guys on idle circuit, tip in, etc. as you said, maybe this variable rpm/surge stuff is just typical of AFB carbs.

[Stuart F.]

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

Mel;

One thing I can tell you about AFB idle characteristics on an L-76 learned over the years: If you use just a single throttle return spring, be it an incorrect 63 white spring or a 64 and up black spring (which is correct), the carb will never find a consistent idle speed. It is just the nature of the beast. What I do for street driving is add a second light weight "booster" spring which helps steady it a lot, but it still is not perfect.

Noting a swing in vacuum of 1" or 2" Hg during idle is actually quite good. That too is just the nature of the beast. By swing, I don't mean a steady swing or jiggle, but rather a little like a momentary stumble. If you want or expect a steady vacuum reading, you'd have to change your engine over to an L-75 version with hydraulic lifters. An 097 or 30-30 cam engine will not idle steady.

Tim; I have switched primary clusters between my 3461S and 3721SB without any change in surge characteristics. There is just something else and I will find it during this forthcoming cool season for sure.

Stu Fox

[Timothy B.]

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

Mel,

FWIW, the NAPA premium brand (Belden 700103) spark plug wires are a nice set. I just installed them on my 67 and they ohm out at approx 3000ohms/ft. They have black plug boots and coil connection caps but the wires are a darker blue color, but good wires are very important and with the shielding installed you can't see the color anyway.

[Richard M.]

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

Interesting problems and creative diagnosis has had me lurking and learning since the start of this thread.

From the very start, your detailed troubleshooting and data collection was a great jump start for the expert analysis by the masses. Since you've made some great progress to resolve what appear to be multiple problems, and as mentioned the intermittent ones are extremely difficult to find, I have a few thoughts to add based on some of my own experience.

- In your data, you said you do not have the spark plug heat shields installed. Do you have a stock radio and are the horizontal ignition shields installed? I'd imagine not installed during your work but need to ask. Here's why. The temperature of the center outlet of the manifold can run 500* to 600*+. It's the hottest part of the manifold. Without the spark plug heat shields installed, the plugs at the #3,#5 and #4,#6 positions will be exposed to extreme heat directly next to the manifold center section when idling at rest, stopping for long durations, etc. At speed, the #5 and #6 positions will be hotter than #3 and #4 as they are directly behind the center outlet and do not get good airflow, and any air they get is heated higher due to their proximity to the center outlet. If there are no spark plug heat shields, and if the horizontal ignition shields are installed it makes matters worse.

From experience, I solved a intermittent miss on my '59 years ago due to not having a "back-ordered" rear heat shield installed. The #6 spark plug boot had deformed and was intermittently arcing to the manifold. The in-the-dark spark test never showed the arcing it as it was out of view. I noticed the boot when I had the car on a lift. Replaced the boot, installed the heat shield......fixed.

Whether this helps solve your last bit of possibly electrically related miss or not, I suggest you reinstall your spark plug heat shields.

- One other long shot, and I've heard it discussed but never experienced it, is the #5 #7 adjacent design oddity. You know, the only two spark plugs that are next to each other in both proximity and firing order. Maybe try relocating the plug wires at those 2 positions in different configurations as an experiment.

Rich

[Mel H.]

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

Thnx Rich.. good point on heat shields (missing on my car) and related impact on plug wires. And horizontal shields ARE in place so trapping heat as you noted. I've ordered new plug shields... recently arrived... so will be installing soon as I get bolts Duh!! Forgot bolts!! !!#?&!! Old age, I guess.

That said, I didnt see any evidence of plug elbow damage/deforming when I removed old plug wires. In fact, they looked great. Course, doesnt mean anything as damage might not be visible. I assumed the wires werent the cause as didnt see improvement after changing. But sounds like you saw very direct correlation to heat damage and missing plug shields so can happen. Gives me encouragement to get them installed! One has to assume shields were put in place for some reason, and your heat impact makes a logical reason to keep them in place. Wonder how many engines are running without them?!

I found my old wires had #5 & #7 criss-crossing all over each other. I suspected wires and replaced them early on with correct placement. I didnt get an improvement but will keep an eye on them anyway. I also want to rebuild that distributor. Archives suggest I need #54 'football' and #724 cam??? If anyone has experiece getting improvements from changing to OE or even different dist shaft, I'd like to hear your story. And I'm still gonna shim that shaft tighter, too.

BTW.. I have a 1.8 ohm ballast resistor vs the OE .3 ohm. Would this be causing low voltage during firing? The coil is within spec, but I dont understand the associated electrical workings very well. Thanks again.

[Joe M.]

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

Direct replacement wires??????? if you mean 'correct dated wires' they are known for their failures.

[Timothy B.]

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

Mel,

If you are sure the primary and secondary ignition systems are in good condition then you may want to try another carburetor as a test to see if the condition persists, at least that way you have narrowed the problem down to the carburetor.

FWIW, with a vacuum gauge, the hot air choke circuit should show engine vacuum so if you only show 1" vacuum check for a leak around the bakelite housing seal. A leak here is hard to find unless you can hear a hiss but sometimes you can feel it if you can get your hand around the housing or maybe a kleenex that will get sucked in to show a leak. The only restriction in that circuit is the choke pull off piston and it easily lets air pass through it but if it's working on the car I don't think it's the surge problem you have.

Your problem may just be a tuning issue, idle air bleed size, transfer slot length something causing the idle circuit to stop feeding fuel before the main circuit starts feeding fuel, I wonder how one of those Edelbrock AFB clones would work.

[Joe M.]

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

Any recentlt replace intake manifold needs to be retorqued, probably twice. Also the carb screws and hold down bolts tend to loosen on a new install.