intermittent Miss at cruise rpm

[Eds56]

Hello Guys,

Working on my 56 BA & I'm trying to eliminate a "slight" intermittent miss / stutter while cruising 55mph @ 2100 rpm and would like your input. It doesn't do it all the time and sometimes you have to really feel for it.

Specs -- 1968 327 with 461 heads, TRW Forged Flat tops, Crane 284 cam, set 4 degree advanced with 1.60 roller tip rockers, NEW 600 Holley Carb, with MSD 6A, Pro Billet 85551 dist. (NO Vacuum Adv.) Blaster 2 coil, NGK V-power xr5 spark plugs (heat range 5) gapped to 45 per MSD. dist. is running the 18 degree bushing with light blue springs & 16 initial timing to put me @ 34 total and all in around 2800--3000 rpm. I ONLY run BP 93 & Car starts great, hot or cold and i don't have any pinging at high rpm. 4- speed tranny, manual Disc brakes, 160 thermostat, 1 5/8" Headers.

Things i have already done--

• lowered timing, seem to make it worse so i put it back.
• changed spark plugs from AC 43 --- the NGK's did seem to do better, also bumped gap UP to 45.
• marked, pulled & inspected plugs, the lower side (closest to piston) is a white/vanillaish clean color.....however, the top side (closest to valves) is dark, black, wettish color.
• Replaced pick up modue, dist. cap and rotor button.
• set air fuel mixture. both are turned OUT 1 turn to get a maximum vacuum of 12-13 inches.
• looked for stray sparks under hood at Night with engine rev'd up pretty good. - none found.
• Checked for vacuum leaks -- none found.
• Checked with timing light, while rev'n up the rpm's.....occasionally the timing line will fluctuate, indicating the miss.....BUT not noticeable by ear, just sitting at idle.

On the to do list--

• GOING to change dist. springs from blue to Silver (the lightest to allow to come in sooner)
• Pulling plugs back out for one more review -- NOTE these have been Fouled Once and were cleaned and re-used.
• Verify line on Harmonic balancer is correct for TDC

This complete MSD 6A ignition system (likely now out dated) was purchased a long time ago and then suddenly the project came to halt, (family reasons). Originally i had planned to just take it drag strip as a race "toy" hence the NO Vacuum Dist. BUT not now, as my intentions are to just DRIVE IT / Cruise around and enjoy..... NO drag strip.... just an "occasional Heavy FOOT between red lights" while the boys in Blue are having doughnuts.

So here are my questions--

• Considering this is now more of a weekend driver and less WOT --- would a vacuum advanced type dist. be a better option? the one i'm looking at is MSD 8361 as this will plug right into my current box.
• Do i even need a MSD box, in other words perhaps go to a small cap version distributor with a Pertronix unit or something similar? I do NOT want points and need a dist. with small style cap to clear firewall.
• increase or decrease spark Plug -- HEAT Range and or GAP??
• could it still be a spark plug wire breaking down, even though i don't see any sparks under hood? The reason i ask, is these are OLD and have a couple nics on them.
• Increased or decrease timing

Sorry for the Lengthy post, but had to get it ALL out there.

Thanks in Advance,
Ed

[markm]

When I have a miss usually at upper RPM on a SBC especially one with Headeraches I change plug wires and usually find the problem.

[Rick_L]

I would suspect the plug wires given what you've done already. You can have a problem without seeing sparks jump in the dark. Actually if you were able to see that, you'd probably have a full time miss, not a part time one.

Another possibility is the MSD box itself. Personally if it's that, I'd repair the MSD box rather than switch to something else.

Another possibility is bad fuel, especially if you don't drive the car often. 10% ethanol in the gas is an additive problem if you don't drive it often.

A vacuum advance distributor is something you should look into, it will give you driveability and fuel mileage, but it's not likely part of your current problem. However the extra timing under a cruise situation may help, especially with the cam you have, which is a touch big for cruising.

I don't think the spark plugs are a problem, except that you should stay away from the cold ones. Big plug gaps when new result in even bigger gaps after some miles - and that can be a problem. I'd go with .040" or so with an MSD.

[55 Rescue Dog]

Definitely need a vac distributor, and I would check vacuum when the problem occurs. Could be pulling open the power valve.

[Eds56]

Guys,

I really do appreciate your time and input, THANK YOU. I will look into getting a new set of wires in the next week or so and probably a set of plugs too. Would you stay with the heat 5 NGK's or jump back to the AC's? Maybe i just had a bad batch on last AC go around? But the V-power did seem better when new.

I'll also be looking into a vacuum dist. -- i was kinda leaning that way, but wanted to try all the other cost effective options first.

I drive it at least once a week and most of the time more and try to keep fresh gas in it. But i understand your point about today's ethanol blend of gas.

[55 Rescue Dog]

Guys,

I really do appreciate your time and input, THANK YOU. I will look into getting a new set of wires in the next week or so and probably a set of plugs too. Would you stay with the heat 5 NGK's or jump back to the AC's? Maybe i just had a bad batch on last AC go around? But the V-power did seem better when new.

I'll also be looking into a vacuum dist. -- i was kinda leaning that way, but wanted to try all the other cost effective options first.

I drive it at least once a week and most of the time more and try to keep fresh gas in it. But i understand your point about today's ethanol blend of gas.

I run the MSD ready to run vacuum distributor on a standard coil. It has a built in rev limiter, and fully adjustable. Works perfect.

[markm]

I have had great success with Autolite 85swith those heads. if not them AC plugs.

[Eds56]

I have had great success with Autolite 85swith those heads. if not them AC plugs.

------ Thanks markm


Would the "extended / projected" Tip spark plug be the Prefered Version VS. non ext. tip?

[55 Rescue Dog]

Light throttle at 2100rpm, with no vacuum advance I think your timing is retarded quite a bit, which will make your plugs run colder too. That's one of the reasons for vacuum advance to keep the plug tips hot at idle, and light throttle. Figured that out from 40 years ago running a 427 with no vac advance. Couldn't get a set of plugs to last a month in town.

[markm]

Summit Racing Part Number:
ATL-85
UPC:
00009100000859
Shorty Spark Plug:
No
Resistor:
Yes
Manufacturer Heat Range:
5
Electrode Core Material:
Copper
Electrode Tip Material:
Copper
Insulator Type:
Projected
Spark Plug Thread Size:
14mm
Spark Plug Reach:
0.375 in.
Spark Plug Seat Style:
Gasket
Wrench Diameter:
13/16 in.
Ground Strap Quantity:
One