Low speed miss, Ignition? Options

[Allan]

I just spent yesterday swapping out a set of 46mm carbs for a set of more streetable 40's. One of the problems I had before the swap was a low speed miss. Once it is at 1/3 to wot it is nice and smooth and pulls really hard. One thing I do know is that the motor has what looks like all original plug wires. The metal shielded oem kind. I'm going to replace all the plug/coil wires as well as replace the points, condenser and distributor cap this week.

I guess my question is, could one or more of the above mentioned items cause this symptom?

Tx again...

[lapuwali]

Also check (very carefully) the low-tension wires to the coil (power and points). At least one club member had a low-speed miss like that cured by simply replacing the old (loose, tired) connectors on the ends, and adding heatshrink as the insulation was so brittle that just bending it slightly broke it. Replacing the wires completely with new wire and new connectors would be the hot ticket.

[J P Stein]

Guys running carbs are well advised to do the following:
Get an MSD 6AL....the 7K pill is good.
High vibration MSD coil
8.5 mm wires...I use Magnacore
Replace the points with the unit of your choice....I use Pertronix
In your case, BP5ES NGK plugs gapped to .045-.050

I dicked around for 6-8 months before eliminating fouled
plugs & general low speed shitty running problems by using this set-up.

If nothing else, this will eliminate the ignition as the source of your problem.

[motorhead]

I had a similar problem on '70 El Camino. Big Holley double pumper, hot cam, etc. Thing ran fine at light throttle/ high vacuum, but start to put your foot in it at low rpms and it would misfire and run real shitty. Obviously it has to be that big carb. Changed jets, power valves, float settings, nothing helped. Finally changed plug wires, and bam. End of problem. It had one plug wire that had broken down enough that it would misfire when cylinder pressures were high, but not at idle or light throttle.

[messix]

plug gaps great than .35-.40 will over heat the coil by having to build to hi of voltage to arc the gap. racing coils can do that but only for short periods or are built purposely for such use [very spendy]. check plug wires with a VOM on the ohm setting and compare to each other. you'll probly find one that reads very high.
5 ohm is close to a short. 5m [mega] ohm has some some resistance, 5k [kilo] is very high resistance.

i hope this can help. plug wires are cheap and easy.

[Twystd1]

QUOTE

plug gaps great than .35-.40 will over heat the coil by having to build to hi of voltage to arc the gap

Hmmmm. I don't believe you can include HEI ignitions into that rule or some after market ignions SYSTEMS for that matter. Cause they are often .40 - .60 thou gaps.....

Depends on the ignition package.... Some gotta be .40 or less to work correctly.

Typically a point set up is a good example of this ...

Some ignition systems are better at .40 PLUS... Think high compression, blown or turbo'ed

Factory late model Vettes run .55 or greater stock....

My plugs on my roundy round with a 16 volt ignition system... RAN .70.. (IMG:http://www.914world.com/bbs2/html/emoticons/wink.gif)

Nascar Style.........................

Bottom line... The plug gap is dictated by the ignition sytem as a package. Not as an individual part.

There I go again... Thinking I know something........

Twystd1

(IMG:http://www.914world.com/bbs2/html/emoticons/wink.gif)

[Dr. Roger]

QUOTE (messix @ Dec 18 2005, 09:37 PM)

plug gaps great than .35-.40 will over heat the coil by having to build to hi of voltage to arc the gap. racing coils can do that but only for short periods or are built purposely for such use [very spendy]. check plug wires with a VOM on the ohm setting and compare to each other. you'll probly find one that reads very high. 5 ohm is close to a short. 5m [mega] ohm has some some resistance, 5k [kilo] is very high resistance. i hope this can help.  plug wires are cheap and easy.

son of a gun. i learned something tonight. i always thought dwell didn't matter that much as long as the coil saturation was enough.

i just got done researching the history of the points ignition and you are exactly right. page is HERE

at low speeds the coil actually has more time to saturate for a stronger spark.

geez, now i'm on ebay for an HEI..... (IMG:http://www.914world.com/bbs2/html/emoticons/laugh.gif) anyone want a recurved Mallory dual point w/vac advance and hot coil for a SBC??? mine's now for sale. (IMG:http://www.914world.com/bbs2/html/emoticons/chowtime.gif)

thanks. (IMG:http://www.914world.com/bbs2/html/emoticons/smilie_pokal.gif)

[J P Stein]

QUOTE (messix @ Dec 18 2005, 08:37 PM)

plug gaps great than .35-.40 will over heat the coil by having to build to hi of voltage to arc the gap. racing coils can do that but only for short periods or are built purposely for such use [very spendy]. check plug wires with a VOM on the ohm setting and compare to each other. you'll probly find one that reads very high. 5 ohm is close to a short. 5m [mega] ohm has some some resistance, 5k [kilo] is very high resistance. i hope this can help. plug wires are cheap and easy.

Don't forget your .0, guys. (IMG:http://www.914world.com/bbs2/html/emoticons/biggrin.gif)

MSD ignition units & coils are are designed to run with these gaps. They also give a multi spark discharge under 3K rpm. They draw about 9 amps directly off the battery.
The coil is under 40 bucks. The Magnecore wires are not cheep at around 130 bucks.....which is less than a set of Berus (sp?) The wires need to carry about 40k volts
so this is not an area to skimp.

Summit Racing has the best prices I've found for the MSD stuff. I've been running this set-up for about 4 years with no failures.....if that means anything.

[Allan]

QUOTE (J P Stein @ Dec 18 2005, 09:53 AM)

Guys running carbs are well advised to do the following: Get an MSD 6AL....the 7K pill is good. High vibration MSD coil 8.5 mm wires...I use Magnacore Replace the points with the unit of your choice....I use Pertronix In your case, BP5ES NGK plugs gapped to .045-.050 I dicked around for 6-8 months before eliminating fouled plugs & general low speed shitty running problems by using this set-up. If nothing else, this will eliminate the ignition as the source of your problem.

I'm headed over to GPR tomorrow to pick up a bunch of stuff. Wouldn't it be prudent to get a decent running baseline on the car using points and then go to an MSD?

[messix]

QUOTE (J P Stein @ Dec 19 2005, 06:15 AM)

QUOTE (messix @ Dec 18 2005, 08:37 PM) plug gaps great than .35-.40 will over heat the coil by having to build to hi of voltage to arc the gap. racing coils can do that but only for short periods or are built purposely for such use [very spendy]. check plug wires with a VOM on the ohm setting and compare to each other. you'll probly find one that reads very high. 5 ohm is close to a short. 5m [mega] ohm has some some resistance, 5k [kilo] is very high resistance. i hope this can help.  plug wires are cheap and easy. Don't forget your .0, guys. (IMG:http://www.914world.com/bbs2/html/emoticons/biggrin.gif) MSD ignition units & coils are are designed to run with these gaps. They also give a multi spark discharge under 3K rpm. They draw about 9 amps directly off the battery. The coil is under 40 bucks. The Magnecore wires are not cheep at around 130 bucks.....which is less than a set of Berus (sp?) The wires need to carry about 40k volts so this is not an area to skimp. Summit Racing has the best prices I've found for the MSD stuff. I've been running this set-up for about 4 years with no failures.....if that means anything.

yea, that .0 i forget all the time. the cops aren't amused when i say .8 is still legal. (IMG:http://www.914world.com/bbs2/html/emoticons/biggrin.gif)

[J P Stein]

QUOTE (Headrage @ Dec 19 2005, 12:49 PM)

I'm headed over to GPR tomorrow to pick up a bunch of stuff. Wouldn't it be prudent to get a decent running baseline on the car using points and then go to an MSD?

According to Wayne of Pelican fame, Summit sells MSDs cheeper than he can buy them from his wholesaler.

Baseline?
No, but do what you want.

[Allan]

QUOTE (J P Stein @ Dec 19 2005, 03:41 PM)

Baseline? No, but do what you want.

I think I'm just going to see if I can get rid of that miss by replacing the basics. The car ran real well for a few days after re-torquing the head. I have this weird feeling that I'm soon going to learn what it is like to rebuild a /6... (IMG:http://www.914world.com/bbs2/html/emoticons/blink.gif)

[sixnotfour]

Does that thing have a stock Bosch 911 CD box?
every 911 since 68 has had a CD ignition.

[Allan]

QUOTE (sixnotfour @ Dec 19 2005, 06:34 PM)

Does that thing have a stock Bosch 911 CD box?

(IMG:http://www.914world.com/bbs2/html/emoticons/confused24.gif) I have no idea.

The car was suppose to be a daily driver when I got it...

[r_towle]

I would go through the basics..

Make sure all the ignition is timed perfectly and all wires are good and sound.

It will run without skippig with all the stock running gear.

If after you do that..it still skips, I would say carbs...

It could be the transition between idle and main jets that is not correct...

I know in IDF duals that circuit transition seems to be hard to get right..

The vents, tubes, and jets need to be perfect...

You may just have a wierd setup on the carbs...

Run the carb setup by a weber pro...there is a guy in pano who rebuilds them...hes up here in Groton MA...he knows the settings by heart...

I am sure an older porsche shop does as well..Otto would also know...

Rich

[goose2]

What engine were the carbs on before yours...and were they working well then? I'd suspect carb jetting, synchronization, or dirt in the idle/transition circuit(s). There's no point in messing with them until you're sure everything else is working right though...you'll just make yourself crazy.