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how to track down a possible random ignition miss??

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Old 04-30-2014, 10:36 PM   #1
Gun Jam
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Default how to track down a possible random ignition miss??

my 302 is actually running pretty well but I can't shake the feeling that its got an ignition miss..its been this way for sometime. It seems most likely to occur at smooth steady rpms around 3k but also under wot acceleration

For example it will purr along at 2800 rpm with A/F ratios indicated about 14.3 to 13.5 well in the acceptable range nothing changes and you'll feel a skip in the beat or maybe 2 or 3.

When checking timing curves the timing light (which is Innova brand digital display "dial" back compatible with MSD ignitions) clearly will miss a flash once in a while especially at 3k. I have not tried it on other plug wires (induction pick up) to see if they skip any flashes.

Are induction pickup digital timing lights that are dealing with an MSD system known to skip flashes??...is this really a valid test in this situation?

Is there a better way to check for a missed spark?

The distributor is in good shape and its running off an MSD a6 box which is several years old

Thanks

-Gun
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Old 04-30-2014, 10:42 PM   #2
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What plug wires, how old? How old is the coil? How old are the cap/rotor?
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Old 04-30-2014, 11:36 PM   #3
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Msd 8.5mm not all that old maybe 2 years replaced identical set...I built the set im using now...dont think I screwed anything up.

coil is about 3 years old

cap and rotor are new. changing them didn't seem to make a difference.

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Old 05-01-2014, 02:23 AM   #4
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What plugs? Are they gapped right? You might want to check the plugs for cracked insulators.

How much do you drive it? MSD wires are ok, but if you drive a lot after 2 years they could possibly be an issue.

Could be timing too. Or AFR, or changes in fuel because of seasonal blends.
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Old 05-01-2014, 06:53 AM   #5
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I'll bet dimes to doughnuts that your coil is breaking down.
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Old 05-01-2014, 06:56 AM   #6
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If your distributor has a "ground wire" that runs to the to the coil, you might want to replace it.
This wire flexes every time the distributor is moved. The copper part of the wire can break and become intermittent while the insulation looks OK.
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Old 05-01-2014, 11:02 AM   #7
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they are autolite plugs. I dont remember the gap for certain but I think they maybe .035. they are about a year old

The vehicle is driven most days.

I myself am not a 100% sure its actually losing spark momentarily...I suspect that it is...how can we test this is the real answer Im looking for. There has to be a better solution than looking to see if a timing light skips a beat....any ideas?
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Old 05-01-2014, 02:02 PM   #8
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Dust off the ol' oscilloscope and see what kind of wave it's making..failing that..I dunno.
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Old 05-01-2014, 07:25 PM   #9
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I could always just wait for it to get to the point where its actually a problem then I'll know !!

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Old 05-01-2014, 08:32 PM   #10
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It could be your imagination too.
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Old 05-01-2014, 09:03 PM   #11
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that certainly had crossed my mind.....but I would like to think I know my horse

I'll keep an eye on it

thanks for your help guys

-Gun
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Old 05-02-2014, 04:06 AM   #12
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hello;

a cheap induction timing lite can definitely skip or miss even though the engine is not missing.

if you feel it then it is missing.


here's some options:

1. remove vac from distributor and make sure there is no vacuum to the port at idle. if there is it should be corrected.

2. plug the vacuum.

3. check idle timing, it should be around 8 for a stock cam and 10 - 12 for a high perf one.

4. hold throttle 1/3rd open. timing should be around 28 - 32.

5. connect your vac to dist and hold throttle open 1/3rd. it should be 42. if it is more it is too much. test drive with it disconnected or adjust it if your vac can is adjustable.

6. try coil as mentioned.

7.remove your msd wires and connect direct to your coil.

8. reinstall your points and condenser.

9. look at your plugs, if they are almost white it is too lean. they should have a light tan color.

10. accelerate wot from a dead stop to 20 mph 3 times times in a row. if it pings your octane is too low and/or your timing is too far advanced.

if it stutters and burbles it is too rich.

if it hesitates for a second like it is out of gas but does not stutter/burble it is too lean.

if your carb is a holley it needs a 6.5 power valve and at least 67 main jets.

if it's a holley, check the gas level, it should be just below the inspection holes. if more than a drop of gas comes out when you remove the gas level inspection screws, it is too high.

either way, if it is lean you need carb tuning.

forget about the AFR numbers.

11. in the dark, move your plug wires around and look for a spark especially near the headers if you have them. increase engine rpm and try it again.

Last edited by barnett468; 05-02-2014 at 04:13 AM.
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Old 05-02-2014, 08:19 AM   #13
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The spark plug boots themselves can be problematic. Sometimes sparks can jump down the side of the plug. If the boots got wet, big puddles etc., they can intermittently provide a path for an arc. Try cleaning the porcelain top of the plug then coating the plug top with dielectric grease before installing the boots. It also makes them easier to remove. with today's ignition systems we forget how prone the old systems were to damp and wet conditions.
Distributor bearing can also be a suspect if it hasn't been rebuilt. If there is pitting on the points change the condenser. Change it anyway.
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Old 05-02-2014, 10:12 AM   #14
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I get that same feeling around 3k with my MSD setup. I wonder if its just the transition from multi spark mode to single spark...
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Old 05-02-2014, 10:52 AM   #15
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Quote:
Originally Posted by 69mach1377 View Post
I get that same feeling around 3k with my MSD setup. I wonder if its just the transition from multi spark mode to single spark...
Simply bypass it as I suggested to gun jam and you will find out.
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Old 05-02-2014, 01:00 PM   #16
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^ Looking at your post it seems to be for an old fashioned points with vacuum setup. My MSD has neither.
Don't know what you mean by 'simply bypass it'.
Step 6 is unclear to me as well.
Are you sure any of this applies to this specific issue at 3k rpm?
It seems very generic...
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Old 05-02-2014, 02:21 PM   #17
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Quote:
Originally Posted by 69mach1377 View Post
^ Looking at your post it seems to be for an old fashioned points with vacuum setup. My MSD has neither.
Don't know what you mean by 'simply bypass it'.
Step 6 is unclear to me as well.
Are you sure any of this applies to this specific issue at 3k rpm?
It seems very generic...
hello;

it is for a pertronix module or other electronic points replacement with vacuum advance.

i have no idea if by passing it will help yours, it is just a test to eliminate or confirm the msd as the problem.

since the cuase of the prob is unknown i check and set everything to proper spec first to eliminate them as a potential cause of a problem because it is easy and logical to do.
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Old 05-04-2014, 12:37 AM   #18
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3) Idle time is 12

4) timing is as follows 12 initial, 20 adv all in at 2500

5) + 10 deg vac adv.

6) dont have spare coil

7) dont fully understand...you mean bipass msd box? My dizzy is capable of this.

8) does not have points

9) plug #1 is in "okay rich" other plugs are "okay lean" range. Plug 4 is carbon fowled it is black with dry soot that wipes off

10) does not ping...ever

11) runs what I would consider pretty good in most cases...Off idle easy accel is not amazing but "ok" vac sags a lot from around 11 to 5 inhg if you dont get enough throttle into it...it seems overly picky about how the clutch comes out.

its a holly based QFT HR600 I have spent about 9 months making very minor turning adjustments and watching fuel milage and other factors. It has 6.5 pv and 68 main jets also is running idle and high speed air bleeds not factory (I forget numbers right now) as well as not factory PVCRs. Full power accels yield AFRs around 13.1 it sounds good under wot as well.

Float levels match arrows on sight glass per QFT specs (mid glass)

11) No visible ignition leaks in dark room from idle to 3k

Okay....Initial reaction was based on the above finding of plug #1 "okay rich" and plug 4 carbon fowled that cylinder 1 and 4 are perhaps low on compression a compression test was made on cylinders 1 , 4 , 5 , 8 for four compression strokes each cylinder was almost exactly 140 psi...That's about what I remember seeing 7 years ago...I think it was about 150 but It also had cast iron 289 heads it now has trick flow al heads with a slightly larger combustion chamber.

It does not burn oil or smoke ever.

But wait there is more!

I made a full power run from dead stop to 70 in a spot where I didnt have to drive much and could look at the gauges AFRs looked good. But vac did not get below 4inhg...I checked throttle plates and they make wot no problem. So I removed the air cleaner and tried again (ran it with no air filter) power gains were "woah" noticeable no second guessing there...Vac now reaches zero.

The air filter is obviously too small and plugged up directly with in 9 months...because I knew it use to make zero vac 8 months ago.

but that doesn't solve the one carbon fowled plug

what do you think?

Thanks

-Gun
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Old 05-04-2014, 01:16 AM   #19
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ok, i'll look at this some more. lots of good detailed info.

where is you pcv connected to exactly?

what is your rear gear ratio?

does it have a "big" cam?

are you saying that you did 3 accelerations in a row from a dead stop up to 15 or 20 mph and there was 0, nada, zilch no burble, hesitation or flat spot?

what type of filter is it?

oil or dry filter?

do you live down a dirt road, lol?


the one carbon fouled plug makes no sense, it doesn't work that way but you have one so there you go, lol.

for now i might switch the carbon plug and spark plug wire with another cyl. even though you did not see sparks in a dark place, it does not mean that the wires or just the wire on that cyl is not breaking down even though they are pricey wires.

i made around $6000.00 off of a car i bought cheap because it had brand new fancy wires that were bad and the seller thought his new $4000.00 engine was bad.

your float level might be a hair high. i might lower it a bit, i've done a zillion of these.

your power valve size is fine and not the prob.

just unplug the msd box and connect your dist directly to the coil. your pertronix does not need it to run unless it is set up for a box only but i have never seen one like that.

you may need to buy a coil. the cheapest one is a stock type p/n 061.


good thinking on the air filter. it should have made a noticeable change in your afr's. are you sure it wasn't simply the louder sound that made it seem faster?

Last edited by barnett468; 05-04-2014 at 01:32 AM.
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Old 05-04-2014, 01:32 AM   #20
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PCV is built into the breather cap it is less than a year old and I need to verify that it is working.....could be an issue I did not check this.

Plug 1 is border line its in the rich okay side of things.

Plugs 2,3 are more of a gray color but not carbon fowled plugs 5-8 are a tan white on the lean okay side but not bright white or ghost white like in a hot or lean situation the electrode and ground strap have have nice edges not melted, eroded or corroded.

I did two full power accels from 0 to 70 with and without air filter. NOT in a row there was a 3 min section of freeway involved. In both cases as soon at the clutch was fully locked up in 1st gear at about 10 mph went wot to 70 in 3rd ran great. Without the air filter it made more power but in both cases accel was smooth no issue of any kind.

If I have time ill post a pic of the plugs.

Thanks

-Gun
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Old 05-04-2014, 01:32 AM
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