Help! I am baffled by intermittant miss , I need XPERTS

Old 06-30-2008, 11:02 AM
  #1  
Pdmracing
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Default Help! I am baffled by intermittant miss , I need XPERTS

Here goes, Bought me a new car off RJ,, a few months ago. Made it to the finals @ the Superchevy its first race! Problem started then & its still here.
Some backround.

Car is back halved monza running a 4:10 w TH 350 w 8 inch conv. Tires are MT 29.5x13.5

358 sbc with Brodix trac 1 , msd 6al , msd billet dist, 4150 holley 750 mech 2nd.

After Dyno tuning (ran great) brought to track. First 3 runs same et, ran great. run # 4 started to break down after 5800 in 2nd to 3rd shift, lost .2 et run # 5 same thing, looked over everything nothing obviouse, last run ran fine, back to original et.

Brought back to Dyno, changed cap, rotor, SP wires(all msd stuff) New Proform 750 carb. Ran both carbs, ran like a beast, all the way to redine in 3rd no problem. Hot day too.

Went to track
3 runs dead nuts. 4th run again with the pop pop pop @ the top of 2nd. this time with the pro form which was jetted rich. sounded like a lean pop, but I know it was rich. Some spectators who knew the car said it sounded like ignition.

I turned up Fuel pressure to 7 from 6 , ran fine next 2 runs then It started again, I raised my dial & shifted early, made many rounds, some great , some broke down. Last 2 runs were perfect & I ran to redline.

Went back & borrowed a msd & coil off a pal. Rewired all the wiring to ignition, grounded to the battery, soldered all connections. Replaced the autolite racing plugs. prior owner had them gapped @ .25 I opened to .40

1st run fine. 2nd run slight break up, 3rd run breakin up again, swapped coil. run #4 run OK.

Regrouped. Put in new msd mag trigger, & swapped a mallory digital 6 al, in place of the MSD just to see. Run # 1 started to break down again @ same place top of 2nd, this time on 1st run not 3rd, so not heated up. all 4 runs the same.

The one thing I did notice was my fuel pressure guage was fluctuating slightly when I reved the engine without load seemed to move (down when reved, but not by much) I may just be paranoid, but I didnt remember any movement prior.
Also when I picked up car from Dyno I noticed the fuel cap wasnt on correctly.
Could this be a fuel vent issue?

Why would it run fine @ the dyno & not on the track(attitude of the car? front end lift @ speed?)

I am stumpped, any ideas will be appreciated.
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Old 06-30-2008, 03:17 PM
  #2  
Scooterz
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Well, I am no expert by any means... but i did the same nature of problem come from a bad distributor gear.
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Old 07-08-2008, 10:13 AM
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Here is what MSD SAid, Ill let you know what I found this weekend

Well its sounds like the problem could be either a fuel delivery issue
or a ignition related issue. To help get to the bottom of the problem
check this stuff out as far as ignition related issues that you may be
having. It sounds like you may be experiencing electrical interference.
There are a number of things that can cause this but here are a few
things to try to see if it helps with your situation.

The first suggestion is to perform a full ignition system tune up. Test
all of your wires for resistance with an ohm meter. Replace any wires as
needed. Pay close attention to the coil wire since it takes 8 times the
abuse of any of the other wires. The resistance is 40-50 ohms per foot
with our 8.5mm Super Conductor wires.

Replace the cap and rotor if there is any signs of wear or burn traces
on the cap. Check and replace any spark plugs as needed as well.

Try disconnecting the Alternator to see if the problem disappears. The
alternator is one of the largest EMI generators in an automobile's
electrical system.

If you are using a magnetic pick up distributor or a crank trigger, you
can try shielding the magnetic pick up harness from the crank trigger or
distributor with some aluminum foil as a temporary test. If the problem
starts to clear up I would recommend installing a PN 8862 shielded
magnetic pick up harness to help dissipate electrical interference to a
common ground. You may also want to keep the coil away from other wires
and the MSD unit by at least 12". If you are using the points/amplifier
wire (white wire) to trigger the MSD unit, try isolating the wire from
the other wires it may be running along with. You can try shielding this
wire with some aluminum foil as a temporary measure to try and isolate
the issue.

Try re-grounding the electrical system of the vehicle. The grounding of
a vehicle should be like a ring or a loop. Try these steps-

1.) Replace existent battery cables with high quality replacements,
an upgrade to 0W or 2/0 gauge is recommended, and the use of fine
stranded cables ensures great conductivity, durability and flexibility.
Strand density is important...regular battery replacements have stranded
copper wires in the tens, while fine stranded cables are in the
thousands. They're more expensive, but worth the investment due to their
low receptivity, flexibility and ruggedness to heat.
2.) When you visualize a typical ground circuit, need to look at it,
as an electrical loop. All electrical/electronics equipment are
sensitive to noise, electrical variations (spikes/dead spots) and will
affect either accuracy (gauges, sensor outputs, ECM), control quality
(DC frequency voltage for actuators like IAC/Injectors/Solenoids), or
the lifespan of the equipment in use (alternator, starter, ECM). The
connections that make up the "ground loop" are:
1.Battery Post-to-Engine
2. Engine-to-Frame
3. Frame-to-Battery Post (ends where it started).

3.) As mentioned above, an upgrade to fine stranded battery cables
is recommended, along with an upgrade to 1/0 or 2/0 gauge. Any contact
point to the engine should be stripped of paint. AN engine block
location is preferred, but the use of the cylinder heads and the timing
cover is acceptable. An addition to ensure a permanent and reliable
metal-to-metal contact, is the use of External Star Washers.
4.) Engine to frame: This is the most important ground connection
for gauges, interior lights, exterior lights...proper operation. It is
recommended that this strap, if it's still in place, be reconditioned or
replaced by a new one, making sure the contact points are stripped from
paint, and the use of the external star washers. It is also recommended
to add a second ground strap at an alternate engine-to-ground location,
as a reinforcement/redundant measure.

Another suggestion I have would be to install a noise filter. The PN is
8830. This is a 26kufd noise capacitor that will filter and regulate the
voltage that may be feeding back through the battery.


You may also want to check out this tech article.

http://www.msdignition.com/pdf/tech%.../tb_top_10.pdf

I would be concerned with the fuel pressure dropping as the RPM
increases. It could cause the bowls to empty out under load. You might
want to start with the filters and make sure that there isn't any kinks
or bends in the lines reducing the volume to the carb.


Thank you,
MSD tech
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Old 07-08-2008, 06:11 PM
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jmarksdragster
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Fuel delivery is my thought as well. MSD did give everyone reading great info for the electrical end. Proper grounds are most important.
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Old 07-08-2008, 10:46 PM
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chevysteve2
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I'm no expert but i would check the fuel delivery, because the chassis dyno does not test the car against gravity caused by acceleration. you can rig a nitrious low pressure cut off switch to a tempoary light on the dash of the car. but I would also check all your ingnition related wiring under the hood that could be moving by air turbulance at high speed.

hope this helps
steve
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Old 07-09-2008, 03:59 AM
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us7race
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I just got over a problem like this. I ended up rewiring the MSD and made a floating ground for everything else going directly to the battery.
I just got my box back from MSD yesterday. I ran 0/1 gauge for the hot and grounds for the battery and wired the MSD directly to the battery by itself on the sideposts of my battery. I also shielded the trigger wire like stated above. With the combination of it all the miss went away. 8)
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Old 07-09-2008, 05:19 AM
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Tod74
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One thing I have learned about MSD is they always seem to blame it on the ground or something YOU did wrong with the wiring ...read the forum on their site. which could be true a lot of the time..but they told one guy on there ,if I remember correctly, to ground both heads to the block (aluminum) and then the block directly to the battery. Wouldn't the solid motor mounts serve as a ground? Wouldn't the head bolts ground the heads to the block? And someone teach me how to keep every plug wire completely seperate on a chevrolet distributor cap? almost impossible with a standard cap..all you can do is seperate em with the loom thingamajiggers but they are still sorta criss crossing at the cap to some extent.( 8 and 4, 3 and 6)

A friend of mine had a 7AL3 in his dragster, it ran fine in the driveway or pits but would miss out down track. Sent it back to them, they said nothing wrong with it and blamed the grounds etc( which were connected properly to the battery)He put it back in...same thing. He sent it back again, they said it was fine. Put it back in..same thing. He got mad and bought a new one. Put it in hooked to the same wires and never had another problem. SO we put it in my car, ran fine in the pits and burnout box but when you hit 6200 it cut out as though it was on the chip( tried several chips even took it out all together)...just like it did to him. Took it out and put mine back in(same wiring ) mine works fine. They still said nothing wrong with it but ended up replacing it to get him off their case.
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Old 07-09-2008, 05:45 AM
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suicidebomb
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Tod74, did the replacement box work OK? Just curious.
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Old 07-09-2008, 08:08 AM
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Tod74
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Originally Posted by suicidebomb
Tod74, did the replacement box work OK? Just curious.
He sold it. It was new in the box. He didn't need it because He had already purchased a new one just to prove to himself that it was the box and to get the car going. Yes it fixed it. All he did was hook the new one up TO THE SAME WIRES the first one was hooked to and it cured it.That was in 2002 and he is still using that box.
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Old 07-09-2008, 08:43 AM
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hammertime
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Default Re: Help! I am baffled by intermittant miss , I need XPERTS

Originally Posted by Pdmracing

Replaced the autolite racing plugs. prior owner had them gapped @ .25 I opened to .4



Could this be a fuel vent issue?

Why would it run fine @ the dyno & not on the track(attitude of the car? front end lift @ speed?)
Have you pulled the plugs back out of it yet ? Maybe the prior owner didnt gap them to 25, maybe they closed thereselves up hitting the piston ?

You do need a fuel cell vent -8 would be good.

Grounds are key 10 gauge wire from each side of the head straight to the battery is darn near a must, I have that and also have 4 gauge wire from each motor mount bolt to the chassis. Running your coil wires next to your dist wires could also cause this. Fuel system not getting enough fuel could also do it, a lot of options to keep looking at.
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