MSD6AL box go bad on a twin plug setup?

[Geneman]

hello braintrust. on a 74 RSR race car i am chasing down a huge power lag/ miss .. which seems to kick in after driving a few track laps..... seems like something is heating up . or maybe being blocked. still cannot yet tell if this is a fuel issue or an ignition issue.

this is a 3.0 SS high comp. race motor by knightrace, 50mm PMOs, Bruns twin plug dizzy, twin older 6AL boxes, brand new blaster coils

i was heavily leaning towards fuel because i had found a mouse nest in the fuel cell ( not kidding), which had chewed up a bunch of the foam. but that was fixed. fuel pump good ., showing 3-5 lbs on the dial, good sight levels in the glass windows. idle jets clear.

i am using one of the tach output wires to connect to the AIM dash./logger system and it seems to be giving intermittent signals. at least on the dash and in the video.

can anyone tell me a good test to distinguish a fuel from an ignition problem? can i test each box separately . what would be the symptoms of a box going bad.?

[dannobee]

Are the boxes wired as two complete separate systems, or are you using only one of the pickups? If two separate systems, it's not likely to be ignition related (unless it's the rotor). The funky tach signal is a cause of concern though.

We'd sometimes see the MSD boxes fail, but they'd usually quit like someone shut off the key. And sometimes they'd cool off and come back to life. In nascar we'd have two separate systems (two pickups in the dizzy but only one plug per cylinder), and a driver operated switch for the switchover to ignition "B."

When you mean fuel pressure is good, the gauge is in your line of sight at speed? And it's plumbed in after the filter(s)?

[rsscotty]

Can you post a picture of the wires that come out of the distributor to where they hookup to the MSD box wiring. Looking for wire color. Usually the JB distributor wires end about 8" from the distributor.

[Tom '74 911]

How's your fuel filter look? I had similar symptoms at the track once with a clogged fuel filter. Car would run fine for 2 laps and then start missing under throttle. Fuel filter was clogged (my mistake, long story)...

As someone who tends to overcomplicate things, I try to verify the easy to check, cheap to replace stuff first...

Best of luck in your troubleshooting.
Tom

[winders]

The coils/MSD issue is fairly easy to diagnose. Start the car with one coil output wire going to ground and then the other going to ground. How does the car run, if at all, in both cases? It should run the same in both cases.

This will not hurt the car just sitting in the garage. You can even rev it to see how it sounds and behaves.

Also, what does the rotor and cap look like? We have seen some cap/rotor interference problems with the JB caps. Not on my car, luckily, but on a friends.

[Jonny H]

The three most common twin plug misfire issues:

1) Sensor polarity. MSD is positive edge, Bosch is negative. Car can stlll run but rotor is out of phase with spark plug post.
2) Two boxes on one sensor. Can cause a misfire as two boxes can affect the shape of the sensor signal causing it to cross the zero volt line twice between cylinders. Depends on the type of box and the sensor.
3) On JB Racing distributors the rotor can be assembled in two ways. They advise that the adjustable rotor tip ‘trails’ the fixed tip with respect to the direction of rotation. Also the plug order is different for CW and CCW rotation.

[Geneman]

THANK YOU EVERYONE.. but crap . still had another awful track day . thats three wasted days in last four...

Another disaster…simply no power…. I found one coil wire with low resistance. ( one coil wire had 4k ohms res., the other showed 0.8 k ohms res. .. changed it..) . had no effect… coils are fine by the primary/secondary ohms test,

Scott the motor will start and run on either MSD box alone when cold, just firing one of the sets of plugs. i did this by simply pulling one of the coil wires from the dizzy. i did not ground it as you suggested.

The MSD boxes are wired as completely separate units to fire a single set of plugs. each box has its own coil. The pig tails coming out of the Bruns distributor have green and orange wires with a plastic connector that snaps directly into the connector from the MSD box... (hard to screw that up). both boxes, for both 12v operational power and the +/- high tension lines are connected at the same places: both high tensions directly to battery. 12v to power the units is separately fused and is run thru the cutoff-switch. ( online it says this may be a problem)

Its the same pattern every time…. First lap is fine, then starts losing power and pedal in second. by third lap, Flooring it down the straight will not go over 3-4 k….its actually stalling when i downshift at corners... Pulling into the pit it dies. This last time I actually I coasted back to hauler. Trying to restart it. Engine turns over fine so battery good but motor does not start .. …. when I get to trailer it still will not start ( i did not check spark output during this no-start period, which i should have done) . i also did not check if either the msd boxes ( mounted on the inside of the cockpit firewall, with good ventialtion and shock protection) or the coils ( mounted inside the engine compartment on drivers side) . were hot. google tells me that these boxes can be affected by heat..

Even when motor will not start, the site glasses in both carbs are fine with fuel…..fuel pressure at the analog meter on the regulator shows 5lbs.. just where i left it. ( i do not have another fp meter)

then, i let it sit for 15-20 minutes. Starts back up fine....

really do not know what to think. hard to imaging both MSD boxes going bad at once ?

thanks for your comments

[rsscotty]

Originally Posted by Geneman

THANK YOU EVERYONE.. but crap . still had another awful track day . thats three wasted days in last four...



The MSD boxes are wired as completely separate units to fire a single set of plugs. each box has its own coil. The pig tails coming out of the Bruns distributor have green and orange wires with a plastic connector that snaps directly into the connector from the MSD box... (hard to screw that up). both boxes, for both 12v operational power and the +/- high tension lines are connected at the same places: both high tensions directly to battery. 12v to power the units is separately fused and is run thru the cutoff-switch. ( online it says this may be a problem)

I thought I would clarify the wiring, since I think it is easy to screw it up since the colors of wiring at the distributor do not match the MSD when hooked up correctly.

[dannobee]

Probably not the boxes, but really sounds like the coils. Which ones are they? We had quite a few problems with the oil filled MSD coils. Even the epoxy filled ones had problems.

Would you happen to have enough parts lying around to convert one system to a conventional electronic ignition system?

The quick and easy way to narrow it down to spark or fuel is when it dies, dribble some fuel down the carbs. If it starts, you know it's fuel. If no joy, it's spark. Also, when cranking with dead spark, you'll get the strong fuel smell out the tailpipes.

Fuel system problems usually cause the engine to just "lay down" and lose power, and often the car won't go over a certain speed, regardless of downshifting. Spark problems generally have some sputtering and bucking.

[Mahler9th]

You have my empathy on this one. I have been having some strange issues with my twin plug set up. Twin MSD units, Blaster 2 coils and a JB Racing distributor.

We may be having similar issues. Longshot, but possible.

Here are some things to think about:

I have used Blaster 2 coils and MSD boxes since 2001. I have had two coil failures in all that time. I carry 2-3 spares. I also carry a spare 6AL, but I have never had one fail.

The oil-filled Blaster 2 coils do not like to be mounted upside down. I learned this in 2002 in the first ever PRC race. Of course the potted coils should eliminate this possibility. Hard to imagine MSD having issues with Blaster 2 coils since they sell so many.

It sounds like you think at least one issue may have a thermal component. If there is any way to confirm or refute that, it might be really helpful.

The caps sold by JB Racing are replica units from a factory in Italy. You can find the same caps at places like Amazon. I think they all come from the same place. I had one fail-- the center electrode just disintegrated. I still do not know why.

I have tried several different caps, and in some instances my rotor has had physical contact with the contacts in the cap. Its just not clear to me that the caps have tight manufacturing quality control and/or tolerances. Not sure how I feel about that.

I have had had quite a few conversations with JB Racing since I bought my distributor in 2017. At first I just could not get it to work. I even sent it back to them and they tested it. I think it is a fine product.

I am really impressed with them-- I feel we are lucky to have them making and selling these distributors.

I would be surprised if they had any manufacturing issues, but I would not be surprised if they have at least some vulnerability with respect to component suppliers. Hard for anyone to avoid that with such a product.

I might attempt to use both pickups in an experiment to see if one might be wonky. I have seen situations where factory distributors developed thermal issues. I am sure that many out there have. I wonder if it is possible for the pickup JB Racing uses to have a similar situation occur. Need to really think more about that-- perhaps a thermal issue where the pickup goes dead or intermittent.

I do not think that picking up the MSD tach output for an AIM system can cause the type of issue you are having.

Plug wires... I started out with a good used set of 993 wires that I re-terminated for the JB Racing cap. These wires had worked fine when I had a 993 distributor (that distributor failed due to a bad bearing-- cost me a crankshaft). As part of my troubleshooting I invested in a set of Clewitt wires, Excellent parts and I do not regret my investment.

There are several ways to wire the MSD's-- there is at least one other thread on this topic in these forums to which I contributed some diagrams/concepts.

My boxes are now connected in series. The tacho output of the first box feeds the points input of the second box. It also goes to AIM and my Haltech ECU.

So for box one, if there is a failure that kills its tacho output, the engine won't run. I will see no rpms on AIM, and neither the second box nor Haltech will not be able to operate.

The tacho output of my second MSD box goes to an rpm switch set at about 2k rpm. So when the revs are below 2k, the NC contacts of this switch light up a small dash light. Above 2k, that light is out.

So I effectively have a system that will warn me if one or both of the MSD boxes tacho outputs stop.

But that only helps if all MSD failure modes result in a dead tach output signal. I have not asked MSD about this nor studied their circuit topology.

I think MSD can test their boxes for a fee. I might look into this.

Let's all work hard to get this sorted.

No fun to troubleshoot stuff like this.

The apparent thermal nature may be a big hint.

Heat gun and distributor testing machine... hmmmm.

[Geneman]

blue alternator wire? hi guys . i just posted this:

alternator indicator light via blue wire?

could this be my problemo?

[dannobee]

Does the alternator charge right now? Does the engine shut off when you power down the ignition?
Put a voltmeter across the battery posts with the engine running. Voltage should be about 14v.
MSD’s do take some power to run, but they’ll usually draw the battery down so low that it wouldn’t crank over if the alternator quit. I think you said that yours will still crank when it quit, right? Was the battery still strong enough to turn it over at normal cranking speed?

[Mahler9th]

Not likely an alternator/charging issue. Yes you need to have a resistor or a bulb for that alternator circuit to work correctly. I will respond with more in your other thread.

[Geneman]

thanks mike and danno.... danno as i remember it, the engine cranked fine but would not start not sure how to measure the alt. output. but will do the test across battery terms. as you suggest,..

meanwhile i will add the bulb to the blue wire. see my query in other thread on wiring

[Geneman]

hi friends . i posted this in the other thread i have going on this topic.... but my explanation is more germaine to this thread.. sorry for dupes.
to wit:


Reporting in from the field. Based on so many reports of MSD boxes overheating/going bad... i bit the bullet swapped them out for brands new ones, ran all new wiring, . also hooked the friggin little blue alt. wire up to a light. This thing is great! comes on with ignition on, ( as i have it wired) . Then when alt. kicks in at start it goes out. . very nice indicator to have on dash! ..

at 3000rpm in my garage i get 14.7 volts across the battery terminals., so to me, the alt. is working.

unfortunately . does not seem to have solved any problem... engine still misses and backfires up thru midrange. i do not yet know if it will cut out after a few track laps... but will see on tuesday.

i also checked fuel flow thru and past the pmo regulator and into the carbs. all is fine. no blockages. good fuel in pmo windows.

at rest in garage, i have AIM afr readings of 16-17. at odles . 1000rpm... they go down to 13-14 when revs are up to 3500 or so. but this still seems high. at that rpm. motor is still backfiring.

These are lumpy race cams along with 12.3 hi comp motor. not sure this makes a difference.

i am at a loss as to what to do next if this does not solve. cap, rotor, wires?

[dannobee]

Did you replace the coils too? We had far more problems with the coils than the boxes.

[Dan J]

Hey Frank
Have you tried a fuel volume test? Do you have a fuel pressure gauge on the dash?
I sent you pm

[stownsen914]

I second Dan's suggestion for a fuel pressure gauge that you can see while driving (get the electric kind so you don't have fuel flow / connections in the cockpit). If you're experiencing any fuel insufficiency under load, you'll see that on the pressure gauge. I had a problem similar to what you describe on my vintage race 911 a few years ago. I had a couple fuel issues, including a restrictive filter before an external surge tank. Turns out that halfway through a session the surge tank was running empty due to the first fuel pump not keeping up with the engine. The pressure gauge showed this immediately.

Not a bad idea to have a spare coil on hand to rule that out too.

Scott

[stownsen914]

One more comment re: Dan's other suggestion to check fuel flow ... I had a separate issue with one of my fuel pumps (the one feeding the engine from the surge tank) not quite keeping up under load. You can check fuel output before heading to to the track. It's best to check it in situ with the filter, pressure regulator in place since those can restrict fuel volume. In my case, the gal/min I was getting from my in situ fuel volume test looked OK to me, but when I finally put the car on a dyno to troubleshoot, I could see fuel running low in the PMO sight glasses. A higher flow pump fixed that problem.

[Mahler9th]

Looks like you are getting some suggestions for checking on the fuel system. Hopefully its performance can be fully verified as a cause or not.

As to spark, I think it may make sense to try NIB coil or coils, or known good used ones. It is more painful than it once was--- when I bought my first Blaster 2 replacement in 2002 they were about $25. Now more of course.

It also pays to check the wires and distributor cap. To make sure that the parts have integrity and are properly and firmly connected.

As for the distributor cap, like I may have said somewhere, a Forum poster has suggested that the has some experience with these types of caps. The current ones are apparently all made in the same place, but that might be difficult to confirm. I had one center button disintegrate from an unknown cause. It made a mess. The speculation is that a Blaster 2 coil failure wiped it out. Cannot confirm that.

I have several different caps, and they are all a little different in terms of clearances between terminals and the rotor. I don't think the distributor shaft is wobbly (it is a new distributor that JB Racing checked out 2x). So keeping an eye on my caps.

I have been through this.