quote:

I don't know why some people can run only a distributor/coil set up and have no problems at all.

If you’ve ever seen the DA from Holley showing a distributor trigger (brand new install) verses a crank trigger you’d learn that the reported rpm can differ over 75 rpm in a .014 time period. Very easy to identify distributor trigger vs crank trigger in acquisitions.

If you are shifting on rpm, using a distributor trigger is a poor choice for shift accuracy imho. It is one of the items I regularly review in DA.

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Try drilling a vent hole in the distributor cap

I like the big distributor caps and everything I use has two (2) 1/4” holes in them located between the towers and centered on the rotor side face. Even my smaller cap efi distributor has these holes in it. Always clean and no rust inside.

are you running a small cap on the new distributor? if so could it be spark jumping where the old distributor was a large cap so less likely to spark jump.

i may have missed it but change the pick up coil and domafloger on the shaft.
Switched power to the box comes through a relay panel that is also new.
is this a k&r and are you 12 volts this is critical to my next response

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Originally posted by rusty:
i may have missed it but change the pick up coil and domafloger on the shaft.
Switched power to the box comes through a relay panel that is also new.
is this a k&r and are you 12 volts this is critical to my next response

No to K&R and yes to 12V system.

ok if you have acces to msd tester check voltage to grid while testing with tester.if the voltage drops much below 10 under load it can cause problems.sometimes misfire sometimes motor shutoff

Does MSD make a tester to check a MSD tester?

lol my point was i ihave worked on two different systems lately that had a break down in voltage feeding the grid ign unit on the big wires.under load the of the system firing the voltage would drop.and when it did even when still running the test plug there would be a slight change in the sound it made.it was enough to cause the engine to break up going down the track.it was detected by reading the feed voltage.on 16 volt systems it can be masked.

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Does MSD make a tester to check a MSD tester?

Check with an Oscilloscope

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worked on two different systems lately that had a break down in voltage feeding the grid ign unit on the big wires.under load the of the system firing the voltage would drop.and when it did even when still running the test plug there would be a slight change in the sound it made.it was enough to cause the engine to break up going down the track.it was detected by reading the feed voltage.on 16 volt systems it can be masked.

This is one reason why MSD recommends that you wire the +/- leads directly to the battery. That's where mine are terminated.

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Originally posted by BD104X:

That sounds like what happens when the red 7AL-3 boxes go bad. If you put a timing light on the car and bang on the box, you'll see the timing jump. Then you send the box to MSD, they tell you there is nothing wrong but they 'updated the board / relay' and then the box miraculously works fine again.

100%

Take the high side chip out and make a pass.

I recently had multiple cars doing the same high rpm breaking up crap...EVERY car needed float replacement. Every car with floats less than a yr old were changed out and situations resolved.

I won't beat down carb guys...BUT the floats sold today S U C K b a l ls IMO. Let's just say Get M isn't currently having this issue...many other brands are. IRONICALLY, many other brands use the same source for floats. RANT over.

quote:

Originally posted by TORQIN:
I recently had multiple cars doing the same high rpm breaking up crap...EVERY car needed float replacement. Every car with floats less than a yr old were changed out and situations resolved.

I won't beat down carb guys...BUT the floats sold today S U C K b a l ls IMO. Let's just say Get M isn't currently having this issue...many other brands are. IRONICALLY, many other brands use the same source for floats. RANT over.

I've been switching my stuff over to braswell floats as needed. So far so good.

Here's an update. Changes were made 1 at a time and passes made to test for improvement.

7AL2 box is now wired direct to the battery - no change.
Changed to an 8251 coil - no change
Tried to hook up a spare 7AL2 box but it smoked as soon as I put power to it.
Took the high side chip out - no change
Unplugged the 2-step module - ran clean
Swapped 2-step modules - seemed ok on 1 pass but started to break up a little bit at the top of high gear.

So seems maybe 2-step or rev-limiter related? Anyone seen this before?

Maybe it's time to upgrade to a new ignition system altogether.


edit:
I forgot to add that I have inspected the cap and rotor as recommended and they look brand new as I would have expected. I did not add holes to the cap though as it is a vented cap from MSD.

This message has been edited. Last edited by: CSRacing,

quote:

Originally posted by TORQIN:
I recently had multiple cars doing the same high rpm breaking up crap...EVERY car needed float replacement. Every car with floats less than a yr old were changed out and situations resolved.

I won't beat down carb guys...BUT the floats sold today S U C K b a l ls IMO. Let's just say Get M isn't currently having this issue...many other brands are. IRONICALLY, many other brands use the same source for floats. RANT over.

Floats are brand new. I just spent a small fotune having Rupert go through the carb so I think it's unlikely a fuel problem.

I still think it is the 7Al-2 box. I have had 3 go bad through the years and one did just like yours, breaking up on top end. Not a dead miss just not 100%.

I also think you need to stage lot higher water temp than 150*. Mine runs cleaner, faster and more consistent at higher temp.

Why did the other 7AL-2 box smoke when you wired it up? Should not smoke unless it was wired wrong?

I had the same symptoms as you describe. I chased it for a full season, it would miss maybe 6 or 8 times going down track mainly in high gear but occasionally in low. It wouldn't do it every pass. The 1st pass of the day was usually the least likely to do it. It ended up being one of the wires to my grid had been crimped too tight and had cut about 1/2 the wire strands. Not a likely resolution but trust me it can be the issue. I also went through changing coils, plug gaps, rotors, caps, plug wires and on and on. MSD told me to pull the grid and send it in and that was when I found the wire issue. I would give all your ignition power wires a good tug and make sure you have good solid connections.
Best of luck.

Same only not, just went thru this with a 7AL3 {I know they are junk} but would run fine till u put the t-brake on would load up the motor sent it to MSD and they said the limiter circuit was bad and x amount of money later its fixed. With a red 7AL3 box if u have 1 you need 2

Try replacing the wires from the distributor to the box-ya I know they are new but several years ago had same type issue-replced wires and problem went awayoh ya had ohmed wires and they checked fine

This one is gonna sound stupid, but had it happen on a BBC a few years back.

Assuming all the electrical parts are in good shape....

Have you checked camshaft end play and distributor gear condition? Mine had waaaay to much camshaft end play due to a timing cover swap and failure to recheck end play, cam was walking forward and backward while under power. Very difficult to diagnose with the car in neutral and checking the timing with a light light, but under acceleration the cam would walk back and forth and change the timing. I swapped to a crank trigger at the racetrack and solved the problem but went home the next week and fixed the camshaft end play to be what it should be. The same thing can happen if the distributor gear is worn out. I've seen some get really sharp and create slack in the engagement which will allow the timing to wander during the run.

Usually I just replace everything when I get to the point you're at. ROI is about 3-4 trips to the track. Maybe the problem is found through replacement and then you have some spare parts.

I'd be looking closely at the fuel system also. Low volume will cause high RPM miss.