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  #1  
Old 10-25-2007, 07:04 PM
Robin Marler Robin Marler is offline
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Default Idle problem

I've had an off and on idle problem with a smal block chevy for some time. The ignition was a used MSD 6AL Box, MSD 85551 and Blaster Coil. The problem acted like a vacuum leak. However, after checking for vacuum leaks, changing the carb, fuel regulator and fuel presue guage, I began to suspect ignition. I tried a new MSD distrbuter to no effect. Finally I tried the igniton from another car (distributor, coil and ballast resistor). That fixed the probem. I reinstalled the MSD distributor with a new 6AL box and the problem was still fixed. Have you heard about this problem before? My associates that have a lot of experience with high performance motors all say that they've never heard of a MDS box idle problem.

Last edited by Robin Marler; 10-26-2007 at 07:58 AM.
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Old 10-26-2007, 08:34 AM
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Usually the box either works or doesn't. However it is possible for other things outside of the box that can affect its performance.
Let's start with some basics.
Where are the Heavy Red and Heavy Black wires of the MSD box connected?
Is the cable that connects the distributor to the MSD box bundled with other wires?
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Old 10-26-2007, 09:06 PM
Robin Marler Robin Marler is offline
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Default Idle Problem

Heavy Red directly to Battery +
Heavy Black directly to Battery _
Wires from box to dist not bundled with anything else.
Small Red wire to box also bundled with tach power.
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Old 10-27-2007, 05:55 AM
Robin Marler Robin Marler is offline
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Default Idle problem

I think I misunderstood "bundled" in my original reply so here's a replacement.

Large red is directly to the + battery cable
Large black is directly to the _ battrery cable
Small red is on a switch with tach and shift light power

As the box came to me used, the wires to the distributor were NOT an original cable and and NOT in a sleeve like ones that came with the new box.
Also, they were bundled together with cable ties with other wires, e.g. tach / shift light power, signal and light.

Also, because it wasn't an original cable, could I have reversed the polarity of the wires to the distributor?

Last edited by Robin Marler; 10-27-2007 at 11:46 AM.
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Old 10-29-2007, 08:09 AM
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It is possible that the mag p/u leads may have been reveresed. On the distributor you will have a black/orange wire and black/purple wire. The box has a green and violet wire pair. The Green will connect to the Black/purple and the Violet will connect to the Black/orange.
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Old 10-29-2007, 09:18 AM
Robin Marler Robin Marler is offline
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Default Idle problem

Thanks for the quick responses. Racing is over her so I'll do a test and re-install the old box with the new cable this winter. That way I'll have confidece that I have a good spare box. Just one more question. How sensitive is the system the having a shielded dist cable and routing it by itself?
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Old 10-29-2007, 10:33 AM
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The box is looking for a specific signal that is generated by the magnetic p/u, if noise is introduced into these leads that may look like that signal the box will attempt to fire.

Any unshielded wire is susceptible to picking up or transmitting interference.
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Old 11-14-2007, 06:12 PM
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what will happen if you hook the heavy black to the frame with a ground strap and not too the negative side of the battery?
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Old 11-15-2007, 07:31 AM
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DO NOT rely on frame or chassis grounds. The unit is a high voltage piece of electronic equipment and relies on the best possible groung to complete the flow of electricity. The battery is the best source of ground found in that vehicle. That is why we stress that the MSD Heavy BLACK and RED be attached directly to the respective battery posts.
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Old 11-15-2007, 03:05 PM
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could this possibly cause a weak spark?
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