Crab distributor triggering MSD.

B

bobscogin

Senior Member
I want to use the dual point crab distributor on my '42 Merc to trigger a CDI box such as an MSD. I know I'll need to remove one of the two point sets. Which set do I remove? Will I need to modify the position of the remaining set to get the timing within range? Any advice is appreciated.

Bob
 
Your choice, but why? Are you chasing 1/0ths at the track? I don't mean to be negative or anything, but sometimes people think these buzz boxes will work some kind of magic.
 
I suggest that you find an old crab distributor out of a 2GA six cylinder motor and swap the breaker plate from that to your V8 crab distributor, they are the same except for the shaft I beleive. The 2GA distributor only had one set of points. This might be easier than modifying the V8 plate. Just a suggestion - not sure if it would work or not but maybe worth a try?
 
I'm thinking of using a 4 lobe Mallory in an OHV and I reasoned this way: 1 set of points triggers 4 cylinders alternating with the second set for the other 4 cylinders, but the end result is all 8 are triggered in sequence and on time. The MSD just uses the point action to initiate and control the coil saturation and ultimately the plug firing. The 8 lobe dual point dist. really provides the same triggering function whether using 1 or 2 sets of points: just set the dwell to the correct number.
 
X38 said:
Your choice, but why? Are you chasing 1/0ths at the track? I don't mean to be negative or anything, but sometimes people think these buzz boxes will work some kind of magic.
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Not to worry about being negative, mate. I'm going negative ground. :D I consider a point triggered CDI ignition to be the best of both worlds. The single point set will last forever because it's conducting milliamps instead of amps, and there's no electronics inside the distributor to be affected by heat. Just a small external box than can be mounted out of sight. Not even a ballast resistor. I don't expect magic, but I do expect to have a simple, low maintenance ignition system with a hotter spark that stays in adjustment .

Bob
 
joel said:
The 8 lobe dual point dist. really provides the same triggering function whether using 1 or 2 sets of points: just set the dwell to the correct number.
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If I understand the operation of the dual point, the leading set of points breaks the circuit thus firing the coil, but it's the trailing set that closes the circuit. If that's the case, then how many degrees of distributor rotation separate them? They could not be exactly 180º apart or both sets would open close simultaneously, emulating a single set. It occurs to me that one could vary the timing by the angle of separation by running both sets in parallel and selecting either set to trigger the CDI unit thus shifting the timing by the angle of separation. Just thinking out loud.

Bob
 
I have an MSD unit to use on my 39 Deluxe, plan is to leave both sets of points in place and not sweat it... MY reason for the MSD box is that the points are only used as a signal-very low amperage-and will last forever... knock on wood... Karl
 
bobscogin said:
I know I'll need to remove one of the two point sets. Which set do I remove?

Bob
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How do you know you need to remove one set of points? All that would accomplish would be to decrease the dwell somewhat and move the timing a little. I don't think that would make any difference in triggering an MSD. Are there specific instructions from MSD to do this?
 
It is not necessary to remove one set of points. I have thousands of miles on a system using both sets.
 
BillM said:
How do you know you need to remove one set of points? All that would accomplish would be to decrease the dwell somewhat and move the timing a little. I don't think that would make any difference in triggering an MSD. Are there specific instructions from MSD to do this?
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In researching point triggered CDI, I found that most manfacturers recommend removing the trailing set of points from dual point distributors. On engines where the distributor can be rotated to change the timing, there's no problem with correcting it if needed, but the crab has very limited adjustment. While it may not be necessary to run only one set of points, I believe in simplifying when possible, and one set is simpler than two. Perhaps that's why manufacturers recommend one set, or maybe there's a technical reason. I guess I'll get out my degree wheel and test light and do some experimentation.

Bob
 
did that on a crab with the MSD 6A on a 59A in my 41 truck. you do not need to remove anything, points, from the distributer. when first point opens and triggers MSD it fires and waits for the points to open again to fire again. the dual points was for coil saturation and is not relavent with the MSD unit. it sees that leading point opening of the points only as a reason to fire. works just fine with the stock distributer as is. the MSD does not care about dwell, points are a trigger to fire MSD. don
 
donald1950 said:
it sees that leading point opening of the points only as a reason to fire. works just fine with the stock distributer as is. the MSD does not care about dwell, points are a trigger to fire MSD. don
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That confirms what I discovered/already knew, and is the reason that the trailing set is redundant. A clarification for those who left both sets of points installed: I'm not "removing" anything from a complete distributor. I'm putting one together and see no reason to install a second set of points if not necessary. Just more to fiddle with and get out of adjustment. As per MSD's recommendation, I'll leave out the trailing (secondary) set and check the static timing to see if it's within range.

Bob
 
Ol' Ron said:
What's interesting here is: The advance curve is more important than all this BS.
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Ron, I couldn't agree more, but my question wasn't about the curve. There's already tons of info/discussion/theory on the advance curve, but none of that answered my question. I was simply asking those who've used points to trigger a CDI unit, which set of points they used. Now I think I have enough info from this thread to arrive at a conclusion.

Bob
 
Ol' Ron said:
What's interesting here is: The advance curve is more important than all this BS.
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While this is correct, its not all that matters.

I have done many point to msd setups and for sure best of both worlds. Much hotter spark, faster starts, ease of maintenence, bullletproof reliablity all factor in too.

Be sure when you do this swap to open the plug gaps up to .050 and you will notice significant gains.
 
In the stock setup, opening of the LEFT set of points triggers the spark. Left means left of car, USA driver's side. If you use only one set, use that one.
Right set ius 10 degrees or so retarded from that, fixable by rotating the plate, but keeping the original trigger one keeps you in center of scale with room to adjust AND allows you to use original methods of all sorts to set basic timing off the engine.
 
Bruce Lancaster said:
In the stock setup, opening of the LEFT set of points triggers the spark. Left means left of car, USA driver's side. If you use only one set, use that one.
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Bruce, once again you've come to my aid with a concise and "to the point" answer to my question. Thanks for taking the time to respond.

Bob
 
ANOTHER thought, if I have an on the road MSD failure, I could with both sets of points in place, put another coil in place and continue... Not real likely, but not unheard of..... Karl
 
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