Crab distributor triggering MSD.

It'll run a normal setup fine without the other point.
The Ford V8 was a very high RPM multi cylinder engine when new, too much for the single point-single coil setups of the day. Other eights were in expensive cars and used dual coil setups to solve the problem...
As a Ford, unnecessary stuff had to be eliminated to keep the car in it price class, so Ford turned to Mallory, using his dual point-single coil patents to extend dwell for the coil and his vac-brake designs to control the problem of advance at both large and small load running. This gave enough single-coil function for a maybe 4,000 RPM eight.
Modern coils, going quite a ways back, are perfectly happy with 4-5,000 RPMS and eight cylinders with single point dwell.
Remember, Ford was able to go to single point in '48-9...they lost performance due to the garbage Holley advance system (Spit on floor, mutter a curse!) but there was plenty of spark for the stocker.
 
Karl Wolf said:
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
Click to expand...

Once I get static timing dialed in with a single set of points, I'm going to try to run the engine that way --- just a coil and single breaker points/condensor --- before I convert it over to a points triggered CDI. Every modern 8 cylinder engine I've owned with 12 volt ignition and 8 lobe points cam, ran just fine on one set of points. I see no reason why my '42 Merc engine wouldn't.

Bob
 
Onward...this, of course, means a lot of our obsessing with combined dwell and phasing is pretty much a waste of time, fine-tuning 1930's tech without a real need beyond the aesthetic one of maintaining cool vintage high tech.
It would not be hard to redrill the plate for a 1960's single point of some sort and save a bunch of time and money in the future...but you'd know something clever and beautiful was missing.
 
Hang on fellas.
There are some points (geddit) not quite being made clear here.
The LEADING set of points does NOTHING as they open. NADA. The TRAILING set of points fires the spark when THEY open.
So either leave both sets in, in which case the leading set are completely redundant, or take out the leading set and keep the trailing set.
The timing will then remain unaltered from when you last ran it. The engine fires on the trailing set of points.
I will now brave the cold to see on which side is the trailing and leading set of points.

Back in 5 mins.

Mart.

EDIT:
Right, just checked a distributor on the bench - the right side points lead, the left set trail.
So, as Bruce states - keep the left set.

Left as in automotive convention as in left and right side of the car as viewed from the drivers seat. Regardless of hand of drive.
 
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Oh yes, Bruce, tis I.

Bruce, from your post above, you are correct in stating the left set do the triggering, but the right set are advanced, not retarded.

Great to have a constructive debate.

Mart.
 
Do i get a "A+" for staying out of this entire discussion ?????

On the note of installing a single set of ignition points ( like a chevrolet) its pretty hard to get the cam/rotor phaseing correct with out some equipment and experience......
 
BUBBAS IGNITION said:
Do i get a "A+" for staying out of this entire discussion ?????
On the note of installing a single set of ignition points ( like a chevrolet) its pretty hard to get the cam/rotor phaseing correct with out some equipment and experience......
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By cam/rotor phasing, do you mean that when the points break to fire, for example, the number 1 cylinder, the rotor is actually indexed to the number 1 terminal on the cap?

Bob
 
bobscogin said:
By cam/rotor phasing, do you mean that when the points break to fire, for example, the number 1 cylinder, the rotor is actually indexed to the number 1 terminal on the cap?

Bob
Click to expand...

Yep the rotor is phased to the trigger point on the distributor cam so the rotor is centered on the cap terminal when the cylinder fires. ( any cylinder) If the location of this trigger is changed ( like the addition of a different part etc) then the rotor fires either early or late making the kv demand so great it may or may not fire with rpm change OR actually fire the wrong cylinder under load etc.
Google distributor rotor phase and look at MSDs page of how to check with old cap and a timing light........
I fought one most of the day today on a old mallory flathead unit ( been apart many times etc) that started good and would idle but not rev up at all ! Rotor phase ( cam position on shaft) was off approx 1/8th inch !!!
Finally after much trial and error got it on the money........:D.
 
Thanks, Bubba's. I'm going to give this single point thing a try, but don't be surprised if you get a call from me! :D

Bob
 
bobscogin said:
Thanks, Bubba's. I'm going to give this single point thing a try, but don't be surprised if you get a call from me! :D

Bob
Click to expand...


Bob,

Mark the cap terminal locations on your housing ( with cap installed in stock position)

Position rotor to point at one of these marks.

Position new contacts ( ford V8 contacts are the right size) with rubbing block to just open at this point. Mark location of hold down screws and drill , tap etc...

Recheck making sure points just open when rotor is lined up.:eek::)
 
A little tip on phasing----Be mindful of the fact that centrifical advance alters the location of the rotor in relation to the cap. I allow for this and bias rotor location to be conducive to high speed operation where demands are higher.
 
JWL said:
A little tip on phasing----Be mindful of the fact that centrifical advance alters the location of the rotor in relation to the cap. I allow for this and bias rotor location to be conducive to high speed operation where demands are higher.
Click to expand...

Perhaps the advance mechanism should be blocked to full advance before checking rotor phasing? If the phasing is off, couldn't it be corrected by rotating the cap on the distributor housing? This would, of course, require that the indexing tab on the cap be modified.

Bob
 
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bobscogin said:
Perhaps the advance mechanism should be blocked to full advance before checking rotor phasing? If the phasing is off, couldn't it be corrected by rotating the cap on the distributor housing? This would, of course, require than the indexing tab on the cap be modified.

Bob
Click to expand...

Yes the cap can be moved ( thats what we had to do with some of the hot rod odd fire Buick V6 engine applications to ge tthe right cap alignment etc.
The rotor cant move away from the contact anymore than the edge of the rotor without causing a larger rotor to air gap. Most advance units wont move any more than the width of the rotor tip.
Heres a couple pics from MSDs tech sheets showing the phasing operation.....
 

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Well, on the crab everything stays in the same place re rotor and cam when you mess with this stuff, so keeping phasing stock if you were to transplant a later point set instead of standard would come down to only one issue...get the new setup drilled and installed so that the new point kicks open very close to same point as the original one did. Unlike many modernish distributors, you aren't likely to alter rotor/cam positions on this thing while messing around. I haven't looked into one in quite a while, but on Chevy and the very similar MSD distributors you could kick up a lot more trouble very easily if not watching out for several aspects of phasing the parts.
 
Bruce Lancaster said:
Well, on the crab everything stays in the same place re rotor and cam when you mess with this stuff, so keeping phasing stock if you were to transplant a later point set instead of standard would come down to only one issue...get the new setup drilled and installed so that the new point kicks open very close to same point as the original one did.
Click to expand...

I'd considered going to a more commonly available set of points. If there's room on the points plate, I like the early GM V8 points that adjust with the socket head screw . However, given that points switching a CDI will probably outlast me, it makes more sense to stay with one set of stock points. They may require an occasional gap adjustment, but the contacts will probably outlast me.
Speaking of point gap, I'd like your opinions of what the gap should be when the points are used to switch a CDI unit rather than carrying current to the coil? Stock specification, wider, narrower? If not stock spec, why different?

Bob
 
The good GM points are probably too big. I would look at 1950's Ford...there are even some of those made in mirror image of each other, 6 and 8 applications, or circa 1962 Mopar V8. Have no clue on points switching electronics, but probably not critcal at all given very low current. I suspect that standard gaps would be fine, as I believe that ONLY the opening moment matters. I think all considerations equivalent to dwell are handled electronically, and as long as firing triggering happens at correct time the electronics don't care when or how long the points close.
 
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