Crab distributor fine adjustment

D

d m moore

Junior Member
Question re crab dist on my 1942 Canadian army truck - '42 Mercury engine. After having pertronic electronic ignition fitted I needed to make some fine timing adjustment using the slotted hex screw on the top right hand side of the distributor (not the vacuum adv/ret adjustment). The bolt seemed very easy to turn and I can see no locking arrangement. Could vibration make this adjuster move? Second question - does a 5 degree (say) adjustment of this screw equal a 5 degree adjustment of the timing? i.e. a direct relationship, clockwise advance, anticlockwise retard?
 
After rebuilding perhaps a 100 Crab dist. I can not figure out WHAT you are trying to adjust. The indexed timing plate on the rt side [pass side] or the vacume piston on the lft side [drivers side]. Explaine better please. ken ct.
 
Yes, direct relationship - advance one way, retard the other. You are moving the points plate only; as opposed to a "normal" distributor (non-flathead) where you are moving the whole distributor.

On a side note: moving the points plate changes the rotor to cap electrode relationship as well.

Also, they do not normally vibrate loose, and do not need to be over-tightened.
 
Again what are you trying to adjust.The screw on the index plate must not be longer than 1/2" or it may contact the metal spring arm on the points 10x32x1/2'with a lock washer.If it wont tighten up the hole in the tab of the contact plate is stripped out. You need a new plate. Middle line on the index plate should get you running. ken ct.
 
Tuning a stock pre 48 distributor takes allot of guess work on the part of the tuner.driver. These distributors are well designed and preform very well when in good operation condition. BUT, ther is nor reference point, as to where the actual timing is. Adding a timing mark and pointer would be my first project. Most of these distributors have 20/24 degrees advance and is controlled by the Vacuum brake and this is a seat of the pants adjustment where you back off the piston to start with and drive the car around, adjusting this to prevent ping. It's nice to know how things work, hope this helps.
 
The screw is the locking mechanism. you loosen the screw and then move it either up or down. Once the correct timing is achieved, tighten the screw to lock it. the plate under the screw has graduations marked on it.

Mart.
 
I obviously wasn't clear - and I'm confused by some of the replies. I think I said I was NOT talking about the vacuum adv/ret screw but the one on the top right looking from the front.(Won't use "passenger side" - this is an army truck designed for the British army 1940 so is right hand drive!)

I think C W Gordon has answered my question - I am moving the points plate. I said electronic ignition hidden inside so no points - but is the plate still used? I didn't do or see the installation of the Pertronic and am not expert enough to know what is left of the original system. Maybe I am adjusting something that is no longer functional? But it did make a difference!
 
That screw sits in a slot in the distributor housing. Loosen the screw and move the screw clockwise or counterclockwise. Moving it clockwise advances the timing and counterclockwise retards it. The screw is the locking mechanism.
 
d m moore said:
Question re crab dist on my 1942 Canadian army truck - '42 Mercury engine. After having pertronic electronic ignition fitted I needed to make some fine timing adjustment using the slotted hex screw on the top right hand side of the distributor (not the vacuum adv/ret adjustment). The bolt seemed very easy to turn and I can see no locking arrangement. Could vibration make this adjuster move?

Vibration normally wouldnt make this adjustment move as the plate is held in the distributor with a large c-ring that doesnt move very easiely.
Q ? Is your engine marked with timing marks so you can measure timing ?
Normally timing doesnt need to be adjusted on this distributor. The addition of the pertronixs may have moved the timing causing this need. However it would need to be measured and checked with a timing light....




Second question - does a 5 degree (say) adjustment of this screw equal a 5 degree adjustment of the timing? i.e. a direct relationship, clockwise advance, anticlockwise retard?
Click to expand...

Each degree here is two degrees engine timing...clock wise is advance.....
 
d m moore said:
I obviously wasn't clear - and I'm confused by some of the replies. I think I said I was NOT talking about the vacuum adv/ret screw but the one on the top right looking from the front.(Won't use "passenger side" - this is an army truck designed for the British army 1940 so is right hand drive!)

I think C W Gordon has answered my question - I am moving the points plate. I said electronic ignition hidden inside so no points - but is the plate still used? I didn't do or see the installation of the Pertronic and am not expert enough to know what is left of the original system. Maybe I am adjusting something that is no longer functional? But it did make a difference!
Click to expand...

Bubba has answered this for you (just above). But, I obviously confused the issue; I should have said "points plate" with the little " " around it, as, yes, you do not have one anymore. The conversion kit replaces the original "plate" with a new plate containing the pickup.

So, "points plate", "pickup plate", etc, etc is what you are moving to adjust the timing. It still has the same effect in that you are altering the plate to distributor shaft relationship, which changes the timing.

Sorry for the confusion.

W
 
Loosening and tightening the screw does nothing,you loosen the screw and move the screw and index plate up or down. Not clock wise or counter CW. ken ct. That moves the point plate adv. or retard.
 
ken ct said:
Loosening and tightening the screw does nothing,you loosen the screw and move the screw and index plate up or down. Not clock wise or counter CW. ken ct. That moves the point plate adv. or retard.
Click to expand...

Ken, Flatjack said loosen the screw and move the screw which is attached to the plate, The plate and screw move together.
Clockwise /counterclockwise ,Up / down, same thing just depends on where you're looking from.
Bill
 
bbrocksr said:
Ken, Flatjack said loosen the screw and move the screw which is attached to the plate, The plate and screw move together.
Clockwise /counterclockwise ,Up / down, same thing just depends on where you're looking from.
Bill
Click to expand...
Well sorry hes right, ive always looked at it as up or down standing on the pass side of the car to adjust it. Its embedded in the bowles of my brain and I don't think it will change. LOL ken ct.
 
So, let me get this clear - the screw that I can see on the top right of the distributor I have to loosen - and then move it (thus rotating its attached internal plate) clockwise or antic/w - then tighten the screw again. Right? I can just about get at the screw with the rad in place - can only just see it - those darned spark plug conduits are in the way- and I guess I can find a way to move it once loosened.

Any idea how many turns to loosen - I would hate to have it come out altogether and fall down into the depths! I guess the sheet metal on these WW2 Canadian army trucks together with the extra deep radiator header tank and twin belts and pulleys makes it even harder than usual to get at.

I must admit it does make sense to have to rotate something rather than just turn that visible screw.
 
D M, yes, you have it now.

The distributor is designed to be set up on a timing fixture off the vehicle, as access is very difficult in anything other than an open wheeled hot rod.

Unless you have a timing light and you have taken the precaution of fitting a timing marker you will just be guessing when you are sliding that screw up and down in it's slot. I must admit I mostly just centre the timing plate behind the screw, in other words set the timing straight up. I have never had to set one at either extreme of the setting.

Just a turn or two to loosen, you will see a plate below the screw, as long as that becomes loose that is enough. The plate covers a slot. The screw moves up and down in this slot. For a crab type, "up" will retard the ignition, "down" will advance it.

The screw will be a bit stiff to move in it's slot, you are rotating plate inside the dist. You will need to carefully tap it with a small hammer and a screwdriver to make it move. If you set the screw with the slot fore and aft you can use the slot as a purchase point for the screwdriver and tap the blade sideways. Gentle taps, though, skill is required.

Easier to do than describe.

Mart.
 
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Hi Mart

Thanks for that - I now get it entirely! I might try today except that its minus 15 celsius today and an unheated garage.....not an easy job with gloves on!

Just one more question while I have your attention. The distributor was originally set up off the vehicle by a professional guy in Florida and it ran fine. I later had it fitted locally with electronic ignition and that's when I believe the timing plate may have been rotated unintentionally a little bit. The truck starts with some difficulty but once started is OK. No timing marks to go by. Can you say if its likely that I should retard a bit or advance? With the old manual advance/retard don't you retard to start? I don't want to get way off.

Thanks again
 
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