Vacuum Question

Yes, the distributor vacuum cannister should hold a vacuum. Using the vacuum pump, and distributor cap off, you can start pumping and watch to see the vac level at which the mechanism starts moving. Keep increasing the vac and note at what vac reading it stops.
You can do the same thing with the engine running and a timing light. Increase the vac and note when the timing begins to advance. Continue in small increments until the timing does not advance any more.
It is expecially helpful if you have a dial back timing light or you have a series of timing scale marked on your balancer.
Sorry I can't comment on the 1940 carb.
Doug
 
OK, here goes:

1) The LOM does not send a vac signal to the dizzy at idle. So your SCV is working right at idle.

2) The LOM dizzy reaches about 10 degrees advance around 4 in. Hg according ot my '65 Shop Manual. If you do not have a SM, go buy one, they are worth every penny. A LOM dizzy does not need lots of vacuum to operate. So 5 inches of Hg to the dizzy would not personally concern me.

3) According to the discription in the '65 Shop Manual, the SCV "picks" or "selects" the highest vacuum between the manifold or venturi (when the carb is flowing), "conditions" it and directs it to the LOM dizzy. I do not know how much vacuum is inside the venturi when the carb is flowing, but I have to believe its plenty high enough. Again, I do not think the SCV is bad in your case.

4) Put a timing light on the balancer and either rev the engine high or use the vacuum pump. Find out how much total advance the LOM has.

5) A bog can be caused by low initial timing or a small and/or delayed accelerator pump shot. My bet is that this is where the problem is. If you have verified the initial timing, then start to play with the accel pump linkage.

Keep us updated!
 
Mugsy, Thanks for the reply. I sure hope you are correct about the fact that there is "NOT" a vacuum signal being sent from the carb to the dizzy at idle. I thought maybe my SCV (spark control valve) might be bad, but possibly it is OK after all. I did find out that my dizzy diaphram (spark advance bisquet) was bad and picked up a new Load-A-Matic dizzy this afternoon. It was almost as cheap to buy a new dizzy as it was to buy just the diaphram. Kragen's had the dizzy on sale on their website for pretty cheap. I haven't installed the new dizzy yet, so I will find out if everything is alright with my carb (and the SCV) once the dizzy is installed. I sure do appreciate all the information that I have received from all of you out there. I feel a little more informed now on how the load-a-matic dizzy and my carb (a Holley 1940 with a SCV) work together. In reading these current posts and going back and reading previous posts on the subject; I feel I now know which carbs, and which dizzys will work well together, and which ones won't work well together. It has been very insightful. Great info on this website!! Thanks to Ya'll Jim
 
Jim,

You're welcome.

The symptom you describe can definetely be caused by the leak/no vac advance. I wasn't too sure if the diaphgram was leaky by your test description. Please note, I bought a LOM dizzy from a very good source of Mustang part, National Parts Depot, and through much trials and tribulation found out the their rebuilder (Cardone Industries) had installed the two springs wrong. They had them swapped. Also, the end play was out of spec too. So the moral of the story is to check the rebuilders work, they can get it wrong too!

If you still have any bog or heisitation look into the pump shot and the intial timing.
 
Mugsy, I know what you mean about checking the rebuilders' work. The new/rebuilt (Cardone Industries also) Load-A-Matic dizzy looked identical to the one I was going to replace, except for one small thing; the new gears were just a bit fatter! I kept trying to slide the new dizzy into the hole and mesh with the cam gear, but the gears just would not mesh. Yet every time I went to slide my old dizzy into the hole, it meshed right up. I took back the new dizzy and re-ordered another; we will see if the next one has the proper gearing. If not, I will take the new vacuum advance diaphram off the new dizzy and install it on my old dizzy. That is what needs replacing anyway (the diaphram). Thanks, Jim
 
Mugsy, or anyone else, I forgot to ask you a question concerning your comment about "Also, the end play was out of spec too"! I am not familiar with the end play on a dizzy. Could you explain this end play and how it is checked; so that when I get the new/remanufactured dizzy today I can check the spec?? Do you have the spec? Thanks, Jim
 
The dizzy is basically a shaft inside a housing. The shaft can move up/down inside of the housing. When you get the new dizzy, grab the housing in one hand, the gear/shaft in the other an wiggle it upand down. That's the end play. There is a spec on it, I *think* it might be in the Shop Manual. One way to check would be to use feeler gauges. Again I *think* that you can measure the gap (basically) from the end of the housing to the top of the gear; its been a long time since I have looked at a LOM dizzy though. The other way would be to place the dizzy in a vice and use a dial indicator. Zero out the dial gauge on the end of the shaft when the gear is closest to the housing and then move the shaft/gear away from the housing.
 
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