I've got one of these currently running a newly rebuilt SBC. Timing is 'close enough' by ear for it to start and run, and run well enough to get the cam broken in. It's now time to start getting it dialed in. Before I get too far into setting the timing and checking total advance, I need to address the fact that this is a vacuum brake setup, not the conventional vacuum advance and make sure I understand how it operates. At idle, the brake sits against the advance plate and holds it in place, right? So no advance? Its not until the RPMs increase and develop enough vacuum to pull the brake off the advance plate does the timing actually advance? I know that Bubba is the Mallory guru, but I also know this is how he makes his living and he's already been so generous with his wisdom on here, I don't want to pepper him for free advice. Has anyone seriously fiddled with the vac advance on one of these? The fact that they address right in the instructions that it isn't adjustable or serviced and can easily be disabled makes me wonder if I wouldn't be better off taking their advice right off the bat and going full mechanical advance. I have another distributor without advance that I would assume has the heavier springs I can scrounge from, but another part of me wants to see if I can get the vacuum advance working in the way that Mallory originally intended. Original instructions are attached at the bottom as a pdf. Thoughts?
Manifold vacuum level is based solely on engine load, not RPM. Seems to me any street driven engine benefits from vacuum advance, and there is no downside. It will tend to run hot and lose 3-4 mpg on the highway without it. There's no practical way to achieve 50° of ignition timing loafing down the highway using springs and weights, that's why it was invented in the first place.
Okay, thanks @Truck64 . I was hoping someone would say that its worth to pull the distributor and get the vacuum advance working.
I have one of these distributors in an Olds Rocket which has not been installed in a vehicle yet. I have rebuilt the distributor, including replacing the leather pad on the piston, which was completely worn out. I plan on leaving it operational when I put this engine in service and plan on fine tuning the vacuum brake by trimming or shimming the spring behind the piston if necessary. My local hardware store has a large selection of springs this about size in different stiffnesses. It should be an interesting experience.
Actually vacuum at idle is high so the plunger is held off the plate. During normal driving it is also off the plate. Upon acceleration, vacuum drops and the plunger contacts the plate, slightly retarding and slowing any advance. The purpose was to alleviate any detonation.
That is not a vac advance at all, it’s a vacuum brake plunger that slows advance rate when vacuum is low. It does not advance or retard the spark, just changes the rate at which it is supplied depending on engine load.
According to the original sheet provided you need a source of vacuum on the carb that increases as engine speed goes up. It used to be called "spark ported" and on Holley carbs it was found above the throttle plates. Conversely, manifold vacuum is high at idle and when used at a distributor (usually OEM stuff) it was a load compensation feature that would retard timing when vacuum dropped. That was stated above already, the 2nd part. However in the case of what you wish to do get a vacuum gauge on the different ports on your carb if there's something other than a big port at the plates. Usually the smaller diameter tubing anywhere above plates increases vacuum as RPM goes up. A gauge will confirm it for you. I can't raise a right hand to the exact design you have there, and my old Mallorys have no vacuum timing features, strictly mechanicals. But if you follow their gig as printed I don't see how you can go wrong. Let us know how you make out.
It looks to me reading the supplied instructions the vacuum advance mechanism is just a different way of getting to the same place. It provides 14 to 16 deg. of advance over and above whatever the centrifugal advance provides, based on engine vacuum. Sounds like vacuum advance to me. When everything is dialed in most OHV V8 want close to 50 deg. timing when cruising. Manifold and ported vacuum signals are basically identical cruising down the highway. They don't specifically mention where to connect it but they note there is no vacuum present at a slow idle obviously the "timed" or "ported" connection, or above throttle plates.
Thanks for that pdf as I have a Mallory for my Olds that Bubba rebuilt and I will be installing it when I get my shop back (construction delays)
Here are the instruction from the PDF for the guys that don't know what a PDF is LOL Click them and they get LARGER.
I stand corrected, I didn’t realize there is an added spring and advance feature in these units that the crab distributor does not have. Learn something new!
Thanks @loudbang ! I thought at the very least if this post went nowhere, I would have at least uploaded a useful document to the HAMB and maybe help someone else down the road.
Thanks @theHIGHLANDER , I have to admit I totally missed the reference to ported vacuum, and it makes more sense now. The Holley does indeed have a couple timed vacuum ports, so I'll use one of those.
Thanks for the info @tubman , I hadn't thought about messing with the spring behind the piston, but that's a great idea. I think I pulled it apart out of curiosity when I first got it, and found the piston to be frozen in the bore, so I just put it back together and forgot about it. What did you do for the replacement leather pad?
I couldn't find the proper pads, so I looked around and was able to buy some 12 oz leather (more than 1/4" thick), so I bought a 5/8" punch and made my own. I made some extras, and if you're not in a hurry, I can send you one next spring (my shop is in Minnesota and I'm in Florida for the winter). In the alternative, the piston is the same size as a pre-49 Ford distributor, and I think some of the usual vendors sell the piston/leather assembly. The leather is not as thick as it should be, but it should work fine. (It'll just wear out in 40,000 miles instead of 100,000.)
Thanks! Let me pull it apart this weekend and see what sorta shape its in before I take one from your inventory. It might be fine, going by the shape the rest of this distributor is in, I'm just trying to think ahead.
Here's the patient: Wasn't as bad as I thought. A quick wipe, light oil on the piston, and she went back together. I noticed this. Looks like it's wearing too low on the gear. I've got .011-.012 clearance between the gear and current shim and the distributor housing. Doesn't seem like I've got enough room for another shim to move the gear down.
This old gem got dredged up at just the right time for me. I just started on a flat top Mallory with this vacuum mechanism for my 330 Desoto. Thanks!
I'm converting one for a Pontiac V8 to use on a Chevy 4 cylinder. I started a few years ago and turned the case down on the lathe. Then I realized that the Pontiac turned the wrong way. I spent a lot of time looking for parts and playing with plates that have RH numbers on one side & LH numbers on the other. The other day it occurred to me just to use the innards from one for a small block and just shorten the shaft and put on the right gear. My problem now is during the time it's been setting I have misplaced the 4 cylinder cap and a few big condensers. I can use a V8 cap until I find it.
Bringing this back to the top to see how all you fellas made out with your Mallorys. Of particular interest is the vacuum advance as I am running one these on my Olds with 4 97's and no ported vacuum of course. I do have it hooked up to manifold vacuum so when the engine starts it advances quickly from initial 10 to 25 degrees. That is with 11 inches of vacuum at idle. At modest rpm when the weights kick in it appears to be right around 50-52. Yesterday on a quick run around town it appeared to be pinging a little on acceleration with pump gas (87 junk octane) All this Mallory stuff is new territory for me so any advice would be appreciated. Should I disconnect the vacuum and see how the engine performs first before making any other changes? @tubman did you have any luck dialing in the spring on the vacuum brake?
That engine was to be a backup for the one in my "Tub" and is still on the stand. I haven't run it yet. The whole process has been complicated with the acquisition of a couple of T89 OD transmissions, so I doubt I'll get back to it soon, if ever (I'm 81). I was mainly concerned replacing the deteriorated leather pads with a proper substitute.
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