Mallory Unilite - Where'd You Hook Your Vacuum Advance To?

Discussion in 'General IH Tech' started by Thomas, Aug 7, 2008.

  1. Thomas

    Thomas Dreams of Cub Cadets

    ---What's the verdict? If you installed a Mallory Unilite, where did you route your Vacuum Unit to? Manifold or Ported?

    ---For a little over a year now I have been running my Unilite off the ported vacuum per Mallory's instruction. I had been fighting with fuel economy since the mishap with the dirt-dauber in my fuel line, the API-M syntheic oil and a pretty tired engine. I was smelling fuel when I'd let off the accelerator to shift gears, I have the .028" Nozzle, #44 jets 7.5 Power Valve and 30cc Accelerator Pump on a Holley 2300 List# 1710. About as lean as one can get without burning a hole in the top of a piston, and I could still smell rich exhaust when idling!

    ---I had been up and down with adjusting the Vacuum Advance, 1/8 turn at a time and never getting rid of the popping/sputtering during deceleration.

    ---Well, the other day I reached the point where I was fed up with StoneThrower's thirst. I had this "oh so" brilliant idea to go back to the old ways... and run the VA off manifold... we all know how for some reason most IH SVs run better off manifold than ported. Low and behold, I can see big hints of the word economy in fuel economy once again. No long trips at highway speeds though, I still have a rear main seal to replace before I can get on the interstate, but I have made five, ten mile round trips and my gauge has moved the same amount as it did when I would try and make one of those trips, prior to the change.

    ---Just wondering if you have tried it and if not, with gas prices the way they are, would you consider trying it? I figured it wouldn't hurt, seeing as how my Scout ran off manifold from the first tune up I performed, the dizzies that I installed were in excellent mechanical condition and the Vacuum Advances were fairly new.
  2. Erik VanRenselaar

    Erik VanRenselaar Y-Block King

    I've been using a 47 series (dual advance) Unilite distributor for about 10 years, and have always used ported vacuum. This is with both the carbs (refurbished Holley 2210C and new 350 cfm Holley 2300 #7448) that I use for the Scout. IIRC, I used a small hex key wrench to turn the vacuum advance adjustment (through the vacuum nipple) and slightly reduce the max amount of vacuum advance.

    I dunno about your mileage gain. Try a few hundred miles and get back to us. I got about 14+ mpg on the last trip of a couple hundred miles. That was running it at about 60 mph on mostly interstate and two-lane highways.
  3. Thomas

    Thomas Dreams of Cub Cadets


    ---You are correct that a (5/32" IIRC) allen wrench is used to adjust the Vacuum Advance. Mechanical Advance was checked and found to be the specified 24° @ 3000RPM and timing was the usual 8° BTDC & around 12° BTDC w/Vacuum Advance hooked up. This is a 550 RPM @ idle carb FWIW.

    ---After the switch from ported to manifold the engine also gained 4" of vacuum @ idle. It's now @ 24", w/ a more steady idle. Nothing is by ear mind you, I haven't timed/tuned my carb without a vacuum gauge since about 12 years ago.

    ---Where did you have your OEM dizzies hooked up, ported? Have you ever tried manifold... like most claim their SV8s ran better off of? FWIW, I found it funny how many people in the past have asked the question regarding manifold Vs. ported. Almost as many who've asked about the "chirping" sound in their exhaust... LOL.

    ---Thanks for the response. Maybe someone else will chime in too. Out of 20,000+ members (allowing for a hundred or so BOTs that slipped by, the few who feel the need to have more than one account, couldn't remember their passwords or whatever and started a new account and those to whom the Unilite are not available), we can't be the only ones running an Unilite.
  4. Erik VanRenselaar

    Erik VanRenselaar Y-Block King

    I was just tweaking with the Scout today (fri.), getting it ready for a smog inspection. I checked the EGR functions and the idle vacuum. The 304 currently has about 16.5-17" of vacuum.

    The main thing I like about the Unilite distributor, is that the mechanical advance has a fast/steep curve; the advance comes on right off-idle, and maxes out quickly. This seems to work quite well for a low-rpm engine, such as the IH SV.
    Sadly, when the full TBI-EFI system gets installed, I plan on using a fuel & spark control system, with my old Holley gold box dist. converted with DuraSpark guts. That means placing the Unilite in storage...
  5. Thomas

    Thomas Dreams of Cub Cadets

    ---Yep, the vacuum on my 304 was about 18 (bounced a tad) prior to hooking it to manifold.
  6. Darrell Tuxworth

    Darrell Tuxworth Binder Driver

    If your exhaust smells rich at idle try leaning out the idle screws, this would also help with the popping in the exhaust during decel at closed throttle.
    The reason your engine runs smoother using manifold vacuum at idle is that you increased your timing the full vac advance amount, 8 deg on my old Prestolite, I bet your idle speed jumped up a couple hundred too.
    I ran 15 deg base and 8 deg vac advances at idle with the Prestolite dizzy for a total of 23 deg at idle for years. It would idle even better at 30+ deg total advance but that far advanced it was hard to start.
  7. Thomas

    Thomas Dreams of Cub Cadets

    ---My 304 ran better off manifold vacuum when I first performed a tune up and ran just as well after my first rebuild. No problem with the carb at that time and since then, I have heard countless others say their VA is off manifold also, because it ran better that way. As for the carb, yes characteristics changed, and they were changed back to specs. Idle Mixture Screws are set about 1-3/4 (rather than 3-1/2) turns out, give or take one or two twelfths, and back to 550RPM with not even a full turn out on the Idle Speed Screw, gaining 5" of vacuum from where it was before. Again, through the history on the BB, many have said that their SV8 ran better when Vacuum Advance was off manifold and when the "Q" arrives, it is often said that it "should" be off ported, but most run better off manifold. FWIW, before I even joined the BB, back in 1996 or thereabouts, I found this to be so when tuning my SV8.

    ---I explained this to the Techs at Mallory and inquired as to where they suggested it be plumbed to and they said Ported. I couldn't understand why the Unilite would be any different than my two excellent condition OEM distributors, but I installed it per their instruction. Well after a year of running it this way, I got :censored: off, tore it off the carb and plumbed it manifold, which after about two days of driving it this way, brought me to posting this thread. I then wondered how many others, who had installed the Unilite, had done the same. and while typing out the thread, I decided to also ask if they, who hadn't run it off manifold, would consider doing so as a test to try and up their fuel economy...

    . . . ... because it is still showing improvements over ported on mine... as it always had in the past, regardless of whether it was the Prestolite or Holley electronic, proved to run better.

    ---FWIW, it didn't matter where I set the Idle Mixture Screws prior to re-plumbing the VA. When I achieved maximum vacuum (18"), the exhaust was rich. Turning it to lower the vacuum would not only be senseless, but would cause an even more erratic idle. Time and time again I was out under the hood attempting to fine-tune and time and time again I ended up where I started off... till I got to the point where I said :whistling: it, ripped the hose off, plugged the ported fitting and plumbed VA to manifold. Viola, problem solved... Coppice?

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