Ported vacuum question

Discussion in 'The Mixing shop.' started by Bogus919, Dec 12, 2020.

  1. Bogus919

    Bogus919 Silver Level contributor

    Ok so a couple weeks back the engine started to overheat regularly and after doing some research here I decided to check out my timing. I discovered after reading through Larry’s PT thread that my vacuum advance was shot so I replaced it and all is well again it seems. Through my reading I saw that there are members on this page that can do burnouts with 350’s...with a 2 barrel Rochester and so it got me wondering if maybe my timing is just off. I can’t so much as get a chirp out of my tires...and have just chalked it up to being a tired old engine. Also I’ve always tuned the carb by watching my vacuum and just getting it as high as I could at a reasonable rpm.

    So to get to my actual question, and what my current setup and specs are.

    -I have a PO block, I think that’s ‘69...350-2
    -3.08 posi
    -235R 15 on all corners
    -automatic transmission
    -crane kit installed with one light and one medium spring
    -canister is also limited
    -aftermarket piece of 2x6 holding up the battery...
    -assumed everything on the engine is from ‘69

    Currently dwell is at 29.5, timing with canister unplugged and line blocked is 8 degrees .. idles at 700 rpm... I’m all in at 2,000 rpm for a total of 32 degrees and total with vacuum advance falls at 37.

    To me, none of this sounds out of the ordinary... except, my vacuum is 21 at idle and actually it only drops a little, maybe to 15 when I move to WOT... and gets to around 25 at WOT. It never deviates beyond that 25 to 15.

    When I found this out, and know that the canister should be on ported vacuum and not manifold, as per the PT thread... I traced all the the lines. I have a vacuum riser or “tree” on the drivers side of the front intake that has three ports on it labeled MT, C, and D. Like you could guess, the C line goes to the front passenger side of the carb and the D goes to the distributor. The MT goes behind the carb and links up to a T fitting that goes to the choke and the other end goes into a fitting at the base of the carb that is shared with a much larger hose from the rear of the intake manifold ... I assume this is manifold vacuum.

    So, I feel that my distributor is connected to both ported and manifold... does that sound right? I’ve included pictures of the riser and the rear carb connection.

    Should I be running it like this?

    upload_2020-12-12_11-6-16.jpeg
    upload_2020-12-12_11-6-51.jpeg
     
    69GS430/TKX likes this.
  2. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    First, all vacuum goes to zero at WOT, regardless if it is ported or manifold vacuum. Second, you should not be using that tree on the intake. It is part of the TCS emissions system. You don't need any of that, all you need is one vacuum hose directly from the carburetor or intake fitting to the distributor. You can use ported, or manifold vacuum for the vacuum advance. I always encourage that you try both and see what you like seat of the pants. At part throttle, they will be exactly the same. Manifold vacuum will give you added advance at idle. That will help the engine to run cooler in stop and go traffic.

    Tell me what the part number of the points distributor you are using is. It should be 1111938. You could easily have a distributor from another year Buick.
     
    Last edited: Dec 12, 2020
    69GS430/TKX likes this.
  3. rkammer

    rkammer Gold Level Contributor

    To add to Larry's suggestions, I see that you have the vacuum canister limited to only 5 degrees. Why so low? While cruising you can run as much as 45 degrees of vacuum so, you can set your canister to limit at about 13 degrees. If you decide to increase you total timing to 34 or 35 degrees like many do, you can the limit the vacuum advance to just 10 degrees. Should you want to be able to adjust the vacuum advance externally instead of inside the distributor, Accel makes one that has an allen screw adjustment inside the vacuum port that works well.
     
    69GS430/TKX, Waterboy and Bogus919 like this.
  4. Bogus919

    Bogus919 Silver Level contributor

    Ok, that’s some good info. I was unsure about that tree because I’ve not seen it on any other engine on this forum but it was there and in use since I’ve had the car so I just felt it was needed I guess. I misspoke about the vac at WOT, I’m under the hood testing it so I never pull the carb arm all the way back, more like I slowly move it to 2k rpm.

    The canister I put on has a plate and screw to limit the throw... I set it to stop at half the throw and tested to what I thought was ten degrees advance. Looks like I was off, I can easily adjust it in car though.

    The distributor is 1111474, I didn’t think about this earlier but I have no real idea what all was used from the original ‘72 engine that was ran without oil.

    So it sounds like I can remove all the lines from the tree and plug what’s not needed and I’ll fiddle around between ported and manifold vac and see what the engine likes, I can also allow more travel on the canister and see where that gets me.

    Thanks for the input guys
     
  5. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    1111474, can't find that number anywhere. Not sure what the specs are. If your measurements are accurate at 8* initial and 32* total, there are 24* of mechanical advance in it.
     
    Bogus919 likes this.
  6. Bogus919

    Bogus919 Silver Level contributor

    Ok, I found some time to get back under the hood. I took all the vac lines off and I just have one line from manifold vacuum to the canister and I blocked off ported vac right at the carb. I’m going to test this setup first. My distributor is real sticky so it’s not easy to adjust timing but I bumped it up to 10* initial and 34* total. I also changed the canister throw to allow for 10* so my total with vac advance is now 44. I even found a vac leak in one of the old lines so it idles better now it seems.

    I’ll test it out tomorrow, thanks for the help.
     
  7. Waterboy

    Waterboy Mullet Mafia since 6/20

    This goes back to your original statement... You see other 2 barrel 350s that can do burnouts. After you get ALL your ignition problems sorted out, if it still doesn't do any burnout...
    My mom had a brand new 72 Skylark 350. She drove it like a mom. I got to drive it when I turned 16. It would NEVER do a burnout. You'd floor it from a start and it would kinda bog until 20 mph, then take off. Had the transmission serviced for some reason. Transmission place said the torque converter was not right. Had a new, stock, torque converter put in it. What a different car!!! Burnouts baby! Seems it had a sluggish torque converter from the start. Just a thought.
     
    69GS430/TKX likes this.
  8. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    John,
    The 1972 Skylark 350-2 was ham stringed from the start with it's ignition timing. The 1112109 distributor that was fitted to that engine had 12-16* of mechanical advance built into it. 4* BTDC was the specified initial timing. That means full advance could not be more than 16-20*, not the 30-34* that we know Buick engines like best. They did the same thing to the 455 that year. 1112110 distributor had 14-18* of mechanical advance with the same 4* initial timing. That means a full advance of 18-22*

    1972IgnitionSpecs.jpg
    Could not imagine why Buick did that unless they wanted the Stage 1 455 to shine that much more. Look at the Stage 1 distributor, 1112016 had 20-24* and a 10* initial timing. 30-34* of full advance for best power.
     
    69GS430/TKX likes this.
  9. Bogus919

    Bogus919 Silver Level contributor

    WOW! ... I just tested out the manifold vac setup... I can’t believe I’ve been driving my car around for 8 years like it was setup and have just been ok with it. I got a little chirp out of the tires on a turn with gas... I’m going to try ported now. The car ran at 190* in traffic which is normal for my antiquated setup so for now I’m ok with that part... if running ported has it heat up more then I’ll just go back to manifold. So far I am super impressed with the throttle response!


    John, I had the transmission rebuild about 2 months ago so it would be fresh for my 455 build. At my request they put a slightly larger converter in it, but nothing fancy... pretty close to stock. The car was a slug before and after.
     
    Last edited: Dec 13, 2020
  10. Waterboy

    Waterboy Mullet Mafia since 6/20

  11. Bogus919

    Bogus919 Silver Level contributor

    And ported is even better... I can squeal both tires now from a dead stop and the throttle response feels slightly better than manifold vac. It does run hotter, about ten degrees more.

    So where can I go from here? How do you know if your setup is the best? I figured I would add a little more to the vac advance and see where it gets me. How do you know when adding initial timing isn’t doing you any better? Just advance it and see how it feels and if it gets worse, move it back?
     
  12. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    Why?:)
     
  13. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    You'll hear the detonation at WOT when you go too far (hopefully). I've read several times that guys don't hear it. I know what it sounds like, and I always listen for it. You can also see it on the plugs if it is severe. That's why I always advise that you tune the mechanical and vacuum advance separately. Leave the VA unhooked and plugged. Then make acceleration runs from dead stop and 30 MPH. Advance it until it talks to you, then back it down. Advancing the initial adds to your total advance. I would not go beyond 34* on pump gas. You can play with springs to bring it in sooner or later. At the track, your trap speed will fall off when you go too far.

    Once you have that set, then add the VA. As mentioned before, mid 40's cruise timing is usually best. If you go too far with too much VA, one of two things will happen. It will either ping when get on it suddenly from part throttle, OR, the engine will surge at cruise.

    If you really want to do burnouts, you have 2 options. One, just get a line lock. Get the tires smoking and let go of the button:D. Or, get a 9.5" converter from Jim Weise. That converter is probably a good 10lbs lighter than a stock one. It will get the engine into the sweet spot much sooner when you get on it, but it won't feel loose during normal driving. They do run up around 600.00 though. You get what you pay for when it comes to converters.
     
    69GS430/TKX and Bogus919 like this.
  14. Bogus919

    Bogus919 Silver Level contributor

    Thanks again for the help everybody. I may play with it a little more but I’m real pleased with today’s results so far.
     
  15. CJay

    CJay Supercar owner Staff Member

    You might want to make sure the carb is opening up fully when you mat the pedal. I cant tell you how many cars Ive performance tuned where the carb was only opening 3/4 when the car was floored. With the engine OFF, have someone put their foot to the floor and hold it there. Look down the secondaries and make sure the secondary throttle plate is verical. If its not, there is a tang on the linkage you can bend to make the secondaries open fully.

    You will also want to adjust the secondary air flaps so they open faster. Theres a spring that you can adjust. Not too loose, not too tight. Theres a happy medium there. You want them loose enough but not loose enough for the car to bog
     
    69GS430/TKX and Bogus919 like this.
  16. CJay

    CJay Supercar owner Staff Member

    I bet you anything the primary choke pull off is dead too
     
  17. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    It's a 2bbl Jason, but yeah, I've seen that as well.
     
    Bogus919 likes this.
  18. CJay

    CJay Supercar owner Staff Member

    Glossed over that on my phone. Damn, a stock cast iron 4 barrel manifold and a Q-jet from everyday performance would be the cheapest bang for the buck there is.
     
    69GS430/TKX and rkammer like this.
  19. Bogus919

    Bogus919 Silver Level contributor

    Thanks Jason, I will check that out tomorrow, hopefully I can recruit the wife..... I might end up with a kid in there mashing on it lol the primary choke pull off has a small leak when I tested it...I don’t have it hooked up as of right now.
     
    Last edited: Dec 14, 2020

Share This Page