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Technical Need help!

Discussion in 'The Hokey Ass Message Board' started by harris73085, Apr 13, 2020.

  1. harris73085
    Joined: Feb 4, 2009
    Posts: 157

    harris73085
    Member

    I am at wits end. I have a new blueprint 355 and I'm having a really tough time getting it tuned. It has an Edelbrock 1906 650cfm carb on top of an rpm air gap manifold. It is pulling roughly 17 inhg at idle. Initial timing is at 16 degrees. It feels fine under light to middle throttle accelerations. When I quickly go to wot it basically goes silent with no acceleration then I get the pops out of the exhaust. In tried going richer per Edelbrock, and leaner. I've looked for leaks, verified timing about ten times. I have absolutely no clue what is going on with this anymore and I'm getting desperate. Any help would be greatly appreciated. Further info... Advance is hooked to manifold vacuum, and the plugs are a little black, hence why I tried leaner once.
     
    Last edited: Apr 13, 2020
  2. Last edited: Apr 13, 2020
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  3. Jalopy Joker
    Joined: Sep 3, 2006
    Posts: 24,906

    Jalopy Joker
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    is this a crate ZZ4 motor from Chev, or was it a special build?
     
  4. harris73085
    Joined: Feb 4, 2009
    Posts: 157

    harris73085
    Member

    I've tried the biggest squirt it will give me. Have not changed out nozzles yet.

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  5. harris73085
    Joined: Feb 4, 2009
    Posts: 157

    harris73085
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  6. Chappy444
    Joined: Jan 27, 2012
    Posts: 1,085

    Chappy444
    ALLIANCE MEMBER
    1. Maryland HAMBers

    What are you running for ignition?
    90% of carb issues are ignition
    Chappy
     
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  7. harris73085
    Joined: Feb 4, 2009
    Posts: 157

    harris73085
    Member

    Msd billet distributor with 6al box. Initial is at 16, full is 34 per the engine manufacturer specifications
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  8. harris73085
    Joined: Feb 4, 2009
    Posts: 157

    harris73085
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    Pretty good article. I've tried the richest seeing in accelerator pump, but haven't bent the rod. Will review it further and get both the bigger pump and nozzles to give them a try.

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  9. jetnow1
    Joined: Jan 30, 2008
    Posts: 1,529

    jetnow1
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    from CT
    1. A-D Truckers

    I wonder on the use on an air gap manifold on the street (esp in Michigan or any where else that is not warm all the time. Not saying this is causing your issue but it might contribute.
     
  10. Can you swap to another carb to try to determine if the carb is a problem? Maybe it is an ignition issue.
     
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  11. harris73085
    Joined: Feb 4, 2009
    Posts: 157

    harris73085
    Member

    Id hate to think about changing it...

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  12. harris73085
    Joined: Feb 4, 2009
    Posts: 157

    harris73085
    Member

    I can see if I can get ahold of one to try. What kind of ignition issue would be leading towards this? Weak coil, to much or little vacuum advance?

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  13. Lloyd's paint & glass
    Joined: Nov 16, 2019
    Posts: 2,229

    Lloyd's paint & glass
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    I'm a painter and a fireman, but isn't vacuum advance supposed to be connected to venturi vacuum instead of manifold vacuum? Manifold is constant vacuum and venturi is vacuum as the throttle blades open? The highest port on the carburetor? Teach me here.
     
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  14. Timing of 16 is with or with vacuum advance hooked up?
     
  15. harris73085
    Joined: Feb 4, 2009
    Posts: 157

    harris73085
    Member

    My understanding, is that Venturi vacuum as you called it, has zero at idle, and some when the throttle is opened, but Venturi, like manifold has zero at wot.

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  16. harris73085
    Joined: Feb 4, 2009
    Posts: 157

    harris73085
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    Without vacuum advance hooked up.

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  17. Lloyd's paint & glass
    Joined: Nov 16, 2019
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    Lloyd's paint & glass
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    What im saying is that constant vacuum at idle means your timing is fully advanced before you even crack the throttle?
     
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  18. harris73085
    Joined: Feb 4, 2009
    Posts: 157

    harris73085
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    Correct.

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  19. Lloyd's paint & glass
    Joined: Nov 16, 2019
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    Lloyd's paint & glass
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    Unhook the vacuum and set the timing at 35 degrees at at around 2500rpm. Won't that work?
     
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  20. Lloyd's paint & glass
    Joined: Nov 16, 2019
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    Lloyd's paint & glass
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    I've had issues like you're saying in the past, but mine was usually from using junk parts lol. Frozen weights, worn out stuff, etc.
     
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  21. Lloyd's paint & glass
    Joined: Nov 16, 2019
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    Lloyd's paint & glass
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    Does it seem to help if you close the choke plate about halfway as you rev the motor?
     
  22. harris73085
    Joined: Feb 4, 2009
    Posts: 157

    harris73085
    Member

    I've checked the weights and the vacuum canister. Both seem to be working well. I have not tried closing the come when revving it.

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  23. I have no idea if this is good information or not:

    This is a great article!!!!I had my vacuum advance hooked up to the ported vacuum port after reading this article I hooked it up to full manifold vacuum adjusted the idle rpm.The car has so much more power now.
    • As many of you are aware, timing and vacuum advance is one of my favorite subjects, as I was involved in the development of some of those systems in my GM days and I understand it. Many people don't, as there has been very little written about it anywhere that makes sense, and as a result, a lot of folks are under the misunderstanding that vacuum advance somehow compromises performance. Nothing could be further from the truth. I finally sat down the other day and wrote up a primer on the subject, with the objective of helping more folks to understand vacuum advance and how it works together with initial timing and centrifugal advance to optimize all-around operation and performance. I have this as a Word document if anyone wants it sent to them - I've cut-and-pasted it here; it's long, but hopefully it's also informative.
      TIMING AND VACUUM ADVANCE 101
      The most important concept to understand is that lean mixtures, such as at idle and steady highway cruise, take longer to burn than rich mixtures; idle in particular, as idle mixture is affected by exhaust gas dilution. This requires that lean mixtures have "the fire lit" earlier in the compression cycle (spark timing advanced), allowing more burn time so that peak cylinder pressure is reached just after TDC for peak efficiency and reduced exhaust gas temperature (wasted combustion energy). Rich mixtures, on the other hand, burn faster than lean mixtures, so they need to have "the fire lit" later in the compression cycle (spark timing retarded slightly) so maximum cylinder pressure is still achieved at the same point after TDC as with the lean mixture, for maximum efficiency.
      The centrifugal advance system in a distributor advances spark timing purely as a function of engine rpm (irrespective of engine load or operating conditions), with the amount of advance and the rate at which it comes in determined by the weights and springs on top of the autocam mechanism. The amount of advance added by the distributor, combined with initial static timing, is "total timing" (i.e., the 34-36 degrees at high rpm that most SBC's like). Vacuum advance has absolutely nothing to do with total timing or performance, as when the throttle is opened, manifold vacuum drops essentially to zero, and the vacuum advance drops out entirely; it has no part in the "total timing" equation.
      At idle, the engine needs additional spark advance in order to fire that lean, diluted mixture earlier in order to develop maximum cylinder pressure at the proper point, so the vacuum advance can (connected to manifold vacuum, not "ported" vacuum - more on that aberration later) is activated by the high manifold vacuum, and adds about 15 degrees of spark advance, on top of the initial static timing setting (i.e., if your static timing is at 10 degrees, at idle it's actually around 25 degrees with the vacuum advance connected). The same thing occurs at steady-state highway cruise; the mixture is lean, takes longer to burn, the load on the engine is low, the manifold vacuum is high, so the vacuum advance is again deployed, and if you had a timing light set up so you could see the balancer as you were going down the highway, you'd see about 50 degrees advance (10 degrees initial, 20-25 degrees from the centrifugal advance, and 15 degrees from the vacuum advance) at steady-state cruise (it only takes about 40 horsepower to cruise at 50mph).
      When you accelerate, the mixture is instantly enriched (by the accelerator pump, power valve, etc.), burns faster, doesn't need the additional spark advance, and when the throttle plates open, manifold vacuum drops, and the vacuum advance can returns to zero, retarding the spark timing back to what is provided by the initial static timing plus the centrifugal advance provided by the distributor at that engine rpm; the vacuum advance doesn't come back into play until you back off the gas and manifold vacuum increases again as you return to steady-state cruise, when the mixture again becomes lean.
      The key difference is that centrifugal advance (in the distributor autocam via weights and springs) is purely rpm-sensitive; nothing changes it except changes in rpm. Vacuum advance, on the other hand, responds to engine load and rapidly-changing operating conditions, providing the correct degree of spark advance at any point in time based on engine load, to deal with both lean and rich mixture conditions. By today's terms, this was a relatively crude mechanical system, but it did a good job of optimizing engine efficiency, throttle response, fuel economy, and idle cooling, with absolutely ZERO effect on wide-open throttle performance, as vacuum advance is inoperative under wide-open throttle conditions. In modern cars with computerized engine controllers, all those sensors and the controller change both mixture and spark timing 50 to 100 times per second, and we don't even HAVE a distributor any more - it's all electronic.
      Now, to the widely-misunderstood manifold-vs.-ported vacuum aberration. After 30-40 years of controlling vacuum advance with full manifold vacuum, along came emissions requirements, years before catalytic converter technology had been developed, and all manner of crude band-aid systems were developed to try and reduce hydrocarbons and oxides of nitrogen in the exhaust stream. One of these band-aids was "ported spark", which moved the vacuum pickup orifice in the carburetor venturi from below the throttle plate (where it was exposed to full manifold vacuum at idle) to above the throttle plate, where it saw no manifold vacuum at all at idle. This meant the vacuum advance was inoperative at idle (retarding spark timing from its optimum value), and these applications also had VERY low initial static timing (usually 4 degrees or less, and some actually were set at 2 degrees AFTER TDC). This was done in order to increase exhaust gas temperature (due to "lighting the fire late") to improve the effectiveness of the "afterburning" of hydrocarbons by the air injected into the exhaust manifolds by the A.I.R. system; as a result, these engines ran like crap, and an enormous amount of wasted heat energy was transferred through the exhaust port walls into the coolant, causing them to run hot at idle - cylinder pressure fell off, engine temperatures went up, combustion efficiency went down the drain, and fuel economy went down with it.
      If you look at the centrifugal advance calibrations for these "ported spark, late-timed" engines, you'll see that instead of having 20 degrees of advance, they had up to 34 degrees of advance in the distributor, in order to get back to the 34-36 degrees "total timing" at high rpm wide-open throttle to get some of the performance back. The vacuum advance still worked at steady-state highway cruise (lean mixture = low emissions), but it was inoperative at idle, which caused all manner of problems - "ported vacuum" was strictly an early, pre-converter crude emissions strategy, and nothing more.
      What about the Harry high-school non-vacuum advance polished billet "whizbang" distributors you see in the Summit and Jeg's catalogs? They're JUNK on a street-driven car, but some people keep buying them because they're "race car" parts, so they must be "good for my car" - they're NOT. "Race cars" run at wide-open throttle, rich mixture, full load, and high rpm all the time, so they don't need a system (vacuum advance) to deal with the full range of driving conditions encountered in street operation. Anyone driving a street-driven car without manifold-connected vacuum advance is sacrificing idle cooling, throttle response, engine efficiency, and fuel economy, probably because they don't understand what vacuum advance is, how it works, and what it's for - there are lots of long-time experienced "mechanics" who don't understand the principles and operation of vacuum advance either, so they're not alone.
      Vacuum advance calibrations are different between stock engines and modified engines, especially if you have a lot of cam and have relatively low manifold vacuum at idle. Most stock vacuum advance cans aren’t fully-deployed until they see about 15” Hg. Manifold vacuum, so those cans don’t work very well on a modified engine; with less than 15” Hg. at a rough idle, the stock can will “dither” in and out in response to the rapidly-changing manifold vacuum, constantly varying the amount of vacuum advance, which creates an unstable idle. Modified engines with more cam that generate less than 15” Hg. of vacuum at idle need a vacuum advance can that’s fully-deployed at least 1”, preferably 2” of vacuum less than idle vacuum level so idle advance is solid and stable; the Echlin #VC-1810 advance can (about $10 at NAPA) provides the same amount of advance as the stock can (15 degrees), but is fully-deployed at only 8” of vacuum, so there is no variation in idle timing even with a stout cam.
      For peak engine performance, driveability, idle cooling and efficiency in a street-driven car, you need vacuum advance, connected to full manifold vacuum. Absolutely. Positively. Don't ask Summit or Jeg's about it – they don’t understand it, they're on commission, and they want to sell "race car" parts.
    https://www.hotrodders.com/forum/va...-manifold-bad-47495.html#/topics/47495?page=1

    Other link in above thread:

    http://www.nationaltbucketalliance.com/tech_info/engine/Dvacuum/vacuum.asp
     
  24. Doublepumper
    Joined: Jun 26, 2016
    Posts: 607

    Doublepumper
    Member

    Just a thought....might verify proper ignition voltage. Low voltage there can cause a similar symptom you're describing. Good luck!
     
  25. saltflats
    Joined: Aug 14, 2007
    Posts: 10,070

    saltflats
    Member
    from Missouri

    Is there a module in the distributor?
     
  26. Have you tried disconnecting vacuum advance?
     
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  27. harris73085
    Joined: Feb 4, 2009
    Posts: 157

    harris73085
    Member

    Not sure what you mean by module. There is a small magnetic pick up in there.

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  28. Lloyd's paint & glass
    Joined: Nov 16, 2019
    Posts: 2,229

    Lloyd's paint & glass
    ALLIANCE MEMBER

    And the springs in the distributor are tunable also aren't they? Determining how easy the weights will move?
     
  29. harris73085
    Joined: Feb 4, 2009
    Posts: 157

    harris73085
    Member

    I just measured .9 volts across the top of the coil while idling. Battery is at 14.2 while idling. I did install a diode when I installed the high torque mini starter... Perhaps that was the wing thing to do?

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  30. harris73085
    Joined: Feb 4, 2009
    Posts: 157

    harris73085
    Member

    I've got the heavy set on the that finish advance by 3500, again per blueprint engines installation guidelines

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