buick 350 jet suggestions

Discussion in 'Small Block Tech' started by IamLumpy, Jan 3, 2022.

  1. IamLumpy

    IamLumpy Member

    My engine is getting tired but still runs decent enough for now. I don't have complete build specs but it does have bigger cam, poston intake, full length headers, dual exhaust with h pipe, and 750 dp demon carb. I used an eBay HEI distributor set at ten degrees btdc. I have played with timing and jets and still have pinging at part throttle at highway cruise. I have adjusted the idle circuit to the highest vacuum I can get which if I remember correctly was about 14. I also have vacuum advance routed to full manifold vacuum.

    I'm thinking the primary jets are still too small. I think I have 68 in there now and before I order more I wanted to get opinions to see if I'm on the right path. Planning on doing power tour this year and want to get the cruising sorted out.

    Any suggestions will be appreciated.

    Dano likes this.
  2. Mark Demko

    Mark Demko Well-Known Member

    What’s your total timing, all in at what RPM?
  3. Reidk

    Reidk Well-Known Member

    does the carb have tunable air bleeds? try 72 main jets...they are stock in a 750. there could be other stuff going on. float level could be too low. secondaries could be opened too far making it lean...but these things would cause other issues too like stumbles and hesitations.
  4. Gary Bohannon

    Gary Bohannon Well-Known Member

    Before tuning the air and fuel ratio, the distributor must be blueprinted.
    You must know the range of timing in your distributor, especially if you have noticed detonation.
    Block off the vacuum.
    Stick a timing tape on the damper.
    Install the weakest springs on the weights.
    Use a good timing light with rpm on the dial, but NOT a dial back type.
    Know exactly how much advance the distributor has in it.
    Determine where the timing can start and end plus initial idle timing for about 34 total.
    The distributor may require modification before you can tune anything.
    Last edited: Jan 3, 2022
    patwhac, techg8, 73 Stage-1 and 3 others like this.
  5. LARRY70GS

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

    X2, you can't arbitrarily run whatever initial advance you want. Mechanical and vacuum advance adds timing as RPM increases. Learn to use a timing light to set the total advance. Then the initial timing ends up where it must depending on how much mechanical advance is in the distributor you are using and what total you set.

    patwhac, techg8 and Dadrider like this.
  6. Gary Bohannon

    Gary Bohannon Well-Known Member

    I have a 1972 350 distributor # 1112109. It has short advance slots and is among the best factory distributors for performance. A 1972 # 1112110 is similar but for 455 engines.
    These are easily interchangable by swaping the gear on the bottom. Use 13 teeth for the 455 and 14 for a 350. This can simplify the timing-tuning problems.
  7. IamLumpy

    IamLumpy Member

    Thanks for the help. I currently only have a basic timing light so it sounds as though I need to purchase a better light along with a jet set and distributor tuning kit. I was also thinking I read the cam card that came with the engine said it was already advanced but I will have to dig that out again and see what it says for sure.
  8. LARRY70GS

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

    No, you don't have to purchase another light. You can make a 30* mark on your balancer and use that to total time the engine. All you would need is a set of light distributor springs. Read the linked thread I posted for you.


    Cut and paste from the above link,

    "Most Buick V8's run best at WOT, with a total timing of 30-36*, all in at 2500 RPM, or less. The easiest way to determine your total advance is to use a dialback timing light. You simply connect the light, plug your vacuum advance, and have a second person slowly rev the engine. With the dial back feature, you adjust the light to keep the timing mark in sight as it rises. When the timing mark stops moving, you hold the RPM's steady, adjust the dial until the balancer mark lines up with the 0 on the timing tab, and read your total advance off the dial.

    To do this with a conventional timing light, you need to make a 30* mark on your balancer. The Buick 350, and 455 balancers are 6 3/4" in diameter. Circumference (360*) of a circle is pi(3.14) X diameter. 6.75 X 3.14 = 21.195"/12 = 1.76" (30*). Looking at the engine from the front, measure exactly 1 3/4" clockwise around the balancer, and make a second mark. This is your 30* mark. Connect up your timing light, and watch your 30* mark as you increase the RPM's. At some point, your 30* mark will stop rising, and move no higher. This is the RPM, where all of your mechanical advance is in.

    At this same RPM, with the distributor loose, adjust it so that your 30* mark lines up with the 0 on the timing tab. You now have 30* of total timing. Line it up with the 2, 32* total, ect.

    Keep in mind that a stock distributor usually has stiff springs in it, that don't allow full advance in until 4000 RPM or more. For best performance, you want your advance in at 2500 RPM, or before. The easiest way to do this is to purchase a Crane adjustable vacuum advance kit. It comes with 3 sets of springs to allow your advance in as early as 1600 RPM, or as late as 3200 RPM, or anything in between. For points distributors (Jegs part # 270-99601-1,) (GM HEI, 270-99600-1). What I did was purchase the kit, and install the lightest springs(2 yellow). I used these springs to adjust my total timing, that way, I didn't have to rev the motor very high to see my total. Afterward, I installed the springs that brought my advance in at 2200 (2 silver)"
    Last edited: Jan 3, 2022
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  9. Reidk

    Reidk Well-Known Member

    Screenshot_20220103-155905_Samsung Internet.jpg you may have an adjustable vacuum advance. Maybe adjust it to where you're only adding 10 degrees of advance and see if that helps. Or dial it down even less. This is quick and easy.
  10. Reidk

    Reidk Well-Known Member

    Oreilly and autozone can get holley jets next day. Maybe try some 72s or 74s . Summit sells a big kit for around $60-$70 but is probably not necessary. but like the other guys are saying...probably a timing issue more than anything.
    patwhac and sean Buick 76 like this.
  11. Gary Bohannon

    Gary Bohannon Well-Known Member

    The 30-degree mark at 1.76" on the damper makes timing advance simple to check. No dial-back needed.
    A budget timing light is fine. Some expensive dial-back timing lights are less accurate in spite of their price.
    I like the Innova with rpm dial best.
    Reidk likes this.
  12. IamLumpy

    IamLumpy Member

    I like the 1.76 inch tape method and I will try that once it warms up. The car is in storage til spring. I do think it is a timing problem. I'm not sure if the vacuum advance is adjustable but if it is that may help dial it in. The pinging tends to lessen if I give more throttle (I'm guessing less vacuum) without the secondarys opening.
  13. LARRY70GS

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

    9 times out of 10, pinging is an ignition timing problem. I wrote an entire thread on optimizing ignition timing because of the confusion about it that I saw so frequently. Vacuum advance is the last thing you tune. Leave it disconnected until you get your total (initial + mechanical) advance set. Then you add the VA, and adjust it. Chances are your Ebay HEI has a stock VA. There are ways to adjust a stock VA, or add an adjustable one. The more load and throttle opening, the lower engine vacuum is. In fact, VA drops out completely at wide open throttle when engine vacuum is at or close to zero.
    techg8 likes this.

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