Engine rattle under acceleration

Discussion in 'Small Block Tech' started by JN_Skylark, Dec 25, 2020.

  1. JN_Skylark

    JN_Skylark Well-Known Member

    I do run ARCO regularly, but on my next fill-up or even 1/2 a tank it wouldn't hurt to try another brand of fuel
  2. Quick Buick

    Quick Buick Arlington Wa

    I my opinion dont even bother tinkering with the motor till you get that junk out of the tank...
  3. LARRY70GS

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

    .035 for points.

    JN_Skylark likes this.
  4. theroundbug

    theroundbug Well-Known Member

    I've been running regular arco gas in all my cars and bikes for 10 years and never had a single problem. Honda, jeep, bmw, toyota, suzuki, aprilia, kawasaki, buick, oldsmobile, dodge, saturn, mazda. The only problem between shell and exxon down the street vs arco was my wallet hurt more. Carb, carbs, efi - didn't matter. Smog checks didn't change. Literally nothing changed. If you think getting shell is going to be that much better than arco I have some fuel additives to sell you that will fix all your problems ;)
    Max Damage likes this.
  5. JN_Skylark

    JN_Skylark Well-Known Member

    Well I backed off the timing to 0* as per the manual and the pinging has magically vanished. I'm going to investigate the up/down play in the distributor since it looks excessive to me and try re timing it. But it feels nice going wide open with no noises :)

    theroundbug, Aus91R and Max Damage like this.
  6. mikethegoon

    mikethegoon Well-Known Member

    Might be best to enjoy your 68 for crusin and leave WOT be.
  7. Fox's Den

    Fox's Den 355Xrs

    The car seems to sound ok, when you take that distributor out the end play at the gear is at .015 and I don't think it is less than that there has to be some play in there. You can move it up and down.

    Just because the timing says "0" does not mean the timing is off, that is the spot where it is going to run the best for now. Add 2 more and see what happens
    You can also play with the vac advance canister get a kit for it so you can adjust it to your needs. This could be the cause of the pinging since it may be coming on too fast.

    You can use a Mighty vac brake pump to suck on the vac advance to see how much the timing goes up to get an idea.
    This would not have a gauge on it so do not pump a lot maybe 2 times. Or get a pump with a gauge to match what your engine is doing Pull 5-10 on the vac gauge and see how much the timing moves.

    If you did what Larry's timing thread said to do then you should have the timing in the ballpark.
    I have 32 total timing on my engine so start there or go to 30 that is a good place to start also.

    The vacuum canister on the distributor is going to add to the total number when you are stepping on it.
    If you have the light springs in to get the timing all in by 2500 when you hit that number and still on the gas the vac advance in the distributor is still pulling the timing more. hence ping ping paping. boom

    If you are getting this try the medium springs in the distributor, you can also try using a medium and a light spring to change that more to speed up, or slow down the total timing.
  8. JN_Skylark

    JN_Skylark Well-Known Member

    With the play in the distributor I agree that the timing might not be 0* but where it's happy for now. For these tests I've had the vacuum advance disconnected, I wanted to find the cause of the pinging before adding vacuum advance to the equation. I did read and follow Larry's timing thread, I even purchased a timing light that allows me to read the amount of advance.

    With the style of driving this car sees having optimal WOT performance isn't entirely necessary, it's a daily driver so it's usually driven at cruise speeds. That being said I want avoid being in a situation where the engine is under load and it still pings. I'll leave it where it's at for the time being and continue tweaking it in the future.
  9. Max Damage

    Max Damage I'm working on it!

    Sounds good and looks like it goes fine too. Your close.
    JN_Skylark likes this.
  10. LARRY70GS

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

    Vacuum advance will not function at high load by design. Vacuum has to overcome the spring in the advance canister to add any advance at all. As you load the motor, vacuum drops, and at WOT, it is at or very near zero. At that point, the spring wins out and removes any advance the canister added.
    JN_Skylark likes this.
  11. Fox's Den

    Fox's Den 355Xrs

    So it is safe to say once you get past the vacuum that pulls the advance the vacuum drops out when there is enough load on the engine and just the advance weights take over sound correct ?
  12. LARRY70GS

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

    The mechanical advance operates independent of the vacuum advance. At any RPM, the total advance is the sum of initial advance + mechanical advance + vacuum advance. Take the Stage1 Distributor,

    Lets see you are tooling along at 1800 RPM. Your total advance is 10* + (9-13) + (14-18*), or about 33-41*. Now you stomp it to floor, and manifold vacuum drops to zero. Your vacuum advance disappears, and the mechanical advance increases until 4600 RPM where it tops out at 20-24*, and your total advance is 30-34*. As soon as you let off the pedal, vacuum increases rapidly, the vacuum advance increases and the mechanical advance decreases with RPM.

    Manifold vacuum is an excellent indicator of engine load. Light load (cruising) and vacuum is high. Large throttle opening and WOT and vacuum is low. The vacuum advance canister has a spring that opposes vacuum pull. In order for the vacuum advance to add advance, vacuum must be high enough to overcome the spring tension and move the link. When vacuum drops, the spring wins out and pulls out any advance that was there.

    walts72 and JN_Skylark like this.
  13. JN_Skylark

    JN_Skylark Well-Known Member

    I wanted to follow up with this thread because I found the problem. It ended up being the distributor, I found a dizzy with the correct curve and stamping/# for my car, did a simple rebuild on it and I haven't had any problems since. For now I'll run the stock curve since the car seems happy but now with a solid baseline I'll have to go back to Larry's timing guide to try and squeeze out some more oomph
    Dadrider likes this.

Share This Page