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Thread: Troubles with SA570 on Mild 318.

  1. #1

    Default Troubles with SA570 on Mild 318.

    Need some help diagnosing what may be one or two unrelated issues on my '67 Dodge. It's got a stock 318 with closed-chamber heads at ~9:1 compression, a vintage low-rise dual-plane intake that is 318-specific (correct small intake runners) intake, and I'm running an aftermarket small-cap vacuum advance distributor firing an HEI and a 570 Holley Street Avenger. I'm fighting a hesitation off-idle when you punch, and also a bad RPM drop at idle when using the power steering. These might be related, or might be two separate issues? The truck is a stick-shift, so I've got it tuned to idle about 750 RPM. 10° initial advance, about another 12° at idle from the vacuum can since it's hooked up to manifold vacuum. I think the carb is on the edge of being too big because I had to completely close the rear idle screws and it's only got the fronts open ~3/4 turn (when I tried to adjust it as a 4-corner idle, all the screws were open less than 1/2 turn all around). The transfer slots are not uncovered more than .020"-.040" in the primaries and the secondaries are completely closed at idle.

    The hesitation is something I've worked through before with accelerator pump changes. If you roll into the throttle the truck is completely fine, but if you punch it (either from a dead stop or even while cruising 60+ MPH) it coughs and hesitates badly for a second or so before taking off. The hesitation is worse when it's cold, so my inclination is not enough accelerator pump shot, but before I go that route I wanted to make sure it isn't tied to my other problem. I'm still running the original #31 squirter and red cam in the accelerator pump BTW.

    The truck has an aftermarket P/S setup which provides some significant drag at idle when maneuvering into a parking space. For instance, and it'll drop the idle by 200+ RPM, even more on a surface like hot sticky asphalt, and can occasionally stall the truck. Is it possible my idle RPM drop and my stumble off idle are actually the same issue, or at least related to some extent? Obviously the squirter can't help my dead idle P/S performance, but at the same time I don't want to go chasing the squirter tuning only to find out later that it was some other issue causing both problems? I've never tuned a carb with P/S before, and it's messing with my head since it's not a consistent load and it can be very abrupt. Some reading has suggested that maybe I'm relying too much on the vacuum advance for idle and that's why it's dropping out under the P/S load, but since I'm not giving it any throttle when steering. The vacuum stays about the same when the idle drops (I can't verify what the timing is doing without a second set of hands, but if the vacuum isn't dropping the timing should be the same as well). Is this an application where ported-vacuum would be better? I suspect both my idle stability issue, my hesitation, and the fact that my idle screws are so far in might all be related somehow?

    Also the engine makes 21 inHg of vacuum at idle so I moved up to a 10.5 power valve, but it ran the same with the original 6.5, so I don't think that's causing my issue? Not sure what to chase first at this point.

  2. #2
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    Your combo doesn't need 22° of advance at idle.
    Whenever load occurs, such as from a P/S pump, manifold vacuum will drop.
    As to how much, you'll need to observe this on a vacuum gauge.
    Then if the advance is dropping from 22° to say 15°, then the reduced timing, reduced vacuum, and load, all contribute to the RPM difference.
    I'd connect up ported spark, run about 12° at idle, check for no ping under moderate to heavy load, etc.
    Get the ignition curve correct for the engine first. I'd also check the mechanical advance curve with a timing light. I chased a fault for some time until I realized the trigger wires ARE polarity conscious.
    Your carburetor size is fine for this engine.
    I think you will find more acceptable carburetor idle adjustments with a fixed timing at idle.
    Gary
    Last edited by Gaz64; 11-02-2020 at 05:49 PM.

  3. #3

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    I swapped to the ported vacuum source for the distributor and bumped the static timing up to 12°. Double checked the mechanical advance and it doesn't kick in at all until ~1200+ RPM (hard to say for sure, trying to watch the tach and the light at the same time) and isn't all-in until pretty high RPM. After switching the advance around the idle screws only required minor tweaking to get a good idle (I'm using a Fluke 88V for a tach so it's pretty sensitive). Didn't get to road test it, but it didn't really do much for my load at idle issue.

    I did find something that might well be important though. On a whim I popped off my PCV setup (aftermarket from Jegs), which was also added at the same time as the recent carb/intake swap, and the idle didn't change much with the PCV disconnected, but dropped almost 250 RPM with the PCV port plugged. I know that there'll be some additional idle air with a PCV setup, but 250 RPM seems excessive? To see what would happen I tuned the idle with the PCV plugged and got it back up to 750 RPM and the load at idle issue was greatly improved. Less than 100 RPM loss with the PCV plugged. I want to road-test the truck with both the ported-advance with PCV and without PCV and see how the stumble is. If it's PCV related maybe I can add an orifice to the PCV hose to reduce the bypass air from the PCV at idle?

    I also just realized my vacuum numbers are way off because I was using the PCV port to connect the vacuum gauge during tuning. I intend to rig up a tee so I can leave the PCV connected and see what the real idle vacuum is with the PCV and without.
    Last edited by JLeather; 11-10-2020 at 10:57 PM.

  4. #4
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    Sounds like you're getting somewhere now. I'd run a fixed orifice type PCV valve, and then you might be able to go back to manifold vacuum for vacuum advance. I thought you had an Automatic, until I see now you have a manual transmission. Gary

  5. #5

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    I pulled the Jegs PCV out of the valve cover and found that it has such a large bypass opening that it was fluttering at idle. I also found by hooking my vacuum gauge to the brake booster port that I was losing almost 5 inches of vacuum with the Jegs PCV. I gutted my Jegs billet PCV so it's just an elbow now and I plumbed a stock 318 PCV in the hose under the carb. The stock PCV doesn't lose any vacuum on the gauge vs. a plugged port, and I'm now only dropping 40-50 RPM when I plug the PCV port vs. the stock PCV. Dead-idle power steering is much improved. Haven't road-tested the new setup yet, but I'm hopeful.

  6. #6
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    Sounds like a happy outcome. Gary

  7. #7

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    Still here and still fiddling. The dead-idle steering performance is better, but the truck still has a heck of a stumble, both leaving a dead stop and also punching it while at a level cruising speed. Gotta be the accelerator pump now, right? What I'm confused about is whether to go up or down, or mess with the cam. I've found conflicting info in tuning guides. Based on the fact that the stumble is worse cold and gets better as the truck warms up (but still large & unacceptable) it should be a lean condition and I should go up a couple sizes on the squirter. However, I've also read that for heavy vehicles like trucks sometimes you have to drop the squirter size to lengthen the charge? My stumble is pronounced & immediate, and the truck recovers very quickly, so I still think it needs to go up a few sizes, but before I end up buying too may sizes can anyone comment on these opposing theories? I've got the red accel pump cam in (I think) the #1 position. I can drill out the squirter a size or two up for free (I have a good selection of orifice bits), but obviously that's irreversible.

  8. #8
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    What size pump nozzle is in it now? You do understand the adjustment of the pump lever? Many think the .015 clearance is at idle. Do you have the pump cam assortment? Red is a baby cam, position #1, 18.5cc per 10 strokes. Gary

  9. #9

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    #31 squirter. I don't have any other cams or squirters to tune with, so I'll have to buy what's needed. The SA570 only came with the red cam. I have ~0 clearance of the arm at idle. Never checked the clearance at WOT, I assumed with this cam and diaphram it would be fine.
    Last edited by JLeather; Yesterday at 11:56 PM.

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