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Thread: Vacuum too low - idle problems.

  1. #1

    Default Vacuum too low - idle problems.

    Holley Sniper Stealth in my '65-StingRay. The engine is a 496 cu in with Comp Cams solid roller XR280 camshaft. The cam is installed "straight up". For the last two weeks I've tried to bring my IAC counts to 2%-8%. I can achieve this, but the primary throttle blades are cracked open a little too much, just a little. No matter how many times I try to get it sorted, I fail. Using either full manifold or ported vacuum. In the past I always ran on full manifold vacuum.

    It begins with a cold engine. She starts up, but won't idle without the gas pedal applied until about 80°F, where it will start to idle on it's own. Idle is 900 RPM for my car.

    Also, I'm running mechanical & vacuum advance distributor. Vacuum when cold is about 7-8" of inHg. Warm it idles at 12" of inHg. Now since my throttle blades are open in response to lack of air it doesn't matter if it's on ported vacuum or full manifold vacuum, I'm getting the same readings on either one. So air is slipping into the ported side at idle and I can't stop it.

    If I try to adjust the IAC to close the blades, the IAC reads about 50% after. Right now I'm on full manifold vacuum to the distributor vacuum canister. The canister is a B-28 comes in at 3-5" all in by 8" with 16° of vacuum advance at idle. It's the only vacuum advance can that works with this engine. Initial is 13° plus 16° vacuum = 29° at idle (my car likes this) and 21° mechanical from blue bushing in MSD distributor to give 50° at cruise and wide open throttle at 34°.

    If there are no solutions other than changing camshafts (not an option) should I try to go with full ignition timing to eliminate the vacuum side of the equation. Again I know it won't cure it, but maybe it will improve the situation. Should I skip trying to fix something that may never be resolved? Wayne 65-StingRay

  2. #2
    Join Date
    Dec 2009
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    You need ECU controlled ignition timing, which can implement vacuum advance without the (fluctuating) vacuum.

  3. #3

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    Danny, thanks for your answer. I was hoping this would be my way out of this. I've read many of your posts, thanks for your good head of knowledge. Wayne '65-StingRay

  4. #4

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    Yes, install a HyperSpark distributor to give yourself some more degrees of freedom. The HyperSpark will allow you to fine tune the timing curve which should help a lot. I also think you'll be able to solve some of your issues by working on the cold start settings. You can adjust the AFR when cold and you can adjust the timing when the engine is cold. An expert could resolve your tuning issues in 30 minutes, so if you ever get frustrated just call in some help. Someone like Rich at FAST Man EFI could have that Corvette running like a Lexus with a few taps on the keyboard.

    Another option is to have an expert dial in your Base Fuel Table so you can run Open Loop at idle. That cam is sort of on the edge of where you need to consider Open Loop at idle. My guess is that you'll be able to solve the problem with timing control, but if not then Open Loop is another possible solution.

    When you install the HyperSpark distributor make sure the gear is compatible with your camshaft. If you screw up the gear choice you'll be paying for a full engine rebuild, which isn't fun.
    Last edited by AndyF; 10-28-2020 at 06:22 PM.

  5. #5

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    Andy, good tips thanks. Adding timing when cold and AFR values to help it breath better down low sounds easily doable. I've been thinking about going to Open Loop down low under 1500 RPM. My neighbor has a tuner doing his car and he swears by it. Myself I like to play with things till I get them right, but it definitely takes longer. cheers. Wayne '65-StingRay

  6. #6

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    It sounds to me that you might be chasing a timing issue.

    I'd find a good setting to park the IAC that allows the engine to idle well and work on the timing curve to see if you could bring the vacuum levels up.

    As you match your timing curve to your engine, you may find that you'll need to adjust the idle speed as things sync up better.

    Depending on the overlap, your vacuum readings might not be able to get much higher then they are. But 8-12 inHg be plenty to get a decent idle.

    Perhaps try blocking off the IAC port for a bit as you fine tune the timing, just to rule out that as a variable. Then once you get your engine to idle right and pull the highest vacuum, then focus on setting the IAC parameters. These might not stay stable, my IAC fluctuates quite a bit from a cold engine to a hot engine and when it's in gear or not. But once up to temp, it typically stays in the 2-8% range at idle. Forged pistons take a bit to warm up, add in loose gaps in the rings for a blower setup and this is what happens, engine takes a bit to warm up and work efficiently.

  7. #7

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    @65-StingRay: If you like to play with things then add the HyperSpark distributor (ignition system is up to you, but the Holley CDI box & coil work pretty well). You'll also want to get a laptop with the Sniper EFI software and buy the connecting wire to go from the laptop to the Sniper. Once you have all of that stuff, you can sit in a lawn chair in the driveway next to your car and tune the engine while it is running. Your neighbor will be impressed.

  8. #8

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    Quote Originally Posted by Gen2Van
    Depending on the overlap, your vacuum readings might not be able to get much higher then they are. But 8-12 in Hg should be plenty to get a decent idle.
    Perhaps try blocking off the IAC port for a bit as you fine tune the timing, just to rule out that as a variable. Then once you get your engine to idle right and pull the highest vacuum, then focus on setting the IAC parameters. These might not stay stable, my IAC fluctuates quite a bit from a cold engine to a hot engine and when it's in gear or not. But once up to temp, it typically stays in the 2-8% range at idle. Forged pistons take a bit to warm up, add in loose gaps in the rings for a blower setup and this is what happens, engine takes a bit to warm up and work efficiently.
    @Gen2Van: More good advice. I've already played with those settings. Tried a couple of different PCV valves to help with the vacuum. Yes my vacuum should be enough to hold a decent idle. My engine responds similar to yours. My IAC fluctuates until warm as well then settles in once 180°F is achieved.

    @Andy: I get the hyper spark but I have two distributors to chose from: 1) MSD with mechanical tach drive (I need). 2) stock distributor with Pertronix (Hall-Effect sensor & tach drive)plus MSD 6AL box. So I'm set for now. If the MSD distributor gives me too much RFI/EMI I will switch to the Pertronix distributor. If all that fails then I will look at the HyperSpark (but I need mechanical tach drive for my stock tachometer. There's things I'm not willing to change for the sake of convenience, unless I absolutely have to. Hopefully I won't, but if I have to I will.

    I already have downloaded the software and have created a a couple of GCFs. I'm using my home PC for now, but have been looking for a decent laptop to have in the car - and yes to impress the neighbors. Wayne

  9. #9

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    If it was my car, I'd install a HyperSpark distributor so I could super tune the engine. Once I knew exactly what kind of ignition curve the engine wanted, then I'd try and figure out if either the MSD or the Pertronix would work. The tach issue can be dealt with down the road.

  10. #10

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    All good points. Winter is coming quick, we've already had snow. It's dry now so I have only a few days to get it operational with full timing advance. I hope I won't have too many problems so I can at least drive and tune a little.

    Thanks again Andy, now back to the garage. Maybe I'll have it running in a couple more hours. Wayne

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