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Thread: Idle Problems With Performance Camshaft

  1. #1

    Default Idle Problems With Performance Camshaft

    Yes, this is another thread about idle problems together with camshaft with big overlap. I have read almost every article and threads on the Internet about this problem, but haven’t found a working solution for my setup.

    My engine is a modified Chevy 350 with ported double hump heads, roller lifters and lots of more performance stuff. The engine also have a very aggressive camshaft (I don’t have any specs of it). Finally the engine is equipped with a brand new Holley 750cfm vac carb (3310-SA) with manual choke.

    I have tried to set the idle mixture screws but they are not responding at all and it feels like it running too rich at idle which for me indicates that the main circuit is in use instead of the idle circuit.

    Here are what I have done so far:
    • Manifold vacuum when idling at 900 RPM in neutral is about 9 Hg.
    • Manifold vacuum when idling in drive is low and jumping up and down (around 5, I guess).
    • I have tried to bump up the initial timing to 16° BTDC.
    • The vacuum advance is connected to the ported vacuum.
    • The carb is new and not modified.
    • I have tried different power valves (6.5, 2.5 and 3.5).
    • I have drilled the primary throttle blades.
    • The transfer slot is not exposed to much on any of the throttle blades (I have checked by having the carb upside down).
    • If I use the choke the engines dies.
    • I have searched for vacuum leaks but not found any (I replaced all hoses and used carb cleaner spray around the intake)
    • I have done all tests with the brake booster disconnected and the vacuum port plugged (if it was a vacuum leak there).
    • I use a PCV valve in the drivers side valve cover and the passenger side valve cover is connected to the air cleaner. I did also the test with breathers instead of the PCV system but with no difference.


    After all the above tests, I finally found some discussions about Holleys running to rich at idle when using cams with big overlap and the suggestions was to restrict the fuel into the idle circuit by putting a thin wire into the idle feed restrictor. So yesterday I also tested this but it didn’t help at all.

    I think my problem is the carb isn’t using the idle circuit or the idle circuit isn’t setup according the low vacuum my engine produces. Should I perhaps try to increase the fuel to the idle circuit instead of restrict it with the wire in the idle feed restrictor?

    Does anyone have any ideas or tips how I should tune the carb to be used at the street with my monster camshaft?
    Last edited by chevycsb; 08-20-2015 at 04:05 PM.

  2. #2

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    Hmm. You say the idle screws are unresponsive but you aren't in the transition slots at idle? That's weird, BUT I had a Carter AFB with that issue, and I had to drop the idle air bleed size from 70 to 55 (required some heavy mods). However, my carb was way too lean and that solved it.
    You say the engine dies when you use the choke, that sounds like it's too rich at idle. How is it off idle? Maybe you need to restrict the idle feed restrictor? Open the rear throttle blades up a bit or more air. Or as a last resort, drill small holes in the primary throttle blades.

  3. #3

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    First off if your cam has a high lift/duration, you need to be running as much initial timing as your starter can handle without dragging. Try running about 21 or 22 degrees of initial timing. It will help with two things. It will stabilize your idle AFR by giving the fuel more time to burn and raise your idle rpms by 100 - 200 rpm. This will allow you to readjust your primary throttle blades and get off the transition. You will also need to recheck your total timing as well. Running a high initial timing may require to adjust your distributor to only giving you 35-38 degrees of total timing.
    With a high lift, high duration cam, your going to have some "stinky" exhaust at idle. It shouldn't make your eyes water but it's going to be pretty prevalent. Just one of the drawbacks of having a high performance camshaft. On my SBC 383 with a hyd. roller camshaft with 580 intake lift, 530 exhaust lift and 292 duration, I had a pretty stinky exhaust. Even with running an idle AFR of 14.5 ish it was way better but that high of an AFR caused me some lean cruise conditions. I was able to correct some of it out with smaller idle air bleeds and higher power valve.

    If your idle mixture screws are unresponsive this normally means your idling on the transition slot. You should be able to close your idle mixture screws by lightly seating them (clockwise turn) and kill the motor. Also, you should adjust your idle mixture screws for the highest vacuum possible while in gear (parking brake set) for an automatic transmission. You shouldn't have more then about 100-200rpm drop going from idle to in gear.

    I have tried to set the idle mixture screws but they are not responding at all and it feels like it running too rich at idle which for me indicates that the main circuit is in use instead of the idle circuit.
    Mains wont start flowing until about 2000 rpm. They use negative pressure to start flowing. The only place fuel comes from during idle is the idle well (controlled by idle mixture screws), transfer slot (to much throttle blade open), curb idle discharge port (if so equipped).

    Manifold vacuum when idling in drive is low and jumping up and down (around 5, I guess)
    If your vacuum while in gear can only get to 5, run the 3.5 PV. Try to get this better with idle mixture screws if possible.

    The transfer slot is not exposed to much on any of the throttle blades (I have checked by having the carb upside down)
    The transfer slot should only be exposed about .020 - .030. Very small amount. less then 1/32nd of an inch.

    Pull out the wire you put in the Idle Feed Restriction. Don't mess with that until you can get your idle mixture screws to be responsive. The wire you put in has to be super thin. You can cause an extreme lean condition while cruising if not careful and understand how much you are reducing the overall size of the IFR hole. its not as simple as "my IFR hole is .031 and I put in a .010 diameter wire so now my IFR hole size if .021" Its an area size reduction that has to be calculated out.

    A lot of carb tuners like for the idle mixture screws to end up about a 1 - 1.5 turns from seated as a final adjustment. If you find that your carb gets best vacuum at only a .5 turn out it is said that your IFRs might be a little on the big side. Likewise if your idle mixture screws have to be more then 2 turns out to get best vacuum it could be be said your IFRs are on the small size. This is a guideline, not a MUST. My 750 Street HP was about 3/4 turn out from seated. I in search of better fuel mileage took my IFR down to a .030 from a .032 to get a better cruise AFR.
    Last edited by 69cam; 08-20-2015 at 08:08 PM.

  4. #4

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    When cruising slow it also feels rich but performing well at WOT.
    I have also tried to open the secondaries (I did that before I drilled the primary), and finally I also tried to restrict the idle feed without any success.

    I heard lot's of people saying that the only way to fix this is to bump up the initial timing, but I'm not sure this is the only solution and not the best, but I will give it a try.

    If i turn the idle mixtures screws in to totally seated the engine dies, and I'm already using the 3.5 power valve.
    The wire I putted into the idle feed restriction was really thin (don't have it's size right now).

    Thanks for your replies! Tomas

  5. #5
    Join Date
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    Quote Originally Posted by chevycsb View Post
    The engine also have a very aggressive camshaft (I don’t have any specs of it).
    • I have tried to bump up the initial timing to 16° BTDC.
    I heard lot's of people saying that the only way to fix this is to bump up the initial timing, but I'm not sure this is the only solution and not the best, but I will give it a try.
    Increase the ignition timing at idle to 25°-30° BTDC.
    May God's grace bless you in the Lord Jesus Christ.
    '92 Ford Mustang GT: 385" SBF, Dart SHP 8.2 block, TFS TW 11R 205 heads, 11.8:1 comp, TFS R-Series intake, Dominator MPFI & DIS, 36-1 crank trigger/1x cam sync, 160A 3G alternator, Optima Red battery, A/C, 100HP progressive dry direct-port NOS, Spal dual 12" fans/3-core Frostbite aluminum radiator, Pypes dual 2.5" exhaust/off-road X-pipe/shorty headers, S&W subframe connectors, LenTech Strip Terminator wide-ratio AOD/2800 RPM converter, M4602G aluminum driveshaft, FRPP 3.31 gears, Cobra Trac-Lok differential, Moser 31 spline axles, '04 Cobra 4-disc brakes, '93 Cobra booster & M/C, 5-lug Bullitt wheels & 245/45R17 tires.

  6. #6

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    Quote Originally Posted by Danny Cabral View Post
    Increase the ignition timing at idle to 25°-30° BTDC.
    If your camshaft is really that big, it will like the high initial timing. If you push it out to the 25°-30° range, disconnect the vacuum advance and test drive. Vacuum advance will typically advance 14°-15° if still stock. Also, your mechanical advance built into the distributor is usually good for another 12°-15° or so. YOU WILL NEED TO LIMIT THE TOTAL AMOUNT OF TIMING THE ENGINE gets. That will cause detonation if it pushes your timing into the 45+ range. Run premium gas if your not already as well. 25°-30° degrees will be nice, but if you don't have a GOOD starter and heat shield, it will drag and possibly not start with that much initial timing.

    One way to get high idle timing & cruise timing, is run the vacuum advance from the "full manifold vacuum port". That way you can run an initial timing of about 20 or so, and when the engine starts, it will instantly advance the timing from the vacuum.

    When you get into certain camshafts that have high duration, you will also hear to "LOCKOUT" the timing as well. Meaning to run 35-38 degrees all the time. Not really practical for a street driven car, but from a performance standpoint the motor will like it.

  7. #7
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    Quote Originally Posted by 69cam View Post
    Vacuum advance will typically advance 14°-15° if still stock. Also, your mechanical advance built into the distributor is usually good for another 12°-15° or so. YOU WILL NEED TO LIMIT THE TOTAL AMOUNT OF TIMING THE ENGINE gets. That will cause detonation if it pushes your timing into the 45+ range.
    I completely agree. The entire distributor centrifugal and vacuum advance curve needs to be modified. I used to be very proficient at this (then I converted to EFI).

    25°-30° degrees will be nice, but if you don't have a GOOD starter and heat shield, it will drag and possibly not start with that much initial timing.
    Use a "Start Retard" module.

    One way to get high idle timing & cruise timing, is run the vacuum advance from the "full manifold vacuum port". That way you can run an initial timing of about 20 or so, and when the engine starts, it will instantly advance the timing from the vacuum.
    Yes, I almost always recommend "full manifold vacuum" for vacuum advance distributors. However, sometimes a radical camshaft can fluctuate the manifold vacuum too much, and exacerbate an erratic idle. Another benefit of EFI.
    May God's grace bless you in the Lord Jesus Christ.
    '92 Ford Mustang GT: 385" SBF, Dart SHP 8.2 block, TFS TW 11R 205 heads, 11.8:1 comp, TFS R-Series intake, Dominator MPFI & DIS, 36-1 crank trigger/1x cam sync, 160A 3G alternator, Optima Red battery, A/C, 100HP progressive dry direct-port NOS, Spal dual 12" fans/3-core Frostbite aluminum radiator, Pypes dual 2.5" exhaust/off-road X-pipe/shorty headers, S&W subframe connectors, LenTech Strip Terminator wide-ratio AOD/2800 RPM converter, M4602G aluminum driveshaft, FRPP 3.31 gears, Cobra Trac-Lok differential, Moser 31 spline axles, '04 Cobra 4-disc brakes, '93 Cobra booster & M/C, 5-lug Bullitt wheels & 245/45R17 tires.

  8. #8

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    Thanks again for you replies. After more investigation, I started to suspect a leak in the intake manifold gasket. So I decided to open it up. I think there have been some leaks, but I also found that the block has been broken before and welded. I think the welding is still OK, but there was also some sort of floating/chemical metal which now have came loose, and perhaps I have may some small rest of it in the oil.

    So I need to check this up first, and take the ignition & carb adjustment little bit later. Tomas

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