Idle Quality Issues with Performance Camshafts
Because of the design of a performance camshaft, its increased lift and duration, and more radical overlap geometry; the engine is going to need more air at idle to run. Most are familiar with the fact that an engine with a radical camshaft idles at a higher speed and has less intake manifold vacuum. Several things happen as a result of this:
- In order to get more air into the engine the throttle plates need to be open more which causes the loss of idle circuit adjustability.
- Power valves need to be readjusted or changed for the correct value.
- Power brakes may not want to work because of lack of engine vacuum.
- Vacuum advance controls may not function properly because of lack of engine vacuum.
- Ported vacuum signals from the carburetor may be present at the throttle opening that you now have at idle.
- So, how to deal with this? If you go to any manufacturer and tell them that the idle screws don’t seem to make any difference in the way the engine runs, whether they are screwed all the way or all the way out, they will all tell you the same thing: The most likely scenario is that you have opened the throttle so far to make the engine run that the engine is now not running on the idle circuit fuel, but on transition circuit fuel. This is not ever going to work. They will give you different strategies to get the throttle plates closed down to where the idle circuits will work again. This problem has been solved in the past by drilling holes through the throttle plates to let more air into the intake manifold instead of changing the throttle plate angle. This is a hit and miss operation, but it works great. The problem is that if you drill the holes too big you either have to install new throttle plates and start all over again or solder the holes you drilled and begin again. And when you get done you’re stuck with those holes because they are obviously non-adjustable.
Barry Grant has fixed this issue with some of his carburetors by installing an adjustable air bleed in the base of the carburetor that can be accessed by removing the air cleaner stud and slipping a thin screwdriver down through the stud hole. You can now adjust the proper amount of air so that the throttle plates are nearly closed, which restores the idle circuit to where it is adjustable. Great invention! I needed to make a similar adjustment on my hot rod because of the large solid lifter cam, so I kind of cheated a little and took the air cleaner stud out and drilled a 3/16” hole straight down through the bowl casting (this is a Holley 750). Then I drilled a couple of small holes straight into the stud housing. The cavity below the stud housing in a Holley is hollow and sits right on top of an equalizing port in the base plate. The equalizing port makes a connection between all the throttle plates for balance purposes. The air from the bleed I just drilled into the stud housing will get to the throttle plates through this port. Now I can use the air cleaner stud to control the air bleed by screwing it in and out. When I get the throttle plates to where the idle screws work well, I then use a lock nut on the stud to hold it in place and use the idle screws to get my target air to fuel ratio.
On carbs with a 5/16” air cleaner stud, instead of drilling the holes in the stud boss I drill a hole straight up through the center of the stud, then drill a hole horizontally to meet the vertical hole. Now I can use a lock nut to control the air bleed instead of turning the air cleaner stud in and out. There is an attachment at the end of this that shows how the process is performed.
Of course there are issues with attaining your target air fuel ratio. The idle circuit air bleeds, if they are too big, won’t let enough fuel siphon from the bowl to get the air fuel ratio you are looking for, so you may have to reduce the size of the air bleeds by replacing them with smaller bleeds. If you have the money to get a performance carburetor body or a really well built performance carburetor, it will come with replaceable air bleeds. If the idle circuit is continuously rich then the air bleeds will have to be opened. Buy a set of small, numbered drill bits for this operation and go one size at a time.
Note: Whenever you change throttle position you change the relationship of the accelerator pump lever to its cam. That can create problems, so whenever you turn that idle SPEED screw, recheck the accelerator pump adjustment.
Note: The very best way to adjust the air fuel ratio is with the help of a tool such as the Innovate Motorsports LM1. It uses a wide band oxygen sensor that gives an exact readout of air/fuel for you. But beware! If you have any kind of a misfire condition the readings you will get will not be accurate. This system uses an oxygen sensor, and if you have misfires for any reason the resultant oxygen content in the exhaust will be high, indicating a lean exhaust condition. Only look at oxygen sensor information if the engine is running correctly on all of its cylinders. A rough engine idle because of a big camshaft will undoubtedly be the result of misfire from an improperly adjusted fuel system. You’ll be surprised at how much milder the engine will idle with that big cam once you get fueling straightened out.
Here are some other articles:
https://documents.holley.com/lit705rev.pdf See pg 14 Idle-Eze
Idle-Eze
The Idle-Eze (Figure B) was created by Demon Carburetion chiefly to ease the complications associated with idle-speed adjustment. This device can introduce an extra source of idle air to the engine without disrupting the critical relationship between the idle-speed adjustment screws, the throttle blades, and the transfer slots.
The Idle-Eze has access to air above the throttle blades and can transfer it beneath them, if necessary. This introduces an extra source of air to the engine that can alter the idle speed by as much as 500rpm without touching the idle speed adjusting screws.
https://www.chevelles.com/forums/13-performance/138912-drill-not-throttle-plates-holley.html
https://www.supermotors.net/articles/nsmc-issue01-a1-1.php
https://nastyz28.com/threads/drilling-throttle-plates.9027/#post-56408
[URL]https://nastyz28.com/threads/low-signal-transfer-slots-holes-in-t-plates.159755/
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