can't get idle lower than 1000

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71dustar

71dustar

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I've been trying to tuning my carb so that the idle is below 1000. I first checked to see if the bowls were set right and then i tried adjusting the idle mixtures. I'm going to check the vacuum to make sure the power valve is the right one. It seems as though when i go below a 1000 it wont run on its own.
 
71dustar said:
I've been trying to tuning my carb so that the idle is below 1000. I first checked to see if the bowls were set right and then i tried adjusting the idle mixtures. I'm going to check the vacuum to make sure the power valve is the right one. It seems as though when i go below a 1000 it wont run on its own.
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are the secondaries open at idle? i know this is what was causing a high idle on my carb before...i had the idle screw backed all the way out and was still idling at 1100rpm
 
Does it refuse to run slower than 1000rpms, or does it stall when you adjust the idle lower than 1000rpms? If it's the latter, then you may have a small air leak.
 
Points or electronic? Sounds like either vacuum leak or points not opening enough.
 
secondaries are closed. almost like it doesnt want to run lower than 1000, if i adjust idle screw to go lower than a 1000 it will slowly die.. points are electric.
 
Do you have a fat cam? I have had cams that wouldn't idle below 900-1000. Check for vacuum leaks spray carb cleaner around suspect areas and wait for idle to pick up, place rag over carb and if it smooths out briefly you might have a vacuum leak somewhere. You might also set the fast idle very high and remove the idle mixture screws and spray carb cleaner into the idle screw holes while its running. You might also not have enough advance to keep it running at low idle.
 
We really should start by knowing just what all this combo is specifically. Cam duration @ .050, type of intake, carb make and size, what booster type is the carb, jetting, power valve, are the throttle plates drilled, vacuum or mechanical, headers or manifolds, stick or automatic. Is this out of gear or just in gear, is this a fresh build or was it a running engine and this just started or was something recently changed, what distributor, vacuum or just mechanical advance, locked distributor, what is your base timing, what is the total timing? There are so many possibilities that without basic information we are just making wild *** guesses.
 
mygwenevere66 said:
i was thinking the same thing.or an idle circuit issue,like the throttle plates are open to the point where its no longer on the idle circuit to stay running.
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That's what it sounds like to me. Not pulling from the idle circuit because the throttle blade isnt covering it.
 
Sounds like the primary throttle plates are not set correctly. Take the carb off and set visually. I think thats what POS Dakota refers to in his post.
 
Guitar Jones said:
We really should start by knowing just what all this combo is specifically. Cam duration @ .050, type of intake, carb make and size, what booster type is the carb, jetting, power valve, are the throttle plates drilled, vacuum or mechanical, headers or manifolds, stick or automatic. Is this out of gear or just in gear, is this a fresh build or was it a running engine and this just started or was something recently changed, what distributor, vacuum or just mechanical advance, locked distributor, what is your base timing, what is the total timing? There are so many possibilities that without basic information we are just making wild *** guesses.
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THe cam is 241in 247 ex at .050.
rpm air gap intake
holley 4160 carb 750 cfm
primary jets are 72 while secondary is metering block
power valve is 65
vacuum secondaries
tti headers
auto 727
msd elec all in dist
mech advance
total timing is 34


this is at idle in park and motor is freshly built.
 
rpms do not change when i turn mixture screws until i lean them out.

hooked vacuum gauge up and i was pulling about 30cm HG (11.8in HG) at 1000 rpms.

I also sprayed around entire intake and carb and no change in rpms.
 
So what you are saying is you can't richen the idle mixture enough correct?

This could be due to a couple factors. Either the carb isn't running off the idle circuit or the idle circuit won't go rich enough.

First take the carb off and turn it upside down (drain the gas first) look at the primary throttle plates from the the rear of the carb. There is a small slot in the throttle bores, this is your idle circuit. If the throttle blades are open past the top of the slot you are not running on the idle circuit. You can crack the secondaries open a little more to compensate so you can close the primary throttle blades in order to bring the idle circuit alive.

If this doesn't work the next step is to reduce the idle air bleed orifice size. Most Holleys come with like a .070 hole or something. You can put a dab of Devcon F epoxy on the air bleed and redrill it to .040 or just get new air bleeds. These are the outboard air bleed holes in the carb air inlet area. Do just the fronts first. You probably won't need to do the rears but maybe. If none of this works the last resort would be to drill the primary throttle plates to get more air into the engine while keeping the throttle blades closed enough to activate the idle circuit. But that will be another post.
 
the primary throttle blades are not past the top of the slot. they are are towards the bottom but the blades are not completely below the slot either
 
Then you'll need to richen up the idle circuit by reducing the size of the idle air bleeds.

Just as a FWIW, I've had problems with those straight leg booster carbs and long duration cams before. Does the car run alright otherwise?
 
I really haven't had the chance to drive the car long enough (need insurance first) but i have put about 5 miles cruising around the block keeping it below 3000rpms since i'm running open headers. It doesn't seem to bog or stumble or anything. the engine runs healthy and strong.
 
You need to reduce the size of the idle air bleed. I explained it in an earlier post.

When you look down the top of the carb you see along the front edge of the venturi inlet two small jets for each venturi. The inboard ones are for the main metering circuit and the outboard ones are for the idle circuit. Most Holleys will have a .070-.080 orifice in the idle air bleed. You need to reduce the size to about .040 by either replacing the air bleed (you can buy air bleeds from BLP or similar carb shops, try a Google search) or just putting a dab of Devcon F epoxy (the black) on the bleed, pressing it into the hole then letting it cure 24 hours and redrilling it to .040 using a drill bit that size and a pin vise. I'm sure Jegs sells small carb drill bits.
 
sounds simple enough. So by restricting the inlet of the air coming through those needles, i will be able to adjust the idle to go lower?
 
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