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Old 10-19-2008, 09:26 AM
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engine idling

I have a question I hope someone can help I'm at a loss.
I have a small block chevy a 377 stroker, 10:56.1 compression, 540 hyd comp cam,750 mighty demon carb, dual plane air gap 202/194 heads. I have 4 in of vacuum at idle I installed a #2 power valve, also installed a heavy spring set up in th msd distributor and installed quickfuel metering blocks, I cannot get this engine to idle with out trying to fall off and die out plus it runs very rich, any adjustments that I make to any of the idle screws does not do very much, when I rev the engine it is not "snappy" it does not come back down to idle quickly. now it does not run as bad as it sounds but it could be alot better, could this be a carb issue being that Im hearing demon carbs a crap any going with a holley hp is better, can someone help

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Old 10-19-2008, 10:07 AM
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big cams won't idle slow
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Old 10-19-2008, 06:36 PM
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Whats the duration @ 0.050" of the camshaft?

Whats the initial ignition advance?
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Old 10-20-2008, 10:38 AM
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If I can remember correctly, the duration was around 240/260 @50, as far as the idle, it will idle at 1000, then fall off, the initial timing is at 18 and total advance is at 36
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Old 10-20-2008, 10:53 AM
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240...260?! 20 degrees more exhaust duration? Is this thing runnin glike 15 psi of boost or something? A 500 shot of nitrous? that is very excessive exhaust duration. bump the initial timing up 2 degrees, get a 4 bbrl carb spacer, and check your cranking cyl. pressure for me- somethign tells me you're not running 10.56:1.
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Old 10-20-2008, 11:11 AM
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I think he is saying 240-260 range not that its a split duration
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Old 10-20-2008, 11:24 AM
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Sorry, mudbogger is right, that is what I ment, I do have a 1'' 4hole spacer,on it, I also used to have a holley strip dominator single plane on it, Than I replaced it with the air gap, for more streetability, the throttle response became better but still, I have all the idle problems and running to rich
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Old 10-20-2008, 11:25 AM
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Oh and also, I did bump up the timing 38 still no help and initial to 20 ive been trying every combination nothing is working, a local speed shop mech looked at this prob and spent a day troubleshooting all he could come up with is "carb issues" I just find that a little hard to believe, noone I know has a different carb to swap to try, but I tried a different demon carb -- a street demon 750 It did the same thing

Last edited by devider440; 10-20-2008 at 11:32 AM.
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Old 10-20-2008, 12:09 PM
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I agree with increasing the initial advance, I'd go higher to 26 or so limiting
the centrifugal for the same total.

Pull the carb. off and check the throttle blade openings, they should not
expose much of the transition slots. If they do that’s what causing the
rich mixture at idle. One fix is to drill 0.100" diameter holes in each throttle
blade. This will allow closing the blades getting the carb. out of the
transition circuit to lean up the idle.

Here’s a test I do to determine if idle is rich or lean. With the engine idling
operate the pump shot manually with out opening the throttle. If the engine
speed goes up its lean, if it goes down it’s rich.

I run a SBC 350ci with 260 degrees duration @ 0.050”/ 0.550” lift; it idles at
800 RPM with 26 degrees initial and 10 degrees centrifugal advance; idle
screws are very responsive.

Last edited by automotive breath; 10-20-2008 at 05:36 PM.
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Old 10-20-2008, 12:56 PM
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In our shop we use the light silver springs in the MSD. I limit to 34deg at 2500rpm. The vacume might be the the peice of info that sticks out here. 4" is not much. ook for a vacume leak. If every thing else checks out- borrow a different carb and see if it is not better. Looking like a carb.
Dave T.
Daves Hot rod Shop
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Old 10-20-2008, 01:46 PM
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cam timing

First from my expierience, I THINK you've got it backwards on the heavy springs...... trust me I just got done going through this, big cams want LOOOTTTSSSSS of advance. If you are keeping the centrifugal advance active (not locking it) you are gonna need LIGHT tension springs on the weights so you can get "all in timing" (total advance) sooner, Id say atleast by at least 1800rpm's you need to be all in.

This is not info that just coming from me, this is from me and a lot of guys the know a H*LL of a lot more than me, some on here, some from lunati, some from comp cams engineering dept, and so on. You get the picture.

I just went through this with my cam: Lunati bracket master II (300 adv. dur. 246/246@50 515/515 lift 108LSA) and yours sounds even bigger. On mine I tried running 20 initial, and it ran OK, but after talking to several people, I decided my best option was to Lock out the centrifugal advance to the fully advanced point and set the base timing to 36 degrees. So im running 36 degrees at idle and it doesn't change with rpm ported vac. adv.

Theres my 2 cents worth hope it helps.... Good Luck!!!
Danny
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Old 10-20-2008, 02:04 PM
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define OK

PS when is said it ran OK, I just ment that it would stay running, not running good.

I would try and lock it out at 34-36 degrees if I were you. I had to put a spark interupt switch on mine wired with a toggle switch through the distributor power wire for hot cranking. Get her spinning then flip the switch and she runs like a dream.... 100 times smoother and more responsive...


Just a couple other things to check.... When you say the idles falling off from 1000 it makes me think of a vacuum leak kinda. So be double sure to track down every vac line and check for leaks.

Check the accelerator pumps or the carb. MINE runs best when the lever is just BARELY touching at idle. Make sure you have no intake or spacer leaks. You gotta get the vacuum up somehow. With only 4" hg the vacuum advance isn't going to work either. Are you even using vac adv equipped distributor???
If you are, all timing/advancing/carb adjusting should be performed with the canister unhooked and plugged.
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Old 10-20-2008, 02:11 PM
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The low vacuum witht he big cam is what leads me to believe there's nto enough compression and taht the 10.56 is a dreamt ratio- not a measured (most people's compression is always what they picture as ieal, not what they measure). If you don't have a vacuum leak- which is a real possibility but should have already been checked then run a cranking compression test as I stated and this will give us an idea of what you're working with. I'm guessing it may be as low as 110 psi- which is basically nothing.
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Old 10-20-2008, 03:59 PM
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As mentioned already be sure and check for any vacuum leaks. Also how much of the transfer slots is showing on the front vs the rear and how far open is the idle-eze? How far out are the mixture screws and where do you have the float level set? Lastly which fitting on the carburetor was used in taking the 4 inches of vacuum reading?
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