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Old 06-18-2011, 06:20 PM
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Idle Issue vs. Cam Selection

Hello again everyone, Jay Gadsby here. Recently, I got the engine running in my boat, all is well there. Got the timing right where it needs to be etc. But, I cant get the idle below 1000rpm no matter how hard I try. The idle speed screw is barely touching (just enough to keep some pressure on it.) As per the holley initial adjustment guide, said to lean out the mixture screws and back off 1.5 turns. Did so, not enough to keep the engine running. A little more fuel and it gets happier. My question is this. I have a Crane Cams p/n 114201 .450 lift on both, 284 duration and 114 separation. This is installed on a SBC bored .060 over, balanced with flat top pistons, vortec heads, and a Holley 4150 600CFM marine carb. According to the local engine builder the CR is about 9.3:1. Ignition is a Thunderbolt IV that requires 8* BTDC, and thats where she sits. Is it possible that its too much cam to get the idle and lower without stalling it out? Is that a possiblity? Oh, and I sprayed the entire silly intake with carb cleaner and so far no idle changes. So I am guessing no vacuum leaks. Is there any obscure areas I need to check for that? Any one have any ideas? Thanks in advance guys (no sort of pun intended there).

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Old 06-18-2011, 06:22 PM
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DId you ck the secondaries idle stop?
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Old 06-18-2011, 06:45 PM
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Yes I did. Does this carb require any sort of vacuum reference or anything. On the backside of the carb, there is a ridiculously large capped off port. It is rather hidden with the coil there and the distributor. The cap slid off rather easily and I gonna put a hose clamp on it and see. The rochester setup on the mercruiser that used to reside in the engine bay had no vacuum lines period. Thanks again!!
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Old 06-18-2011, 08:14 PM
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Remove the carb and flip it over look at the idle transfer slot exposure under the throttle edge at idle and reset the pri and sec throttles so the idle transfer slot expose at idle is right. .020"ish +/-

reinstall the carb. This is your baseline idle throttle opening setting. Give it all the idle timing it wants. (more than 8deg)

That camshaft wants more than 8 deg timing at idle. rev up the engine and set the timing at full maxed out advance at 36deg BTDC. What is the idle timing now, in gear? Needs to be 14 to 18deg at idle while idling in gear. (around 700rpm) Check the idle timing while idling in gear.

Make sure max timing at high rpm does not exceed 36deg BTDC.

Using This amount of WOT max timing in a boat needs the best available high octane street premimum pump gas. 92+octane

If you want to run it on crap marine gas use less WOT timing (30-33deg)

87 octane may only allow 28deg timing at WOT.

8 deg inital base timing at idle is not enough . I believe the V6 mercruiser thunder bolt ignition has a shorter internal electronic advance curve (and a higher rev limit) which will allow more idle timing, while avoiding excessive WOT spark advance.

The other option is to convert to a GM HEI distributor and dial in the mechanical centrifugal advance. The vacuum advacne will not be used on a boat.

Warning::: if you insist on running it on 87 octane fuel don't get nuts with the WOT timing.

Last edited by F-BIRD'88; 06-18-2011 at 08:20 PM.
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Old 06-18-2011, 08:28 PM
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Thanks for the advice. As far as the 8* initial timing, I set it to the Mercruiser standard and supposedly it is going to add 22* at max that should be all in by 3600rpm from all I have gathered. I will look into all that very soon. Right now it is idling at 900rpm (750 is the high side of stock). I will have to pull off the carb tomorrow and get it adjusted up. Thanks again!
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