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Old 12-05-2013, 05:33 PM
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tuning inline carbs

Hey guys I got a 61 chevy pickup with a 1968 396 I bought the motor from some guy who said it was a stock rebuild but the intake, its got inline 500 cfm carbs I set initial at 12 with vacuum plugged off but when I plugged it back in there was no change which raised a red flag, I kept on advancing the timing all the way to 25 and it sounds alot better, but hesitates to start I'm starting to wonder if maybe the timing chain is a tooth off? or maybe the distributer? what do ya think? I did check the plugs after a run and they seem to be burning good
maybe I'm just being paranoid.

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Old 12-05-2013, 06:39 PM
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is the timing mark on the balancer varified to the covers marker?
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Old 12-05-2013, 07:21 PM
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is was all assembled when I bought it, its a aftermarket balancer with timing marks on it and the timing cover just has one of those cheap chrome tabs, also I noticed the motor runs alot hotter at idle when its at 12 degrees then 20 degrees
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Old 12-05-2013, 11:39 PM
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It will run hotter with retarded timing. What Vinnie meant was have you verified the TDC and pointer are aligned when the piston is at TDC on the compression stroke.
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Old 12-05-2013, 11:51 PM
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About the timing not changing when you reconnected the vacuum advance hose, is it connected to manifold vacuum? If so and it didn't change, I'd check out the vacuum canister/diaphragm and hose, pull a vacuum on it and see if it holds.
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Old 12-06-2013, 05:30 AM
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I dont know exactly how to find tdc with the engine assembled I can get it close by watching the rocker arms I think
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Old 12-06-2013, 06:28 AM
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What 68Nova is saying is taht you are connected to 'ported' vacuum and you probably aren't getting enough of a vacuum signal to affect the timing at idle, esp with more than one carb as you imply. Unhook the hose from the carb with engine idling and use a mightyvac to add vacuum to the distr and see if the timing changes. generally, if the vacuum module goes bad it becomes a vacuum leak and you'll have a time of it to get the idle adjustments correct, like the idle richness screws will be ahead of the idle speed etc.
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Old 12-06-2013, 09:44 AM
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what cam is in this motor.
A hiperf cam will want a good bit of initial timing @idle
25ish deg

thats why it runs hot @ 12 deg
Recurve the distributer to allow the yet maintain 36-38 deg total timing

Those chrome timing tabs are always out

Find true tdc with a piston stop


then you can sync both carbs (throttle opening and idle mix adj))

then use ported vac advance on the primary Carb
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Old 12-06-2013, 02:08 PM
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Quote:
Originally Posted by 23 tbucket View Post
I dont know exactly how to find tdc with the engine assembled I can get it close by watching the rocker arms I think
http://www.crankshaftcoalition.com/w...op_dead_center

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Old 12-06-2013, 05:49 PM
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so I was messing with the truck tonight and I was on ported vacuum on the secondary carb, so I switched to full vacuum on the primary and I still have no change on the timing if I unplug the distributer
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Old 12-07-2013, 03:23 PM
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How much manifold vacuum do you have at idle, with distributor disconnected?
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Old 12-07-2013, 05:34 PM
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Quote:
Originally Posted by 23 tbucket View Post
so I was messing with the truck tonight and I was on ported vacuum on the secondary carb, so I switched to full vacuum on the primary and I still have no change on the timing if I unplug the distributer
What PatM asked, what is your vacuum at idle?

If you have decent vacuum, and you're sure you're getting manifold vacuum from that port, it should be altering timing when reconnected, unless the pot is not working or the distributor is limited internally, maybe with a screw, spot welded, rusted or corroded. Are you sure the hose doesn't have a hairline crack in it? Get a cheap vacuum pump and test the hose and pot, see if both will hold a vac.
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Old 12-09-2013, 05:17 PM
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bought a mighty vac today to check the distributer and its not holding any vacuum so hopefully thats the issue of course the part wasn't in stock
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Old 12-09-2013, 07:01 PM
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So, connecting your distributor with the vacuum line was essentially creating a leak in the vacuum line, and NOT advancing the timing. Good find.
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Old 12-10-2013, 04:07 PM
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put my new vacuum canister on tonight reset initial timing at 12 hooked up the vacuum advance and it jumped to 30, and at 3000 rpm goes all the way to 51!
its pulling 16hg at 800rpm how do I get the timing down to 36 at 3000?
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