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Old 05-28-2012, 06:05 PM
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new cam wont idle sbc

hey guys...I got a 68 Impala w/ a 350 V8, TH-400, and 4bbl carb. I wanted to add some horsepower and underwent my first engine build. I put a new cam/intake/carb combo (Edelbrock 2102/2101/1901). I broke in the cam but you have to give it gas or the engine just dies out. Starts up cold good (2-3 minutes), but as engine warms and cold choke opens, the min. rpm's it will operate is 1500 without stalling/dying. Can't get it to idle like before...so I am stumped. I lined dot-dot during install, then rotated crank 360 before putting in dizzy. I adj. valves, put new exhaust springs and valve seals. I recently took out dizzy again, added accel point eliminating kit, found tdc w/ thumb, and installed dizzy. Same problem, cant time since it dies as I try to adjust dist. cap and bring near tdc. The closest timing is >14 ATDC when it first starts. Did I need to degree the cam before installing? I put a edelbrock timing chain set during cam install and instructions said straight up timing is achieved. Any help appreciated, thanks!

Cam Style: Hydraulic flat tappet
Basic Operating RPM Range: Idle-5,500
Intake Duration at 050 inch Lift: 204
Exhaust Duration at 050 inch Lift: 214
Duration at 050 inch Lift: 204 int./214 exh.
Advertised Intake Duration: 278
Advertised Exhaust Duration: 288
Advertised Duration: 278 int./288 exh.
Intake Valve Lift with Factory Rocker Arm Ratio: 0.420 in.
Exhaust Valve Lift with Factory Rocker Arm Ratio: 0.442 in.
Valve Lift with Factory Rocker Arm Ratio: 0.420 int./0.442 exh.
Lobe Separation (degrees): 112

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Old 05-28-2012, 06:18 PM
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The spark timing is too retarded. Dis connect the vac adv. rev the motor while watching the timing. Set the timing at 34-36deg btdc while reving the motor high enough to max out the timing curve. 3000+ rpm It should idle at 14-16deg.
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Old 05-28-2012, 10:02 PM
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thanks...do I need an adjustable timing light to do this? I checked the compression and it ranges from 120-130 for all cylinders.
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Old 05-29-2012, 05:56 AM
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Vacuum Gauge

I like using a vacuum gauge connected right off the manifold for setting timing and adjusting the carb.

It always seems for me to work the best and it's simple.

Jimbo
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Old 05-29-2012, 08:10 PM
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I bought an adjust. timing light and set advance to 34...then timed @ 3000 rpms. It sounds good at high rpm's but as I let off the gas and try to idle it dies/backfires out of the carb. I've tried messing w/ air/fuel mixtures screws but no help. Damn...I am about ready to give up and pull the cam out! Any help out there truly appreciated bros. Thanks!
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Old 05-29-2012, 10:36 PM
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does anyone know what the symptoms are for the cam being off one tooth?
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Old 05-30-2012, 01:34 AM
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Try backing off the valve adjustment 1/2 turn and see if it changes anything.
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Old 05-30-2012, 09:38 AM
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Quote:
Originally Posted by reyes134
does anyone know what the symptoms are for the cam being off one tooth?
For now, forget about the total timing. JUST for now.

Set the initial timing (with vacuum advance disconnected) to 14 degrees BTDC. Hook the vacuum advance back up to full time (aka "manifold" vacuum). Readjust the idle speed to around 800 rpm or so, or to where you like it. BTW, the vacuum advance can may be giving too much advance- you only need about 10-12 degrees from it, an adjustable can may be the ticket- but for now, run what you have.

Reset the idle mixture screws to give the highest idle speed, and readjust idle speed if needed.

Now, see how the idle and off idle response is. No need to worry about the total timing being too high- you're not going to be driving it yet. Don't run the engine under load until you have seen the total advance is 34-36 degrees or less w/the initial set to 14 degrees.

To get the right timing curve, the distributor may need modifying. But if the engine refuses to run right w/these settings, the cam may be out of phase.

You can check the cam phasing by using a dial indicator and a timing tape on the damper, but before doing that, see what you get by doing the above.
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Old 05-30-2012, 07:31 PM
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Quote:
Originally Posted by JeffB
Try backing off the valve adjustment 1/2 turn and see if it changes anything.
I readjusted valves...only did 1/4 turns this time. Still same thing. I am thinking about pulling off timing cover to verify dots/tdc...guess that's my only option now.
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Old 05-30-2012, 09:36 PM
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Quote:
Originally Posted by reyes134
I readjusted valves...only did 1/4 turns this time. Still same thing. I am thinking about pulling off timing cover to verify dots/tdc...guess that's my only option now.
If you believe there's any chance the dot-to-dot alignment (aka "cam phasing") is off, that's a good move- providing the firing order is correct and you are timing the engine off of cylinder #1 (or #6) and not another cylinder. Double check everything before pulling it back apart.
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Old 06-01-2012, 01:57 PM
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So i pulled off the timing cover and here's the deal. dot-to-dot lines up at crank (12oclock) and cam (6oclock)...this is tdc @ exhaust.
rotate crank till air pushes my finger thru spark plug#1 so tdc @ compression...crank gear and cam gear both at 12 oclock w/ dist rotor pointing to #1 spark plug which is #1 cylinder that starts firing order. Harmonic balancer was pointing to tdc.
This all seems correct...can you verify this? I gotta be missing something...why wont it idle at low rpm's?
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Old 06-01-2012, 02:07 PM
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Have you sprayed around checking for a vacuum leak. either that or the carb is out of wack has anyone been adjusting on it ?

Cole
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Old 06-01-2012, 05:28 PM
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That camn sounds in phase. Move on to something else. What is the carb situation?
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Old 06-01-2012, 07:14 PM
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Does the crank gear have multiple keyways for advancing and retarding the cam? If so, attention has to be paid to which keyway is used along w/the one correct dot that sets the alignment.
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Old 06-01-2012, 07:24 PM
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Quote:
Originally Posted by cobalt327
Does the crank gear have multiple keyways for advancing and retarding the cam? If so, attention has to be paid to which keyway is used along w/the one correct dot that sets the alignment.
X2 this could very well be the problem.


Cole
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