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Old 05-25-2012, 05:56 PM
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SBC Timing Questions

Ok guys. Have a feeling that I might get burned here for being a noob but here it goes. Just got back from FL a few days ago to NJ from picking up a 71 Chevelle. It has a SBC but the motor was supposedly "built" for street strip.

Drove it today to and from work 20 miles each way. Car will idle ok in N but is high - over 1200. In D feels like it wants to stall sometimes. Has a stock rear and is not geared for racing or torque. If I keep the RPM's up and I manually shift the trans it runs ok but feel like there is power missing. If I leave it in D the RPM's are low and I was getting a pop from time to time.

Also, when I arrived home today it "dieseled" for about 10 seconds when I shut it down.

I felt like it was getting too much fuel at first. I adjusted the screws to about 1 3/4 turns open from completely closed and it brought my idle to around 900. Took it for another drive and still felt some hesitation.

Got out the timing light and checked and at idle I'm sitting at 0 degrees with no vacuum. At 3500 RPM I'm at 20 degrees.

I was thinking that I should hook up the vacuum advance because it seems that my main problem is at cruising speed. I'm just a little gun shy because I'm being warned that I might blow the motor and in the fall I blew the top end on my last SBC.

I hooked up a hose from the dist to the vac on the carb and the idle did climb and I got nervous so I pulled it off.

I am completely self taught from the internet and you tube and just want to know how to do this right. I am not a mechanic by trade obviously just want to learn. I work on electronics for a living.

Here is what is written on the hand written "build" sheet:

750 Holley DP
Jets Front - 76, Rear - 86
P.V. 4.5

CAM 538 - 540
I-20 EX-22
Fuelie Heads

Timing 36 at 11-1200 Idle
RPM Range 3800-6600




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Old 05-25-2012, 08:54 PM
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I'm amazed it ran with the timing that low.
With vacuum advance disconnected and plugged, set the timing to 12 degrees before top dead center. Reconnect vac advance to a manifold source which is a full time source that pulls vacuum at idle. Recheck timing. It should be 20 to 24 degrees before top dead center.
Now the idle is going to go up a ways and this is what you want. Where the idle is set now the butterflies are off the idle circuit slots and pulling fuel from the main circuits. This is what's causing the engine to run on after you shut it off. Move the idle speed back down to around 750 in park, then check it in gear, in gear it should idle around 650 to 700. Next you'll want to adjust the idle mixture. If you don't have a vacuum gauge set the idle to where it idles the highest. If you have a vacuum gauge set it to where it pulls the best vacuum. Next, test drive. You'll be surprised at the power it picks up.
If detonation is detected, then you'll need to disconnect the vac advance and plug it, back the timing off 2 degrees then retest drive. Repeat until it no longer detonates.
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Old 05-26-2012, 05:08 AM
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Thanks for the reply. I'm gonna give this a shot today. I just need to check out there because I believe that there is no vacuum on the intake (victor jr). I'm guessing that I'll have to add one.

Question though. Do I have to take the vacuum off of the manifold instead of the carb due to the size of the cam? I just really would like to understand why I'm doing everything that I am and I know no one nearby that has a clue how to do this stuff.
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Old 05-26-2012, 06:37 AM
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Quote:
Originally Posted by jp1130
Thanks for the reply. I'm gonna give this a shot today. I just need to check out there because I believe that there is no vacuum on the intake (victor jr). I'm guessing that I'll have to add one.

Question though. Do I have to take the vacuum off of the manifold instead of the carb due to the size of the cam? I just really would like to understand why I'm doing everything that I am and I know no one nearby that has a clue how to do this stuff.
You can use the vacuum port on the carb that is BELOW the throttle plates. That will be full manifold vacuum.
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Old 05-26-2012, 06:52 AM
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Ok. It's 8:53 am and I'm sure my neighbors aren't going to appreciate this but I'm doing this at 9am.

I'll post my results.
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Old 06-04-2012, 08:15 AM
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[QUOTE=DoubleVision]I'm amazed it ran with the timing that low.
With vacuum advance disconnected and plugged, set the timing to 12 degrees before top dead center. Reconnect vac advance to a manifold source which is a full time source that pulls vacuum at idle. Recheck timing. It should be 20 to 24 degrees before top dead center. QUOTE]

Ok, I tried doing all of the above and felt like it was going well. After a few minutes I went to start the car again and got a small fire in the top of the carb. Blew it out and called a guy I know and he said to bring the car in.

I had it towed there. Now here is what he wants to do and I'm not sure I'm ok with this.

First he says I have a blown power valve and wants to rebuild the carb and put in an anti-backfire kit in it. (I'm ok with that but I think if I follow the directions, I am capable of rebuilding it).

Secondly, he wants new wires and plugs for it and then he says he will fully advance my timing to 36 and shut it down. Then he wants to drill and tap for a fitting so that he can use ported vacuum, not manifold vacuum.

From everything that I've been reading on the boards here I'm thinking that I should bring it home and start over. I'm just another guy on a budget who has been trying to learn for the past few years and when I saw that fire I started to think that I was going to destroy the car.

Any ideas or suggestions?

I ordered the carb rebuild kit and everything from summit and it will be here tomorrow so I could have the car towed back home easily.
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Old 06-04-2012, 08:29 AM
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I'm curious, what is an "anti backfire kit"?
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Old 06-04-2012, 08:36 AM
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Quote:
Originally Posted by 68NovaSS
I'm curious, what is an "anti backfire kit"?
I guess it is a check-ball that goes in place to prevent blowing another PV.

I'm sure there is another name for it though.
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Old 06-04-2012, 01:53 PM
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Not sure what that cam is all about, I assume 538/540 lift and what about that duration 20-22? what is the advertised duration and the @ 0.050 duration?

With that kind of lift you most likely have a healthy duration in the 230-240 @ 50 range. In that case to get it to idle correctly in drive as compared to park you will need a minimum 2800-3000+ stall speed on the trans. other wise there will be a big drop in rpm when you put it in drive. Double pumpers like four speeds and gears. 4:11 + gears out back will be the best money spent if you keep that cam.

To correctly set timing >>> Verify TDC first, you can have slipped balancer or worse, do this FIRST:

http://www.crankshaftcoalition.com/w...op_dead_center

Drill for a ported vacuum advance connection? that sounds like a folly in the making, where abouts will this be located above the throttle plates? you cant just drill a hole in the carb

The engine will most likely run better with manifold vacuum advance anyhow to the tune of 10-14 deg added by vacuum advance. The thing could end up with 35 deg btdc at idle, so be it give it what it wants, but we need cam specs

Yeah start over, do the TDC verification and post what you find, also get a vacuum gage for tuning purposes and make a timing tape for the balancer if you dont have a dial back timing light.

http://www.crankshaftcoalition.com/w..._a_timing_tape

Do it, spend the time make the effort and then once you master it you can be the go to guy in the area
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Old 06-05-2012, 05:31 AM
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Thanks for all the help. I'm going to get my car today and I'm gonna do this right.
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Old 06-05-2012, 05:42 AM
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I had a pretty healthy cam in my sbc with a stock converter and I could not get the thing to idle correctly between park and drive. Only way I could get the curb idle speed where it should be was by having idle in park be like 1250-1300rpm.

I agree with the other guys on here. chances are with those lift figures you have quite a bit of duration along with it which will require more base timing and more converter and rear gear.
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