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Discussion Starter #1
I'm trying to start a motor rebuilt 10 years ago. It will briefly run, then back fire through the carb. I have checked and rechecked the timing. I have it set to fire a little after TDC. The Motor is a 350, AFR heads, 11 to 1 comp. Holley 700 cfm with a performance cam. I'm using 91 octane gas, Stock point distributer. I'm beginning to pull my hair out! Help me out guys!
 

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I have it set to fire a little after TDC. The
This is the problem. If your timing is too retarded, it will run rough and may want to die near idle. Usually, you have to keep fooling with the throttle to get it to run good.

First off, make sure you know your timing indicators are properly aligned. There are a few variations in pointers and dampeners. Someone recently found theirs was nearly 15* off. So pull the plugs, insert a piston stop in #1. Rotate the engine clockwise until the stop is met and make a mark. Then rotate the engine the opposite way until the stop is met and make another mark. 1/2 way between the marks is TDC. I like to use a piece of 1/4" copper tubing as a piston stop.

Base timing should be set with vacuum advance disconnected and engine idling properly. 8*-12* is the ballpark for base timing.

Let us know if that does anything for you. Good luck with it, sounds like you're almost close. Keep your face and arms away from the carberator :eek:
 

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kdaniel,

you said you have it set to "fire a little after tdc". all motors are supposed to fire BEFORE tdc. set you timing to about 6-8 degrees BEFORE top dead center.
 

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Discussion Starter #4
Thanks guys, In an old Chiltons book I bought for a 71 Camaro it listed the timing for a '69 350 4bl Manual at TDC. I wanted to be alittle safe and start a few degrees retarded. I took your advice and advanced it and I got it going for about a minute. I like your idea using the copper tube. I'll try it tomarrow. What changes the timing on different motors? Is it the cam? I also have a Fluidampr for the harmonic balancer and I noticed it has two degree spacing from 50-30*BTDC and only 10* marks from 30-0*BTDC and 10*ATDC. I don't know why the fine lines on the 50-30?
 

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The timing marks changed on both the damper and the timing chain cover between 1969 and 1970. If you mix them up you can end up with timing marks that are off by about 20 degrees. Depending on what you mix, they can be off either advanced or retarded.

The timing marks get finer so you can setup your total mechanical and vacuum advance. Most mechanical advances add around 30 degrees of advance by 3000 RPM, so you take the 30 plus the 10 to 12 you will start with (car runs lousy with the stock timing of 4*BTDC) and you end up around 40*. I think 38* total advance, all in by 3000 RPM is about what you want.
 

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Discussion Starter #6
I got the engine running, but now have a problem getting it to idle. Advancing it made quite a difference.
The motor seem to like a more advance timing. As I try to lower the Idle adjust screw, it starts to lower, then wants to die. I have to work the throttle to keep it going. I am a little worried that I have radiator fluid leaking from a few of my head studs(not bolts), but other posting made me feel this is somewhat normal for head studs and it just takes work to seal them. Vacuum seems low around 10 inches. Too many problems
ahhh!
 

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trace all your plug wires from cap to plug, make sure they are wired for correct firing order. make sure number one wire is in the correct tower and go from there. one tower off and all kinds of neat stuff can happen. also with engine off rotate the crank to 0 deg. tdc for number one. pull your cap and see if the rotor lines up exactly with the electrode in the cap. your distributor may be off one tooth. if so your dist. will be turned quite a bit. port on vacuum adv. pod should point almost straight forward (parallel to valve cover) with the rotor dead on number one tower in the cap. also you said it was a rebuilt motor. she'll be tight and may not want to idle down yet. is your vacuum gauge jumpy or rock solid?? cam specs may be cause of low vacuum, big cam-low vacuum. and last but not least your cam may be off one tooth (timing chain) to your crank. you would have to pull left side rocker cover and check intake valve opening on number one cyl. in relation to you crank timing marks. and also check your harmonic balancer, sometimes the rubber ring slips and the timing marks are no longer in the correct place. eliminate all the easy fixes first. then dig deeper. good luck!!
 

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Originally posted by kdaniel:
I got the engine running, but now have a problem getting it to idle.
Are you using vacuum advance? I'd recommend it if not. Base timing between 8 and 10* is a good starting point. Vacuum advance will help the idle some. I think it's time to start tweaking the idle mixture screws. Also, make sure it's warmed up. Choke adjustment will also play into this. Use a vacuum gague to adjust the idle mixture. Start with each needle valve out about 1 and 1/2 turns. Start backing each side out 1/2 turn at a time while watching the vacuum guage and observing idle speed. If the idle speed gets too high, back off the speed adjustment and continue with the mixture adjustment. When you get too rich, the vacuum guage will drop slightly. Turn the screws back in until the vacuum is peaked and the engine idles best. Then after the motor is shut down, I like to run the needles in counting turns on each. I then back both out the average of the two settings (which should be very close). Oh, and never tighten the needle valves. They are only meant to shut off fuel and should only be in contact with the seat at 0 turns.

If this doesn't work, I'd suspect a blown power valve from back-firing. Does it run stinky rich?
 

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Discussion Starter #9
Wow, thats a lot of info. My problem is, I have too many variables. I've never work on a "modified motor", so I'm not sure about their differences from stock. The motor, body, distributer, carb have never been used together. Iv'e tripled checked the plug order. The dist. vacuum is angled toward the #4 cylinder, so I maybe off a tooth, but I don't think that should matter. Running at about 1000rpm the vacuum is around 10 and goes up upon acceleration.(Fairly solid). The motor was professionaly built. The harmonic balancer was new 10 years ago so I can't imagine it's off.

Dnut: I have not been using the Vac adv. I'll try it. No choke. The mixture screws seem to have little effect, but isn't that because I have the rpms past the idle stage? Any lower and it dies. Blown power valve? In the secondaries? What would that do? I did have a lot of back firing initially. What should the vacuum read on a modified motor? Isn't it relative to rpms?
 

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What's your cam specs? 10" of vacuum at 1000 RPM is awfully low unless you have a huge cam.

What kind of timing pointer do you have on your timing cover, the kind that is welded to the cover, or the one that bolts on using two of the covers bolts?

Is then timing tab really close to the damper, or is there a good sized gap between the outer circumference of the dampener and the timing tab?

Where is the engine timed at the moment 12*BTDC?

Seems like you need to advance your timing a bit to bring vacuum up.
 

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that carb, after sitting ten years may just need a rebuild. all the gaskets and diaphragms could be dried up and stiff.
 

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True about the rebuild, I hadn't considered the age. But what you are experiencing sounds normal. I'd say you aren't far off course so hang with it. You just identified another problem - the mixture screws have little effect. Vacuum advance will help this. It will raise the idle RPM and allow you to drop the primaries with the idle stop so the idle circtuit works again. After vacuum advance you may need to open the secondary throttle blade stop 1/2 to 1 full turn. You have to remove the carb to get to it though. Its on the right rear throttle shaft boss.

My edelbrock performer RPM cam gives me about 10" also so I can imagine just about what you're faced with. 700 CFM is a fairly large carb and it's going to be tweakier than say a 600 or 650 CFM on a 350 engine. But you can make 700 CFM work fine. Go to the root URL of this site and search the tech reference. Series II I believe has a Holley car & feeding article you might enjoy.
 

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Discussion Starter #13
Here is the specs on the cam: lift .440" intake
.414 Exhaust. [email protected] .050" 266*intake, 256 exhaust. Intake open @ 24* BTDC @ .050"lift, close @61* ABDC @ .050" lift. Lobe center 108*.
Exhaust open 58* BTDC @ .050, close 18 ATDC @ .050". Lobe center 111*. Solid Roller. Victor Jr. Manifold.

I did do a recent rebuild on the carb. The timing is reading somewhere around 20, but I think the mechanical advance is kicking in.
I'll need to bone up on the secondary throttle blade to understand what you are getting at.
 
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