Initial timing carb popping when cold - Team Camaro Tech
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post #1 of 8 (permalink) Old Dec 30th, 01, 08:55 AM Thread Starter
Join Date: Jan 2000
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I just finished setting my timing with a dial back light. I set it at 16 BTDC which is about 40 degrees less that what it was at previously. Before doing this I verified the timing mark on the balancer with a piston stop, since I was going to make such a big change in the timing. Now when the engine is cold I get an occasional pop through the carb when I give it a quick throttle from idle. Here is some information on the engine, BB427, Mechanical cam with .024 valve clearance, Holley 850 DP 76 Pri 78 Sec, Single Plane Intake, Mechanical advance distributor Mallory Comp 9000, Taylor wires, Accel coil. The cylinder pressure is 130 on all cylinders. I have about 36 degrees total advance around 2800. How much initial advance should I be running. The car starts easily, but the popping concerns me. I can advance the dist and the idle increases dramatically without any other changes. The car ran fine before I set the timing back to 16 BTDC, but was really hard to start. Any suggestions.

67 SS 427
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post #2 of 8 (permalink) Old Dec 30th, 01, 01:00 PM
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does it have a choke? if not, maybe it needs one. how much mechanical advance does the dist have in it? how fast does it come in? what is the fast idle set at? idle mixture screws? i would baseline everything. idle mixture screw 1.5 turns out, choke set properly. i know i aint much help this time, but maybe if you start over it will sort itself out.

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post #3 of 8 (permalink) Old Dec 30th, 01, 01:35 PM
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Colder air is more dense so a cold cubic foot of air would need a little more fuel than a warm cubic foot of air. This is why cool air makes more power. This colder air is causing your engine to run a little leaner than it would in the summer time. I had a double pumper that did the same thing in the winter time. You might want to try turning the choke toward the richer side (if you have an electric choke) or, if you don't have a choke, go up a couple of sizes on the mains. That is what I had to do with my double pumper.
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post #4 of 8 (permalink) Old Jan 3rd, 02, 02:53 PM Thread Starter
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Here is the latest on the problem. I went through the carb today and set everything back to the specs from the box. 80 jets in primary and secondary. The power valves are good. I did notice that the primary throttle blade was exposing about a 32nd of an inch of the idle transfer slot. I just got it back together and it is still popping. The lag is not there, so the jetting must be going in the right direction. I will try opening the secondaries a little bit to allow me to close the primairy blade and keep the transfer slot unexposed. I am wondering if the pump shot is not long enough to allow the main system to start and that is why it is backfiring. Could it be too lean between the idle transfer and main circuits. What really stumps me is that when I had way to much initial timing 40+ degrees it never backfired, it was just almost impossible to start.

67 SS 427
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post #5 of 8 (permalink) Old Jan 4th, 02, 05:05 AM
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I would try opening the secondaries up just a little to try and keep that idle transfer slot from being exposed at idle like you said. You can also drill small holes in the primary throttle blades to accomplish the same thing. If that doesn't work, start playing with the accelerator pump. Your getting closer, just keep trying.

What is your timing set at now? If your motor can handle it, you might want to try advancing it a little.
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post #6 of 8 (permalink) Old Jan 4th, 02, 10:08 AM
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Have you adjusted the idle mixture screws for most vacuum or highest idle? Don't forget to reset or at least check your timing after making carb adjustments. 16 degs initial when idling at 1000 rpm will equate to less initial at a lower rpm. Idealy you want to set the total timing for optimal performance without detonation. If 36 total is all your engine will take without pinging and 16 inital is what that equates to you don't want to increase it with mechanical advance. You are disconnecting the vacuum advance and plugging it when setting the timing I assume.

One way to increase your initial without effecting the mechanical and total is to move the vacuum advance to a manifold source (one that provides vacuum at idle). Of course it would be best to have an adjustable vacuum canister so you can limit the amount of vacuum advance it pulls into the equation...

A little cold stumble is tollerable if runs well when warm. It will keep you from jumping on it when cold as well! You do want it drivable within a min or three of starting unless it's very cold out!!

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post #7 of 8 (permalink) Old Jan 4th, 02, 12:22 PM Thread Starter
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I think that I found the problem. I opened up the secondaries a little bit and readjusted the idle mixture screws. I need to get my vacuum gauge back from my brother and set it with the gauge. I also changed the primary pump cam to setting 2 it is the red cam with three settings. I also found that the accelerator pump adjustment spring was compressing instead of providing a pump shot if the trottle was only moved a little bit about a 1/4". I bought a spring from the hardware store that was a little longer and readjusted the pump. It seems to be running better now. I drove it for about a half hour with no problems. I pulls much harder down low than when it was jetted lean. I hope that it is fixed. I will let you know if the problem comes back.

67 SS 427
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post #8 of 8 (permalink) Old Jan 4th, 02, 01:20 PM
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A few things come to mind that might help..
I would double check the TDC index once more with the piston stop..even though you did this EXTRA thorough and force yourself to check this once again and this time put on some degree tape on the balancer if not already have it..
I assume you have NO vacuum advance on distributor...
I assume you degreed the cam on install and it is not off-set installed??
Check all vacuum points for air leaks...such as intake manifold gaskets ect...
Do you have EGR plate or other emisions stuff??
Make sure to loose the PVC if you are using one..this WILL dilute the intake charge with crankcase fumes of oil and gases....put a breather in that hole instead then plug the hole at the carb..
Make sure base of carb to intake gasket is clean, flat and properly well as all screws on carb are secured..such as base-plate to body...
Check SECONDARY throttle plate clearance closed...there is a screw adjustment for this on the side...normaly this screw faces down and tough to get at..I reverse the screw so I can get it from top-side...make sure the secondary throttle plate is fully closed at rest turn the screw till it just makes contact then make a 1/8 of a turn more..
WHat brand of plugs are you using and what is the # of the plug ???
Also..Make sure your ignition (spark-plug) wires are properly routed and fitted in the seperators...if you get cross-coupling from poorly drapped wires, this can cause spark and poping problems....also check for bad wires in the dark with engine reving..
What vacuum are you getting at idle and what size power valve is in the carb???
Make sure the timing light is indexed to zero degrees on the back dial..
Here is a base-line starting pont for setting up your ignition..
Set the initial crank base timing to 12 degrees BTDC...and try get this car to idle in the 700 RPM will now need to get the idle screws adjusted with vacuum gage at max carefull NOT to use to much idle screw.. also choke should be set when motor is dead-cold have it open about 1/8" ...
Set distributor springs to a start curve at 1000 RPM and finish at 2800 RPM ...steep slope..
Set the advance stop so overall timing is maxed-out by can increment that up later when you test drive it and listen for detonation...ultimately you need to max that till pinging..then back off typically when using pump gas... normally with track gas you back off when you feel loose of power when you go over the power curve..
Now there is much more in the way of fine tuning after this...plugs, jetting, timing..ect
But this should give you a baseline to start from with no back-fireing from the carb..
If you still experience this poping from the carb after re-setting things...check back and hopefully someone here can provide additional suggestions...


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