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253 backfiring through carb

Hi I have put together a warm little 253 over the past couple of years. It's got all new parts and had a full machine and rebuild. The engine is installed and the timing is roughed in with the vac advance blocked off. The engine will idle a little at about 400rpm for a few seconds but then it stalls with a backfire through the carb. It has a 570 vac sec street avenger on it full msd ignition, warm hyd cam with roller rockers, big valves (1.82" inlet, standard exh), and is running about 10.5:1 comp. does anyone have any idea what might be causing the backfire. I have tuned the idle on the carb a bit to get it to idle as it is. Before that it wouldn't run below 2000rpm. Any help would be great.

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idle with standed engine with manule gear box is about 850 rpm with vac line hooked up. yours with warm cam and 10.5;1 comp should idle at about 1000 rpm with engine temp warmed up. with high comp you might have to use the higher optane fule and retard your timing a tad to stop egnine knoking or pinging.

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Holden fan it is running on bp ultimate 98 it isn't pinging or anything. As to the idle rpm, the 400 or so rpm I'm getting is all it will idle at even if I wind the curb or fast idle up. At one setting it would only run on the secondary's 2500+rpm and it was crisp and sweet to rev from there. When I gently let it idle down with the pedal as soon as it hit 2000 it stalled. Where I assumed it came back into the primary's only.

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is your primarys geting fule, are your idle screws ajusted out about 1.5 turns. i dont know the holley you got but some carbys have a 12v fule cut. but i never seen one on a holley but then again i havent see any new types of holleys of late. might pay to check

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Hi,have you checked your vacume ,set timing at 10 degrees.
A healthy Standard engine will run 16 to 20 inches of vacume.

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I haven't checked vacuum a I don't have a gauge. I will get one and give it a try and see what I find. Any suggestions in brands of gauge?

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Sometimes there is more than one thing causing these sort of problems, maybe check that you have wired the ignition switch to coil wiring the right way for electronic ignition (see Holdepedia).

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Grammar: The difference between "knowing your shyte" and "knowing you're shyte".

The vacume will not stop it backfiring,but you have to get it idling to troubleshoot,you need vacume at idle to get the primary circuit of the carby working.My history for a engine backfiring was timing out,check dizzy position on #1 TDC,check leads ,check valve clearances.

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I have confirmed that tdc on cyl 1 lines up with the timing mark on the balancer aswell as rotor is pointing at lead 1 on the cap. It has a msd streetfire ignition so it's all simple wiring. The only thing the car brings basically is the plus and minus from the dizzy and 12v constant and through the key. I have taken the msd out and run it as a standard ignition and it does not make any difference. This is what has lead me to it being a carb tuning issue

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but is #1 at TDC on the compression stroke?

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Grammar: The difference between "knowing your shyte" and "knowing you're shyte".

Hi yes I have checked it's not 180 out. I put my finger over the plug hole and wound it to tdc and it's blowing air out past my finger

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but you will get air over your finger on both strokes, compression stroke is a strong puff and exhaust stroke is a lighter puff.

If you want to be sure pull the rocker cover off and check to see that both valves on #1 are shut, rocker arms right up.

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Grammar: The difference between "knowing your shyte" and "knowing you're shyte".

Can be caused by Crossfire in the Distributor Cap, Air Leaks or worn out Cam Lobes.

Some model NGK Plugs suffered from Centre Electrode breakdown which also caused Backfiring.

Also V8 Plug Lead related Problems are worse if the narrow Distributor Cap Distributor is used as opposed to the larger Cap Distributor.

Photos ...

http://holdenpaedia....
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From this page... http://holdenpaedia....

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T

My Shed

Holdenpaedia, they're the brakes Mate.

Ok so I have pulled the rocker cover off of cyl 1 and at tdc both valves are closed and when I continue to turn in the direction of rotation the exhaust is next to open. Which is right. I'll get a vac gauge on pay day to check for leaks. Does anyone in Brisbane (Logan area) know of any good carb tuners to check this thing over without costing an arm and leg?

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The Plug Leads may be in the wrong locations ... http://holdenpaedia....

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T

My Shed

Holdenpaedia, they're the brakes Mate.

I assume you fitted a new (good quality) timing chain set, or gear drive, and a set of new, or reconditioned, lifters with your "warm" cam? You didn't mention fuel pump? Holleys, including Street Avengers, needle & seat can only handle up to 8 p.s.i.- If you've fitted a high volume pump that puts out more than that, without a regulator, your Avenger will be drowning. When you do get it idling, you'll need to check your float level too.

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Yes everything on the engine is new quality parts except for the block, crank, rods and heads which are all reconditioned and machined. I am running a aeromotive electric pump regulated at 6psi. The leads are in the right places. I have checked it a few times to the manual.

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Is it manual or electric choke?

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It's electric choke

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Is it connected and operating/opening correctly?

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I'm not 100% sure. It has voltage and seems to move a bit in the short time the engine will run for but I have never had a carb with electric choke on it before to know what to compare it to.

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The manual choke may have been a better choice..... Check that the choke air-intake (directly behind the thermostat) isn't blocked off. The thermostat needs a full 12 volts to function correctly, and must not be connected via the coil or ignition. It needs to go to another 12 volt source which is activated when the ignition is on. You may need to adjust the choke "fast-idle" screw (below the thermostat). You can also adjust how soon the choke opens by loosening the three screws on the thermostat cover and rotating the cap: clockwise for sooner/anti-clockwiise for later. I always remove the choke assembly, but I'm not in a cold climate. It could be that your "warm" cam thinks that your electric choke isn't so hot!? Good Luck!

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PS. Depending on the specs of your cam, it is also possible that the Power Valve is opening prematurely. The "Catch 22" is that you need it idling, at operating temp, to get a vacuum reading. The stock size in the 570 Avenger is an 85. If your idle vacuum is below 17"Hg, you'll need to go to a lower number Power Valve, eg. 75 for 15-16"Hg; or 65 for 13-14"Hg, etc. If it's less than 13, you may need to advance your timing, or look for a vacuum leak.

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I think I have figured it to be an air leak. I put my quadrajet which I know works on this afternoon and it still does the same. Will the quady act the same as the Holley with a manifold air leak? It fires like it wants to start but go past the first sputter.

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