350 sbc wont idle

I Have A 1972 Nova 350 Sbc 600 Cfm Carb Four Barrol Carter Electronic Fuel Pump Competition Cam I Have No Specs On The Cam So This Is My First Post First Time On A Thread I Read Them All The Time Tho Im 17 Years Old And Baffled That My Beautiful Car Has An Issue So My Issue Is My Car Wont Idle And Its Weak So I Have To Hold The Gas To Keep It Running When I Put In Drive And Start To Go It Backfires Out The Exhaust As I Go And Has No Power On The Freeway Ask Me More Details If Needed Ill Give What I Can


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Have you checked your firing order and timing? What's your initial timing?

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''Life's tough, Pilgrim, and it's even tougher if you're stupid.''---John Wayne

Midnight Sun Street Rod Association

Knowing th cam specs, engine compression, who's ignition are baseline pieces of data needed.

Living in Albuquerque you've got quite a bit of altitude which is going to make any problem from leaky spark plug wires to an off tuned carburetor just a lot worse than at lower altitudes.

The cam will fall into this same rabbit hole where the roughness and upscale torque and power peaks will add to the difficulty of keeping the engine running sharp.

Backfiring out the exhaust which is more properly after firing, where back firing comes out the carburetor. It's important to distinguish between these as the causes are a lot different. There are a lot of reasons why either happens but basically and toward your problem it's the result of mixture ratio problems. Lean (too little fuel) tends to be explosions and spitting out the carb, or injection. Rich (too much fuel) tends to be to be rumbling and explosions out the exhaust pipe.

Life in high altitude places requires that the jetting on carbs be set "leaner"
because there is less air available as altitude increases. So the correct jetting at Edlebrock's factory in sea level Los Angeles, will be rich in places like Albuquerque or Denver.

The cam if it's very long in timing and big on lift will make this even worse because it pumps a lot of mixture back into the intake at RPMs under the torque peak because the late closing intake valve allows the piston to push mixture backwards when the velocity in the ports is low. This also adds to richness as the carb happily adds fuel to the passing air whether its going in, coming out, then going back in again. Plus some mixture leaks straight through the open valves during the overlap period of the cam. All this tends to gang up to make a low number of air molecules in the cylinder with a lot of fuel. Additionally with a lot of unsued fuel inside the exhaust system which will burn when a cylinder that hasn't completed combustion dumps the flaming stuff into the exhaust system. Lots of reasons for this to happen, you have several with what little I know of your situation, I see that.

Another set of problems with a big cam made worse by altitude is compression and ignition timing. Especially at high altitudes but present everywhere with a long timed cam is the need for more compression to squeeze more power out the troublesome lower RPM mixture densities and the use of more initial advance than would be usually required to get enough burn time before the exhaust valve opens.

The carb also comes into play with idle characteristics of a large cam in that often to get enough air to idle the engine at 800-1000 RPM to keep it running would cause you to open the stopped throttle position to where not only the idle discharge ports are open to engine vacuum but so are the idle to main meterng transfer ports. Again another source of too much fuel.

Things that need to be identified:

1) Camshaft timing characteristics if you can't find a maker and their part number, you'll have to extract the data with a degree wheel, positive piston stop, and a dial micrometer on a stand to measure valve lift. How to do this is am many of the camshaft makers sites in technical reference sections about degreeing that cam, Comp has instructions as to many others.

2) Fuel pressure, you've got a wiz-bang pump that might well deliver much more fuel than the carb can manage. 6 PSI is plenty, 4 is adequate for most normal situations short of full time racing.

3) Compression is very important especially with a long winded cam, you need to know the heads, their combustion chamber volume, the piston preferably by part number and maker as this can be translated into the needed data of compression height (pin center distance to crown edge) for SBC this is 1.56 inch for original equipment. What causes a lot of trouble is the production rebuilders usually zero deck the block, that is mill the decks from the normal .025 inch between the piston crown edge when its a Top Dead Center to the blocks head mounting surface (deck). At zero deck the .025 is reduced to zip, to keep stock compression there are rebuilder pistons that reduce the compression height by .020 inch to 1.54 inch. If these get used in a standard height block, and it happens a lot, your compression is way low. All of this rolls up to calculating the compression ratio.

So you have to know:
- the chamber volume of the head.
- thickness of the head gasket
- the afore mentioned characteristics of the piston, plus the type crown:
- flat with valve reliefs
- dish which will be circular or D shaped and what volume of the dish
- dome and what volume of the dome.
- whether the block was mill decked and if so by how much.


4) Ignition, this needs to start with the quality of parts, high altitude reduces what's called the dielectric strength of insulators, which is a way of saying that spark plug wires will leak high voltage electricity if they can and those that don't at wea level very well might at altitude. Best way to determine this is to run the motor in the dark and look for sparking.

Timing will also be an issue with altitude and with a big cam the motor will want a lot of initial advance. Of course there is a limit as to how much total advance can be tolerated usually somewhere between 34 and 40 degrees depending on the cylinder head, mixture ratio and temperature as well as altitude, cam and some other stuff. So with a big cam and high altitude the ratio of base of static timing to variable timing changes where more base is used with less variable. Here the variable (centrifugal system unless electronic) has to be stopped early with a collar, screw, or weld so that the total timing doesn't get to be too much. Static/base advance increases will also mess with idle speed (higher) which then pushes back on having to make another round of carb adjustments.

This is probably a lot to handle already so I shut it off here for now and send you out to gather info. Work in a methodical way don't do multiple changes the combinations and perturbations quickly become overwhelming resulting in you not knowing where your at nor how you got there. Simple tests first like checking fuel pressure, rechecking float levels, running the engine to look for leaking electricity in the spark plug cables, plug insulators, distributor cap, etc.

Bogie

Wow I Am Impressed With That Information So Basically I Will Start Where You Told Me So I Have A Fuel Pressure Regulator Set At 5.5 Psi On It Carb Is An Eddy My Dad Said Cam Is The Next One Up From Stock Ehich Is No Help I Assume And Toying With The Idle Mixture Screws Can Get It To Idle But Its A Poor Dissapointing Idle From What It Used To Sound Like At Idle I Have Suspicion My Cam Might Have A Flat Lobe Due To The Fact A Rocker Arm Nut Backed All The Way Off And When My Dad Had A Friend Put It Back On He Over Tightned It And Drove It About 250 Miles And It Snapped The Studd Replaced Studd And Nutt Now Ive Been Driving It Sense September Running Good Untill A Few Days Ago Wont Idle Anymore Very Weak Power Have To Hold Gas Pedal To Keep It Running Throw It In Drive Start To Go Backfires Out Exhaust Which I Say That Because I Hear It Behind Me Sounding Like Popcorn Then It Gets Over The Babackfiring And Into Higher R's It Goes Away But Its Slow