Hot Rod Forum banner
1 - 20 of 34 Posts

·
Registered
Joined
·
11 Posts
Discussion Starter · #1 ·
I can't figure out what's happening with my sbc 350. It idles smoothly and revs fine in park. Under load it backfires through the intake. It has ported 882 heads that are freshly decked with new valve guides, valves and springs, new cam and lifters and new intake with a Holley 675 carb. I checked for a flat cam lobe, vacuum leaks and made sure the firing order is right. Can't figure it out. All I know is when I adjust the mixture screws the rpm doesn't change until the screws are all the way in but I have them out about 2 ¹/² turns.
 

·
Registered
Joined
·
15,620 Posts
Either the carb isn’t keeping up on fueling or the ignition is getting wonky.

Idle screws have little if anything to do with fueling past transition. Once the throttle blades are higher than the the transition ports all the fuel is supplied by main metering with acceleration covered by the acceleration pump and approaching WOT the power system turns of to richen the AFR at max power.

Except for the accelerating pump all of this is covered by some form of vacuum. At idle the manifold vacuum is quite high with the throttle blades mostly closed. As the throttle is opened the throttle blade moves away from the idle port lowering the force on it thus flow from it but at the same time the throttle exposes more of the transition port which draws fuel from the idle circuit so this covers the fueling needs as the idle port shuts off and main metering picks up. An oddity in this stage is generally the manifold vacuum takes a slight rise but it’s effect on the uncovering idle port is mostly lost but this does increase fuel flow from the transition. So this is a touchy spot especially once the intake cam timing starts creeping over 218 degrees measures from the .050 inch open datums.

Main metering uses a venturi to develop a vacuum signal but this is the reverse of the manifold vacuum in that the constriction of the venturi speeds the air flow passing through it which reduces the air pressure (which is the creation of a local vacuum) the faster the air flow through the venture the greater is the vacuum this is the force that creates the main metering fuel flow which is roughly proportional in mass to the air flow with the jetting and emulsion jetting maintaining the proportionality.

As the throttle is advanced further the manifold vacuum continues decrease this is used to trigger the power valve. Whether the carb uses a power valve or a piston operated metering rod the power system uses a countering to manifold vacuum to either open a power valve (Holley and their clones) or move a piston that is connected to a metering rod (Edelbrock or QJet)

The accelerating pump covers the lean drop when the throttles are opened. Opening the throttle increases the induction area in the throttle body, this results in a momentary velocity decrease through the venturies which drops the main metering fuel flow. So the accelerating pump provides a fuel shot through this moment to allow the air velocity through the venturi to catch-up and return to normal metering for the new conditions.

Secondary actuation, aside from competition carburetors that only use mechanical linkage, is controlled by venturi vacuum in the case of Holley and its clones or by a mass flow flapper valve as in the Edelbrock or QJet. The Edelbrock uses two systems which date back to the original Carter carbs they derive from the original AFB known as the Edelbrock Performer series places a counterweighted air valve between the secondary venturies and their throttle blades. The Carter AVS now known as the Edelbrock Thunder series uses an adjustable spring loaded air valve above the secondary venturies. The QJet uses a similar air valve with an additional duty of operating secondary metering rods that the AVS does not, it uses a plane jet on the secondary side similar in fashion to the Holley types.

This obviously goes on snd on into metering air bleeds and emulsion tube designs, the forever problems of metering idle air with big cams, to choke or not on the air horn secondary accelerating pumps and power valves and on. So there is plenty of room to expand this discussion snd to get lost in it or by it.


Plus I haven’t even gotten to ignition. I’ll make that a separate dissertation. Let it set on the presumptions snd assumptions of the relationships of the base setting, the vacuum if used and the centrifugal where how much base is sufficient, where the cut over point is for the vacuum coming down versus the centrifugal going up and by how much.

Bogie
 

·
Registered
Joined
·
11 Posts
Discussion Starter · #13 ·
timing late, too lean? did you mean 670 carb? is so toss it out or get the tool box out and spend a day on it. is this a 275 hp 350?
It was a stock 74 block four bolt main with 2 barrel carb that came from the factory in my 74 c10. Don't know the horsepower. The carb is a Holley 80670-3 so yeah a 670. Timing is at factory 8 degrees with an advanced light.
 

·
Registered
Joined
·
11 Posts
Discussion Starter · #16 ·
8 degrees intitial seems low, 34 total not too bad, if confirmed with a light.
Yes confirmed with a light at about 3000. My guess I leaning to is the heads, they are ported pretty significantly and the valve seats are thin but even and consistent. Heads came off of a dirt track car that was wrecked.
Wood Automotive tire Auto part Circle Automotive wheel system
 

·
Registered
Joined
·
783 Posts
at the top of the engine page there is a link to fixing a 670 (not my choice for anything) carb. Its too lean.
The 882 heads are not going to make any power but they will run ok.
Bolt on a 750 X and 90% finished in 40 minutes.
 

·
Registered
Joined
·
11 Posts
Discussion Starter · #18 ·
at the top of the engine page there is a link to fixing a 670 (not my choice for anything) carb. Its too lean.
The 882 heads are not going to make any power but they will run ok.
Bolt on a 750 X and 90% finished in 40 minutes.
Can't find the link but my power valve in the carb came out in pieces
 

·
Registered
Joined
·
1,641 Posts
You are more than likely experiencing a "Lean Pop"
As others have stated the 670 Street Avenger is not the best choice of carb, but can be worked around with some tuning.

If your power valve is in three pieces then you have a good place to start right there by correcting that problem.
 
1 - 20 of 34 Posts
Top