Has anyone ever heard of running 26* of timing at idle?

[Incognegro]

I have been having a stumble issue on my small block Chevy when I stab the throttle wide open. It happens from idle, happens from 1st gear and happens from cruising under 4000 rpms. If I ease into full throttle its fine and the car rips, but if I stab it fast the car falls on its face then takes off.

I have a guy giving me advice that my initial timing needs to be 26* at idle and curved to run 36 by 3000...I have always ran 12*-16* on this motor and have it set at 12 initial right now and all in 36 by 3000, all I changed was the Carburator from an 850-to a more streetable 750 double pumper. Im thinking my pump shot nozzles are just too smal and the carb needs to be jetted properly, but one guy keeps giving me this timing advice and Im skeptical to follow it.

Here is the break down of the engine itself:
355 SBC
10.3:1 compression
64cc heads
Lunati 550 lift 288 degree cam
Weiland 17530 manifold
Holley 750 CFM double pumper
456 gear
powerglide
Car weighs 2800lbs
28" tire

[itsabird]

It's not unheard of, consider guy's running locked out timing, before i went there, i would try larger squirters in the carb.

[Incognegro]

It's not unheard of, consider guy's running locked out timing, before i went there, i would try larger squirters in the carb.
Im with ya on this one....Went from 28 to 31 got a tad better...gonna jump right to 37's and see where it falls..Do you think the pump cam would help too?

[itsabird]

I think you are on the right track, with nozzles.

[TheYellaBrick]

It's definitely an accel pump issue early in the stab.

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[mopar1968]

I agree Double D i would be looking towards the pump :!:



Mark

[Incognegro]

It's definitely an accel pump issue early in the stab.

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what size pump and nozzles are common for a motor of this size?

[mopar1968]

Some might disagree with me but i always change my carbs over to 50cc pumps on both ends :!: I would start out with a 32 to34 nozzle primary and a 36 on secondary :!: JMO


Mark

[jmarksdragster]

50cc pumps supply enough fuel to feed a methanol or E85 Dominator. Why do you think any 4150 would need that much fuel for gas? Make the engine burn the fuel correctly and you don't need to add that much fuel. Sufficient timing, the correct heat range spark plug, the right fuel for the compression ratio, a quality ignition system... all these things make the engine you have burn the fuel best. We can add that more compression helps, better cam, better cylinder heads, but that adds a bit more money. Do the things you can to make the engine work better at burning the fuel.

It's hard to say without knowing your .050 cam info, if 288 is advertised then probably around 230. Converter should probably be a 10" and stall will need to be around 4000 RPM . And most likely it will also need to have the timing locked to perform best. What plugs are you running and what heads?

[TopspeedLowet]

Before you do anything to the shooters, you more likely have slack in the accelerator pump linkage or a way too weak spring, but more likely slack in the linkage. While shut off and at minimum idle, barley move the throttle blades and see if you have a pump shot with the least movement of the throttle blades. It you don't you need to adjust the spring tension at the pump lever to take up that slack. Then after making sure this is responsive you can begin changing shooters and cams in that system.
Float level being low can also cause low speed stumbles.......
Bruce

[itsabird]

Not to hi-jack, thank's for the emulsion info jmark.

[Scooterz]

Just a few things I have noticed & though about...

As Mark cited, check float level... had that happen & cause some stuble b4.

Street use... I wonder if the 750 is still too big?? Maybe 650-700CFM DP?

Does it have primary PV's? Checked it for damage?

Have you had a vac/gauge on it yet?

has it ever been responsive correctly w/ this carb? Is this a new issue or are you just trying to get this carb dialed in still?

BTW: hows things after the wreck??? Did the insurance company come through & help? Good luck. Scooter

[mopar1968]

I have changed all my carbs over to 50cc pumps and haven't had a problem with any of them as of yet :!: :!: Been doing it for about 30 years now :!:


Mark