350 SBC. 300 hp. Edlebrock 650 cfm carb. I’m chasing my tail. I get the carb cough randomly on acceleration, especially rapid acceleration. Plugs read pretty good. I cannot find a vacuum leak. Timing set at 12 btc. I’ve tried setting the timing to 10 btc. No help, actually seems worse. I’ve adjusted the idle air screws by tach and with a vacuum gauge. I can get it to run pretty damn good…except the cough returns at random. Would re-jetting or metering rod spring swap help? Ideas?
Couple things accelerator pump, make sure it is spraying fuel the full range of throttle movement most important at the beginning. What type of choke electric or manual if manual pull it out part way maybe a third way. See if that helps if electric loosen up the spring a bit so choke stays on a bit to test. Sounds like when you open the throttle it’s a bit lean to me. Hope that helps
To be clear just using the choke to trouble shoot not a permanent fix. Check the accelerator pump with engine off eyes on discharge nozzle hand moving throttle.
The most likely scenario is when you open the throttle and let in that extra gulp of air, the carb goes lean until the fuel catches up. I don't know a thing about Edelbrock carbs, but if it was a Holley, I would tell you to put the next size up squirter in it.
Yup, check the vacuum advance diaphragm. My 327 coughed on acceleration. I found the diaphragm ruptured.
Edelbrock makes different squirter nozzles, I think they call them clusters. Several above have touched on this issue and I too believe it is a case of the carb going lean on acceleration. A larger accelerator squirter nozzle will let more fuel into the primaries to help "cover the hole" of missing fuel when the throttle blades are snapped open sharply. Also, the pump arm had three positions which control the duration of the fuel going into the squirter nozzles. Edelbrock last time I had looked had all of their manuals online, so you should be able to find the carb tuning section in them, but be forewarned it is written for very beginners and for knowledgeable souls, but not really for the guys in between in my opinion. Back-ordered at Summit: https://www.summitracing.com/search/department/air-fuel-delivery/section/carburetors-accessories/part-type/carburetor-discharge-nozzle-kits?N=brand:edelbrock But this is what you are looking for. They are a pain to change with the choke plate in the way (do not attempt to remove it though)....don't drop the screws down the carb. it is possible to lay a piece of paper towel below the chock plate, just don't forget to use some needle nose pliers to remove it before startup. The vacuum advance as mentioned could rupture the diaphram, but how it operates also depends on how you have it hooked up. it can help with tuning, all carb tuning takes time. Having it hooked to ported or manifold vacuum makes a big difference in how the car can perform. There are so many factors that go into tuning, the carb is just a small part....but to get you going, I say swap the squiter nozzle to a larger size first and a longer duration setting on the pump arm.
Accelerator pump linkage has three settings. I'd try #1 and see if it covers up the bog, if it's not there now. Come stock in 2nd position. Joe
Put a vacuum gauge on it, then check the color of the power piston spring. There are charts that show the inches to color. You can change the metering rods to tune it in.
90 % of carburetor problems are in the ignition! The tune must be perfect before you adjust carburetor at all . I insist all techs follow a full procedure every tune and tick it off at each step. includes oscilloscope to check ignition pattern ...probably not possible at home but the basic procedure is . Start with oils and coolant level, battery condition and charging, air leak and compression tests, and all ground and power connections (voltage and voltage drop, resistance across ground), make sure all cables and hoses are in correct place, not leaking .....check all plugs , plug leads, (check resistance... ohms). Air cleaner element (clean or new?) Start engine ,... air leaks, again ? hoses for cracks including vacuum modulator diaphragm and hose , booster diaphragm and hose ... Recheck timing and advance , both vacuum and mechanical, fuel delivery volume and pressure , float level, observe inside of carb while running , making sure there is no fuel pouring through mains when car is idling ( indicates faulty float valve). Once all is correct to spec then its time to adjust the carburetor. There is even more to do if the car is going on the dyno!
Something else I also thought of, I am assuming if it's a 650 CFM then you are dealing with a AVS2 style Edlebrock. You might also be experiencing the secondary air door opening to quick. This is pretty common on alot of Edelbrocks I find. If it is a AVS2 or Thunder series, you can adjust the opening rate of the air door and slow it down. Again, refer to the carb manual. It will warn of tightening the adjustment too much, but I have successfully ran them tighter than specified without any ill effects.
The 350 in my Nova would backfire through the carb on acceleration and lacked power. I chased the problem for a good while until I found a few loose rockers. Three lobes on the camshaft were wiped out.
Sounds more like a fuel pressure problem than a carburetor problem. Edelbrocks are very sensitive to fuel pressure. Installing a fuel pressure gauge would be a good idea.
This is exactly what my SBC 300hp 350 with Edelbrock 650 cfm carb is doing! Thank you for the info, I love this place!! Edit: the engine in my '62 Chevy II..
At 12 degrees btdc the timing sounds a little retarded to me. What's your 'all in' timing at 2500+ or so rpm (vac disconnected and plugged)? I had a similar issue following a distributor swap that I did in a hurry and subsequently learned I had it retarded some, but only a little. Thought I had carb issues, chased my tail a bit, then did a careful retiming and problem ta-da gone! Chris
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