I have a quickfuel carb on my 400ci. I had a stumble right off idle so i changed to a bigger accelerator pump nozzle and cam which helped and seems to have the issue resolved. However i now have a stumble right after that. So i push the pedal down and the accelerator shot seems to work perfectly but if i continue to keep the throttle where it is (cruising) it seems to stumble. If i mash it to the floor the carb works as it should. What should i adjust or change out to get rid of the cruising stumble?

 

How long has this been occurring? New carb install, or existing? What size carburetor?

 

750 cfm slayer series. Just occuring but i just did a manual swap is why im thinking its happening. Has been happening since i swapped over

 

If you had a AF/R gauge you'd know for sure, but it was obviously lean. Since putting in a bigger squirter helped but didn't cure, now you should probably try a 50 cc accelerator pump so that squirt lasts a little longer until you fully transition to cruise. And if it's still stumbling at cruise, you need larger primary jets. I'd go up 2-4 jet sizes

 

If it was running fine before, how long did it set during your manual swap? Fuel gumming up the one of the jets can cause similar issue. I've had to tear down & refresh the same carb 2x in a 6 month period just b/c it sat during a bunch of upgrades.

 

I really dont think the carb is plugged up, the swap took about a month and i had driven it for about 40 minutes afterwards with the stumble. Seems like itd take longer to plug up than a month. I think it is from improper tuning. But im not sure what else to adjust

 

Make sure the vacuum advance is working correctly. As far as lean fuel, a 750 slayer has #72 primary jets and a 65 power valve. That should be plenty of fuel. Check fuel levels (float settings).

 

Did the problem start as your weather turned colder? I'm still leaning toward a lean condition. Cold air is denser than warm air. If the carb was tuned for "lean best" during the summer, it'd most likely go too lean once temps dropped. Check some of the previous posts, and look at dates and people's geographic locations. You'll see others with this issue as their weather changed.

DB, I sure don't want to contradict you or anyone, but stating that a certain jet is enough doesn't help. If all else is good, and he's too lean at cruise, then jetting up is in order. Checking the float setting is definitely called for though. The power valve isn't providing fuel at cruise. And a 6.5 power valve won't add fuel at all during light acceleration, unless the engine has very low vacuum. Step-up springs & metering rods on Edelbrocks serve the same function as power valves on Holley's. They are what I change most during fine tuning and weather condition tuning.

All of this can be figured out quickly if you install an Air Fuel Ratio gauge. And a vacuum gauge will tell you exactly when your power valve is opening. I install vac gauges along with AFR gauge, but you don't necessarily need to install one, you can just hold it down with the windshield wiper while you're driving and tuning.

 

So i currently have my vac advance unplugged since it made it drive better after my manual swap. I just installed a 5.5 power valve from a 2.5 (had very low vac with auto). Seems to make it drive just a bit better but not much. It is sneezing out through the carb under light accel. I have yet to pull a plug but ive been meaning too. Last time i pulled one it looked rich. I was hoping the powervalve would fix my issue because now i really dont know what else i can adjust to fix this issue. Cureently my jet size is 69. It came wity 72 or 71s original and it was putting so much fuel into my truck that after awhile the oil started to smell like gas

 

Wow, you definitely have other issues. Backfiring through the carburetor points to either way too lean, and or timing issues. Disconnecting the vacuum advance to get home is one thing, but I wouldn't drive it after that until I found and fixed the problem. Pre-detonation is Bad!

You can bounce around here looking for the answer forever. Or you can start verifying the points brought up here. AFR gauge will tell you if you're lean or rich. Vacuum gauge will tell you if you have other problems like burnt valves, timing chain jumped, vacuum leaks, more.

Sounds like you also need to verify valve timing. That's not hard but requires a little basic understanding of how the camshaft works. Clean your harmonic balancer. Use a silver Sharpy to highlight the timing marks. And if your balancer doesn't have it, mark 90, 180, & 270 degree locations. Pull all the spark plugs. Remove valve covers. Have the firing order written down, have someone rotate the crank with breaker while you verify valve opening by watching the appropriate rocker arms. Also pay attention to how much the rockers move. A short or no stroke means you've got a bent pushrod, dead hydraulic lifter, or wiped out camshaft lobe.

 

Can you remember the daily temperatures just before this problem started? And what is it now?