Carb Problems Help!!

[Machinist1972]

I seem to be having a similar problem when tuning carbs regardless of the carb type Holley, Rochester, or Carter. On hard acceleration it seems to stumble/hesitate or bog, I keep thinking this is caused by to much fuel but i'm not sure anymore. Could it be that it isn't getting enough fuel, or is it something completely different that i'm overlooking. Help would be great thanks.

[Bus Ted Knuckle]

Yes it very well could be fuel...but if it did the same with all three carbs, I would start looking at timing first. What motor are we talking about here? I guess a small block, so try setting your dwell at 30, and your timing at 8 deg BTDC. That should wake it up pretty good. You can play with both to get in a "sweet spot".

[Sweet72]

Sounds like you're not getting enough fuel on acceleration. Have you checked the accelerator pump in the carb in question for a worn/torn diaphragm or pump linkage out of adjustment/alignment. If you've had this same problem with multiple carbs on the same motor then you more than likely have a ignition timing issue as stated above.

[68gmsee]

Lots of things can cause stumbling or bogging from fuel starvation, ignition components and/or vacuum leaks.

Need a little more info like, does it do it when cold or hot? And if you gradually depress the pedal, does it hesitate, miss or backfiire?

Also, don't overlook vacuum advance leaking.

[Machinist1972]

I'm working with a Chev 350 in particular, but this has happened in many cases with other 350 engines. I have looked for vacuum leaks but none, I'm running it at 8 degrees BTDC, 38 degrees total. I've checked for fouled plugs, bad plug wires seems to be no ignition problems. It won't hesitate or stumble when I gradually step into the throttle only on hard acceleration for passing gear or just when I want it to haul ass off the line. As for hot or cold this has happened when the engine is hot or at operating tempature.

[mtndewmaniac66]

With having the same problem with multiple carbs, I will say that it is not in the carburetors, unless you are one who is that unlucky.

I could surmise that your problem lies within the fuel delivery system. Fuel pump could be at fault, not enough pressure or fuel flow. Fuel filter could be partially plugged, or even the fuel filter inlet/outlet size, or fuel line size may be restricting fuel flow. Also something as simple as having several bends in a fuel line will restrict fuel flow. Another problem to look into is the fuel line between the tank and the fuel pump. It could be collapsing from a restriction in the line, or could be sucking in air from a connection.

Another thing to look at is: timing, camshaft condition. This alone can cause stumbling. perform a timing check, look at the tining marks and see if advance is working properly, like stated above. (We need more info)
Good luck!

[jayharrell]

I agree. Probably a stoped up fuel fliter or something with fuel delivery. I actually had an old locking plastic gas cap in my tank causing the same kind of issue. Guess the PO lost the key and just hacked the cap apart, with most if not all of it falling into the tank.

[bryanw1968]

Originally Posted by Machinist1972

I'm working with a Chev 350 in particular, but this has happened in many cases with other 350 engines. I have looked for vacuum leaks but none, I'm running it at 8 degrees BTDC, 38 degrees total. I've checked for fouled plugs, bad plug wires seems to be no ignition problems. It won't hesitate or stumble when I gradually step into the throttle only on hard acceleration for passing gear or just when I want it to haul ass off the line. As for hot or cold this has happened when the engine is hot or at operating tempature.

What size holley? I'm assuming it's a vacuum secondary? What jets are you running in the front and do you have jets in the back or a metering plate? What color vacuum secondary spring are you running? What cam and heads are you running?

I had the EXACT same issue with my motor and after trying 4 different carbs I finally figured out that a vacuum secondary wouldn't work with my particular setup(heads). A double pumper fixed the stumble. It would literally fall on it's face under hard excelleration but it was fine if you slowly go into it.

[mtndewmaniac66]

Originally Posted by bryanw1968

What size holley? I'm assuming it's a vacuum secondary? What jets are you running in the front and do you have jets in the back or a metering plate? What color vacuum secondary spring are you running? What cam and heads are you running?

I had the EXACT same issue with my motor and after trying 4 different carbs I finally figured out that a vacuum secondary wouldn't work with my particular setup(heads). A double pumper fixed the stumble. It would literally fall on it's face under hard excelleration but it was fine if you slowly go into it.

I seem to recall, deep within the recesses of my failing memory, about an issue in the past about stumbling carburetors. Turns out that a carburetor with mechanical secondaries is recommended for a vehicle UNDER 3000 (or 3500) pounds, as to where a carburetor with vaccuum secondaries is recommended for anything OVER 3000 (or 3500) lbs.
I do believe that this COULD be looked into further, if someone has the time. I think that I saw this in an Edelbrock manual, but I am not sure.