Hesitation Issue 84 K10 w/ 350

[SourTooth307]

I noticed when at cruising speeds and light throttle I'm getting a hesitation, almost like a sputtering or a popping from the engine. Pushing the gas down will give me the usual acceleration, and letting off it entirely just makes it coast normally. I don't know what it could be, I have a new fuel pump in there, new plugs, wires, cap, rotor, and the fuel lines are all intact with no splitting or dryrot. Any thoughts what I should look at?

[rich weyand]

To verify, this is a carbureted engine with no ECM (federal emissions, not california emissions), correct? And it's a 350 with a TH350 trans?

Couple questions:

What plugs are you running? What is the gap set at?

Where is your vacuum advance connected? On the carb, I understand, but which port?

Does it have the double delay valve setup on the vacuum advance line to the distributor?

What is your base timing set at?

[SourTooth307]

It's a 350 with an Edelbrock carb, as far as emissions go, I only have cats on it, nothing else. I have the OEM AC Delco plugs in there gapped correctly. Anything beyond that I don't know.

[motornut]

I'd check your timing.could be your vacume advance is sticky?

[SourTooth307]

Originally Posted by motornut

I'd check your timing.could be your vacume advance is sticky?

As far as the vacuum advance goes, where would I find the component that might be sticking?

[SourTooth307]

I'm really thinking the vacuum advance could be the issue. After driving the truck yesterday it seems like it's the higher ranges where it seems to be having an issue. If you're just crusing it's fine but when you call in for a good chunk of power it seems to fall flat. Can anyone tell me where to look for the issue?

[geezer#99]

It's not your vacuum advance. When you step on the pedal the vacuum drops and any timing supplied by the vacuum pot goes away. Sounds like you need more initial timing.
What's your initial timing set at?

[notallthere]

1406 Edelbrock carb? If so they are set up for economy right out of the box but a mild engine may experience a stumble. I have found that replacing the Accelerator Pump with a slightly larger one will cure the stumble.

http://www.summitracing.com/parts/edl-1475

[rich weyand]

Try changing to the 8" step-up springs. This will richen the mixture sooner as vacuum decreases. These come in the spring kit. http://www.summitracing.com/parts/ed...Fe47MgodqmAAtg

Another thing you can do is move the linkage from the throttle from the center hole to the inner hole on the actuator for the accelerator pump. This will increase the shot when the pedal is depressed.

Try these first. I think the step-up spring is the best likelihood. I had to change mine.

[motornut]

Originally Posted by SourTooth307

As far as the vacuum advance goes, where would I find the component that might be sticking?

I tried cleaning but ended up replaceing,works better

[Dead Parrot]

The diaphragm in the vacuum advance can go bad and leak. Not only do you not get any vacuum advance, you also get a small intake air leak.

Another thing to check is to make sure the centrifugal advance is working properly and not sticking. If it sticks at full advance, you can have too much timing advance at low rpm. Or if it was stuck full advance when you set your timing, you now have too little timing at higher rpm.

Make sure the power pistons in the carb are moving freely. They gum up after a while and can stick.

[SourTooth307]

Originally Posted by rich weyand

Another thing you can do is move the linkage from the throttle from the center hole to the inner hole on the actuator for the accelerator pump. This will increase the shot when the pedal is depressed.

Try these first. I think the step-up spring is the best likelihood. I had to change mine.

Good plan, I'll start with these and try everything else suggested if it continues. I remember it driving fine until one day this started happening......

[rich weyand]

Originally Posted by SourTooth307

Good plan, I'll start with these and try everything else suggested if it continues. I remember it driving fine until one day this started happening......

OK, that's new. That sounds like sticky step-up pistons. Remove the air cleaner, loosen the two torx screws on either side of the bores, flip the little covers out of the way, and remove the rods+pistons and clean the pistons. Flush the bores with some WD-40, and replace the rods+pistons. Make sure they move up and down freely. Do not over-torque the little torx screws.

[SourTooth307]

Originally Posted by rich weyand

OK, that's new. That sounds like sticky step-up pistons. Remove the air cleaner, loosen the two torx screws on either side of the bores, flip the little covers out of the way, and remove the rods+pistons and clean the pistons. Flush the bores with some WD-40, and replace the rods+pistons. Make sure they move up and down freely. Do not over-torque the little torx screws.

Awesome, I'll check them out. No adjustments will be messed with while doing this, correct? I don't have any of the metering stuff to make any adjustments.

[rich weyand]

Originally Posted by SourTooth307

Awesome, I'll check them out. No adjustments will be messed with while doing this, correct? I don't have any of the metering stuff to make any adjustments.

No, the adjustment would be if you replaced the rods with rods of a different diameter than what is in there now.

[SourTooth307]

Originally Posted by rich weyand

No, the adjustment would be if you replaced the rods with rods of a different diameter than what is in there now.

Gotcha. Thanks a lot, I'll have a look.