[L31 Vortec swap]

454 with Holley carb and mild cam. Backed by a TH350 auto.

If I floor it from idle, it takes off and runs up through the gears fine.

If I floor it from a roll at, say, 2,000 rpm, it revs a few hundred rpm and then bogs badly -- feels like the engine cuts out for a few seconds (feels like a really long time when it's happening), then revs up fine until redline.

Still happens after this:

Spark: New plugs, wires and HEI. Ran a 12V line directly from battery, still happens (so it's not a switch or wiring issue). I've played with timing (both advancing and retarding, and the bog is still there.

Air: Checked and/or replaced all vacuum lines. Engine dies when covering primaries on carb with my hand. No vac leaks.

Fuel: Have tried 2 known good carbs, and it still happens. Played with delaying secondaries and still no difference. New fuel filter. Fuel pressure is reading about 6 at idle.

Any ideas? The thing that throws me is that it happens regardless of engine temp. Only happens under a load in 2nd or 3rd gears. And again, it runs great hammering it from idle.

I've ruled out air and spark, and am down to fuel.

Could it be bad fuel pump? Some other restriction in the fuel line?
Anyway it could be a flat cam?

Thanks for the help!

 

[64nailhead]

It's either fuel or spark.

To determine if fuel, then have someone follow you and recreate the bog situation. If it smokes heavy black when the bog occurs, then it's too rich. Why is it too rich? Too much accelerator shot, too large of squirters, something giving too much fuel. If this is the case, then try a smaller accelerator cam or move the current/size back a hole to come in later. If that has no improvement, try unhooking and plugging the vac advance line and seeing if the same bog occurs. If it goes away, then you are crusing with not enough timing at 2000 rpms and the vac advance can is hiding the problem, or the vac advance can is lazy(moving too slowly.)

If it doesn't smoke black, then it's too lean and it needs more accelerator pump shot meaning you need a quicker shot of accel pumpshot or a bigger squirter.


Either way, the first step is to determine if it is too lean or too rich. Also, let us know what your timing curve looks like to make sure that it is close.

 

[L31 Vortec swap]

Thanks for the reply, I appreciate it.

Timing is all in by 3,000 - totaling 34-36 degrees.

I believe it is a lean bog (occasional pop thru the carb), but will test it later as suggested and look for black smoke.

Quick question: isn't the vac can irrelevant at full-throttle (I thought vacuum went to 0 in such conditions)?

Again, I appreciate your help.

 

[JJLT1]

when you say 2000rpm roll,, and under a load in 2nd or 3rd


is the trans not downshifting??

 

[ssimpala]

You don't say what kind of vehicle it's in. Does it accellerate fairly quickly? Could it be something as simple as fuel slosh in the tank causing the engine to go lean?

 

[L31 Vortec swap]

1974 Chevy C10 pickup with 3.08 gears.

It happens when the tank is nearly empty or completely full. Also, increasing the float levels on the carb doesn't seem to make a difference.

If it is in 3rd gear, the trans is on the verge of downshifting when it happens, so when the engine recovers. the trans downshifts and powers on down the road. It still happens even if I downshift manually, and still bogs if I'm in second at 3,000 rpm and holding it in 2nd manually.

The thing that throws me is that it cuts off pretty hard, then recovers and revs away and pulls strong. And it always hauls straight off idle. But the bog is like hitting a hard rev limiter where the engine cuts out and goes silent for a few seconds.

I'm thinking fuel, but I don't get why it runs strong from a stop. Also, if the fuel pump couldn't keep up, wouldn't it not rev so hard after it recovers?

 

[L31 Vortec swap]

Exhaust is loud, but I haven't heard any knocking--I've been listening for it. I've had base timing between 8 and as high as 20--still bogs (and I can't hear any knocking).

The other carb was an Edelbrock, and I tried a lot with that one, too--rods, springs, more aggressive accelerator pump, etc. It ran better at part-throttle with more pump-shot, but the full-throttle bog was still there.

Will try to pull a plug later. IIRC, the plugs that came out looked fine, though.

 

[astroracer]

You've ruled out most everything but the pump. What are you using? It may be that the pump is going south. It can keep up at lower RPM launches but the higher RPM stuff may be caused by mostly dry fuel bowls if it can't pump as much on the high end... Causing the bog. The RPM drops and the pump catches up so the engine takes off again. I dunno...
Mark

 

[BogiesAnnex1]

454 with Holley carb and mild cam. Backed by a TH350 auto.

If I floor it from idle, it takes off and runs up through the gears fine.

If I floor it from a roll at, say, 2,000 rpm, it revs a few hundred rpm and then bogs badly -- feels like the engine cuts out for a few seconds (feels like a really long time when it's happening), then revs up fine until redline.

Still happens after this:

Spark: New plugs, wires and HEI. Ran a 12V line directly from battery, still happens (so it's not a switch or wiring issue). I've played with timing (both advancing and retarding, and the bog is still there.

Air: Checked and/or replaced all vacuum lines. Engine dies when covering primaries on carb with my hand. No vac leaks.

Fuel: Have tried 2 known good carbs, and it still happens. Played with delaying secondaries and still no difference. New fuel filter. Fuel pressure is reading about 6 at idle.

Any ideas? The thing that throws me is that it happens regardless of engine temp. Only happens under a load in 2nd or 3rd gears. And again, it runs great hammering it from idle.

I've ruled out air and spark, and am down to fuel.

Could it be bad fuel pump? Some other restriction in the fuel line?
Anyway it could be a flat cam?

Thanks for the help!

Possibly needs the ignition advances adjusted, could be at 2000 RPM the advance is mostly vacuum and the centrifugal is sprung too stiff so when you open the throttle the vacuum goes away taking the advance with it while the centrifugal isn't enough and the engine stumbles till it picks up some RPM and pushes the centrifugal back up the advance curve.

Bogie

 

[L31 Vortec swap]

astroracer: I agree that it might be the pump. The reason I posted is that it confuses me the pump would then "recover." But maybe that's what happens with a worn diaphragm under sudden load? Or maybe a bad check valve allows the fuel in the line to flow back (but I don't see how this would happen at a 2,000rpm cruise). Would putting a fresh filter before the pump make any difference in preventing backflow, or would it just further strain an already worn pump?

oldbogie: That's what I thought, and why I tried a lot of base timing and why I ultimately replaced the entire HEI distributor. With 2 dizzys and lots of base timing, I'd at least expect it to have continual acceleration (even if it felt a little slow while the centrifugal was kicking in). It happens from light to full-throttle at 3,000rpm or even higher, too.

I searched the forums pretty thoroughly, and couldn't find the same symptoms I've been struggling with.

Am I missing anything obvious? I have no experience with flat cams, could this be a symptom?

I appreciate everyone's input.

 

[ssimpala]

We're probably all missing something simple...try the pump.

 

[astroracer]

A flat cam will be flat all the time. You would know if that is what it was. Ticky lifters and no power would be the main symptoms. If you think this may be an issue pull the covers and watch the valves as the engine is running. Flat lobes will produce zero to not much rocker movement on the bad lobe/s.
Mark

 

[tresi]

You have a heavy vehicle with tall gears and I'll assume a stock torque converter. What are you calling a mild cam or did I miss the specs?

 

[L31 Vortec swap]

hcompton: Tried 2 new sets of wires. Currently have new Taylors. Plugs are Autolite 104s. No vac leaks. Checked grounds and ran a jumper directly from the battery to the HEI, so I doubt it it's pulling a ground, but thanks, I'll check.

 

[L31 Vortec swap]

tresi: Cam specs:
Cam Style: Hydraulic flat tappet
Basic Operating RPM Range: Idle-5,500
Intake Duration at 050 inch Lift: 214
Exhaust Duration at 050 inch Lift: 224
Duration at 050 inch Lift: 214 int./224 exh.
Advertised Intake Duration: 288
Advertised Exhaust Duration: 298
Advertised Duration: 288 int./298 exh.
Intake Valve Lift with Factory Rocker Arm Ratio: 0.472 in.
Exhaust Valve Lift with Factory Rocker Arm Ratio: 0.496 in.
Valve Lift with Factory Rocker Arm Ratio: 0.472 int./0.496 exh.
Lobe Separation (degrees): 112

 

[T-bucket23]

You mentioned you have vacuum advance. Did you disconnect it while setting the initial and where is it attached. ported or direct

 

[lmsport]

Put a fuel pressure gauge on it that you can see when driving. Fuel sloshing away from the pickup is my guess.
Have you tried it with the gas tank almost full?

 

[64nailhead]

Do you have an air gap manifold? I ask as you are describing a fuel puddling issue due to a cold intake.

You've been asked a couple of times but are yet to answer - have you ran it with the vac can unhooked and the line plugged?

And I agree with others to get a check on the fuel pressure.

 

[L31 Vortec swap]

Timing set with vac advance disconnected. Have tried it with vac line disconnected -- bog still happens.

Fuel pressure is pulsing at idle, but settles on 6 as soon as the engine is shut off.

Yes, it's an air gap intake. Could you please explain the puddling?

Thanks to everyone for the help.

 

[L31 Vortec swap]

Vac advance connected to ported location on carb