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.

 

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.

 

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


is the trans not downshifting??

 

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?

 

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?

 

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.

 

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

 

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

 

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.

 

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

 

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

 

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?

 

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.

 

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

 

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

 

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?

 

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.

 

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.

 

I'll bet you're issue is due to the air gap. The issue is caused because the intake isn't warm enough and you do not have enough air flow when you step on the gas. So what happens is when you step on the gas at low speed conditions the fuel vapor from the carb enters the cold intake at low air speeds and the fuel drops out of the vapor and puddles on the manifold until the air flow increases enough to evaporate the puddling fuel.

The air gap manifold claims cooler intake temps make more power due to keeping the intake charge cooler, and the manufacturers are correct. But max power is not found at 2000 rpm's where you're cruising. A former member here would be telling you to swap that thing off there as fast as possible. He'd mentioned many times that the air gaps created more issues than the extra bit of hp on the top end are worth.

 

Vac advance connected to ported location on carb

 

Fuel pressure at idle means or with the motor off means little. you need to see the fuel pressure when it bogs. Cam specs look OK for the application.

 

Makes sense. But it still bogs when the engine is up to normal operating temp (and the intake is hot to the touch).

Could the puddling still be the issue then?

 

Did I miss what size the carb is, what spring in the secondaries? What is the spring setup in the distributor and have you tried a larger squirter? I agree it could be the intake.

 

The Edelbrock was a 600cfm. Richend that one up a lot, increased pump shot, no difference. The Quick Fuel on it now is a 780 that is calibrated for my setup. It has adjustable vacuum secondaries (and I tried adjusting for them to come in later, no difference).

Stock advance springs are on the HEI.

Can the puddling / bog still happen on a hot intake?

 

How hot is the intake? Over 120? I wish I had more experience witha big block Air Gap, but if you air flow is low (high vacuum due to mostly closed throttle plates) and you dump gas into an intake that is below 100 degrees, then you can expect puddling and what your describing.

A way to check this is to drop a gear and get cruising at 3000 or so and then lace into it and see if it reacts the same or if the bog is better or gone. If it is, then I'm confident you have a puddling issue (also called XTau). When you try this attempt to have the vacuum about the same. If it goes away, then it is due to the extra air flow that the engine is consuming (more rpm's implies more air is passing through the manifold.) Also, an AFR gauge would identify the problem. I truly hope that puddling isn't the issue so that another intake isn't the only way you can resolve this.

Good luck - Jim

 

Thank you, Jim.

I haven't measured intake manifold temp, but have tried the 3,000rpm cruise to WOT without much difference.

From 4,000rpm it's more of an extended hiccup than a bog.

 

For C>/st's sake I just tyoed a long response to this and closed the window by accident:spank:

With this information in hand you are ready to start making changes. Try a different squirter, larger or smaller is irrelevant at this moment, and then try a different pump cam. Make only one change at a time and drive like you described above and record the info (a notebook works great for comparing changes.) Like I mentioned previously, an AFR gauge can really speed up this process, but isn't necessary if you're willing to put the time into messing with this.

I've been in your shoes with a puddling dilemma and trying to tune it out - it's a PITA. And again, FWIW, it's why I'm such a fan of EFI with a fully controllable ECM. I can make the changes I'm describing in seconds and test out 10 different changes in less than 15 minutes.

Also, keep in mind that the AFR gauge can be used to dial in cruising fuel economy. And when you're done you can pull the sensor out of the exhaust and put in a plug and save the gauge for your next build/tuning challenge.

FYI - I was able to tune out the puddling (mostly) when I ran into this - different engine and intake setup. And I'm still not 100% convinced that it is your issue, but I think your about to find out and become a Holley 'expert' in the process.

Good luck - Jim