Carb tuning: Bog off idle.

[Jonas Nordstrom]

With that carburetor being vacuum secondaries could the bog be as they are starting to open. One other question do they open?

They do open fine. Though right now I have the secondaries disabled when hunting the bog.

It sounds like the first step is to get an even four corner idle and raise the float a bit.
The acc pump is triple checked and working instantly.

 

[Jonas Nordstrom]

With 4-corner idle mixture a adjustment I’m assuming this is a mechanical secondaries carb. Am I wrong? I didn’t catch that earlier thus my comment on adjusting the secondaries which would only apply to a vacuum secondary carb.

It is a vacuum secondary carb.

 

[451Mopar]

Quick question, Is that a new carb right out of the box?
Last few New Holley carbs I got were dirty inside and I needed to take them apart and clean all the passages before using them.

 

[Jonas Nordstrom]

Quick question, Is that a new carb right out of the box?
Last few New Holley carbs I got were dirty inside and I needed to take them apart and clean all the passages before using them.

Brand new carb, though I built it myself from different parts. CCS body with annular boosters 650. Qft base. Proform 650 calibrated metering blocks.

 

[Jonas Nordstrom]

More testing today:

- Adjusted all four idle screws the same: No bog change (But got a smoother idle.).
-Raised float level to top of sight glass: No bog change.
-Tried manifold vacuum adv: No bog change.
-Tried without vacuum adv: No bog change.
-Tried 65 idle air bleeds: No bog change (But afr went down a bit.).
-Tried 75 idle air bleeds: No bog change (But afr went up a bit.).
-Tried upping the curb idle: Engine recovers better from the bog but its still there.


Next up Im gonna bolt on my trustworthy Holley classic that has always run great to see how it runs on this intake.
If it runs without bog then Im gonna swap parts one by one to see if I can find something

 

[69sunfireGTX]

Give it more initial timing (18-20) and see what happens

 

[Geoff 2]

Might be time to get rid of the POS Holley & use a Carter/Edel 750 AFB or Edel 800 AVS2.

 

[Jonas Nordstrom]

Might be time to get rid of the POS Holley & use a Carter/Edel 750 AFB or Edel 800 AVS2.

Tried it allready, the Avs2 had really good mid range though it couldnt hold the gas as long as a Holley and the Holleys gave more power up top.

The carb in the first pic is not a Holley, it is a carb built on the holley modular style but has all the characteristics that I wanted to bring from all the carbs Ive tried, including the annular booster from the Avs2.
It runs great at starting (Starts on first turn cold without choke, just a little pedal feathering) idle ,cruise, wot is great.

So, I just have to figure out this bog then it will be a really good carb.
I built the carb myself so I accept some hickups in the beginning.

To give up and bolt on some out of the box carb Eddy, Qft etc. Well Ive been there allready.

 

[hunt2elk]

You mentioned in the post above that you set the fuel levels at the top of the glass. That is to high and if anything, I would think make your issue more pronounced as well as richening up everything. I would not go higher than midway in the glass. I run all of mine so the fuel is just above the bottom of the hole. Sure sounds like a different accelerator pump cam or squirter change is your issue. Or could be the power valve/PVFR jetting. When I got serious about tuning my carbs, I bought a couple of these AEM gauges. https://www.summitracing.com/parts/avm-30-0300 They really help a lot to determine what is going on. That and run a hose into the cabin for a vacuum gauge. Just temporarily zip tie them up during tuning and when it is dialed in you can remove them if you want. At this point, we are all just guessing....

 

[62 Dart Convertible]

You mentioned in the post above that you set the fuel levels at the top of the glass. That is to high and if anything, I would think make your issue more pronounced as well as richening up everything. I would not go higher than midway in the glass. I run all of mine so the fuel is just above the bottom of the hole. Sure sounds like a different accelerator pump cam or squirter change is your issue. Or could be the power valve/PVFR jetting. When I got serious about tuning my carbs, I bought a couple of these AEM gauges. https://www.summitracing.com/parts/avm-30-0300 They really help a lot to determine what is going on. That and run a hose into the cabin for a vacuum gauge. Just temporarily zip tie them up during tuning and when it is dialed in you can remove them if you want. At this point, we are all just guessing....

OP ain't guessing, he has an AFR meter.

 

[hunt2elk]

OP ain't guessing, he has an AFR meter.

Whoops, I missed that.

 

[Jcharger]

Might have missed something because I didn't read through the whole thread. You have AFR. Is it just the gauge, or are you logging it with RPM or time? Since you have the gauge, if you are not logging, I suggest figuring out how to log the AFR as the car bogs. If you are not logging it, you could be losing critical information. Seeing the curve can tell you a lot. You want to find out if it is dropping to too low AFR (flooding) or too high AFR (starving) and where that is happening (pump shot or transition to the boosters). This can tell you where to look to make changes.

The rest of this is just a list of things I try to keep in mind, but I think you already know them. It is possible to solve the problem one way and end up with something that really doesn't work. For instance, lean bog might be fixed by increasing the size of the main jets at the expense of the high RPM fuel curve. Logging the full curve lets you check everything. You also want to make sure you are not identifying something that really doesn't matter for any driving you would normally do. Wiring the secondaries shut will give some information, but you may find you have to start over once you allow them to open. Advancing initial timing with some cams can make a difference, but you will need to recurve the distributor so total advance doesn't make it ping. Ignition can also make problems worse but I do not get the impression that is what you have.

 

[Jonas Nordstrom]

Whoops, I missed that.

No worries, I have people asking me if I am out testing the flux capacitor when out driving with all the temp cables zip tied to the car.

For float level that was just a test, Ive tested having it at the bottom too, now its back at slightly below middle of sight glass.
Ive tested all PVs and nozzle/cams, no major bog change sadly.

 

[Jonas Nordstrom]

Might have missed something because I didn't read through the whole thread. You have AFR. Is it just the gauge, or are you logging it with RPM or time? Since you have the gauge, if you are not logging, I suggest figuring out how to log the AFR as the car bogs. If you are not logging it, you could be losing critical information. Seeing the curve can tell you a lot. You want to find out if it is dropping to too low AFR (flooding) or too high AFR (starving) and where that is happening (pump shot or transition to the boosters). This can tell you where to look to make changes.

The rest of this is just a list of things I try to keep in mind, but I think you already know them. It is possible to solve the problem one way and end up with something that really doesn't work. For instance, lean bog might be fixed by increasing the size of the main jets at the expense of the high RPM fuel curve. Logging the full curve lets you check everything. You also want to make sure you are not identifying something that really doesn't matter for any driving you would normally do. Wiring the secondaries shut will give some information, but you may find you have to start over once you allow them to open. Advancing initial timing with some cams can make a difference, but you will need to recurve the distributor so total advance doesn't make it ping. Ignition can also make problems worse but I do not get the impression that is what you have.

Yes, only afr gauges atm.
I have a tiny spot with afr 17-18 right at the beginning of throttle travel, so my money is on lean.

Yes, Im ready to start over when I have found the bog, no problem.
Timing is the last thing I am going to try as the engine ran great with the old carb and intake.

 

[Jonas Nordstrom]

Ok, more testing today.
Did put on my old 650 Holley, started and ran like a champ.

Now, the bog was still there. I played around with lots of pump nozzles and cams, some slight variations on the bog but it still stalled the engine with a quick blip.

So....intake?
Vacuum leak or does my engine not like the CH4B plenum notch?

 

[Jcharger]

I do not understand your message. Ran like a champ with the old Holley but still had the bog, or ran like a champ without the bog and then the bog came back after returning to the new carburetor.

 

[hunt2elk]

After all you have done so far with the carb, maybe you should play with the timing a bit? That really should be set before fine tuning a carb. Try 18 initial and 34 total all in by 2400. Leave the vac advance plugged off. See if that does anything for the bog as well as seeing if your detonation is still there.

 

[Jonas Nordstrom]

I do not understand your message. Ran like a champ with the old Holley but still had the bog, or ran like a champ without the bog and then the bog came back after returning to the new carburetor.

Sorry, the old one started right up and ran like a champ but it did also have the bog.
This means that both carbs are prolly ok and I need to look into the manifold or timing. (Does the pcv valve have anything to do with idle transition to cruise vacuum?)

 

[Jonas Nordstrom]

After all you have done so far with the carb, maybe you should play with the timing a bit? That really should be set before fine tuning a carb. Try 18 initial and 34 total all in by 2400. Leave the vac advance plugged off. See if that does anything for the bog as well as seeing if your detonation is still there.

Yep, soon time to play with timing, ive ordered a new timing light that also can show rpm.

 

[Cranky]

Yeah, wouldn't hurt to push the timing up some more just as a check but if it likes it and you're getting some ping up top, you might have to recurve the distributor to pull back some total.

And do you have yet another known good carb to stick on and see if it still has the bog? If so, I'd start looking at an intake leak especially if you advance the timing and it doesn't do much....and it wouldn't hurt to block the PCV as a test.