Carb tuning: Bog off idle.

[Jonas Nordstrom]

It is either a lack of fuel or too much air, maybe disconnect and plug the PCV for now to eliminate it.
If you have rectified the root cause you can put it back on again.

Since it is just on idle circuit it does not have anything to do with jetting or power valves so no point checking that out.
I know my car needs a rich fuel mixture for idle, you can try fatten it up to 12.5 AFR and see what happens as you mentioned when increasing just a slight amount of more throttle the lean spot disappears which makes me think your transition slot starts feeding at that point.
This would indicate the idle circuit is ok to idle but not feeding enough to get going until the transition slot gets active.

I plugged the pcv completely and then it got harder to start the car.
I tried to fatten up the mix and also raised the curb a bit.
Same bog.

Its boiling down to ign timing or manifold leak.

 

[Geoff 2]

If it idles well I doubt it is an air leak. Have you got another carb to try?

 

[4speed68rt]

Maybe it's the gas. Old or low octane? Mine will barely run on regular unleaded. Just a thought.

 

[Jonas Nordstrom]

If it idles well I doubt it is an air leak. Have you got another carb to try?

Ive tried the new carb and my "old" trustworthy one, they both behave the same so I dont think its the carb.

Maybe it's the gas. Old or low octane? Mine will barely run on regular unleaded. Just a thought.

New gas 98 Oct with octane booster added, been running this for years on this engine.

 

[Jonas Nordstrom]

Update.

I got my timing light and my timing at idle was at 22. (Wonder when that happened?)
I set the timing back to 14.
(Starting point for 413 should be 10 but the engine struggled at 10. At 14 it really wants to run.)
I double checked TDC with timing mark.
I have over 20 vacuum at idle, 750 rpm.
Overall the engine feels happier with a snappier throttle response.

The bog is still there though.
Now the engine recovers easier from the bog but I can still stall the engine by doing a deep blip to the throttle.

It feels like the bog is not when pressing down on the pedal, it is when you quickly let go of the pedal.

Something that still bothers me is that when you press the pedal just a tad down and holding, going forward like walking speed then the afr is really lean at 16-17.
Press the pedal just slightly more and afr jumps to 13.5.
Is that normal?

 

[Wietse]

Something that still bothers me is that when you press the pedal just a tad down and holding, going forward like walking speed then the afr is really lean at 16-17.

It stays at the 16-17 AFR if you do not change the throttle, right?
If you don't move the throttle it stays consistently lean?
Because if it recovers after several seconds it has to do with the pump shot, if it stays lean over longer time it has to do with the idle circuit fuel feed.
Because if you increase the throttle slightly it becomes good so that must be the transition slot that starts flowing and adding the needed fuel.
You can try open the curb idle more and reduce engine rpm with the timing, if the throttle starting point is a little higher, the transition slots starts flowing sooner as well.

 

[Jonas Nordstrom]

It stays at the 16-17 AFR if you do not change the throttle, right?
If you don't move the throttle it stays consistently lean?
Because if it recovers after several seconds it has to do with the pump shot, if it stays lean over longer time it has to do with the idle circuit fuel feed.
Because if you increase the throttle slightly it becomes good so that must be the transition slot that starts flowing and adding the needed fuel.
You can try open the curb idle more and reduce engine rpm with the timing, if the throttle starting point is a little higher, the transition slots starts flowing sooner as well.

Thanks for the tip, you are prolly spot on about curb idle as I think I have missed the elephant in the room.
My new timing light shows Rpm and showed 730 at idle in park while my tach showed 1100.
I guess I fooled myself thinking the rpm was high as the engine sounds angrier now with the new intake and carb.

And yes, it stays lean if I dont move the throttle.

Tomorrow I will up the curb to 1000 in park and see how that goes.

 

[451Mopar]

Check the size of the idle restrictions in the metering blocks (circled in yellow.)
The brass plug from this Holley Ultra Avenger metering plate is more like a cap or cup.
The hole size you see on the outside is much larger than the hole drilled through the bottom of the plug. I measured this one and it is really small at about 0.026"

 

[Jonas Nordstrom]

Check the size of the idle restrictions in the metering blocks (circled in yellow.)
The brass plug from this Holley Ultra Avenger metering plate is more like a cap or cup.
The hole size you see on the outside is much larger than the hole drilled through the bottom of the plug. I measured this one and it is really small at about 0.026"
View attachment 1285889

Allright. I dont have those adjustable on my block but will check size. (Need to get those measuring pins.)
By increasing size will it only affect amount....or will it also change when it starts flowing?

 

[451Mopar]

The restrictor at the top right (and left) on your metering plate. If you make it larger, it will richen the fuel in the transition circuit.

 

[hunt2elk]

The restrictor at the top right (and left) on your metering plate. If you make it larger, it will richen the fuel in the transition circuit.

And richen up the idle A/F. I would almost think that is ok since his idle is good? Be interesting to know what op's low rpm cruise A/F is. Under 2500 rpm or so would be affected by these jets, like you stated.

 

[Jonas Nordstrom]

And richen up the idle A/F. I would almost think that is ok since his idle is good? Be interesting to know what op's low rpm cruise A/F is. Under 2500 rpm or so would be affected by these jets, like you stated.

Idle is good and smooth.
At low cruise I am now at 13.4 A/F.

Now I tested to up the idle rpm, up to 1000 and the bog is still there.
Two full tanks of gas now only for testing and I am running out of options.

 

[Geoff 2]

Not reading through 5 pages & not that interested in H carbs [ & clones ], but I would increase pri main jets by 3 sizes. Quick/easy way to determine if it needs more fuel under load. If this fixes or improves the problem, the correct permanent fix is go back to the original jets & increase the size of the PVCR.

 

[451Mopar]

Are you testing with the vacuum advance connected?
Description sounded like idle/transition was lean, or accelerator pump not discharging on light throttle (is the needle under the discharge nozzle sealing?)
Maybe ignition timing curve / vacuum advance.
You did mention the engine has 20" of vacuum? Me, living at mile high altitude, I never see that much engine vacuum

 

[Jonas Nordstrom]

Are you testing with the vacuum advance connected?
Description sounded like idle/transition was lean, or accelerator pump not discharging on light throttle (is the needle under the discharge nozzle sealing?)
Maybe ignition timing curve / vacuum advance.
You did mention the engine has 20" of vacuum? Me, living at mile high altitude, I never see that much engine vacuum

I´ve been testing vacuum advance both disconnected and connected.
Now I am at nozzle size 31/orange cam and here the car pulls away fine, no hesitation.
Manifold vacuum is 20" and steady.

The "bog" that I will rename to "Engine Stalling" is happening when I make a quick blip to the throttle close to standing still.
Stand still in Drive, whack the pedal and let go = the engine dies. Stand still, whack the pedal and hold = car takes off like a beast.
So from this (Plus that I have tested another reliable carb) makes me believe that it is not the carbs fault.


Now I am wondering if the Distributor could cause this?

 

[Geoff 2]

Another possibility, pretty rare, but this is a pretty rare problem.

One or both idle ccts are restricted. I mean the passages, not the jets. Leftover machining swarf or passage not large enough. Here is what happens: normal operation, there is enough time for a/f to get through the restriction, no problem.
When you blip the throttle & lift off, more a/f is reqd over a shorter time & the carb cannot deliver.

 

[Charlie Brown]

Here's a no cost item to check - your fuel pump push rod - is it worn down? Probably not the problem, but it seems like you've looked like a ton of other issues. I'd check it so you could tick that item off of your list.

 

[Jonas Nordstrom]

Another possibility, pretty rare, but this is a pretty rare problem.

One or both idle ccts are restricted. I mean the passages, not the jets. Leftover machining swarf or passage not large enough. Here is what happens: normal operation, there is enough time for a/f to get through the restriction, no problem.
When you blip the throttle & lift off, more a/f is reqd over a shorter time & the carb cannot deliver.

Ive tried other carbs and the engine behaves the same.

Here's a no cost item to check - your fuel pump push rod - is it worn down? Probably not the problem, but it seems like you've looked like a ton of other issues. I'd check it so you could tick that item off of your list.

Hm, the carb has lots of fuel for that 1 second blip, though as you say, an easy thing to check, thanks.

I am planning to dive into the Distributor, prolly no fault there....though the Timing was at 22 at idle now when I had set the timing at 14 last autumn. This is a bit suspicious.

I would like some input about too big Intakes:
Can a intake (CH4B) flow too much for my stock 413 and thus create the stalling of the engine? (This sounds strange to me as the CH4B is just a tad bigger than stock.)

 

[Runner71]

How do the plugs look? Have you checked your TDC and confirmed your timing?

 

[4speed68rt]

It seems like you are expecting alot out of your carburetor. They are not perfect. Its hard to expect a carb to just take off when you floor the accelerator pedal, without a hesitation. In my experience, you have to work the gas pedal a little on a hot rod car. Fuel injection would probably work better for 0 to WOT, without any bog or hesitation. Just my opinion again. Im not a super tuner...