Carb tuning: Bog off idle.

[Jonas Nordstrom]

Tuning the carb and I have a bog off idle I cannot get rid off. (413/650 carb)

Standing still making a quick blip to the throttle and the engine almost dies.

With the afr meter im at 14 idle, cruise 13.6. From standing still I can move the throttle slowly just a bit and I reach 13.6.
The strange part is where I just put my foot on the pedal ever so slightly so the car creeps away then the afr meter shows and holds on 17 until I press the pedal just a tiny bit more.

Ive played around with different pump nozzles/cams, float levels, but no change.


Timing is at 14 (Too advanced?)
Could it be too big idle air bleeds?

 

[Don Frelier]

If anything advance the timing some more

 

[Jonas Nordstrom]

If anything advance the timing some more

I do run ported now, will try moving it to manifold vacuum.

I have some pinging at wot...so was planning to dial back a step,

 

[WileERobby]

Can you visually observe the pump shot ? Can you verify you're getting an instant fuel shot ?

 

[70bigblockdodge]

I had a old Holley with the pump check valve out off whatcha and it bogged constantly. Looks like your vacuum advance is to manifold vacuum never a fan of that I know some swear by it. You need enough initial timing to be able to get off idle, vacuum to manifold is working against you.

 

[Jonas Nordstrom]

Can you visually observe the pump shot ? Can you verify you're getting an instant fuel shot ?

Yes, pump shot is instant and strong.
Though I can touch the pedal softly and let the car creep away and the afr sits at 17 for as along as I drive at that speed. And keeping the foot still then I dont use the pump shot.

 

[Jonas Nordstrom]

I had a old Holley with the pump check valve out off whatcha and it bogged constantly. Looks like your vacuum advance is to manifold vacuum never a fan of that I know some swear by it. You need enough initial timing to be able to get off idle, vacuum to manifold is working against you.

The check valve is brand new and I have an instant pump shot as soon as I move the throttle. (green cam, 28 nozzle size.)
My vacuum is now on ported, will try moving it over to manifold to get a little more timing.

Am I looking at the wrong place, should I hunt for mainifold vaccum leaks? Am a bit lost here...

 

[WileERobby]

As it transitions to the main metering circuit, it may not be supplying enough fuel. What is the status of the carb: new, rebuilt, ?

 

[Jonas Nordstrom]

As it transitions to the main metering circuit, it may not be supplying enough fuel. What is the status of the carb: new, rebuilt, ?

Yes, thats my suspicion too as it can creep forward with the slightest opening of the throttle and the afr sits at 17, just a tad more throttle and it shows 13.6.
Carb is brand new.

 

[bolson6333]

mechanical advance working?

 

[Jonas Nordstrom]

mechanical advance working?

Have to check again but worked before I changed the carb.

 

[dvw]

When the throttle is moved only slightly the transfer slot to throttle plate position is changed. At this point in time you are still running on the idle circuit. I would suspect your issue lies there. Maybe the idle feed restriction is too small? Transfer slot length is wrong? Another guess is the throttle shaft holes are worn and the shaft moves in the hole exposing a leak? How far are the idle screws turned out?
Doug

 

[70bigblockdodge]

My vacuum is now on ported, will try moving it over to manifold to get a little more timing

It was just how the hose is routed, it should be fine ported by it never hurts to try. Easy and free.
Do you have a vacuum reading at idle? May help with diagnosis.

 

[pnora]

Try a different power valve.

 

[Jonas Nordstrom]

When the throttle is moved only slightly the transfer slot to throttle plate position is changed. At this point in time you are still running on the idle circuit. I would suspect your issue lies there. Maybe the idle feed restriction is too small? Transfer slot length is wrong? Another guess is the throttle shaft holes are worn and the shaft moves in the hole exposing a leak? How far are the idle screws turned out?
Doug

Idle feed restrictions are tunable, would need to find some bigger jets to test.
Throttle plate is brand new, though its aftermarket and Ive learned not to trust those.
Front Idle mixture screws are 1,5 turns out, rear are 1/4 turns out. 19hg vacuum at dle.

Im starting to think vacuum leak, maybe try the carb cleaner spray method.

 

[Jonas Nordstrom]

It was just how the hose is routed, it should be fine ported by it never hurts to try. Easy and free.
Do you have a vacuum reading at idle? May help with diagnosis.

Yeah, sorry, its a QFT plate that has ported and manifold right next to each other.
Yes, 19Hg.

 

[Jonas Nordstrom]

Try a different power valve.

Ive tried PV 10.5 all the way to 5.0. No change.

 

[pnora]

Ive tried PV 10.5 all the way to 5.0. No change.

What size pump?

 

[6872n73]

What cam?

Could try 13-13.5 idle. That may give enough fuel to get you through this lean spot. That would confirm idle and transfer area.

 

[Jonas Nordstrom]

What size pump?

Pump Nozzle? Tried from 21 to 37. Tried green, orange, pink cams. Slightly better with green cam compared to pink cam, but not much of a change.