Carb tuning: Bog off idle.

[pnora]

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.

35 or 50 cc pump?

 

[Jonas Nordstrom]

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.

Dont know what came with the engine but prolly stock. Have a very smooth idle.

 

[AR67GTX]

Have you adjusted the accelerator pump linkage to make sure it’s firmly in contact with the pump lever at curb idle so that it pumps at first touch of the throttle? There’s a dimensional tolerance to the adjustment at full throttle to prevent bottoming the pump - somewhere around .010 to .015” clearance I think.

 

[Jonas Nordstrom]

35 or 50 cc pump?

Ah, 35.

 

[Jonas Nordstrom]

Have you adjusted the accelerator pump linkage to make sure it’s firmly in contact with the pump lever at curb idle so that it pumps at first touch of the throttle? There’s a dimensional tolerance to the adjustment at full throttle to prevent bottoming the pump - somewhere around .010 to .015” clearance I think.

Yep, adjusted at full throttle to .010. Have instant shot as soon as I touch the throttle.

 

[pnora]

Ah, 35.

I would do this first. Cap the vacuum advance. Set the timing 36 all in. Then see what it does. Second step would be to upgrade the front pump to a 50cc pump.

 

[Jonas Nordstrom]

I would do this first. Cap the vacuum advance. Set the timing 36 all in. Then see what it does. Second step would be to upgrade the front pump to a 50cc pump.

Right now I am at 37 all in without vac adv.

The pump is an idea....though I dont touch the pump when holding the pedal still pressed down only the smallest tad...and here the afr shows 17... thats not normal, right? (Idle standing still is 14, slight throttle/cruise 13.6)

 

[AR67GTX]

Did you examine how much primary side transition slot was exposed at curb idle setting below the blades before installing the carb? Maybe the secondaries need to be opened or closed a little to get an approximately square exposure below the blades.

I installed an FBO advance limiter plate in my GTX to hold advance to around 18 deg centrifugal and run 15 to 16 deg initial. It comes off the line hard now and solved my pinging issues.

 

[Jonas Nordstrom]

Did you examine how much primary side transition slot was exposed at curb idle setting below the blades before installing the carb? Maybe the secondaries need to be opened or closed a little to get an approximately square exposure below the blades.

I installed an FBO advance limiter plate in my GTX to hold advance to around 18 deg centrifugal and run 15 to 16 deg initial. It comes off the line hard now and solved my pinging issues.

The primary side transition slot i did set to a square before installing the carb. The secondaries are fully closed.
In the beginning after installing the carb I did try opening the secondaries just a tad but that made the idle really hard to dial in so i closed them again.
(Note that this is my first carb that has four corner idle, so no expert on that.)

 

[4speed68rt]

What about the jets? Have you tried to lean it out or richen it up a couple of jet sizes? How do the plugs look?

 

[WileE]

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

Well, you make a very good point, he says its a new carb... machining is off ?

 

[Jonas Nordstrom]

What about the jets? Have you tried to lean it out or richen it up a couple of jet sizes? How do the plugs look?

Yep, been up and down on jets, stopped on 67 for primaries at 13.6 afr in cruise.
No change on the bog.

 

[Jonas Nordstrom]

Well, you make a very good point, he says its a new carb... machining is off ?

Yes, I also think there is something fishy with the transfer slot/idle passages.

How about too big idle air bleeds, would that cause a bog?
Mine are size 70, will try size 65 tomorrow. Just for test as I have them on another carb. Running out of options.

 

[WileE]

Yes, I also think there is something fishy with the transfer slot/idle passages.

How about too big idle air bleeds, would that cause a bog?
Mine are size 70, will try size 65 tomorrow. Just for test as I have them on another carb. Running out of options.

Good idea, check the alignment of the throttle blades as well.

 

[hunt2elk]

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.

Curious as to why you have the rears shut down so much? I always set all 4 of mine the same.

 

[Jonas Nordstrom]

Curious as to why you have the rears shut down so much? I always set all 4 of mine the same.

As earlier said, first time with a four corner for me.
Will test an equal set up tomorrow.

 

[451Mopar]

Sounds like idle / transfer slot tuning issue?
Why idle only 1/4 turn out on the rear metering block?

Small throttle changes off idle is usually idle/transfer slot, and partly accelerator pump. Throttle too far closed for main metering to be active.
Accelerator pump should add a small amount of fuel, check that the pump arm clearance to the accelerator pump diaphragm.

If at light cruse, throttle just slightly cracked, no accelerator motion, if he AFR is lean, increase the idle fuel restriction size.

 

[dvw]

Main jets wont effect the idle circuit. For sure the rear side should be lean at that screw setting. Don't be afraid to fatten up the idle circuit front and rear. Unless you looking for the best fuel economy. Low float level will delay the main circuit. But it sounds to me like the issue is earlier than the main circuit.
Doug

 

[pnora]

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

 

[AR67GTX]

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.