Dual Weber IDF 40s, Carb Rookie question... Options

[malcolm2]

Update: added 1/2 a turn today. Total of 2-1/2 turns in now. Still getting some pops. It was said here that it might take 3 turns to make a difference.

Still the plan here? Seems to be about 12 threads showing on each.

[930cabman]

Update: added 1/2 a turn today. Total of 2-1/2 turns in now. Still getting some pops. It was said here that it might take 3 turns to make a difference.

Still the plan here? Seems to be about 12 threads showing on each.

Is this with the accelerator pumps? If I recall correctly the spec calls for 7mm of threads showing (70 y/o memory) might want to double check

Where are your idle mixture adjustment screws?

[porschetub]

Update: added 1/2 a turn today. Total of 2-1/2 turns in now. Still getting some pops. It was said here that it might take 3 turns to make a difference.

Still the plan here? Seems to be about 12 threads showing on each.

No expert but thinking 2.5 turns is pretty much the norm then change idle jets up a small amount ,as mentioned you have a lean condition under load which isn't good for your engine,maybe try 52's,if you have rich backfire on overun bring the screws in by 1/4 turn
Try that and see how you go,I'am presuming your engine is well tuned with recent valve set and ignition spot on,cheers

[malcolm2]

Update: added 1/2 a turn today. Total of 2-1/2 turns in now. Still getting some pops. It was said here that it might take 3 turns to make a difference.

Still the plan here? Seems to be about 12 threads showing on each.

Is this with the accelerator pumps? If I recall correctly the spec calls for 7mm of threads showing (70 y/o memory) might want to double check

Where are your idle mixture adjustment screws?

New carbs set up by AC.net started with 10 threads of the accelerator pump arm visible on both carbs. I am up to 12 threads visible now.

That equaled about 2-1/2 turns IN.... no idea about millimeters. but would guess 1 per thread.

Mixtures screws started at 3 turns OUT from seated. I went thru the Best Lean Idle process several times and ended here.... Best guess on the screws would be 1-1/2 to 2 turns OUT, as the BLI process has you turn the screws IN.

[malcolm2]

Update: added 1/2 a turn today. Total of 2-1/2 turns in now. Still getting some pops. It was said here that it might take 3 turns to make a difference.

Still the plan here? Seems to be about 12 threads showing on each.

No expert but thinking 2.5 turns is pretty much the norm then change idle jets up a small amount ,as mentioned you have a lean condition under load which isn't good for your engine,maybe try 52's,if you have rich backfire on overun bring the screws in by 1/4 turn
Try that and see how you go,I'am presuming your engine is well tuned with recent valve set and ignition spot on,cheers

Well it is 2.5 turns from where I started... not the tip of the rod.

been talking Lean backfire on here...... now you mention rich backfire.

How do I tell the difference?

I did buy 2 sets of jets.... one up from here and one down.

here is a pic of 1 side of the accell pump rod.

[rfinegan]

Did you try to pull the main stack, and drive around on the idle jets? Conservative Throttle/driving should not need an accelerator pump.
Did you check the sync of the carbs at 1500 rpm or 3000 rpm? The last video seemed to pop/fluter on cracking the throttle? If you want to know what it sound like if the carb cable is out of sync, just crack the slave carb a bit, and it will flutter and pop, as it will not drive the master card. I have the tangerine cables and not the sync link, but the theory is the same. I seem to recall you had issues with the mounting of the cams binding? They do need to be tight and no play to the throttle shafts, (like the original arms that came on the carbs) I chased a lot of sync issues here till I got me in order...Great progress by the way
Little tweaks here and there is common. Once you find your sweet spot you can drive with out touching them for miles and miles.... and any and all the time you have spent setting these up and be reproduced in about 30 mins or so..Cheers

[nditiz1]

You will never know whether you are lean or rich without an AFR gauge.

When are experiencing the backfire again? Is it only at right off idle, i.e. taking off from rest?

[malcolm2]

You will never know whether you are lean or rich without an AFR gauge.

When are experiencing the backfire again? Is it only at right off idle, i.e. taking off from rest?

The pop-pop comes in a couple places. But not consistent. taking off from rest, but usually with a higher load, say up a hill. And Shifting from 1st to 2nd. But it seems to be just those type conditions. And as things warm up, I don't hear it.

A friend does have, or DID have a AFM set up with a tail pipe sniffer. I used it on my 914 before I welded in a bung. Been bugging him to find it and loan it to me again. Not sure the cord will be long enough for the bus. Might have to set up a video to record it going down the road.

Also have been looking online to buy one.

Any thoughts on Brand?

[malcolm2]

Did you try to pull the main stack, and drive around on the idle jets? Conservative Throttle/driving should not need an accelerator pump.
Did you check the sync of the carbs at 1500 rpm or 3000 rpm?

Have not done that yet.... set-up wise, what is removing the jets gonna tell me. I understand that it will give me a FEEL for where the MAINS kick in, but what else can I learn there??

I did CHECK the flow on the carbs at 1800 rpm. But I only checked it, did not adjust anything at 1800rpms

Drivers side was 11 on the front barrel and 12 on the rear.
Pass side was 10 on the front and 10 on the rear.

So is it recommended to re-sync at that RPM? Kinda go back and forth from 900-ish to 1800-ish adjusting so all 4 barrels are flowing evenly at both RPMs??

[nditiz1]

AEM UEGO wideband is the one I use in almost every car.

If it only pops when the engine is cold, don't worry about it. Carbs are cold blooded. My carbs pop when cold too, as soon as the engine is warmed up, smooth and no pops, well some on decel, but that's to be expected, no load, burning what's left.

[r_towle]

I’m a big believer in ensuring the carbs are perfectly balanced side to side.

I went through something like 6 different x-bar linkages until I went to cable actuated.

If one carb opens slightly slower than the other from idle to just off idle, you will get popping. It goes away once you open them both a bit more.

Because we all spend a lot of time in stop and go traffic, tuning for that off idle response, and making sure the linkage is exactly the same on both sides is paramount.

Having two different readings side to side brings me back to the linkage adjustment, and finding the flaw in the design.

Secondarily, an AF mixture in the tailpipe will be meaningless for you for this problem because that will only test a mix of all four cylinders.
It will not help you isolate why the two sides are different.

Back when Chris Foley was racing a lot he invented some cool exhaust port standoffs for his header with integrated exhaust gas temp sensors.
That is how you really tube carbs.

For your issue, I’m still wondering why the two sides do not match.
Once they do match, I suspect the popping will disappear all together.

Rich

[malcolm2]

I’m a big believer in ensuring the carbs are perfectly balanced side to side.


For your issue, I’m still wondering why the two sides do not match.
Once they do match, I suspect the popping will disappear all together.

Rich

OK. so my flow at idle (950) was
DS front 5.25 (needle was touching the 5, but slightly higher)
DS rear 6 (needle was smack on the #)

PS front 5.25
DS rear 5

Some folks say "close enough" some say get them dead nuts....

I will take a look at the cables today. I am pretty sure I have no extra binding, I did have that earlier in this process, but I never really looked at the timing of the pulleys.

[r_towle]

Watch the movement of both carbs just off idle.
That is the key moment.
Slowly pull Accel cable until 1/4 to 1/2 open
Notice the difference side to side

[r_towle]

Did you try to pull the main stack, and drive around on the idle jets? Conservative Throttle/driving should not need an accelerator pump.
Did you check the sync of the carbs at 1500 rpm or 3000 rpm?

Have not done that yet.... set-up wise, what is removing the jets gonna tell me. I understand that it will give me a FEEL for where the MAINS kick in, but what else can I learn there??

I did CHECK the flow on the carbs at 1800 rpm. But I only checked it, did not adjust anything at 1800rpms

Drivers side was 11 on the front barrel and 12 on the rear.
Pass side was 10 on the front and 10 on the rear.

So is it recommended to re-sync at that RPM? Kinda go back and forth from 900-ish to 1800-ish adjusting so all 4 barrels are flowing evenly at both RPMs??

10 on one side, 12 on the other.
That stuck in my head.

[nditiz1]

Oh wow I missed this, yeah you definitely need to make sure they are close at all stages of the throttle. Also, if at idle front and rear are in sync there should never be a front at 11 and back at 12. One reason why I hate crossbar linkage. At least for a bus you can look directly at the carbs instead of a top view like in a teener

[malcolm2]

Did you try to pull the main stack, and drive around on the idle jets? Conservative Throttle/driving should not need an accelerator pump.
Did you check the sync of the carbs at 1500 rpm or 3000 rpm?

Have not done that yet.... set-up wise, what is removing the jets gonna tell me. I understand that it will give me a FEEL for where the MAINS kick in, but what else can I learn there??

I did CHECK the flow on the carbs at 1800 rpm. But I only checked it, did not adjust anything at 1800rpms

Drivers side was 11 on the front barrel and 12 on the rear.
Pass side was 10 on the front and 10 on the rear.

So is it recommended to re-sync at that RPM? Kinda go back and forth from 900-ish to 1800-ish adjusting so all 4 barrels are flowing evenly at both RPMs??

10 on one side, 12 on the other.
That stuck in my head.

interesting thought here.... the Drivers side is the SLAVE. So if there is slack in the cable, wouldn't the flow on the DS be lower than on the Passenger side?

[rfinegan]

I bring this up as it seem to be a lot of talk up changing the idle jet sizes...Driving on the idle jets circuit will give a better idea if you rich or lean and can tune that first before moving to the transition circuit, main jets and accelerator pump. The AFM will be best but reading the plugs will get you there too, one step at a time.

General rule here for the mixture screws: Plates below transition holes
1/2 turn from closed or less = jet too big
1/2 to 1 1/2 from closed = jets ok
1 1/2 or more from closed = jets too small
You can have slight variations between mixture screes but they should be close each other
Jets size will be tuned when you get to the transition tuning stage. (step 5)below

Syncing the idle is EZ, as you have a throttle plate screw on each side. The final speed can be adjusted with air bleeds and timing if needed.
You can follow along as there are a few write up on this :
Example:
https://www.aircooled.net/how-to-use-a-wide...-flat-4-engine/
snippet :
The carburetion adjustment is carried out as follows:

1. Adjust the position of idle throttle just below the machined progress holes in the body (using a vacuum hose).

2. Synchronize the second carburetor.

3. Adjusting the idle speed with the idle screws to a value at the bottom of the 14 / 14.7: 1 range a little richer than the stoichiometric point for good idle stability. The idle speed should be around 800 RPM.

4. Adjust the idle speed to the desired value by adjusting the idle by-pass screws, or by increasing the idling advance value, check that the maximum advance does not exceed 28 / 30 ° before TDC.

5. Idle jet selection, main nozzle assemblies / emulsion tubes / air nozzles removed. We aim for an AFR of 16/17 in order to obtain a low consumption in cruising. Re-adjust the idle speed at each idle jet change.

6. Selection of the air nozzle, main jet assemblies / emulsion tubes / return air nozzles, with oversized main nozzles. The hole between the progression circuit and the main circuit must disappear by gradually increasing the size of the air nozzle.

7. Select the size of the main jet by accelerating at full load, aiming at an AFR of between 12.5: 1 and 13: 1, ideally 12.75: 1.

8. Reassemble the pump stem nuts and screw them in until the slightest hesitation during a very sudden acceleration disappears completely.

9. ENJOY YOUR “NEW” TOY!

Did you try to pull the main stack, and drive around on the idle jets? Conservative Throttle/driving should not need an accelerator pump.
Did you check the sync of the carbs at 1500 rpm or 3000 rpm?

Have not done that yet.... set-up wise, what is removing the jets gonna tell me. I understand that it will give me a FEEL for where the MAINS kick in, but what else can I learn there??

I did CHECK the flow on the carbs at 1800 rpm. But I only checked it, did not adjust anything at 1800rpms

Drivers side was 11 on the front barrel and 12 on the rear.
Pass side was 10 on the front and 10 on the rear.

So is it recommended to re-sync at that RPM? Kinda go back and forth from 900-ish to 1800-ish adjusting so all 4 barrels are flowing evenly at both RPMs??

[malcolm2]

Oh wow I missed this, yeah you definitely need to make sure they are close at all stages of the throttle. Also, if at idle front and rear are in sync there should never be a front at 11 and back at 12. One reason why I hate crossbar linkage. At least for a bus you can look directly at the carbs instead of a top view like in a teener

The Weber book, or some book I had, had me start the flow check using the FRONT barrels. It mentioned to use the barrels closest to the shaft controls. One reason why the FRONTs match. But I think you mis-read, I do not have a match on a CARB at idle. table below might help.

Also had replies that said, "if the flow is close, leave it..." Some said, "they never touch the bypass screws...." Some gave a range to adjust to, like the 0.25.


Anyway, It seems that after I VERIFY the cable sync is activating both carbs at the same time......

i should go back and putz with the bypass screws and

1. get all 4 equal at idle.
2. then tweak a bit at a higher rpm
3. then tweek a bit at idle again.... back and forth until both speeds have very close flow across the barrels.

Should I try even higher RPM, like 3000?

[rfinegan]

If you have big differences in from and back on a carb make sure the plates are closing at the same time. It is very common to have twisted throttle shafts on used carbs. These are new so it should be fine?

[malcolm2]

Watch the movement of both carbs just off idle.
That is the key moment.
Slowly pull Accel cable until 1/4 to 1/2 open
Notice the difference side to side

WELL DAMN>>> the slave carb was a bit behind the master. The Sync-Link is very easy to adjust that.

Video shows the slave pulley turning at the same time as the cable moves. It WAS NOT doing that prior. The cable was loose.

Rest of the vid is the engine running and me reving it. I hear no "flat spot". Do you, or anything else??



https://www.youtube.com/watch?v=_4DanFqkpYc





Test DRIVE: I did hear just one POP this time. It moved. It was as I shifted 2nd to 3rd. But it only did it that one time.... FINGERS CROSSED

Since that was an issue and I have been adjusting the Accell pump and now have them adjusted IN 2.5 turns from new. Should I consider returning the nuts to where they were?