I have been doing some searching about the Pump adjustment. I seem to be leaning toward the CC measurement as well. Gonna be tricky getting a bunch of syringes and wires etc....

Do you feel that is necessary? Even with that, the "nut turning" is required.

I counted threads and got 10 threads or so showing on each.

More threads will equal more PUMP, right?

So i figured I would turn 1/4 turn and drive. Hitting the same stop sign and load point each time until I get no popping.

Sound logical?

TO BE CLEAR.... when you say LINKAGE above, you mean the accel pump linkage, right?

Yes, sorry for lack of clarity -- I meant the accelerator pump linkage. At this point, don't mess with your main carb linkages.

I never did the CC measurement thing. Can't be a bad thing. I just never found the need.

I think you'll find that you will need more like 3 (or more) turns (in) to make a meaningful difference that you'll perceive. Also be aware that adjusting the nut has a limited range of increase / decrease. It is usually enough to move the needle so that you can learn what the engine wants. However, to make bigger changes, the accel pump bypass jet (bottom of float bowl) and/or the nozzle will need to be change. Not likely you'll need to do this but if you find that moving the nut in helps but eventually it doesn't help moving it in further, you've reached the limit of what can be done via the nut.

Moving the nut is just tweaking how far the accel pump diaphram is allowed to move as it sucks fuel in.

The accel pump bypass jet varies how much fuel is allowed to flow backwards back into the bowl when the accel pump is activated. I forget how Weber sized these by number. A zero bypass does what it implies, basically a one way check valve that allows fuel in but no flow in reverse direction back to the bowl.

In High School, I had a buddy.... used to buy and trade Trans Ams..... He would go drive one and tell me "I'm not gonna get too excited about it...."

Well, I am pretty EXCITED about it tonight.

I moved each side's pump nut in 1 turn..... big help

then a 1/2 turn..... maybe a bit more better

+ another 1/2 turn (2 total)..... No POPS.

And I was driving with the hatch off so the engine was roaring right there with me. Remember this is a TIV 2.0 in a 72 bus.

These were short drives, from cul de sac to cul de sac not really any big hills. Enough load to make it pop before, but not now.

Tomorrow I will travel a bit further.... This is great.

Now, who is gonna tell me what adjustment or tweeking will be next??? Is there a next?

Thanks again. Longer drive is coming!!!

I think you got it. Button it up and drive around. I would not stray too far. Carry the appropriate tools. Have fun!

This is a great experiment for us carb guys. I have lived with pops, sneezes, and the like for many years simply by being lazy. At least now I have one less excuse

A big shout out to all contributors

T oo US Carb Guys

Carbs are better than FI

I'm a carb guy and do not think carbs are better than MFI, EFI, CIS

1) carbs are better than FI
2) you cannot turbo a 914
3) it’s not a real Porsche

@clay what some of the other longer term “laws” regarding the 914?

After driving some more - keep monitoring plugs to determine if running rich or lean.

Really suggest that you find a nice flat stretch of road where you can get up to about 60 mph for a bit. Pull the main jet / emulsion tubes. Super easy to do. Get a feel for how much time is spent driving on idle circuit and where that transition to the mains occurs. Without the mains in place, that transition will be where the car just begins to fall flat the more you open throttle and the higher the RPMs. Don’t drive in that condition for long (you’ll be running lean) but find it then back off / slow down - back to running on the idle circuit.

That transition to mains is dependent on speed, load, throttle opening, and RPMs. It really helps to get an idea of just when you need the mains.

When you’re done playing - just put the main tunes back in and you’ve now learned more about your carbs and tuning them - for free!

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?

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

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.

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.

Click to view attachment

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

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?

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??

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.

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

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.

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

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

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

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?

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!

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?

Click to view attachment

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?

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?

Sounds much better...did you sync with the snail or just first movement of the slave?

I double checked with the snail after I tightened the cable.... All flow numbers FOR IDLE still match the table I posted above. made no changes there tonite and no testing over idle.

But I do want to know about the accell pumps I adjusted. Thoughts on returning those?

That sounds better, much better.
Go drive it

I would stop adjusting now.
I would swap in new plugs, drive it till hot, try to shut it off while revving with no idle
Then read the plugs

You are down to just reading the plugs now without a EGT sensor on each port….and don’t worry, reading plugs is how it is still done.

Google how to test and read plugs
Specifically the process.

For me, when I think I am right, I take a highway run.
I line up an exit at speed then shut it down and glide into the rest area.

Plan ahead, find a simple safe place todo this

The idle circuit is naturally very rich, so it will make the reading of the plugs false if you idle too long after the run.

Hard to explain.

But, before you adjust anything you now need data.

Not likely the throttle shafts are twisted due to them being new.still a good point as if the arm or cam wheel is not supported when tightening this will happen ,had that on one of my Zenith carbs and had to replace on spindle and the jointer coupler...so you never know.
Anyway I degress,have read this one over and over and IMO there is a vacuum problem caused by valves that aren't seating correctly or a leak @ the manifolds/carbs,reason for saying this is;
incomplete combustion as seen on the plugs ?,could be ignition issue also ?,
uneven vacuum reading well out of specs and not correcting with tuning,
uneven setting "tip in" idle on throttle stops,
valves not seating when closed ,could be correct lift not dialed or geometry wrong ?,
piston rings haven't "bedded in " so a compression test will reveal that ,
What we don't know is with other variable's but I would be a doing a compression test first,think you are over carb tuning as the vacuum is the issue here.
good luck and cheers.

Pretty sure a compression test was one of the early things that he did, maybe at the beginning.

Thanks Rich ,missed that...not good for a "fresh" build ,even with low comp pistons it should be 125 psi even ish if run in properly ? the variation between cyl's 1 and 3 exceeds what is the norm even for a used motor .
Think the OP should do another compression test when engine is warm and if result is the same put a squirt of oil down the plug holes,if a retest shows an increase thats the issue,if as I suspect you will need to do a "leak down" test on your valves also.
Don't for a minute trust the manifold gaskets with the common intake manifold kits around,the EMPI ones for example are very soft and too thick causing leaks after they are fitted ,ask me how I know,most likely better check that first.
I persist asking about this but fitted 36dlra's to my last carb case from 411,NOS raised top pistons and 1.8 barrels,fully rebuilt heads,my carbs had little done to them but setup on a bench and cleaned,motor ran unreal almost out of the box as they say,so without being a dick something is wrong here reason I'am asking,cheers.
Click to view attachment
Click to view attachment

Something could have gone wrong. The mechanic here is me. Even doing the BEST LEAN set-up in the beginning was challenging to me. The Weber to-do list was not clear for a real carb beginner, like me. Poorly written, if you ask me.

Part of the reason I paid AC.net for the cleaning, set-up and jetting, etc..... I figured I could follow directions to get the damn thing on the road.

This process has taught me a good bit, but not near enough yet.

It is running nicely now, or at least I would rank it as NICE. Ran some good hills etc... this week. Bus is a different animal vs the 914, wow is it different.

Gonna put some more miles on it and take the plug reading recommendation after I fix a few other items.

Thanks again everyone.... Stay tuned.

Clark

Congratulations.
remember...same (or less powered) motor...yet add in terrible aerodynamics, and about 1500 lbs.

The great thing about a bus is it teaches you how to plan when you want to pass someone, or go up a long hill.

momentum is your friend.

Rich

hey guys,
Kevin here,
been lurking here ,working on a friend 914, fresh rebuild, trying to carefully break in the motor,, even added zinc for peace of mind.

I'm a Fuel injection guy, absolutely hate carbs on a stock 914 motor.
so you know where I come from.

anyhow, helping my friend here, and willing to learn about carbs and their adjustment stuff.

I had dual carbs on my Type III awhile back, so familiar with them.
(I even put back the fuel injection the ole Type III had.)
another story.

After fidgety with the 40's Weber carbs,
get them settled in,
removal of throttle linkage info was valuable !
(someone else had rebuilt motor and install the carbs)

experiencing constant backfire issues.
someone mentioned the exhaust may be leaking?
I wouldn't doubt because the improper used of washers on the exhaust studs,
I corrected em,

should I remove the exhaust to inspect for leakage or is there any other suggestions for me?

Me and Joe, his 914, are trying to get his 914 ready for Red Rocks.
will appreciate any info or leads.
Thanks!

oh yeah,
his motor is a 2056

Where is the backfire occurring? Out of the intake manifold or in the exhaust system?

No point in messing with the exhaust if it’s backfiring out of the intake

yes, back firing through carbs.

Verify plug wires vs firing order.

Verify timing

Verify valve lash - especially on intake valve but do both intake and exhaust while you’re there. This should be done before adjusting carbs if there is any question about what valve clearances are.

SuperHawk has good advice:




If ignition timing is correct, advances working then.

Compression test, leak down test, R&R heads to repair poor sealing on intake valves would be my next steps.

If comp/leak down were good I'd be confused.

Thank guys!!

I was wondering about the valves too,
Previous mechanic was a lil out there, and I inherit this mess.