Factory Dual Carbs No Idle

[Mase_00_7]

Hi Colin,

I did post on The Samba but thought you'd see this sooner.

Basically I am trying to work through your instructions but unfortunately I cannot get my bus to idle properly.

I have backed out both volume mixture screws to 3 1/2 turns out, the idle mixture is out by 4 turns and the ICS screw is out by 4 1/2 turns.

It now *almost" idles (it chuggs and chuggs before finally dieing) but I can't get it up to anywhere near 1,000rpm.

I have timed it with both vac hoses off to 30* BTDC (was aiming for 28* but missed Very Happy) and I get backfires through the carbs after revving a bit.

Any ideas?

Cheers, Paul.

[Amskeptic]

I cannot get my bus to idle properly.
volume mixture screws to 3 1/2 turns out,
idle mixture is out by 4 turns
CIS screw is out by 4 1/2 turns.
*almost" idles (it chuggs and chuggs before finally dieing)
backfires through the carbs after revving a bit.

Try closing the Central Idling speed screw, the big one, with the retard hose off.
Can it idle now? Fuel supply to central idling circuit may be obstructed.

Still can't idle? Block off the intake manifold take-offs for the brake booster with little sections of 13mm breather hose that have bolts screwed into the ends of them. Idles now?
Massive vacuum leak in booster circuit.

Still can't idle? Fuel delivery issue.
I will check back in in a day or so.
Colin

[Mase_00_7]

Awesome, cheers Colin I will try that all in the morning and let you know how I get on.

Thanks, Paul.

[Mase_00_7]

Try closing the Central Idling speed screw, the big one, with the retard hose off.
Can it idle now? Fuel supply to central idling circuit may be obstructed.

Still can't idle? Block off the intake manifold take-offs for the brake booster with little sections of 13mm breather hose that have bolts screwed into the ends of them. Idles now?
Massive vacuum leak in booster circuit.

Still can't idle? Fuel delivery issue.
I will check back in in a day or so.
Colin

Closed off central idle and plugged retard hose and still won't idle.

I removed the vac hose from the booster bar and plugged it from where it exits behind the tin (elbows are all new and very tight) and still no difference so it sounds like a fuel delivery issue.

To get it to "idle" and I say that in the loosest sense of the word, I have to back all the mixture screws out, but then I pulled the plugs and they are wet/oily black, yet it doesn't smoke at all when running (either blue, white or black)

When the chokes are on (i.e. cold start) and the throttle valve is being held slightly open on the fast idle cam, it will fire up off the key without any touch on the accelerator pedal and rev to about 1500rpm before coming down and dieing.

What do you think? I have had the carbs apart a few times, cleaned them out with carb cleaner and blown them through with compressed air...

[Amskeptic]

Try closing the Central Idling speed screw, the big one, with the retard hose off.
Can it idle now? Fuel supply to central idling circuit may be obstructed.

Still can't idle? Block off the intake manifold take-offs for the brake booster with little sections of 13mm breather hose that have bolts screwed into the ends of them. Idles now?
Massive vacuum leak in booster circuit.

Still can't idle? Fuel delivery issue.
I will check back in in a day or so.
Colin

Closed off central idle and plugged retard hose and still won't idle.

I removed the vac hose from the booster bar and plugged it from where it exits behind the tin (elbows are all new and very tight) and still no difference so it sounds like a fuel delivery issue.

To get it to "idle" and I say that in the loosest sense of the word, I have to back all the mixture screws out, but then I pulled the plugs and they are wet/oily black, yet it doesn't smoke at all when running (either blue, white or black)

When the chokes are on (i.e. cold start) and the throttle valve is being held slightly open on the fast idle cam, it will fire up off the key without any touch on the accelerator pedal and rev to about 1500rpm before coming down and dieing.

What do you think? I have had the carbs apart a few times, cleaned them out with carb cleaner and blown them through with compressed air...

OK, we have established that you do not have a nasty vacuum leak.

Ignition-on only, check that all cut-off jets are clicking as you remove and replace the wires to them. If no, replace. If yes, unscrew both cut-offs approximately 1/4 to 1/2 turn from seated. Start engine and allow to come off the chokes. Does the engine continue to run? You might even have to lean out the mixture screws a bit! If it runs well with the cut-offs unscrewed, you have fuel delivery failure in the idle circuits. You can remove the cut-offs and clean their orifices with carb cleaner, blast the openings into the carburetors where the cut-offs screw in, and you can remove and disassemble carbs making sure that the base gasket between the body of the carb and the throttle body is correctly installed. The idle circuits have very small little pin holes in the throttle bodies that are either under the throttle plate or just above the throttle plate. The ones under the throttle plate draw in fuel/air. As you open the throttles, the other little pin holes successively go under the edge of the throttle plates. These give you your transition to the main jets at higher rpm.
The other symptom that allows you to "fix" the idle by unscrewing the cut-oofs a bit, is a very low fuel level. It is supposed to be 13mm from the top of the main carb body wioth the top off. Some people accidentally st them to Type 1 level, which is 17mm.
ColinAin'tThisFun?

[Mase_00_7]

Colin,

If there wasn't thousands of miles of open ocean between us I would hunt you down and kiss you!

All cut-off's work but unscrewing both idle cut-off's (not the central idle one) revealed a flawless idle that was rock steady with no hunting/popping/hissing etc etc, and the engine rev'd nice and freely and then came back to idle in a very nice and controlled manner =D>

I am going to remove and strip both carbs (again!) so what in particular should I look out for, and how can I go about ensuring that the cut-off's aren't blocked, that they are "working" correctly (i.e not just clicking) and that the relevant passages are open properly.

I have rebuilt these carbs twice now, and each time I have been careful to clean everything properly and blown it all out with compressed air, but something has been missed here.

I would like to make a contribution to this site or to a chosen charity to say thank you for taking the time to help not just me, but a lot of other enthusiasts with their problems. =D>

I think it's fair to say that without your help I would be using my bus as a bonfire very soon as I had come to the end of my tether with it! Anyway, let me know what I can do :)

Cheers, Paul.

[Mase_00_7]

Well pulled the cut off's to clean and test them and found that one is a 55 and the other is a 57.5. Make much difference?

Also, the holes are arranged differently on each as shown in the below picture. The 57.5 is on the right and the holes are in the "step" which comes further out towards the tip, and the 55 has the holes the same distance from the tip but the "step" is further back so the holes are drilled in a narrower section. The 57.5 came from the left carb:



I tested both with a bit of fuel hose attached and filled with carb cleaner. Activated the solenoid by giving it 12v and both then allowed fluid through.

Do you think my problem is related to these mismatching cut-off's or is it more likely to be crap in the carb passages?

Cheers, Paul.

[Mase_00_7]

Ok so all passages in both carbs are definately perfectly clear (I have extensively tested each carb and all the little holes flow carb cleaner etc as they are supposed to).

I also went and bought a new cut-off solenoid which didn't make any difference. The only way I can get the engine to idle is by unscrewing the cut-off's.

Float level is bang on (13-14mm) so that isn't the issue, it just seems that the 55 jet is too restrictive.

What about opening up the volume mixture screw to something like 4 turns out? Feasible or is this not the best way to go about fixing this issue?

Cheers, Paul.

[Amskeptic]

Ok so all passages in both carbs are definately perfectly clear (I have extensively tested each carb and all the little holes flow carb cleaner etc as they are supposed to).

I also went and bought a new cut-off solenoid which didn't make any difference. The only way I can get the engine to idle is by unscrewing the cut-off's.

Float level is bang on (13-14mm) so that isn't the issue, it just seems that the 55 jet is too restrictive.

What about opening up the volume mixture screw to something like 4 turns out? Feasible or is this not the best way to go about fixing this issue?

Cheers, Paul.

So you have ensured that the little bitty itty bitty holes that are arrayed around the throttle plate in the throttle bodies are all clear?

We had an issue with poor acceleration and stumbling yesterday with a '73 bus that was improved markedly by above cleaning of little holes in throttle bodies and jacking up the main jet sizes to 137.5.

It is possible that a lifetime of screwing in those cut-offs has drilled a deep well in the pot metal of carburetors that blocks the proper fuel flow down to the transition ports. I leave it to you to figure out how to increase flow.

[Mase_00_7]

I will remove both throttle bodies for further cleaning just to be sure!

Am I right in saying that when closed the throttle plate should cover not only the vac advance port, but it should also be over the top of one of the transition ports just above the port that has the volume mixture needle poking through?

Also, would richening up the central idle circuit help compensate for lean conditions at idle? I assume that moving over to the main jet once rpm's increase would mean that the engine wouldn't be running lean once moving?

[Amskeptic]

I will remove both throttle bodies for further cleaning just to be sure!

Am I right in saying that when closed the throttle plate should cover not only the vac advance port, but it should also be over the top of one of the transition ports just above the port that has the volume mixture needle poking through?

Also, would richening up the central idle circuit help compensate for lean conditions at idle? I assume that moving over to the main jet once rpm's increase would mean that the engine wouldn't be running lean once moving?

Your throttle plates should be adjusted to the 1/4-1/2 turn or so from fully closed regardless of the transfer ports. They come into play when the throttles are moved. As it happens, yes the idle screw ports are visible under the plates when at idle. You adjust the central idling circuit to its favorite spot + a little. Unfortunately, it ducks out of the picture pretty quickly when you just crack the throttle plates, so it cannot help your issue of transitioning.

What are your main jet sizes? What are the air correction jet sizes? What rpm do you get with retard off and central disconnected?
Colin

[Mase_00_7]

Ok, well in that case both throttle plates are adjusted properly, and when I managed to get it to idle with the pilot's unscrewed I measured the airflow and both carbs were fairly well sync'd.

Jets are standard for a '72 1700 manual except for 135 mains in place of the 137.5 (so 155 air correction, 55 idle, and a non-removable jet directly above the main nozzle with a 50 on it)

In all cases these jets are larger than specified for the 1800, so I am now wondering whether my air correction jets are adding too much air to the mixture or would that not make much difference at idle?

[Amskeptic]

Ok, well in that case both throttle plates are adjusted properly, and when I managed to get it to idle with the pilot's unscrewed I measured the airflow and both carbs were fairly well sync'd.

Jets are standard for a '72 1700 manual except for 135 mains in place of the 137.5 (so 155 air correction, 55 idle, and a non-removable jet directly above the main nozzle with a 50 on it)

In all cases these jets are larger than specified for the 1800, so I am now wondering whether my air correction jets are adding too much air to the mixture or would that not make much difference at idle?

I would stick in 140 air correction jets. Do not assume that the specified jets for the 1800 means that those jet sizes are for a bigger engine. That is not the way it was actually going for VW. They were strangling the engine to allow them to sell in rigid emissions markets. The '72 jets were Let Er Fly jets.
Colin

[Mase_00_7]

Ok I will keep an eye out for some 140's then.

One thing that now has me really puzzled is that I pulled my plugs today and replaced them with a new set of NGK's (I originally fitted Bosch but these are going to be spares now) and they were completely black and wet.

The engine doesn't give any puffs of smoke of any colour but it does smell rich to me, however if I try leaning anything out then it won't run at all.

I'm about ready to give up on these things so what carbs would be a good replacement for fuel economy and reliability over all else?

Cheers, Paul.