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

[930cabman]

How does she run? WOT? seems to idle ok

[Superhawk996]

Still sounding a bit like it’s bogging a little when throttle begins to open before it picks up RPMs. You are on 28mm venturis so should be better than that.

I think you’re getting a bit distracted by fuel pressure. To be honest, carbs don’t care much and will run in a wide range of pressure between 1-4 psi. Don’t believe me. Try this - run your fuel pump till bowls are full - then disconnect and plug the fuel line. You’ll be amazed how long it will idle / and do light transitions before the bowls run low and start running really poorly.

You’re expecting a 0-15 psi gauge to be accurate in the 1-3 psi range. I think you’re probably seeing gauge “sticktion” at those low pressures where the needle temporarily sticks from internal gauge friction with just barely enough pressure to even begin to move the gauge. As someone said earlier you want a gauge that is about 0-5 psi to have an accurate indication.

Depending on the range that the regulator is designed for it may be that the “sticktion” is in the regulator if that was designed for 0-15 psi. Edit: just looked at your JEG’s link - you should be peachy!

Bottom line - I suspect you’re chasing your tail on fuel pressure.

Barrels to barrel airflow is pretty good. Edit: after watching a 2nd time - I’d say that one cylinder is 6 vs all the others at 6.5. Might bring that one up to 6.5 with air bypass. Then recheck idle mix.

Are you sure your idle mix is tuned properly for best lean idle? I know you said you’re about one turn out which is probably about right but you should be getting an idle speed drop if you open or close the idle mix from that setting if you’re at best lean idle. Is that the case?

What is your idle speed set at? The tach is not the best indicator if true idle speed. I use a digital engine multimeter to set idle speed. Idle sounds sort of low. Once you have best lean idle set, you should only be using the carb linkage, idle screw to tweek the idle speed. After you tweek that, you need to go back and recheck best lean idle mix - it’s an iterative loop.

Also what distributor set up are you using? You may not be getting enough advance early enough.

Final edit - I just compared 1st video to latest. Defiantly improving. Good job. (IMG:style_emoticons/default/smilie_pokal.gif) Keep tweaking it and once you have the idle speed and mix finessed - go back and recheck carb to carb sync and how equally your linkages are moving. You’re getting there

[Jack Standz]

Yes, fuel pressure could be the problem, if you truly are having little or none.

I recently had a problem where the fuel pressure gauge was showing zero and the regulator made no change to the reading turned all the way in or out. The motor would run. But, it wouldn't run right or wouldn't take a tune. Since the motor would run (some), I just couldn't believe the gauge.

Completely new fuel delivery system, new carbs, new motor. Turns out the fuel line was kinked under the tank (914 versus bus). Seems that some fuel could make it through. At least the first carb in the set was gettingsome fuel. Bus fuel lines are different, but I suppose something could pinch it.

Probably not your problem, but worth checking. Especially since you said the gauge goes to zero. But, it could be just the gauge. I bought a second gauge ($15 on Amazon) that also showed zero before I figured out the fuel line was pinched. When you get it figured out, run with 3 psi or less, as long as fuel isn't dribbling out the auxiliary venturis. My 2056 likes about 2.5 psi give or take.

If you want to tune these carbs, suggest getting a colortune spark plug (Google it) and a wide band AFR gauge (you decide whether to permanently mount it or not. If permanently mounted in a bus, you'll need the longer wiring harness.

[malcolm2]

All day run with new plugs. Lots of adjusting etc.. but not DRY JET BLACK.

[Superhawk996]

You’re running way too rich

Have you verified that the throttle plate is closed, covering the 1st transition port when the idle speed screws aren’t even touching the linkage?

This is Hugely important.

If you have done that then I suspect the idle mix isn’t being adjusted properly for best lean idle

OR

You will need to go down on idle jet size

[malcolm2]

You’re running way too rich

Have you verified that the throttle plate is closed, covering the 1st transition port when the idle speed screws aren’t even touching the linkage?

This is Hugely important.

If you have done that then I suspect the idle mix isn’t being adjusted properly for best lean idle



You will need to go down on idle jet size

Only way to verify now is that the speed screw is hitting the bracket. I did look at it when it was on the bench. Fully closed. Several little holes in the barrel under the plate. Seemed closed to me.

Remember, this pic is from idling all day and setting and resetting screws. All that. They look a bit lean to me. Gray??

[Superhawk996]

look a bit lean to me. Gray??

There are also a series of small holes (the transition ports) that should closed and a few that are open or just obscured above the plate when the throttle plates are closed.


It is critical that you’re not running idle such that the transition ports are uncovered by having the idle speed screw set too far open. You want to make sure you don’t have the condition shown in illustration B when idling.

With respect to the plug condition, that plug is no where near a lean condition even when looking at only the insulator.

Here’s a plug visual reference

[malcolm2]

Are you sure your idle mix is tuned properly for best lean idle? I know you said you’re about one turn out which is probably about right but you should be getting an idle speed drop if you open or close the idle mix from that setting if you’re at best lean idle. Is that the case?

What is your idle speed set at? The tach is not the best indicator if true idle speed. I use a digital engine multimeter to set idle speed. Idle sounds sort of low. Once you have best lean idle set, you should only be using the carb linkage, idle screw to tweek the idle speed. After you tweek that, you need to go back and recheck best lean idle mix - it’s an iterative loop.

Also what distributor set up are you using? You may not be getting enough advance early enough.

Here is the LBI method I was using, per http://www.redlineweber.com/html/Tech/idf_...nt_controls.htm

DO YOU SEE ANYTHING ELSE I SHOULD DO, OR DO DIFFERENTLY??

Linkage removed
Speed Screw 1/2 turn in after touching
Bypass screws closed
Mixture screws out 1 turn

Engine runs poorly as expected.
1. turn IN mix screw until the engine runs worse... i would turn each screw about 1/4, then move to the next one, and so on. 2nd time around (1/4 at a time), as I got to the 3rd barrel, the engine would run worse.
2. turn OUT mix screw until the engine runs better. Again 1/4 turn at a time, til I got no reaction. Turn them back IN until they are BEST.

Then I set the Speed screw to 950 to 1000rpm. I have an analog meter.... one lead on batt- one lead on coil-. using the V8 scale and doubling it. I am sure you remember those old timers.

I placed the snail on the front barrel of one side then the other. adjusted the High flow barrel down to the low flow barrel with the speed screw. then adjust both back to 950 -1000, matching flow using the snail.

From there, i went to the bypass screws. checking flow front barrel to back barrel and adjusting the LOW flow up to the high flow. Leaving the high flow CLOSED. Then again, matching the right to left carb's flow to the idle rpm with the speed screw.

I have a new pertronix SVDA disty with a P3 module in it. I dismantled it, and cleaned and lubed it after arrival.... another AC.net item.

[malcolm2]

If you want to tune these carbs, suggest getting a colortune spark plug (Google it) and a wide band AFR gauge (you decide whether to permanently mount it or not. If permanently mounted in a bus, you'll need the longer wiring harness.

Was thinking I should put an A/F on the bus and eventually I will.

I have a A/F on my 914. It has a blue tooth adapter that allows me to use my phone for the readout. Unfortunately, they have been out of stock for over a year. Guess I need to find another vendor....

https://www.plxdevices.com/ProductDetails.a...de=897346002931

[Jack Standz]

You’re running way too rich

Yes, +1.

[rhodyguy]

Now you’re seeing how small adjustments can result in BIG changes. Remember to pause a bit after every adj. Bosch plug #?

[Superhawk996]

Linkage removed
Speed Screw 1/2 turn in after touching
Bypass screws closed
Mixture screws out 1 turn

Generally I think you’re on the right path and doing reasonable things. You don’t mention readjusting mixtures after resetting idle speed. Idle mix affects idle speed and vice versa. Same again after setting an air bypass - recheck mix all the way around again after that. Everything affects everything else.

I might go only 1/4 turn on the initial idle speed screw just to ensure that transition port is fully closed when you are adjusting mixture initially.

And probably just slow down on the idle mix screw adjustment - there is always some time delay between turning the screw and observing the effect when it finally gets to the engine.

Maybe smaller 1/8 turn changes as you get closer. It really is a multiple iteration process between mixture, idle speed, air bypass. Each change affects the others. Agree with Rhodyguy that small changes have big effects as you get closer and closer.

Check ignition advance and make sure you’re starting to begin a good advance by 1500 rpm. As noted on NGK chart retarded timing (not enough advance) can manifest as running rich. Basically just verify SVDA is working properly.

And finally, it isn’t unheard of to have to go down an idle jet size. Too big of a jet really jacks up the speed & mixture effects of the needle mix screw adjustments. .

[malcolm2]

Now you’re seeing how small adjustments can result in BIG changes. Remember to pause a bit after every adj. Bosch plug #?

NGK BPR6ES

[malcolm2]

How does she run? WOT? seems to idle ok

I would not say WOT, but fast enough for me to be a bit worried.... 55mph in a bus is kinda scary. But engine-wise it felt nice.... a bit up hill, accelerating all the way thru 4th gear.

I live on a hill with 4 speed humps. The trip on the day that crap started raising it's ugly head, I did not feel like I had much power.

TODAY, I went up the hill 2 times, 2nd gear, pulling hard after the speed humps to the next hump.... also felt good. Enough power for a 72 bus with a 2.0 liter, bus pistons and camper special heads!!! Much better. Drove about 8 miles today around the hood and down to the elementary school on the long straight is where I got to 55. (IMG:style_emoticons/default/piratenanner.gif) (IMG:style_emoticons/default/piratenanner.gif)

[malcolm2]

You’re running way too rich

Have you verified that the throttle plate is closed, covering the 1st transition port when the idle speed screws aren’t even touching the linkage?

This is Hugely important.

If you have done that then I suspect the idle mix isn’t being adjusted properly for best lean idle

OR

You will need to go down on idle jet size

I found a way to MAYBE verify your concern about the throttle plate and maybe you are right, but how do I deal with it? As you can see from the last vid, all 4 barrels are very close in flow. So here is what i found.








It was obvious that the tip of the speed screw shows more threads on the PASS Side than on the Drivers Side. Wouldn't that mean that there is a chance that the plate is open past the port you showed me???

What can I do to make it close more and still run nicely?

[Superhawk996]

I wouldn’t judge by that 1.3mm variance in the amount of screw showing. You have casting variance, stamped linkage variance, and variance in the length of the screw. That 1.3mm variance is sort of normal when you synch carb to carb.

Especially since you seem to be happy that it’s running better and improving.

I’d just stay focused on trying to lean out idle mixture and given where you are in the process, I’d just swap the idle jet and keep moving forward.

I’ve always made a point of being sure that the transition ports are closed when it is on the bench for initial bench settings. It’s much harder to do when the carbs are installed.

What I’d probably do is shine a light down the throttle bore and verify that the throttle butterfly is hardly open when it’s setting at rest on the idle speed screw stop. Throttle butterfly plates should basically be closed with only a tiny sliver of a gap. Maybe you might be able to see the top of one of the transition ports sitting ABOVE the plate that would be good. If the butterfly plates seem to be lifted significantly when sitting in the stops, I might pull them and bench check where they are sitting. Just pulling them and replacing them won’t mess up too much of the tuning you’ve done but you’ll have to verify and then re synch again.

I would really hate for you to pull them again which is why I’m suggesting you go down in idle jet 1st. It’s easier and is probably going to be more of a sure thing vs the chance that the transition port is uncovered at idle.

[r_towle]

The throttle plates should be visible from above to let you see how they are sitting at idle.
Idle is zero throttle
They should match.

Like Stated above, the screw position can be different
The throttle plates should not be different

To make yourself comfy with how they work….I might suggest you remove the throttle linkage
Start car
Turn screw out (close throttle plates)
Listen to the difference on each side
Then check again with the air flow meter, adjust as needed.

You are getting close, and thankfully once you get them setup perfectly for your engine, you won’t need to adjust them anymore.

Get the plugs brown, not black
It can be done, and you will be thrilled once you nail it

Then you can answer all carb related questions here (IMG:style_emoticons/default/smile.gif)

I listen to the motor, your ear can tell once it purrs

[malcolm2]

OK so I should get smaller Weber IDF Idle Jets.... CB has 0.45 and 0.40. figured I would get 4 of each. $4.50 each.

I have 0.50

Seem correct?

https://www.cbperformance.com/product-p/639...=1&CartID=1

[930cabman]

OK so I should get smaller Weber IDF Idle Jets.... CB has 0.45 and 0.40. figured I would get 4 of each. $4.50 each.

I have 0.50

Seem correct?

https://www.cbperformance.com/product-p/639...=1&CartID=1

Where are your idle mixture screws? If I recall correctly they should be in the range of 1 1/2 turns out from seated. If less the idle jets are too large, if more the idle jets are too small. PLEASE check the previous statement. It's one way or the other

Don't throw new jets in her just for the heck of it

[nditiz1]

50 idle are fine. I doubt you will need to go lower.