76 Westy with dual Webers--popping sound at decel

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thesamwise4
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76 Westy with dual Webers--popping sound at decel

Post by thesamwise4 » Sun Mar 16, 2014 4:52 pm

Hey all,

My bus has a type four engine with dual Weber IDF 40s. It's been running really well lately, for the most part, but it's developed a consistent popping (sounds like it's coming from the exhaust system) when I take my foot off the accelerator at higher rpms. Sometimes it will pop like this while idling, but not as often.

My understanding of this is that this probably means that the mixture is lean on at least one of the barrels, but I haven't really adjusted my carbs before and would feel a lot more comfortable if someone could provide some input/guidance on how to move forward.

Do I just back out all mixture screws a half turn? If not, how do I locate the problem area?

Full disclosure: I've had the bus for a while and have always been a bit intimidated by the carbs (I'm not super mechanically inclined...) but I'm tired of driving this thing with a sense of fear and confusion, so I want to rectify this and learn how to maintain the carbs. I have the Bob Tomlinson book on Weber carbs and have read through it. That's more or less the extent of my knowledge, but I would like to learn.

Thanks for any advice you can provide.
-Dave

1977 Westy Deluxe--The Green Lantern

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sped372
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Re: 76 Westy with dual Webers--popping sound at decel

Post by sped372 » Mon Mar 17, 2014 6:11 am

Check for exhaust leaks too... look for soot at connection points.
1971 Karmann Ghia - 1600 DP
1984 Westfalia - 1.9 WBX

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Re: 76 Westy with dual Webers--popping sound at decel

Post by thesamwise4 » Mon Mar 17, 2014 6:56 pm

Thanks for the reply, Sped.

I adjusted the air/fuel mixture today and it seems to have taken care of the problem, but we'll see.
-Dave

1977 Westy Deluxe--The Green Lantern

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Re: 76 Westy with dual Webers--popping sound at decel

Post by Amskeptic » Tue Mar 18, 2014 6:13 pm

thesamwise4 wrote:Thanks for the reply, Sped.

I adjusted the air/fuel mixture today and it seems to have taken care of the problem, but we'll see.
And . . . . HOW . . . did we "adjust the air/fuel mixture today" . . . . ?
Colin :cyclopsani:
BobD - 1978 Bus . . . . . . . . . . .111,130 miles
Chloe - 1970 bus . . . . . . . . . . . 206,787 miles
Naranja - 1977 Westfalia . . . . . 93,996 miles
Pluck - 1973 Squareback . . . . . 55,510 miles
Alexus - 1991 Lexus LS400 . . . 72,113 miles

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Re: 76 Westy with dual Webers--popping sound at decel

Post by thesamwise4 » Tue Mar 18, 2014 9:19 pm

Haha. Hey, Colin. Hope you are doing well. Great to hear from you.

Here's what I did the other day:

I let the bus warm up a little and then turned each mixture screw in until the engine started protesting vigorously. I then backed each screw out to close to 2 and 1/2 turns (I was doing this in a parking garage at school, so it was a somewhat quick job). The engine stopped popping and ran smoother (jerked around less). I drove it for a day and half and had no small pops.

I noticed that turning some of the mixture screws to 2 1/2 significantly reduced engine function, so I had to just adjust by engine smoothness, if that makes sense.

I went out this evening to do a more thorough checking/adjusting.

Here's what I did this evening:

-used timing gun to track rpms as I further adjusted the mixture screws
-tuned for smoothness/no pops (RPMs went up significantly)
-adjusted idle speed screws (to around 920) after adjusting mixtures (after adjust mixture screws, RPMs were around 1000)

One thing that did grab my attention is that adjusting the mixture screw on one of the barrels (the one closest to cylinder 4) seemed to have very little effect on RPMs until the engine started jerking around.

What I mean is that for the other screws, slight turns raised or lowered RPMs quickly. For this one, it didn't seem like turning the screw changed the RPMs until it was nearly all the way in (when the engine started bucking).

I pulled the spark plug cap off 4 while the engine was running and it didn't have nearly the same effect that it did when I pulled the plug on 3. I'm wondering if this spark plug might be dead or if there's another issue.

Here are some questions I have:

1. Does it make sense that small turns in the air mixture screw could have little to no effect on RPMs but turning it all the the way in could cause the engine to buck?
2. When adjusting carbs by RPMs, should you go for maximum RPMs or a moderately high RPM number and a smooth engine?
3. Should I start by replacing the plug(s)?

I'd be happy to clarify anything that doesn't make sense. Thanks in advance for your help, guys/gals.
-Dave

1977 Westy Deluxe--The Green Lantern

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Re: 76 Westy with dual Webers--popping sound at decel

Post by Amskeptic » Sun Mar 23, 2014 11:12 am

thesamwise4 wrote: Does it make sense that small turns in the [fuel?] mixture screw could have little to no effect on RPMs but turning it all the the way in could cause the engine to buck?

When adjusting carbs by RPMs, should you go for maximum RPMs or a moderately high RPM number and a smooth engine?
If it is already unnecessarily too far out, then you will waste a couple of turns to find the "lean drop-off" point. Once the engine protests, then as you guessed, you are cutting off needed fuel.

Adjust idle to 950-1000 as your baseline air flow from which each carb delivers fuel. You turn each mixture screw to the lean drop off point, then back out to "best idle" + 1/4 turn. Each cylinder will affect all the rest as it contributes more effectively, so yes, you have to continually reset the idle speed to your baseline if it changes while adjusting mixtures. Otherwise, a change in air flow due to some cylinder coming on line will also change the previous mixture setting. A little like rounding up frogs.

If you have a screw that is not responsive to your ministrations, there may dirt playing fuel keeper.
Colin
BobD - 1978 Bus . . . . . . . . . . .111,130 miles
Chloe - 1970 bus . . . . . . . . . . . 206,787 miles
Naranja - 1977 Westfalia . . . . . 93,996 miles
Pluck - 1973 Squareback . . . . . 55,510 miles
Alexus - 1991 Lexus LS400 . . . 72,113 miles

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Re: 76 Westy with dual Webers--popping sound at decel

Post by thesamwise4 » Sun Apr 20, 2014 9:05 pm

Colin,

Happy Easter, my friend; I hope you are doing well (that goes for everyone, not just Colin!).

Thanks for your reply. I have been busy lately but had a little time earlier today to take another look at this. I adjusted the carbs per your instruction right after your posted and have been meaning to write back to you with two follow up questions.

1. I've noticed that the shift linkage binds a bit. It's worse on the passenger side carb. I can use my finger to push the throttle level back into place but it won't go there on its own. How can I go about making sure that the throttle level snaps back and sits snug against the end of the idle speed screw?

2. I am still concerned about mixture screw #4 (rear driver's side). I can turn it several full turns with almost no change in RPMs. When I do finally get a change, it's not RPMs but in serious exhaust popping. It is an outlier among the other mixture screws (which behave in the way I would expect them to--turn a few times and I notice serious RPM changes). Any suggestions on how I can trouble shoot this?

Thanks in advance for any guidance you all can provide. I very much appreciate it.

--Dave
-Dave

1977 Westy Deluxe--The Green Lantern

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Re: 76 Westy with dual Webers--popping sound at decel

Post by Amskeptic » Mon Apr 21, 2014 8:01 am

thesamwise4 wrote:Colin,

Happy Easter, my friend; I hope you are doing well (that goes for everyone, not just Colin!).

Thanks for your reply. I have been busy lately but had a little time earlier today to take another look at this. I adjusted the carbs per your instruction right after your posted and have been meaning to write back to you with two follow up questions.

1. I've noticed that the shift linkage binds a bit. It's worse on the passenger side carb. I can use my finger to push the throttle level back into place but it won't go there on its own. How can I go about making sure that the throttle level snaps back and sits snug against the end of the idle speed screw?

2. I am still concerned about mixture screw #4 (rear driver's side). I can turn it several full turns with almost no change in RPMs. When I do finally get a change, it's not RPMs but in serious exhaust popping. It is an outlier among the other mixture screws (which behave in the way I would expect them to--turn a few times and I notice serious RPM changes). Any suggestions on how I can trouble shoot this?

Thanks in advance for any guidance you all can provide. I very much appreciate it.

--Dave
Lubricate throttle linkage at appropriate spots. Yes, make sure that the return springs do their jobs. Replace them as necessary.
A mixture screw that does not react to your ministrations tells us that crap in the carb is acting as your unintended mixture screw.
Colin
BobD - 1978 Bus . . . . . . . . . . .111,130 miles
Chloe - 1970 bus . . . . . . . . . . . 206,787 miles
Naranja - 1977 Westfalia . . . . . 93,996 miles
Pluck - 1973 Squareback . . . . . 55,510 miles
Alexus - 1991 Lexus LS400 . . . 72,113 miles

72Hardtop
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Re: 76 Westy with dual Webers--popping sound at decel

Post by 72Hardtop » Fri Jun 02, 2017 2:12 am

thesamwise4 wrote:
Sun Apr 20, 2014 9:05 pm
Colin,

Happy Easter, my friend; I hope you are doing well (that goes for everyone, not just Colin!).

Thanks for your reply. I have been busy lately but had a little time earlier today to take another look at this. I adjusted the carbs per your instruction right after your posted and have been meaning to write back to you with two follow up questions.

1. I've noticed that the shift linkage binds a bit. It's worse on the passenger side carb. I can use my finger to push the throttle level back into place but it won't go there on its own. How can I go about making sure that the throttle level snaps back and sits snug against the end of the idle speed screw?

2. I am still concerned about mixture screw #4 (rear driver's side). I can turn it several full turns with almost no change in RPMs. When I do finally get a change, it's not RPMs but in serious exhaust popping. It is an outlier among the other mixture screws (which behave in the way I would expect them to--turn a few times and I notice serious RPM changes). Any suggestions on how I can trouble shoot this?

Thanks in advance for any guidance you all can provide. I very much appreciate it.

--Dave
Old thread but here goes....

The offending idle jet is dirty. You must remove the idle jet on the offending cylinder and clean it with some carb spary (carefully). Also insert the tube from carb cleaner into the idle jet port (jet removed) and give a few short shots of cleaner. Then reinsert idle jet and readjust mixture lean best idle.

The idle jet/s on IDF's effect A/F from idle all the way up thru progression then the main jets will kick in. Once the main jets are in at high RPM's such as 3500+ the air corrector jets will determine AFR (once main jet size is determined best). This is where an LM-2 wide-band helps greatly.

The air corrector jets can delay and/or increase when the main jets come into play. They also play a role in overall AFR with the main circuit.

Popping on decel....is an inherent side affect of going to aftermarket carbs such as IDF's or Dells. If the exhaust is plumb and you have no vacuum leaks you likely will still get subtle popping during high decel moments. It's most notable when exiting freeway off ramps (downshifting or in gear) and/or coasting down carport ramps while in gear with foot off gas. Perfectly normal behavior for these engines combined with aftermarket performance carbs and/or exhaust.
1972 Westy
2056cc T-4 - 7.8:1 CR
Weber 40mm Duals - 47.5idles, 125mains, F11 tubes, 200 Air corr., 28mm Vents
96mm AA Biral P/C's w/Hastings rings
42x36mm Heads (AMC- Headflow Masters) w/Porsche swivel adjusters
71mm Stroke
Web Cam 73 w/matched Web lifters
S&S 4-1 exhaust w/Walker 17862 quiet-pack
Bosch SVDA w/Pertronix module & flamethrower 40K coil (7* initial 28* total @3200+)
NGK B5ES @ .028
002 Trans
Hankook 185R14's

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Re: 76 Westy with dual Webers--popping sound at decel

Post by Amskeptic » Fri Jun 02, 2017 7:16 am

72Hardtop wrote:
Fri Jun 02, 2017 2:12 am
Old thread but here goes....

The offending idle jet is dirty. You must remove the idle jet on the offending cylinder and clean it with some carb spary (carefully). Also insert the tube from carb cleaner into the idle jet port (jet removed) and give a few short shots of cleaner. Then reinsert idle jet and readjust mixture lean best idle.

The idle jet/s on IDF's effect A/F from idle all the way up thru progression then the main jets will kick in. Once the main jets are in at high RPM's such as 3500+ the air corrector jets will determine AFR (once main jet size is determined best). This is where an LM-2 wide-band helps greatly.

The air corrector jets can delay and/or increase when the main jets come into play. They also play a role in overall AFR with the main circuit.

Popping on decel....is an inherent side affect of going to aftermarket carbs such as IDF's or Dells. If the exhaust is plumb and you have no vacuum leaks you likely will still get subtle popping during high decel moments. It's most notable when exiting freeway off ramps (downshifting or in gear) and/or coasting down carport ramps while in gear with foot off gas. Perfectly normal behavior for these engines combined with aftermarket performance carbs and/or exhaust.

Thanks for the tips. Do you notice "dead cylinders" at idle that go away when you rev the engine? Some of the big four-throat carb set-ups I have run into seem to suffer this only at idle.
Colin
BobD - 1978 Bus . . . . . . . . . . .111,130 miles
Chloe - 1970 bus . . . . . . . . . . . 206,787 miles
Naranja - 1977 Westfalia . . . . . 93,996 miles
Pluck - 1973 Squareback . . . . . 55,510 miles
Alexus - 1991 Lexus LS400 . . . 72,113 miles

72Hardtop
Old School!
Location: Seattle, WA./HB. Ca./Shizuoka, Japan
Status: Offline

Re: 76 Westy with dual Webers--popping sound at decel

Post by 72Hardtop » Fri Jun 02, 2017 9:02 pm

Amskeptic wrote:
Fri Jun 02, 2017 7:16 am
72Hardtop wrote:
Fri Jun 02, 2017 2:12 am
Old thread but here goes....

The offending idle jet is dirty. You must remove the idle jet on the offending cylinder and clean it with some carb spary (carefully). Also insert the tube from carb cleaner into the idle jet port (jet removed) and give a few short shots of cleaner. Then reinsert idle jet and readjust mixture lean best idle.

The idle jet/s on IDF's effect A/F from idle all the way up thru progression then the main jets will kick in. Once the main jets are in at high RPM's such as 3500+ the air corrector jets will determine AFR (once main jet size is determined best). This is where an LM-2 wide-band helps greatly.

The air corrector jets can delay and/or increase when the main jets come into play. They also play a role in overall AFR with the main circuit.

Popping on decel....is an inherent side affect of going to aftermarket carbs such as IDF's or Dells. If the exhaust is plumb and you have no vacuum leaks you likely will still get subtle popping during high decel moments. It's most notable when exiting freeway off ramps (downshifting or in gear) and/or coasting down carport ramps while in gear with foot off gas. Perfectly normal behavior for these engines combined with aftermarket performance carbs and/or exhaust.

Thanks for the tips. Do you notice "dead cylinders" at idle that go away when you rev the engine? Some of the big four-throat carb set-ups I have run into seem to suffer this only at idle.
Colin
It's most notable at idle if the idle jet is plugged. Once it transitions it will run fine. It's very rare if ever to see a main jet plug due to their size. If one has plugged main jet/s you're likely getting large enough debris from the tank that's plugging them when the main comes into play (High speed/cruise). But that is very rare.
1972 Westy
2056cc T-4 - 7.8:1 CR
Weber 40mm Duals - 47.5idles, 125mains, F11 tubes, 200 Air corr., 28mm Vents
96mm AA Biral P/C's w/Hastings rings
42x36mm Heads (AMC- Headflow Masters) w/Porsche swivel adjusters
71mm Stroke
Web Cam 73 w/matched Web lifters
S&S 4-1 exhaust w/Walker 17862 quiet-pack
Bosch SVDA w/Pertronix module & flamethrower 40K coil (7* initial 28* total @3200+)
NGK B5ES @ .028
002 Trans
Hankook 185R14's

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Re: 76 Westy with dual Webers--popping sound at decel

Post by asiab3 » Sat Jun 03, 2017 10:02 am

Amskeptic wrote:Do you notice "dead cylinders" at idle that go away when you rev the engine?
I had this on a 2110 with dual IDFs or the EMPI clones, I can't recall. The carbs actually had a tiny brass "air bypass" screw locked in with an 8mm nut like a mini valve adjuster. All the shiny online tuning instructions I could find said "these are factory set and are for expert tuning only."

So I cracked one to bring up the idle airflow in the dead cylinder and adjusted the fuel to match. It worked quite well, but I still had a hard time finding documentation on why no setup literature online mentions those options for synchronizing idle airflow. Cylinder #4 would NOT contribute under 1200 RPM, and that cured it right then and there.

Robbie
1969 bus, "Buddy."
100k miles with me.
279k miles on Earth.

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Re: 76 Westy with dual Webers--popping sound at decel

Post by 72Hardtop » Sun Jun 04, 2017 12:36 am

asiab3 wrote:
Sat Jun 03, 2017 10:02 am
Amskeptic wrote:Do you notice "dead cylinders" at idle that go away when you rev the engine?
I had this on a 2110 with dual IDFs or the EMPI clones, I can't recall. The carbs actually had a tiny brass "air bypass" screw locked in with an 8mm nut like a mini valve adjuster. All the shiny online tuning instructions I could find said "these are factory set and are for expert tuning only."

So I cracked one to bring up the idle airflow in the dead cylinder and adjusted the fuel to match. It worked quite well, but I still had a hard time finding documentation on why no setup literature online mentions those options for synchronizing idle airflow. Cylinder #4 would NOT contribute under 1200 RPM, and that cured it right then and there.

Robbie
The general rule with the air by-pass screws is start with them closed. If the barrels of 1 carb are no ore than 1 point off from one another on your snail that's fine. They are used to balance the barrels between one another on said carb.

If they are more than 1 point apart between barrels the shaft is likely twisted slightly or the butterfly's differ enough from one another causing the excess reading. Or you may have a vac leak or a combination of more than 1 issue.
1972 Westy
2056cc T-4 - 7.8:1 CR
Weber 40mm Duals - 47.5idles, 125mains, F11 tubes, 200 Air corr., 28mm Vents
96mm AA Biral P/C's w/Hastings rings
42x36mm Heads (AMC- Headflow Masters) w/Porsche swivel adjusters
71mm Stroke
Web Cam 73 w/matched Web lifters
S&S 4-1 exhaust w/Walker 17862 quiet-pack
Bosch SVDA w/Pertronix module & flamethrower 40K coil (7* initial 28* total @3200+)
NGK B5ES @ .028
002 Trans
Hankook 185R14's

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asiab3
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Re: 76 Westy with dual Webers--popping sound at decel

Post by asiab3 » Mon Jun 05, 2017 12:19 am

Our particular case was a low compression cylinder that just wouldn't contribute to idle any other way. Uni-syn readings weren't even on the same planet until we got the cylinder to fire at 950 rpm. The carbs were new, so a bent shaft or misaligned butterfly could have been possible. We didn't SEE it, but we didn't measure for it either.

Robbie
1969 bus, "Buddy."
100k miles with me.
279k miles on Earth.

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