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virusdoc
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Joined: August 13, 2018
Posts: 525
Location: Durham, NC, USA
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 Posted: Wed Aug 04, 2021 9:20 am Post subject: One cylinder does not participate in idle during IDF tuning |
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I’m trying to set up new Weber IDF 40’s using the lean best idle method. All four barrels are well synchronized with a snail gauge. Engine will idle at 700-800 but is a little irregular. When adjusting the mixture screws, all cylinders except #4 behave as the CBP install guide indicates—when the mixture screw is all the way in, idle gets very slow and rough, and then smooths out and speeds up around 1-1/4 to 1-1/2 turns out.
Cylinder 4 is not responding to the mixture screw at all. Even when it is bottomed out, there is no change to idle speed or quality. I can open it through 4 turns and nothing about idle changes.
Troubleshooting thus far:
-all four cylinders have good compression. 130-135 cold, 120-125 warm.
-no detectable vacuum leak on cylinder 4. New gaskets on carb and manifold and torqued to spec.
-spark is being delivered. tested at distibutor cap/#4 wire. About 1/2” long and bright blue. Interestingly, the only time cylinder 4 seems to effect idle speed is in the second or two right after the plug wire is reconnected. Idle speeds up briefly at this time, then falls down again.
-plug is not visibly fouled. Cleaning it gently with solvent and a nylon brush doesn’t help
-spraying carb cleaner into the barrels for cylinders 1-3 noticeably slows idle for several seconds, but spraying into #4 barrel does nothing.
-idle jet for #4 is clean
-timing is static at 7.5 BTDC, confirmed with a strobe
-points gap confirmed with feeler gauge.
I feel like the slight bump in idle speed just as I plug in the #4 wire is trying to tell me something—and that the problem here is actually spark or timing related—but I’m too inexperienced to figure it out. Thanks for any advice!
_________________
1976 Convertible Super Beetle, "June Bug".
FI-->34PICT3 carb conversion-->now dual 40 IDFs
Self-rebuilt 1904 (after totally botching a 1641 rebuild, cussing a lot, and throwing lots of cash in the fire) |
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evanfrucht
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Joined: July 24, 2016
Posts: 2180
Location: Laurel Canyon, CA
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| Posted: Wed Aug 04, 2021 9:25 am Post subject: Re: One cylinder does not participate in idle during IDF tuning |
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I would say your idle jets are too lean based on a similar problem I had recently. Or maybe it just needs to be adjusted more. Sometimes 1 screw needs 1 turn out and the other screws need 3 or 4 for whatever reason, or visa versa, etc.
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1967 Bug ( the daily rod )
1964 Fury Wagon ( the pavement shredder ) |
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virusdoc
Samba Member
Joined: August 13, 2018
Posts: 525
Location: Durham, NC, USA
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| Posted: Wed Aug 04, 2021 9:32 am Post subject: Re: One cylinder does not participate in idle during IDF tuning |
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| evanfrucht wrote: |
| I would say your idle jets are too lean based on a similar problem I had recently. Or maybe it just needs to be adjusted more. Sometimes 1 screw needs 1 turn out and the other screws need 3 or 4 for whatever reason, or visa versa, etc. |
Thanks for the reply. Wouldn’t it be odd for 3 of the four jets to be appropriate, spot in the middle of the target range, and one to be that far off? Brand new carbs set up any ACN.
_________________
1976 Convertible Super Beetle, "June Bug".
FI-->34PICT3 carb conversion-->now dual 40 IDFs
Self-rebuilt 1904 (after totally botching a 1641 rebuild, cussing a lot, and throwing lots of cash in the fire) |
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flat4Imp
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Joined: October 14, 2010
Posts: 48
Location: Pennsburg, PA 18073
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| Posted: Wed Aug 04, 2021 9:51 am Post subject: Re: One cylinder does not participate in idle during IDF tuning |
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I'd swap 3 and 4 plugs and see if it moves to #3? If that shows nothing try swapping 3 and 4 wires just as process of elimination? I'm not a fan of cleaning a plug with solvent when trying to diagnose a problem.
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"Often immitated, never duplicated." |
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evanfrucht
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Joined: July 24, 2016
Posts: 2180
Location: Laurel Canyon, CA
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| Posted: Wed Aug 04, 2021 10:13 am Post subject: Re: One cylinder does not participate in idle during IDF tuning |
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Swap carbs side to side and see if the problem moves with the carb.
Just because the carbs were "set up" they still might need to be tuned for your engine. I'm not sure what ACN does to the carbs before they send them off or how they decided what size jets to put in, etc.
I'm suggestion that maybe all the jets are lean even tho it seems like 3 of them are acting properly. They might be just lean enough that it's not allowing enough room for adjustment... so you have 3 that are slightly lean (beyond what you can notice) and then 1 that is just barely acting different... and no amount of mixture screw adjustment can fix it...
Hard to say without knowing more.
Is it makimg popping, coughing, or burping sounds from carbs or backfiring at all?
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1967 Bug ( the daily rod )
1964 Fury Wagon ( the pavement shredder ) |
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virusdoc
Samba Member
Joined: August 13, 2018
Posts: 525
Location: Durham, NC, USA
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| Posted: Wed Aug 04, 2021 10:32 am Post subject: Re: One cylinder does not participate in idle during IDF tuning |
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| evanfrucht wrote: |
Swap carbs side to side and see if the problem moves with the carb.
Just because the carbs were "set up" they still might need to be tuned for your engine. I'm not sure what ACN does to the carbs before they send them off or how they decided what size jets to put in, etc.
I'm suggestion that maybe all the jets are lean even tho it seems like 3 of them are acting properly. They might be just lean enough that it's not allowing enough room for adjustment... so you have 3 that are slightly lean (beyond what you can notice) and then 1 that is just barely acting different... and no amount of mixture screw adjustment can fix it...
Hard to say without knowing more.
Is it makimg popping, coughing, or burping sounds from carbs or backfiring at all? |
No popping or backfiring. This is a 1904 with a 2239 cam and stock rockers. Jets are:
F11 emulsion, 50 idle, main 120, air 200.
I swapped the plugs and wires from 3 and 4, and the problem sticks with cylinder 4. I suspect the idle circuit for that barrel is clogged, although the jet is clean. I’ll disassemble and blow the entire circuit out and report back.
_________________
1976 Convertible Super Beetle, "June Bug".
FI-->34PICT3 carb conversion-->now dual 40 IDFs
Self-rebuilt 1904 (after totally botching a 1641 rebuild, cussing a lot, and throwing lots of cash in the fire) |
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evanfrucht
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Joined: July 24, 2016
Posts: 2180
Location: Laurel Canyon, CA
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| Posted: Wed Aug 04, 2021 10:42 am Post subject: Re: One cylinder does not participate in idle during IDF tuning |
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Ya its tough to say. Swapping carbs is an easy way to either diagnose or rule it out as being carbs.
If not carbs check compression or do a leak down test and check the valves.
New spark plugs can't hurt.
How many turns out are each one of the mixture screws at?
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1967 Bug ( the daily rod )
1964 Fury Wagon ( the pavement shredder ) |
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virusdoc
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Joined: August 13, 2018
Posts: 525
Location: Durham, NC, USA
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| Posted: Wed Aug 04, 2021 11:17 am Post subject: Re: One cylinder does not participate in idle during IDF tuning |
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| evanfrucht wrote: |
Ya its tough to say. Swapping carbs is an easy way to either diagnose or rule it out as being carbs.
If not carbs check compression or do a leak down test and check the valves.
New spark plugs can't hurt.
How many turns out are each one of the mixture screws at? |
Compression is good in all cylinders, and all mixture screws are in the 1-1/4 to 1-3/4 turns out at best idle (see OP for numbers). Heads are brand new Mofoco 041’s and valves are appropriately adjusted.
I can swap the carbs side to side but since that would involve switching fuel inlets and linkage sides I’m going to first try cleaning the #4 idle circuit.
_________________
1976 Convertible Super Beetle, "June Bug".
FI-->34PICT3 carb conversion-->now dual 40 IDFs
Self-rebuilt 1904 (after totally botching a 1641 rebuild, cussing a lot, and throwing lots of cash in the fire) |
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mcdragracer
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Joined: December 30, 2007
Posts: 714
Location: Cally
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| Posted: Wed Aug 04, 2021 12:09 pm Post subject: Re: One cylinder does not participate in idle during IDF tuning |
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| Have you tried contacting ACN about your concern? |
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virusdoc
Samba Member
Joined: August 13, 2018
Posts: 525
Location: Durham, NC, USA
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| Posted: Wed Aug 04, 2021 12:14 pm Post subject: Re: One cylinder does not participate in idle during IDF tuning |
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| mcdragracer wrote: |
| Have you tried contacting ACN about your concern? |
Yes. I sent in a number of questions around carb setup. I got answers to some, but this question did not receive a response. I imagine John gets a ton of emails so I am trying to teach myself and eliminate some obvious solutons before going back to him.
_________________
1976 Convertible Super Beetle, "June Bug".
FI-->34PICT3 carb conversion-->now dual 40 IDFs
Self-rebuilt 1904 (after totally botching a 1641 rebuild, cussing a lot, and throwing lots of cash in the fire) |
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Ohio Tom
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Joined: February 09, 2006
Posts: 1660
Location: Marshallville Ohio
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| Posted: Wed Aug 04, 2021 12:48 pm Post subject: Re: One cylinder does not participate in idle during IDF tuning |
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Once clue you gave: pull the plug wire and then re-connect, she hits for a second then dies out again.
That clearly says to me that your idle jet is plugged.
Either that, or there is blockage in the fuel path of the idle circuit.
pull the idle jet and blow compressed air in the hole, then try again.
Another trick, is to just crack the idle jet loose when idling.
If it picks right up when you crack the jet loose, that confirms idle fuel metering issue. |
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virusdoc
Samba Member
Joined: August 13, 2018
Posts: 525
Location: Durham, NC, USA
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| Posted: Wed Aug 04, 2021 4:21 pm Post subject: Re: One cylinder does not participate in idle during IDF tuning |
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| Ohio Tom wrote: |
Once clue you gave: pull the plug wire and then re-connect, she hits for a second then dies out again.
That clearly says to me that your idle jet is plugged.
Either that, or there is blockage in the fuel path of the idle circuit.
pull the idle jet and blow compressed air in the hole, then try again.
Another trick, is to just crack the idle jet loose when idling.
If it picks right up when you crack the jet loose, that confirms idle fuel metering issue. |
Good suggestion! Definitely a clogged idle circuit. I blew it out with carb cleaner at both the idle jet and the mixture screw, and it idles smoother now and that cylinder’s mixture screw can impact idle quality. It’s still not fully cleaned out but it is responding. So I’ll disassemble and clean it fully. I think the culprit was a shredded o-ring on the idle jet. The o-ring stayed in the carb body when I took the jet out the first time, so I didn’t notice it was damaged. But I took it out with a small pick this time, and it is sliced up pretty badly. I suspect I have some rubber debris in the circuit. Hard to imagine this was caused by removing the jet only twice, but maybe they are that fragile? Can I substitute any appropriately sized o-ring, or does it need to be a special fuel-resistant o-ring?
_________________
1976 Convertible Super Beetle, "June Bug".
FI-->34PICT3 carb conversion-->now dual 40 IDFs
Self-rebuilt 1904 (after totally botching a 1641 rebuild, cussing a lot, and throwing lots of cash in the fire) |
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udidwht
Samba Member
Joined: March 06, 2005
Posts: 3779
Location: Seattle, WA./ HB, Ca./ Shizuoka, Japan
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| Posted: Mon Aug 09, 2021 1:56 am Post subject: Re: One cylinder does not participate in idle during IDF tuning |
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| virusdoc wrote: |
| Ohio Tom wrote: |
Once clue you gave: pull the plug wire and then re-connect, she hits for a second then dies out again.
That clearly says to me that your idle jet is plugged.
Either that, or there is blockage in the fuel path of the idle circuit.
pull the idle jet and blow compressed air in the hole, then try again.
Another trick, is to just crack the idle jet loose when idling.
If it picks right up when you crack the jet loose, that confirms idle fuel metering issue. |
Good suggestion! Definitely a clogged idle circuit. I blew it out with carb cleaner at both the idle jet and the mixture screw, and it idles smoother now and that cylinder’s mixture screw can impact idle quality. It’s still not fully cleaned out but it is responding. So I’ll disassemble and clean it fully. I think the culprit was a shredded o-ring on the idle jet. The o-ring stayed in the carb body when I took the jet out the first time, so I didn’t notice it was damaged. But I took it out with a small pick this time, and it is sliced up pretty badly. I suspect I have some rubber debris in the circuit. Hard to imagine this was caused by removing the jet only twice, but maybe they are that fragile? Can I substitute any appropriately sized o-ring, or does it need to be a special fuel-resistant o-ring? |
Got to a hardware store such as Ace hardware they sell varying size Viton O-rings.
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1972 Westy Hardtop/Type-4 2056cc
96mm Biral AA P/C's~7.8:1CR
Headflow Masters New AMC 42x36mm heads w/Porsche swivel adjusters
71mm Stroke
73 Web Cam w/Web solids
Dual 40mm IDF Webers - LM-2 - 47.5 idles/125 mains/190 air corr./F11 tubes/28mm Vents - Float height 10.45mm/Drop 32mm
Bosch SVDA w/Pertronix module (7.5 initial 28 total @ 3400rpm)
Bosch W8CC plugs
Pertronix Flamethrower 40K coil
S&S 4-1 w/Walker QP 17862
3 rib 002 Trans
185R14 Hankook tires |
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