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jaket3 Samba Member
Joined: December 26, 2018 Posts: 125 Location: UK
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Posted: Wed May 05, 2021 11:39 am Post subject: Weber IDF Mixture Screw has no impact |
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Hi,
I have Type 4 Motor in a bay window. 1.8 Litre Engine.
Dual 40 IDF's
They are balanced at idle. And at 2000 rpm on the snail guage.
However when I adjust the mixture screw on number 1 cylinder there is no response in RPM. I can screw it all the way in. No difference.
On all the other cylinders it's easy to find the sweet spot as the RPM changes. And engine dies as it they get screwed in.
These are new IDF's from Weber. Spanish ones.
I have a fuel pressure regulator. 2.5 psi.
Bus actually runs really well. Picks up well and cruises at 60mph no problem.
I verified spark is getting to number 1 and I changed the spark plug, no difference. Plug was a little black but no different to the others.
I also removed the idle jet and verified it's clean. And same for the main jet stack.
Any ideas what I should try next please!
Thanks |
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mark tucker Samba Member
Joined: April 08, 2009 Posts: 23937 Location: SHALIMAR ,FLORIDA
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Posted: Wed May 05, 2021 11:47 am Post subject: Re: Weber IDF Mixture Screw has no impact |
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1 dead cylinder, 2 cloged idle jet.3, vacume leek on that cylinder,4 needs valve adjustment,5 twisted throttell shaft,6 dead plug,7 dead or dying camshaft.8 air bypass screws adjustment afu. |
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74 Thing Samba Member
Joined: September 02, 2004 Posts: 7393
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Posted: Wed May 05, 2021 2:12 pm Post subject: Re: Weber IDF Mixture Screw has no impact |
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My guess is a plug in the idle jet "circuit." The jet may not be plugged, but there may be an obstruction in the port that the fuel flows. Use carb cleaner and air to blow it out.
If you spray carb cleaner down the carb throat when at idle and rpms pick up then you have an obstruction or an intake leak (since carb wants more fuel-eg rpm picks up). |
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jonny1309 Samba Member
Joined: February 16, 2005 Posts: 680 Location: Oside
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Posted: Fri May 07, 2021 11:12 am Post subject: Re: Weber IDF Mixture Screw has no impact |
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I was dealing with the same thing, a non-responsive mixture screw on one of my IDFs. After a friend went through my carbs he mentioned that one mixture screw did not feel right when he was re-installing it. After further inspection, he determined that the seat for the mixture screw was messed up. He replaced it and it's all good. Now, the carb responds to adjustments with the mixture screw.
Replacing this seat/nozzle is a delicate process so be gentle. _________________ Jonny
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scottvw wrote: |
kiss my ass Tom....I'm without centercaps right now. |
OgCalLook wrote: |
nothin' wrong with: Nose down - Ass up!! |
Ragman wrote: |
Did you tell her that Al Reed painted them? |
lonslo wrote: |
My tailbone still hurts like hell. |
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jaket3 Samba Member
Joined: December 26, 2018 Posts: 125 Location: UK
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Posted: Fri May 07, 2021 1:03 pm Post subject: Re: Weber IDF Mixture Screw has no impact |
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Thanks for the replies.
I took the carb apart and cleaned / checked it all. Double check float.
Same problem.
Compression is good (About 110 on every cylinder)
I swapped plugs round, no change.
Tomorrow I'll check valves first, the check for vacuum leaks and then if that hasn't sorted it I thought I might switch the carbs round and see if the problem moves with the carb.
I doubt it's the cylinder. I wouldn't expect it to drive so nice if it was that?
Thanks |
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jaket3 Samba Member
Joined: December 26, 2018 Posts: 125 Location: UK
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Posted: Sat May 08, 2021 7:20 am Post subject: Re: Weber IDF Mixture Screw has no impact |
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So I checked the valve clearances and for manifold leaks. All OK.
So then I swapped Carbs from side to side and the problem followed the carb.
So it's definitely the carb then.
I cleaned and verified that air can get through from the idle jet holder down the tube to the mixture screw. And clear to the brass hole in the carb throat. However I'm a bit unsure how the fuel gets to that point. (The mixture screw hole). Maybe the air is getting through and no fuel. Any ideas which bits to clean out.
Presumably there is a pathway for the fuel to get into the tube before the mixture screw.
Thanks
Jake |
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APPLEGREENVW Samba Member
Joined: November 30, 2003 Posts: 2388 Location: Seekonk,Massachusetts USA
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jonny1309 Samba Member
Joined: February 16, 2005 Posts: 680 Location: Oside
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Posted: Sat May 08, 2021 9:39 am Post subject: Re: Weber IDF Mixture Screw has no impact |
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Great video. Supports the fact that it's difficult or not probable that you will clear an idle jet with the carb installed. _________________ Jonny
Barndoor Mafia wrote: |
Add a cop mustache, some cholo shades and a Doyers cap to the guy in the suit .......... Tocayo. |
scottvw wrote: |
kiss my ass Tom....I'm without centercaps right now. |
OgCalLook wrote: |
nothin' wrong with: Nose down - Ass up!! |
Ragman wrote: |
Did you tell her that Al Reed painted them? |
lonslo wrote: |
My tailbone still hurts like hell. |
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mark tucker Samba Member
Joined: April 08, 2009 Posts: 23937 Location: SHALIMAR ,FLORIDA
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Posted: Sat May 08, 2021 10:25 am Post subject: Re: Weber IDF Mixture Screw has no impact |
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if there is a chunk out of a intake gasket causing a leek many times that air screw can still adjust that barrell to read the same as the other cylinders...and...result in idle mix screw not working...was it ok and nobody tried to adjust the air bypass screws? if that cylinder running at idle??( check all pipe temp's after a 10-20 second cold start/stop, finger check is ok, you will know if any are different.unless you have a dead finger too (dont burn your finger and blame me for your slow response times) |
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jaket3 Samba Member
Joined: December 26, 2018 Posts: 125 Location: UK
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Posted: Sat May 08, 2021 10:31 am Post subject: Re: Weber IDF Mixture Screw has no impact |
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Super, thanks. I hadn't appreciated that the fuel is drawn in by the idle jet. I thought that was just air.
All makes sense now. I'll give it another good clean and test!
Thanks |
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jaket3 Samba Member
Joined: December 26, 2018 Posts: 125 Location: UK
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Posted: Sat May 08, 2021 10:34 am Post subject: Re: Weber IDF Mixture Screw has no impact |
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jonny1309 wrote: |
Great video. Supports the fact that it's difficult or not probable that you will clear an idle jet with the carb installed. |
Yes glad it's on a bus. 5 minute job to pull carb off. On my Ghia it is a PITA.
Cheers |
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jaket3 Samba Member
Joined: December 26, 2018 Posts: 125 Location: UK
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Posted: Sat May 08, 2021 10:41 am Post subject: Re: Weber IDF Mixture Screw has no impact |
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mark tucker wrote: |
if there is a chunk out of a intake gasket causing a leek many times that air screw can still adjust that barrell to read the same as the other cylinders...and...result in idle mix screw not working...was it ok and nobody tried to adjust the air bypass screws? if that cylinder running at idle??( check all pipe temp's after a 10-20 second cold start/stop, finger check is ok, you will know if any are different.unless you have a dead finger too (dont burn your finger and blame me for your slow response times) |
I pulled the intake manifold off to double check the gasket and all was good with that. The gaskets between intake manifold and carb are good too. I had a squirt around with carb cleaner on the manifold joints and makes no difference to engine speed.
Air bypass screws are screwed in.
I'll check the temps next time it's running.
Pulling the number 1 plug wire made no change to idle speed. All the others do.
So looks to me that since other carb worked fine I must be getting next to no air/fuel from the idle circuit at idle. After watching that video I now understand the circuit so will give it a better clean on the bench tomorrow and report back.
Whilst it drives fine I suppose at load it's on the main circuit so can still feel OK.
Thanks for the input. |
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APPLEGREENVW Samba Member
Joined: November 30, 2003 Posts: 2388 Location: Seekonk,Massachusetts USA
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Posted: Sat May 08, 2021 11:47 am Post subject: Re: Weber IDF Mixture Screw has no impact |
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jaket3 wrote: |
Super, thanks. I hadn't appreciated that the fuel is drawn in by the idle jet. I thought that was just air.
All makes sense now. I'll give it another good clean and test!
Thanks |
Check out the other weber videos, Vic has for more tips. _________________ Parts for sale https://www.thesamba.com/vw/classifieds/search.php?username=APPLEGREENVW
02/76 Beetle sedan |
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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: Sun May 09, 2021 8:43 pm Post subject: Re: Weber IDF Mixture Screw has no impact |
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Pull the carb and remove the carb top and be sure the air passage is clear and that the top gaskets are not obstructing flow.
How do the a/f mix screws look compared to one another? Also carefully check the seats.
What intake gaskets are you using? _________________ 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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mark tucker Samba Member
Joined: April 08, 2009 Posts: 23937 Location: SHALIMAR ,FLORIDA
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Posted: Mon May 10, 2021 11:00 am Post subject: Re: Weber IDF Mixture Screw has no impact |
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when I got a cloged idle jet ( about 3 times in 15 years)I would also pull the mix screws and flush out them as well. from bottom to top then from top to bottom.( top is idle jet hole) try adjusting the air mix screws out so the throttel baldes can be closed farther thus closer to the transfer/progression ports. if no use try inward... be sure thse arnt pluged..and be sure you are not missing a gally plug in the carb...most leek,I usuely epoxie them.( from my old days with Qjets that were always leeking down to a empty fuel bowel and flooded engine....very common on those. you can tap in the aluminum plugs some too.just a little if you suspect a leek there. |
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74 Thing Samba Member
Joined: September 02, 2004 Posts: 7393
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Posted: Wed May 12, 2021 1:05 am Post subject: Re: Weber IDF Mixture Screw has no impact |
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Did you resolve the mixture screw problem?
Maybe change the idle jet on that circuit with another. Maybe the jet is sized different-just a suggestion.
Last edited by 74 Thing on Thu May 13, 2021 10:47 am; edited 1 time in total |
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wheel607 Samba Member
Joined: May 30, 2004 Posts: 1847
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Posted: Wed May 12, 2021 2:33 am Post subject: Re: Weber IDF Mixture Screw has no impact |
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I had the exact same problem with a 42DCNF carb. It was the number 4 screw as I remember. It ran really well and I could never find the problem. It bothered me so much, the running really well and such that the only way I got rid of this condition, after doing all and more that has been mentioned, is to change out carbs with Dellorto 45s (NOS) Believe me when I tell you, IT RAN GOOD...could NEVER find the problem. |
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modok Samba Member
Joined: October 30, 2009 Posts: 26789 Location: Colorado Springs
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Posted: Wed May 12, 2021 3:17 am Post subject: Re: Weber IDF Mixture Screw has no impact |
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I would try more fuel and less fuel.
Unscrew the idle jet or choking it to give it more fuel.
Pinching the fuel hose to stop flooding if it's flooding.
The more things you try the more info you get.
It is not always necessary to understand a problem to fix it, but it sure can help. |
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tattooed_pariah Samba Member
Joined: April 24, 2006 Posts: 2047 Location: El Cajon, CA
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Posted: Wed May 12, 2021 12:02 pm Post subject: Re: Weber IDF Mixture Screw has no impact |
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20 years of running a 1776cc with dual 40IDFs an this is the FIRST time I've ever seen someone explain where the clogs normally are.
It's totally logical and could be considered "obvious" but I've always made the mistake of just pulling the jet, at best blowing air in the jet hole, but as this video explains, that just makes the clog worse..
GREAT link! thanks for sharing it! _________________ -pariah (just a novice from "that other VW site")
'72 Volksrod "Effigy"
"Never worry about stepping on people's toes. People who get their toes stepped on are either sitting down, or standing still on the job."
-Admiral Arleigh Burke, USN(ret.) |
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