Weber IDF Mixture Screw has no impact

[jaket3]

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

[mark tucker]

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.

[74 Thing]

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).

[jonny1309]

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.
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Jonny

[jaket3]

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

[jaket3]

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

[APPLEGREENVW]

https://www.youtube.com/watch?v=fzf18S-HVOg

Check out this video.
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[jonny1309]

[mark tucker]

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)

[jaket3]

[jaket3]

[jaket3]

[APPLEGREENVW]

[udidwht]

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?
_________________
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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
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[mark tucker]

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.

[74 Thing]

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.

[wheel607]

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.

[modok]

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.

[tattooed_pariah]