Unresponsive idle mixture screw...

[type149doug]

   I have a problem with my engine which is a 2276 and running IDAs.  I have an idle mixture screw (#1) that is unresponsive when trying to adjust the idle mixture.  I've done a bunch of trouble shooting and am at a loss for what could cause this.  I've changed the manifold base gasket and checked for vacuum leaks with an unlit propane torch.  No change of engine speed with the propane torch trick. With my SK synch tool (snail synchrometer) I noticed that the vacuum signal is a little higher in that barrel as well. I was thinking I have a twisted throttle shaft, but it checks out to be ok.  I'm stumped.... Any ideas without doing an engine teardown?  Thanks DS

[Zach Gomulka]

Step one- swap the carbs from side to side and see if the problem follows.

[benlawrence]

Step one see if the cylinder picks up on the mains.

[type149doug]

   OK I'll try that...  Thanks.

[type149doug]

    benlawrence, how do you mean "see if it picks up on the mains"?

[benlawrence]

Rev it or drive it, if you can stutter around on 3 cylinders, mash the throttle , if it picks up onto 4 cylinders and runs hard it means you have an issue with fuel supply ( blocked idle jet ) on the idle circuit on one cylinder. If it doesnt pick up you either have an ignition issue, a combo of fuel and spark or worst case a mechanical issue.

Start with the basics, if the engine runs up on 4 clys when you mash it , you have a jet or mixture problem idle side, if it doesnt check the spark to the plug, if you have spark swap out or check the plug. A

[type149doug]

   I got the offending carburetor reinstalled and synched up.  I checked each throat with my SK synchrometer and found that 2,3 and 4 are idling nicely at 6 on the scale.  #1 is idling at 7 on the scale.  The idle mixture screw is now functioning and I'm able to detect a difference in the idle speed when I screw it in or out.  I'm still a bit concerned about the #1 barrel being higher than the other three.  Right now the engine idles at about 1250 rpm. What are your thoughts regarding my idle speed?  Thanks

[henk]

i think 1200 is a bit high on idle.

henk!!!

[Sam K]

If I were you, I would adjust the valves. A tight exhaust valve can cause carb tuning issues.

[type149doug]

    Adjusting the valves was one of the very first things I did in my troubleshooting quest.  I will go pop the valve cover and check again.  Thanks for the input.  I'm aching to drive this car.  It's been throwing be a curve ball since April.

[Jim Ratto]

every blue moon or so one of my idle jet holders will loosen up in carb and back out.

[type149doug]

   Thanks Jim, I verified the idle jet holders are snug.  I swapped the carbs from side to side and the problem didn't seem to follow, but the carbs seem to be in balance on all four barrels now.  Could my engine just prefer a particular carb on a particular side of the engine??? 

[Jim Ratto]

maybe a rogue spark plug too. I've had it bite me.

[type149doug]

   Thanks Jim, I have new plugs in it too.  I've been trouble shooting it for a few weeks now.  I'm close and think it may be a twisted throttle shaft or maybe the engine prefers a particular carb on one side rather than the other. I swapped the carbs from side to side and the imbalance seems to be less. Any other ideas are greatly welcomed.

[Zach Gomulka]

That's odd for sure... I would expect the problem to either follow or stay, not be fixed...

[type149doug]

  Tell me...  I'm gonna swap the carbs back to their original positions and see if I have anything different again.  While I have them off I'll see if there is a difference in clearances between the gaps of the throttle plates and carb body.  That will show if the throttle shaft is twisted or not.  Thanks

[Fritter]

I'd say some sort of vaccuum leak, these engines are very sensitive to leaks.  Also, if your idle is too high, the distributor advance can start to come in, which further muddies the waters/confuses.

[type149doug]

    I agree on the idle speed/ignition curve theory.    Let me ask if you use any kind of sealant on the manifold base gasket?  And what that sealant might be.....?

[Fritter]

No sealant for me.  Just make sure everything is flat and seating correctly.  Also the manifold nuts have a tendency to back off from engine vibration, especially if you don't use lock nuts.  I use nylocs on all studs except #3 which is too tight to get a 13mm nyloc on.

[type149doug]

   Ok, at one time I think I used hylomar.  I usually put a thin coat of bosch grease on the intake gaskets and tighten the manifolds down as tight as I can get them.   I use an 8mm wharpy washer under the nut. 

[Fritter]

Grease isn't a bad idea to prevent the gasket from sticking and tearing.  Yes, use 8mm wave washer and nyloc nuts where you can. 

[Jim Ratto]

Thing is, when you swapped carbs side to side, did you lift them as manifolds/carb assemblies from engine or just unbolt the carbs and switch them? Reason I ask is that if you unbolted the manifolds and then swapped carbs, then bolted them back down, the chance of you having the same vacuum leak as before the manifolds were removed, is slim.

it's easy to see if that one throttle butterfly isn't closing. Remove carb and insert like a 0.008 feeler blade between butterfly and throat on one butterfly and adjust idle stop screw so the feeler just drags... then try it on the suspect barrel. Does it just fall through?

How much differently does the mixture screw in question react /not react? By more than 1/4 turn?

About 15 years ago I built a nice 90.5 x 84 for a for a friend's convertible, all new parts, almost all new. Carbs were his exisiting IDF Webers and we used his existing 009/Pertronix. Everything else was brand new, inspected etc. After the engine ran through its cam break in I went to set mixture and idle speed, but the engine was exhibiting a classic IDF "plugged idle jet" issue... limpy idle and non responsive mixture screw on #1. Everybody knows plugged jets are a part of life and a bitch with IDF's so I said a few choice curse words and removed car to clean it and move on. Installed carb and same issue, same screw. Checked for vacuum leaks. No luck. My friend Tony (I was working for him at the time) asked if I used a new cap and rotor. I hadn't. We threw a new Bosch 04033 rotor and magically the down cylinder came to life. Would have never guessed. Just FYI

[type149doug]

  Thanks for the info Wignition.   I removed just the carbs.  I will now install another new intake gasket and go from there.  I was using a complete Mallory unilite, cdi and coil. I removed all that and installed a freshly rebuilt Bosch 010 that I got from Glen Ring for a different project.  I'm confident that the ignition system is now functioning correctly. I'm going to also try the feeler gauge trick.  I'm missing something and I'm sure it's a small detail.  With your suggestions It'll surface and this thing will be back on the road.  Thanks Doug S.

[Martin S.]

Hmmmm, on a stock bug engine, an unresponsive idle mixture screw means only one thing: the idle jet is too lean and you need a bigger jet. You have to drill out the jet.

[type149doug]

    I put some compressed air to #1 cylinder and did not get any indication of air coming past the intake or exhaust valves.  I'll install a new intake gasket and go from there.  Still stumped....

[type149doug]

   I swapped the carbs back to their original positions and I have the unresponsive idle mixture screw again...  Let me pose this question....  If I have a cylinder that has a bad ring seal that would mean less vacuum.  Therefore the fuel wouldn't be drawn from the idle passage....Correct?

[type149doug]

   I did some redneck engineering and did a simple flow check on the offending carb.  I used some duct tape, shop vac and my SK flow tool.  I found that the barrel over #1 flowed 5.5 and the barrel over #2 flowed 3.5. Twisted throttle shaft...  Here's what I'm thinking is going on... There's not enough vacuum on the bottom side of the throttle plate of barrel #1 therefore more flow but not enough to pull fuel from the idle outlet orifice. This will cause a lean miss.  What fuel is getting into the cylinder is not being ignited bot going into the exhaust where it is then lighting off which is popping out the exhaust.   Sound like a logical theory???

[MC Dyno Don]

My VW brother, I feel your pain... The best way for you to check your complete system is to do a Leak Down test with a leak down tester. This will be able to determine the percentage of ring and or valve wear. A very useful device and will perhaps save you a lot of further headaches.  Hope this helps in some way for you..?    Good Luck, Dyno Don

[type149doug]

  Thanks Don... Point taken.

[Martin S.]

Dirt in the idle jet will do that.