Webers: 4 Mixture screws have no effect? (Running on 2 cyls?)

[belling]

Hello. Trying to get my Webers dialed in.

The car runs pretty rough, but ok. Lots of popping out the carbs.

Trying to follow the procedure for setting up the carbs, but only mixture screws on 2 & 3 have any effect. I can screw the others all the way in (or take them all the way out!) to no effect.

Actually, if it's idling at around 1000, I can pull the right 3 plug wires and it doesn't change anything!

Compression is 150 on all cyls.

Spark at all plugs.

Quintuple checked plug wire order (1-6-2-4-3-5 off the top of my head).

Used the float gauge, all floats are at the right level.

Carbs were just overhauled, all the passages are clean as I could get them.

[belling]

One more thing. I am running the stock fuel pump, but have replaced the Zeinth carbs with the Webers. Could this be an issue of low fuel pressure?

I read somewhere that you have to upgrade the pump with the Webers, so I bought a new pump, but it wasn't a drop-in replacement so I put off installing it.

[2.7RACER]

When you assembled the engine you timed the right side cam the same as the left side. That is when number one is at TDC compression, number 4 is also at TDC compression instead of at crossover.
Easy mistake to make for a first time rebuild.
To check what I'm saying rotate the engine by hand with a compression guage in number one. When number one is coming up on compression the guage needle will move up indicating compression. Stop when the crank indicates TDC. Now move the compression guage to number four. If the right side cam is correct, it will come up on compression exactly ONE complete turn of the crank back to TDC. If it is wrong it will not come up on compression until you rotate the crank TWO complete turns.
In other words, one and four are compressing at the same crankshaft TDC, instead of 360 degrees apart. Think about this. I can explain further if you need.

[belling]

I just tested it and it is timed correctly. Cyl 1 is 360 deg from cyl 4. Also, I took of the distrib cap and could see that it was pointing up at compression of 1, and down on compression of 4.

This afternoon I disconnected the throttle linkage between the two carbs, and when I pushed the throttle on the carb over the cyls that don't seem to be working, the engine revved, so they are working, just not at idle, or maybe not at full power.

[TimT]

Sounds like you have some clogged up idle circuits.
If turning mixture screws yieds no change in engine operation, Id bet you have some gunk, a small particle etc clogging something in the carb

Webers need a good clean supply of fuel. Dont skimp and buy some cheap fuel filter.

A quick way to slean up the carbs is just pull the idle jets, and see if you can see light through them. If not, a shot of carb cleaner usually will remove any offending bits.

[cgarr]

I think Tim has it, have had the same problem with my 912 running weber carbs I bet if you pull the idle jet you will find a peice of ***** in it, its just like you have a dead (cold) cylinder at idle, what size jets are you running in idle?

Craig

[jryerson]

Check your ignition timing.

Make sure your floats are the right height, clean your idle circuit, might want to check and clean the mains also.

Check for air leaks in the carb bases.

Make sure your idle set screws are not too far in as you will be starting to get into the progression holes in the carbs.

Synch each of the carb thoats.

[belling]

I put in the new fuel pump, it diddn't change anything.

I think the advance mechanism is hosed in the distrib. I set the timing at 900 rpm to 5-6 BTDC. At around 1100 RPM it advances to 20 or so, but never advances past that. So at higher RPMs the timing is too low.

But I don't think that's important for this idle adjustment.

It's really strange, the mixture screw for cyl 3 works just as described, close it up and the engine stalls, pull it out and RPM's increase until backfiring out the exhaust.

None of the other mixture screws do anything. I've pulled all the idle jets and they're clean.

I just had the carbs totally apart and blew all kinds of cleaner through the passages, so I'm pretty sure they're clean.

I'll make sure the carb bases are sealed up tight, that seems like it could have this effect.

The #5 idle set screw is kind of tight and doesn't seem to work, so that cyl's a bit out of sync with the rest. I'll chase the threads and try to get it cleaned out.

[jryerson]

You can spray some carb cleaner on the base or propane and check for any idle increase for leaks, also check the throttle shafts for leaks.

Pull some plugs, if they are black its probably timing if idle circuit is clear.

I don't know on early 911s if they will run 180' off on the dist but you may verify you are at TDC at no.1 on the rotor.

[campbellcj]

Several of the tuning write-ups mention worn/leaky throttle shafts as a cause of what you describe.

My Webers have been a big PITA too. I finally have them set up better than ever before but I still don't think they are "right". It takes a lot of practice, I guess.

[belling]

"Several of the tuning write-ups mention worn/leaky throttle shafts as a cause of what you describe."

Hmmm. I noticed when the engine was idling, there seemed to be some motion of the throttle shaft. I wonder if they're loose and seeping air. I guess that would lean out the mixture and make the mixture screws ineffective...

Doesn't explain why on the other carb one of the three carbs works fine.

I like the idea of checking for leaks with propane. I'll try that.

[jryerson]

If you can attach a small rubber hose to the propane bottle you can isolate leaks further.

[belling]

I used propane and carb cleaner to look for leaks and I diddn't find any. Which is good.

Here's the situation though:

Cyls 2, 3 and 5 are working fine. Unplug any of the plugs and the engine stops.

Cyls 1, 4 and 6 don't appear to be idling at all. Unplug them and nothing happens. Even all 3 at once.

If I spray a bit of carb cleaner into 1, 4, or 6, the idle speeds way up. So it sounds like the problem is fuel, not spark or timing.

So the idle circuits on those cyls don't appear to be working at all. I'll pull them apart and check them out tomorrow. Also, I'll try swapping jets from one carb to the next and see if that changes anything.

[jryerson]

You might try removing the idle jets, mixture screws and aux air bleeds and spraying them with carb cleaner and let them soak for a bit them blowing out the passages with air. You should see air coming out each of the ports and out the top through the air corrector via the emulsion tube.

[belling]

I think the problem was that the air adjustment screws were too far out. As I turned them in slowly on the cyls that were not firing, the idle picked up. If I leave them there, the mixture screws come into effect.

Most of the stuff I read said to start with them at 1/8 turn out, at which point the mixture screws don't seem to work either.

I have them about 2 turns out, and things work pretty well. Just a small ammount of popping.

It's very drivable now so it makes for a good starting point

[Wayne 962]

If you think a cylinder is not firing, get an infrared thermometer and check the exhaust pipes to see if they are as hot as the others.

I also had a problem like this that I couldn't track down. Turned out it was the distributor. With the old one, the timing light was flickering on and off - with the new one, the signal was rock solid.

-Wayne

[911pcars]

I realize you performed some of the following already and it sounds like you have it under control, but this would be my checklist:

1. Engine:
- Perform a compression test. The carbs are the very last thing to do when performing basic settings. If each cylinder is good, set the ignition timing, then the carbs.

2. Ignition:
- Check the spark plug wires with an ohmmeter to verify there is continuity. Check from the bottom of the dist. cap to the end of the cable.

- Check the spark plugs. Sometimes they get installed w/o any gap.

- Check for spark at the end of each plug wire, especially the non-responsive cylinders. The spark should have a loud "snapping" sound. Perhaps a faulty rotor isn't distributing correctly or the cap is installed ca-ca.

- Does your ignition system run on points? If so, check the distributor cam lobe. Make sure it opens the points for each cylinder. A worn-flat lobe will not open the points. A too small or large gap may not fire the spark plug consistently.

3. Fuel:
- Remove each idle mixture screw (6) and inspect. They should be smoothly tapered w/o any grooves worn in them. Later smog-controlled engines w/Webers (IDT-3CI) use mixture screws w/a very sharp taper. Make sure the tip isn't broken off inside the carb (maybe the PO replaced the mixture screw but left the broken tip inside). Blow carb cleaner through to verify.

- Are all the throttle blades on each carb synchronized? That is, there's a coupling on each 2-piece throttle shaft (accessible at the bottom of the carb) that allows all three throttle blades to operate in unisyn. To do this, back off the idle speed screw. All throttle blades should be closed and should open at the same time. Adjust the coupler if necessary. Perform this on each carb.

- Disconnect the cross shaft and adjust the idle speed screw on each carb so the engine idles around 1000-1200 rpm and each bank draws in the same amount of air. A Unisyn or other air flow indicator is needed to verify this.

- There will be throats/cylinders that might draw in less air. Balance with the idle air bypass screw. Begin with all air bypass screws closed - lightly. Initially, only use this screw to increase the air flow of a throat to match the highest flowing throat on a carb.

- If the carbs are spitting back, give each mixture screw about 1/8 more turn out, or try just the throat/cylinder that's spitting. Maybe you can pinpoint using this method. Check for any leaks in a vacuum hose/connection.

Hope this helps,
Sherwood

[belling]

Wayne - the distributor is definitely in need of a rebuild. It advances beween 900-1200 rpm about 10deg, but then never advances again. I need to find that advance chart someone posted, but it's supposed to advance all the way to 35 at 6000, which it does not. That said, I don't think the advance is the issue here since the timing is ok at idle.

The light does seem to flicker a bit though...


Sherwood - Thank you so much for the rundown. These things are priceless for us newbies. There are some things in there I missed, like the syncing of the throttle blades.

The only strange thing I see is that on some cyls I have to have the air bypbass screws pretty far out (2 turns) for the cylinder to work at all, while others are ok with it closed or at 1/8.


One thing I've found to be helpful is a little puff of carb cleaner into the carb tells me how functional the cylinder is. Working cylinders will sputter and the engine will stall (due to flooding or fouling I guess) while dead cylinders will cause the idle to rev a bit.

I've got it to the point where 5 seem to be working well, one is in between. It idles pretty well and doesn't pop or backfire much. Time to put some doors on the car!

[rw7810]

Actually, I think the distributor advance should only show half that of engine advance. I think you will see that in the charts.
Also, total advance at 35 degrees is more important that what ever the advance shows at idle. I would try setting total advance, then go back and let off of the idle adjustment screws to bring it down to about 900-1000rpm and see what that does for you.

[belling]

I don't understand what you mean about showing half of engine advance.

This thread has the chart for reference.

I'm getting that first steep part of the advance (1100-2000 in the T chart), although it's happening too soon (900-1500 or so).

After that, there is no more advance at all, the 2000-6000 part is flat.

Aren't there are two centrifugal weight/spring sets in the distrib? One for each part of that curve? I bet one is not working. I'll pull it apart and have a look.

Also, the fact that the first advance is too low (<900 rpm) makes setting idle difficult, since the timing tends to go up and down as the idle changes slightly.

Can you buy distrib rebuild kits?