Retitled - 'sneezing' misfire under load

[Who.Me?]

What might cause this uneven scorching of the terminals on a new distributor cap?

This is the cylinder 2 terminal after a 15 minute test run...

The scorching looks unusual to me and particularly so compared to the other terminals that all look relatively clean like this...

It's a 1600 dual port engine with an original Bosch SVDA distributor. I've just changed the oil, points, distributor cap and rotor arm before taking it out for it's first run of the year (the drive train has been out while I repaired the transaxle).

The engine developed a slight missfire under acceleration as it came off choke that worsened as the engine warmed up. By the time I'd done a few miles and got it fully up to temperature the missfire became severe and an intermittent 'sneezing' backfire developed.

I will do a proper tune up over the weekend when the weather cools down (we've had freak heatwave here), but in the mean time I checked the parts I've changed.

Pulling the distributor cap, I noticed that the rotor arm seems to have 'clearanced' some of the plastic. I wiped the plastic dust off before I thought to take any pictures. More significantly, I noticed that unusual scorching on the cylinder 2 terminal. The other three terminals look OK.

I checked the distributor shaft in case there was any play that might cause the rotor arm to come closer to that terminal than the other. There is no lateral play (there is about 1mm up/down movement).

The cap and rotor arm I recieved from the supplier have slightly different part numbers to my old ones. (New cap = VK1061, Old cap was VK106BE. New rotor arm = EVL0851EF, old arm was EVL085BE).

The cap was seated correctly, as far as I can tell.

Any ideas why that terminal would show unusual scorching after just a few minutes? And could it relate to the worsening missfire?
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Andy


Looking for info on my truck's history. Are you from Campbell California or nearby. Do you recognise it? ... http://www.thesamba.com/vw/forum/viewtopic.php?t=636786

[my3bugs]

pretty obvious the rotor did that ? but was it a cap made off center ? looks like its like one or the other . was the rotor installed all the way down and in the notch ? my guess is likely cheap aftermarket parts ? ill often pay more for Bosch .

[SGKent]

silicone grease (dielectric grease) put onto the tip of the rotor helps protect it. That is just a trail of arcing. I suspect that when the industry went from brass embedded caps to aluminum embedded ones that type arcing became more prevalent.
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[Who.Me?]

Just seems weird that only one of the contacts is like that and there is such a clean swipe and that I had an increasing missfire.

The cap I took off had the usual marks on the contacts and the other three terminals new cap look to be forming normal marks.

The parts are all Beru. I didn't cheap out.
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Andy


Looking for info on my truck's history. Are you from Campbell California or nearby. Do you recognise it? ... http://www.thesamba.com/vw/forum/viewtopic.php?t=636786

[oprn]

Put the old ones back on and try it. If it runs fine... chuck the new ones out.

I live by the thought that just because you can buy some part new does not mean that you need to.
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[heimlich]

Some of those Beru parts are made with aluminum inserts. Yours might still be fine. Buy a Bosch one and see if it fixes the issue. If not you have a good spare.
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[Who.Me?]

Swapped the cap and rotor arm back to the old ones (but forgot to swap the points back) and did the rest of the tune up. It seemed better, but I'm still getting intermittent 'miss' under acceleration. Sometimes it's fine, then it suddenly becomes a huge issue stuttering and, if I push it, a 'sneezing' misfire, almost like it stalled momentarily and restarted.

Checked for vacuum leaks with carb cleaner (none).

Was going to swap out the condenser, just in case, but the one I've received is wrong (rectangular terminal block, instead of square).

Will pull the carb off tomorrow to check and clean it, but fuel here is good. I replaced the tank ten years ago with a band new WW one, so I don't suspect rust.

Have added some fuel dryer because it was easy since November, but I use E5 (and haven't had any issues with water in fuel before).

I did notice that there was some spark scatter (it drifted maybe 10mm either side) but I've noticed that before without this problem.

Any ideas?
_________________
Andy


Looking for info on my truck's history. Are you from Campbell California or nearby. Do you recognise it? ... http://www.thesamba.com/vw/forum/viewtopic.php?t=636786

[BoneStorm-zZ]

I had the same sneeze. Few things you could check. One is a vaccum leak. Where the intakes sit, boots if you have them, base of the Carb, and the throttle bushings. Where the accelerator arm attaches.

Spray your carb cleaner religiously. Hold it for a few seconds. Go through a can or two. Both sides of intakes, around the entire base of the Carb, both bushings.

Do you have ALL your gaskets in place? Are all the nuts tightened down?

Then make sure you get good fuel from your pump. Get to idle, pull the lever back and look for a GOOD squirt into the throat. Make sure the little brass nozzle leaves gas directly into the throat. Down the middle! The tiniest pull should give you some gas.

If it doesn't? Adjust the fuel pump. There's a small cam lobe with a screw in it. Right side closest to the alt/gen. The same little lobe has a + or -. Go "-" for more gas. If you max out and still nothing, freshen up the pump. Or buy a new carb, idk.

Also, don't slam the accelerator pedal. Go through the gears a little slower and ease into each gear. Something about appropriate rpms for the gear you're in. Hope this helps!

-Nate

[vamram]

Have you tested your coil? I've had similar symptoms w/bad coils, and w/a dirty carb where the bowl was drying out under load due to junk in the needle valve, idle and main jets.
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[Who.Me?]

[SGKent]

my vote is bad condenser or cracked dual port boot
_________________
Canned Water - the new California approved parts cleaner (except in a drought in which case rub it with sand).

George Carlin:
"Most people don't know what they're doing, and a lot of them are really good at it."

Skills@EuroCarsPlus:
"never time to do it right but always time to do it twice"

[vamram]

[Who.Me?]

[johnnyvw164]

Try an old trick: at night when you can run the engine in the dark, spray some water on the ignition wires and look for arcing to any metal. The symptoms sound like the insulation is breaking down and allowing voltage leakage.

[Who.Me?]

Condenser.

Strange. I wouldn't have thought it could cause symptoms like that. I always thought they either worked, or didn't.

The timing scatter has also cleared up. I'm wondering how old that condenser was, because I can't remember the timing ever being as steady as it is now.
I was starting to doubt the accuracy of my venerable timing light, but there's not much that can go wrong with those which would cause them to trigger at the wrong time.
_________________
Andy


Looking for info on my truck's history. Are you from Campbell California or nearby. Do you recognise it? ... http://www.thesamba.com/vw/forum/viewtopic.php?t=636786