Stumped by rough idle

[maboyce]

(Broken out of this thread to get more exposure.)

I'm having trouble getting my newly rebuilt engine to idle smoothly - it acts like not all the cylinders are firing all the time, and is consequently very shaky. At speed it runs just fine.

Video (but it doesn't sound nearly as bad on video as it does in person).

It's a 1967 1500 engine, all stock. The pistons are real 83 mm units, NOS by Vicsa. The heads are German 373A units. They were advertised as rebuilt, but weren't. I did some test running with them as-is, then had them flycut slightly and the valve guides and valves done. It made things a lot quieter, but there was no effect on the smoothness of the idle.

Compression is exactly the same on all four cylinders, 800 kPa (116 psi), and builds up at the same rate for each.

I don't see evidence of any exhaust leaks. The system, which is stock German, was installed with new gaskets of the standard type with the crushing metal ring, as well as with high-temperature RTV.

The manifold has been changed out for a different one during the testing phase, again with no effect on the idle.

Fuel system tests

The carburettor is a German 30PICT-1 with power fuel system. I had it rebuilt (cleaned, hardware re-plated, shafts re-bushed, jets checked).

After setting the timing, I adjusted the carburettor with the engine warmed up by setting the idle to 900 rpm with the screw on the throttle arm, then turning in the mixture screw until the engine started to bog. Then I turned the screw back out until the idle smoothed out again.

I have removed the carburettor and blown out the idle circuit with compressed air, with the adjusting screw and idle jet / cutoff solenoid removed. I blew out the idle jet itself while applying 12 V to it to keep the valve open.

In case the jet orifice was crushed and too small, I tried screwing out the jet a small amount to allow fuel to flow around it. That made the idle worse, indicating the jet is doing its job.

The choke is opening properly.

I've been around the engine with propane from a torch to check for vacuum leaks, but I couldn't find any. Of course, I wasn't able to hear any RPM change when shooting propane down the carburettor throat either, so it may not be a conclusive test. I hear smoke does a better job, but I don't have a smoke generator.

The fuel pump, an original Pierburg unit, gave me a lot of trouble when I tried to rebuild it. I had the Brazilian rebuild kit, but it would leak and the springs were far too stiff, so that it was putting out over 70 KPa (10 psi). I switched back to all the original springs, and it came down to 15 kPa (2.3 psi) where it's supposed to be. Later I replaced the diaphragms with original units with new material from VintageWerks.

The engine runs with fuel from a can. There is an inline filter between the can and the hard fuel line that goes through the front tin.

Ignition system tests

The distributor is a 205 K vacuum-only unit, with new cap, rotor, points, and condenser. The points gap is 0.4 mm, and I verified the dwell as being 45 degrees using a dwell meter.

The coil is an original 'short' 12 V coil.

The spark plugs are new, are all gapped to 0.6 mm, and give a blue spark when tested out of the cylinder.

The wires are also new. I have extra short wires, so I tried bypassing the short wires for cylinders #3 and #4, with no effect.

I have the wires installed as follows:

Cylinders
(front of engine)
3 1
4 2

Distributor cap
(front of engine)
3 2
4 1

I first timed the distributor statically with a test light. I turned the engine until the left timing notch (7.5 degrees) on the crank pulley (I have the two-notch pulley) was aligned with the case seam, then grounded the coil negative terminal to the case with the test light. With the ignition on, I turned the distributor clockwise to close the points, then back until the light just turned on.

Later I used a strobe to check the timing, at 900 RPM with the vacuum line removed and plugged. Neither method made a difference.

Supposedly this distributor was tested on a distributor tester, sold as 'rebuilt', but I had to change the shaft out for one from another 205 K unit because the rotor didn't fit properly. There could be other problems as well. There isn't any detectable shaft wobble.

I've tried moving the distributor around at idle speed to see if there is somewhere that smooths things out, but there really isn't. I can get a small improvement by adding a huge amount of advance.

When idling, I can pull the wire from cylinder #4 and hear no change to the running of the engine. I do hear a change when I pull any of the other wires. When I pull wires at about 2700 RPM, I hear the same speed drop with all cylinders.


I'm not really sure what else to check. If I had extra parts, I would start swapping systems out to eliminate variables. The other possibility is that the idle is actually normal. It doesn't sound like my last engine, but that was dual port. Do SP engines just have a rougher idle?

[glutamodo]

I've found that using a can of ether starting fluid with a spray tip with a "straw" on it works best for checking for vacuum leaks.

Whenever I hear of such things, I always think right away about the crush seals on the intake ports. I do note that you've changed them. However are you certain there weren't any old ones still crushed in down there? I've seen it more than once - they will get left in there, (they can be quite difficult to pry out of there) then when the head gets bead blasted when rebuilt, it turns them to a dull gray color that makes them look like part of the head. Stack another on top and you don't get a good seal....

Single ports should have a smoother idle than dual ports. The smaller intake ports result in a higher velocity of fuel through them than dual ports have, which is a good thing for low RPM running. Heck, it was 1966 or so that VW changed their minds on idle speed, upping it from 550RPM to 700-800RPM. And the 1500SP should idle fine in the 700-800 range.

Sounds like you've done just about everything I would have done - except for putting in a plain jet in place of the idle cutoff solenoid.

-Andy

[maboyce]

I am sure about the sealing rings - there was definitely nothing there but the machined recess in the head.

Does starting fluid attack paint or leave any residue? Would water do the trick, in briefly obstructing the flow of air through any leak?

[Boom]

Water will work but I have found that sometimes it doesn't find the leak as good as wd40 but starting fluid or brake clean works better.
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[maboyce]

Update:

I went ahead and tried both a different carburettor, another rebuilt 30PICT-1, and another distributor, a 205T unit, to no effect.

I also swapped the cylinder heads left-to-right to see if the miss on #4 would move or not - and it didn't. Pulling #4 plug wire at idle still failed to cause any RPM drop, while any of the other three wires would.

I stripped the engine to minimal tin and cobbled together a smoke machine to test for leaks, but couldn't find any that way. I did find an air leak at the manifold by squirting water at the joint, but I had created it by re-using the seal rings when I removed the cylinder tin. Putting the engine back together completely with new seal rings made that leak go away again, but didn't fix the idle. I couldn't find any other air leaks with water.

At this point I happened to check the fuel pressure again, and found it was high (4 psi). I had lapped the bottom of the pump, and failed to realise that that would change the pump stroke. I added two gaskets to bring the pressure down to 2.8 psi...which didn't fix the idle. It did remove some popping at the exhaust at speed.

The idle has also started to dip low and shudder when the throttle is closed, which it wasn't doing before. I feel like the problem gradually developed over the last several test runs. The Bentley manual says that a rich idle causes that problem, but I can't adjust it any leaner and keep the engine running.

I'm going to double-check the needle valve and float in the carburettor, but otherwise I'm out of ideas. I've found a thread with a similar problem, but it was never resolved.