S3 V12 rebuild engine misfires?

[Danetype3]

Hi All,

The engine had 2 bend valves when I got it, so that needed fixing. The heads did not look great so I had them resurfaced as well, new valves, replaced the timing chain and tensioner, new SNGB ignition. Well I rebuild most, but let the pistons be because those looked good.

Started the engine first time and noticed that the A side is running much hotter than B. Exhaust gases also much hotter from the A side pipe. A1 and A6 spark plugs had what looks like dry black carbon deposit on them (easy to clean) after running for maybe 5 minutes. All other plugs looked good.

Checked compression on a couple of cylinders, about 150 psi, also on A1 and A6. Ran the engine with the spark plug out and A1 and A6 has spark. Exchanged A1 and A6 ignition leads with some other, still runs hot on A1 and A6. The A side carbs draw clearly less air than the B side.

Engine seems to run fine at 1000 rpm, but below that it stutters. Timing is also close, but a bit hard to check precise because of the rough running / stalling near 600rpm.

So
1) compression good,
2) A2 - A4 sparks plugs look ok, so A side carbs ok, so we have fuel on A1 and A6 I assume.
3) timing close to ok.

Any help with trouble shooting is much appreciated

Thanks
Paul

[DWW]

Which cylinders had the bent valves, were they A1 & A6? were the cams checked and the valve gaps set?

[Danetype3]

Bend valves were at A6 and B6. Valve gaps all set. I did not have the cams checked. Do you mean the bend valves might have damaged the cam? The valves were lightly bend and there was a small mark in the cylinders. So it was not too bad. I have no idea how that may have happened. It might have been during hand turning. I tried that myself softly, turning the crank while forgetting that the cams are not connected.

[DWW]

I am just trying to apply some logic to your issue as the issue is on A1&6 it can't be the carbs and unlikely to be ignition. I am not sure whether a slight deviation in timing can affect things in that way, I am sure that when you set static timing you used the cam locking tool. The only member who can possibly offer an insight would probably be Marek......

[Danetype3]

Yes, I used the cam setting tool. Logic does not seem to apply, but I sure it does once the issue is sorted.

A possible, yet unlikely, explanation would be a (considerable?) air intake leak at the inlet manifold near A1 and A6. It explains the lower airflow into the A carbs and the wrong air/fuel mixture would enter those cylinders. But from the looks of it the manifold sits nice and tidy on the head. New seals. All cleaned etc.

Just thinking of there have been different cams that could cause this? The engine is put together from different SN*s. I would not be surprised a wrong cam was put in place. But I wonder if that could explain this issue.

[MarekH]

From the description, you sound lean and retarded. One or the other, or a bit of both.

So the whole A bank has been dismantled.... I'd take the throttle linkages apart and make sure the all four of the throttle butterflies are synchronised to the same gap and that they open up the same amount per side, then that each side moves together with the other. Then adjust the mixtures, then check the chokes are synchronised.

The inlet manifolds have unequal length inlet runners. The ends, in this case 1a and 6a, are the longest so the air pressure is lower on these cylinders - thus they run leaner than the middle cylinders.

You have an aftermarket ignition. I'm not familiar with the SNGB system, but the original was a vacuum retard which only retarded the signal at very low/zero throttle opening. A retarded ignition is less efficient than an advanced ignition and it generates more heat than the same engine at more advanced timing. You should check your timing with a timing light. If the vacuum feed is from the bottom of the left rear carburettor, then it is a retard system.

The engine idles best when slightly rich. Looking at the AFR versus temperature graph, being lean will thus make it run hotter. A leaner mixture is harder to ignite.

The camshafts didn't change timing over the v12 production run

All of this points to your being lean and retarded. If richening the mixture and/or advancing the timing at idle helps, then that'd confirm this view.

kind regards
Marek

[Danetype3]

Thanks Marek, I think you are spot on.

I had also wondered if the intake length difference could be a factor. I had adjusted the carbs equally before starting, or so I thought, but it was not good enough, clearly. My limited experience is with K-jetronic and that does not help at all.

Today I tried again after re-adjusting the carbs (retard vacuum not connected). Still no good. But then I gave the carbs mixture screw a full turn to the rich and then it came alive. Even puff from the exhaust pipes and now the coolant began to flow because now also the A bank got warm. Then I had to stop because of a minor coolant leak that needs fixing.

So too lean indeed. Checked the plugs again and still a bit blackish on the A1 and A6, but I think it is just a matter of getting the carbs adjusted better. I ordered a flow meter because this four vacuum dials device I bought was no help at all.

Thanks!
Paul

[DWW]

Glad to hear some progress has been made, before that I was never aware of the length differences in the intakes….

[Danetype3]

So I have been balancing the carbs and I can get it to run as low as 500rpm when the engine is warm but it is not very steady and something does not add up. I have (on all 4 carbs)

1) carbs in max rich needle position.
2) when I lift the air valve rpm goes down, it likes to stall
3) turn the idle set screws all the way up, not or very little touching of the throttle link.
4) retard disconnected (rpm goes down when connected)
5) timing close to 12 BTDC

1) means I can not make it more rich.2) suggest it is still too lean, 3) kind of suggests it is too rich?

So is something off with my carbs? I have rebuild the carbs and I did my best to do it correctly.
a) New needles (Stromberg B1BH),
b) new jets, 2.4mm down relative to inner surface of venturi area. This is what I could find what it should be.
c) adjusted floaters as best as I could
d) in general made sure the exterior parts seal properly

So I would think the fuel metering of the carbs should be according to spec?

The hoses are new so the vacuum should be ok. I opened the vacuum retard connection (was plugged off) at some point and the effect was quite clear (A side running lean again, smoke form A exhaust) confirming the vacuum lines are ok.

The temperature compensator can make it more lean, but I doubt if it could be the cause of my leanness.

Your thoughts are once again appreciated I think I should be able to get the engine to a state where lifting the air valve will increase rpm. Let it run too rich. But I seem to be running into some limit before.

Paul

[MarekH]

Dear Paul,
you don't want to set up the engine to settle at 500rpm. This will present a harder load to the engine and make it more likely to stall when it gets there. The manual probably tells you to aim for ~850 rpm as a default. By comparison, to land at 500rpm, you'd have to choke off the air supply, stiffle it by an overly rich mixture or retard it to make it settle so unnaturally low.

You might get away with something like this on a fuel injected engine where you can meter the fuel precisely, boost the ignition advance to stop it stalling and control the airflow with electronic air valves, but none of this is available on a carburettored engine, so the fueling is dictated by airflow over the bridge in the carburettor and ignition advance is fixed at the static timing, so you have to do what makes the engine happy.

What is happiness? Vacuum.

Set up your engine so it idles at the recommended rpm and adjust things to the point of maximum vacuum.

kind regards
Marek

[Danetype3]

Thanks Marek,

I meant the 500 rpm as a figure of speech. It is a good sign that it is possible, I think. But it is not stable and too lean. The manual says 650 to 750 (it does not mention the retard, but I assume it is with).

Your input is clear, more vacuum at the carbs is more fuel at a given rpm, hence richer. I'll be looking for leaks first and take it from there.

Paul

[MarekH]

No, "more vacuum" means the engine considers it a lighter, easier load.

If you run at 800rpm and the air pressure in the manifold is 50kPa, and then you make a change and it runs at 800rpm but at 45kPa, then the latter is the better setup, whatever that may be.

kind regards
Marek

[Danetype3]

Got it Marek,

I reattached the 4 vacuum gauges at the manifold and I get about 42-46 kpa when I adjust the carbs with the flowmeter. So the flowmeter and vacuum correlate pretty well.

I wanted to get a better idea of lean or rich, so I bought a Hantek HT25 probe and connected to the scope to see the waveform. Below the results. RPM was about 1000, idle, no load. The first image is with the probe between coil and distributor. The second close to the spark plug. There is a factor 10 difference in signal strength and the burn line is upward only when measuring near the plug. But the burn length is the same, so it does not look like the engine is running lean?

I do hear an irregular plop / misfire and the carbs are max rich, so something is still off, but no luck finding it so far.

Paul

[DWW]

Don’t profess to be an expert but my straight six use to ‘plop’ when it was running on the rich side. Not sure this has any bearing on your problem though.