Valve Stem Seals, or maybe throw in the towel

[Nessism]

With 15 psi compression, it's a waste of time squirting oil in the cylinder and monkeying around like that. So sorry...but, rebuilding engines is FUN, so not sorry. Enjoy the journey!

[Rich82GS750TZ]

I know, not changing my plans, but curiosity got me. Was just putzing around the garage anyway, so no time wasted, that I wasn’t already wasting.

[pdqford]

thinking that a bad coil would effect both the cylinders it served as its wasted spark. But nothing seemed to be making a difference, so to eliminate the coils I decided to swap the 1 and 4 leads. As a further test I pulled the lead that was feeding the problem cylinder off the plug and got this serous arching at the coil. I think pulling the plug lead increased the load on the coil and it discharged through the fault (path of least resistance).

As you probably know that in a wasted spark ignition system both cylinders fire at the same time. One spark plug fires the cylinder under compression and the companion cylinder's spark is "wasted" as that cylinder is on the exhaust stroke..

But the high voltage spark always travels in the same direction: Out of the coil, through the spark plug wire, jumps from the center electrode to the ground electrode, through the engine metal, to the ground electrode of the companion cylinder, across the companion cylinder"s spark plug gap to the center electrode, etc. back to the coil. (Back in the day, Ford even used a different spark plug for the forward firing cylinder from the companion cylinder, since one cylinder's spark traveled from the center electrode to the ground electrode and the companion cylinder's spark traveled from the ground electrode to the center electrode.)

When you removed the spark plug wire from one cylinder you effectively interrupted the current flow for the secondary circuit of that coil. The voltage really wants to complete the circuit and get the high voltage back to the coil. As you said, the high voltage will take the path of least resistance to find the ground side of the coil secondary. It found the weakest link in your ignition's secondary circuit.. If the high voltage can't find a way back to the coil.. the coil is likely to eventually become fubar.

Sorry Rich

[Rich82GS750TZ]

No worries at all, Jim. All good info.

[rphillips]

I'm curious, Just for me, as soon as the cover comes off, ck. the valve clearances on # 4.

[Rich82GS750TZ]

I promise to reveal all that I find.

[Rich82GS750TZ]

Hey, i wonder if your camera probe thingy have ability to be turned and look back up at the valves.

Yes, I can see a bit of the exhaust valves that are open in cylinder 4, a bit out of focus, but enough to see that one of them is pretty clearly deformed.

[KiwiAlfa156]

As you probably know that in a wasted spark ignition system both cylinders fire at the same time. One spark plug fires the cylinder under compression and the companion cylinder's spark is "wasted" as that cylinder is on the exhaust stroke..

But the high voltage spark always travels in the same direction: Out of the coil, through the spark plug wire, jumps from the center electrode to the ground electrode, through the engine metal, to the ground electrode of the companion cylinder, across the companion cylinder"s spark plug gap to the center electrode, etc. back to the coil. (Back in the day, Ford even used a different spark plug for the forward firing cylinder from the companion cylinder, since one cylinder's spark traveled from the center electrode to the ground electrode and the companion cylinder's spark traveled from the ground electrode to the center electrode.)

When you removed the spark plug wire from one cylinder you effectively interrupted the current flow for the secondary circuit of that coil. The voltage really wants to complete the circuit and get the high voltage back to the coil As you said, the high voltage will take the path of least resistance to find the ground side of the coil secondary. It found the weakest link in your ignition's secondary circuit.. If the high voltage can't find a way back to the coil.. the coil is likely to eventually become fubar.

Sorry Rich

Sorry not trying to hijack the thread.

I might be missing something. It's a single coil with two leads. Essentially it's an HT voltage connected to two loads in parallel. Taking one lead off doesn't isolate the secondary windings as it's firing to ground through the other attached lead and plug.

It's a circuit of two 'more or less' equal resistors in parallel between two nodes. The voltage seen by two parallel resistors is the same, but the total resistance seen by the coil is divided across the equally between the two resistors effectively providing half the resistance of the single resistor.




If you remove one resistor from the circuit it 'doubles' the resistance seen by the coil's secondary windings so is an increase in load but not a disconnection. The coil question had an internal weakness that blew when high rpm/high compression raised the density of the fuel air mix between the spark plug electrodes. While agree running a coil without a load will destroy it, this wasn't the case here.

[KiwiAlfa156]

Yes, I can see a bit of the exhaust valves that are open in cylinder 4, a bit out of focus, but enough to see that one of them is pretty clearly deformed.

Looks like its melted.

[93Bandit]

Yes, I can see a bit of the exhaust valves that are open in cylinder 4, a bit out of focus, but enough to see that one of them is pretty clearly deformed.

Is that deformation or is it wet with oil? Oil globs/drips can make it appear deformed, especially if the picture isn't clear.

But with compression that low, I would suspect valve issue as well. Usually worn rings will still have some compression. 15psi is essentially 0 in my opinion, which would indicate an apparent leak, like damaged valve or a hole in the piston (which you don't have thankfully).

[Rich82GS750TZ]

Won’t know for sure till I pull the head off. Next week. Maybe Sunday evening. I’ll be away all weekend at kid’s college parents weekend stuff.

[93Bandit]

You'll get it fixed

Just take your time and be methodical with disassembly. Makes it go back together easier.

As previously mentioned, rebuilding engines is fun. Part of the fun of an old bike if you ask me.

[Rich82GS750TZ]

In my destroy-rebuild thread, I tell of how i got this '81 engine from Norm storm 64 when I broke all the rockers off the intake side on the '82 engine. When I showed up at Norm's shop, he had the head off an was milling out some broken exhaust bolts and chasing the threads for me. Glad he did too. Saved me a ton of work. It also meant that I had to take off the jugs and replace from the base gasket on up. One piston's rings were stuck fast, so rather that fool with it, I took the pistons out of the '82 with their rings and put them in the '81 engine. I did put a fresh hone on the cylinders. There was nothing that I could see that could have been wrong with the head/valves at that time. Says the inexperienced guy who had never gone past valve adjustment into an engine before. If I had taken the time, at that time, to pull the valves and replace seals, inspect seats, I may not be where I am today. But here we are.

I did enjoy that work, and I'll enjoy doing a better job of it this time,

I'll get it done. Not in a hurry this time. Thanks for the vote of confidence.

I'm test riding a Triumph next week. Don't tell my GS.

[Rich82GS750TZ]

And now back to our regularly scheduled program:
Had a few hours in the garage tonight to pull valve cover, exhaust, airbox, and finally, the head

Tadahhh! # 4 exhaust.

[Nessism]

Were you keeping up with valve adjustments? It's unusual for a valve to burn and break apart that way under normal circumstances.

You may be able to use a valve from one of your other cylinder heads. Or, use the best parts from all and make one good head.