Ignition coils wired improperly? One cylinder not firing

[Sterminare]

The odo shows 5600, unless that means 105600 (I hope not!). But my registration is 5600 not exempt/overflow (like my car is listed "exempt" on the title because odo spun over at 99,999 and it's at 107k). I've only had the bike 1.5yr, no idea how many previous owners.

It's possible the engine is swapped. the serial matches the year (2nd year- 1984) but not the VIN, but from what I could find online the engine doesn't necessarily match the frame VIN from the factory.

Im confident the oil level is proper now. Who knows if a previous owner ran it low/dry. For all I know they ran it on no oil, wrecked the camshafts/bearings and that's why they tore the engine apart. Everything I can see (besides the head-crankcase mate actually) has red silicone gooped all over so someone's been in here before.

I have the valve cover off now, best I can tell the valves are moving up smoothly and completely. I understand I may have carb issues additionally but I feel that the blowback is unrelated to the carbs since it happens still with the carbs removed. Happens only on this cylinder. And with no combustion happening, cranking with the starter. I might try bleeding the tappets maybe they werent probably done? I'm trying to think of what the symptoms are of an unbled tappet- they push on the opposite end of the rocker arm, which would tend to open the valve. But the camshafts are doing the actual opening, what is the tappet doing? Dampening the motion to prevent bouncing? Would a valve even bounce at this low RPM (cranking)?

[Bob H]

I have the valve cover off now, best I can tell the valves are moving up smoothly and completely.

Awesome. If that was my bike, I might double check the valve timing if you suspect someone has opened that engine up before. Kind of a shot in the dark, because if the timing was off a couple teeth it would be all cylinders not just #2.

... I might try bleeding the tappets maybe they werent probably done?

That is not a common problem. It just never comes up. I'm no expert, but it seems if you had a collapsed hydraulic lifter it would manifest when you just turned the engine over. Also if the hydraulic tappet had an issue, it would likely be a failure to PUSH IT OPEN and the spring would work to close it - and you wouldn't be getting air pushing out the intake.

Now that you have it down this far, I believe double checking the compression and doing a leak-down test may provide more answers, or at least more confidence that the bones of the engine are OK.

[Sterminare]

I'm spinning the crankshaft by hand and putting my hand on one intake at a time. Bare head no carbs/boots, valve cover off im watching the camshafts compress the valves.

Cylinder 4: my hand is covering the intake. The intake valve opens, I feel strong suction on the palm of my hand. I pull my hand away and release the suction. I put my hand back and continue cranking. The intake closes, exhaust opens, and I feel no suction or force on my hand.

Cylinder 2: again, intake valves open, I feel strong suction. I pull my hand off to break the suction after intake valve is closed, then put my hand back. Piston is moving up for compression stroke, With intake and exhaust closed. It's blowing air by my hand, as if the intake valve were open. The intake valve cannot be fully closed/sealing properly. How else could this be possible?

I know that's a contrived way of describing it, the short version is: cylinder contents are being pushed out the intake during compression stroke on only cylinder 2.

I'm going to redo compression test.

[Bob H]

...Piston is moving up for compression stroke, With intake and exhaust closed. It's blowing air by my hand, as if the intake valve were open. The intake valve cannot be fully closed/sealing properly. How else could this be possible?

Yeah that is odd. I'm not saying it is possible - but the only other source of compressed air I can think of is something like "positive crankcase ventilation" hosed to your #2 carb by psychotic prior owner. But that's not realistic.

...I'm going to redo compression test.

Sounds like a plan.

[Sterminare]

Ok so.. bad news. I have no idea what happened the first time I did the compression test. I got 120psi on all cylinders.

I just did it again, carbs and headers removed. Cylinder 1,3,4 are reading 175psi and cylinder 2 is reading 30.

If it were head gasket bleeding into cylinder 2, cylinder 1 or 3 would also be low. You're in a bad spot when you're hoping for a bad head gasket 

Given that I can see the intake valves returning to position and not sticking, it's probably not bent. My theory then is it sat for a number of years under previous owner without being cranked, and the valve was open letting moisture in and the valve/seat is rusted. That or it's burnt a hole somehow. I don't really know what the next steps are, I guess pulling the head off and seeing what the valve looks like and either getting a new valve and lapping it in or sending off to get the head machined. Either way it looks like I'm probably not riding this year...

[draser]

Cylinder 1,3,4 are reading 175psi and cylinder 2 is reading 30.

\

Well, that's starting to make sense. I'd turn cyl 2 down to BDC then fill with kerosene or MMO or mixture of acetone and ATF, let it soak for a few days, ensure all fluid has seeped down in sump or crank a few time with plug off, then repeat compression. Thinking frozen rings if the 30PSI is good.

[Bob H]

Thinking frozen rings if the 30PSI is good.

Yeah, but that wouldn't explain his odd symptom of air pushing out of the intake during compression stroke.
If he puts a teaspoon of oil down the spark plug hole, and gets the same result, that would rule out rings-piston-cylinder.
If no change on retest with a teaspoon of oil down the spark plug hole, then it has to be the valves.

[Bob H]

I did some searching, apparently it is possible to have intermittent compression. You had 120 psi, later test 30 psi.
This article is boring and technical, relating to a vehicle we are not interested in - but toward the end he mentions:
"Intermittent compression is almost always a valve-sealing issue. If there were a hole in a piston, we'd have no compression at all. If piston rings were broken or cylinder walls extremely scored, we'd have consistently low compression.

So it's relatively safe to say that we have some type of valve-sealing issue. The tough part of the question is determining what's causing it.

What could cause intermittent valve-sealing issues? Valve-to-seat issues, broken valve springs, valves sticking in their guides, or pressed-in valve seats that are loose, just to name a few."
https://gearsmagazine.com/magazine/intermittent-compression/
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He shows how the failing cylinder can eventually pump up to normal pressure on the compression test, yet some of the strokes could be very low.
Again some oil down the spark plug hole and retest compression would rule out piston-cylinder issues. Most articles mention one teaspoon of oil.

[Larry Fine]

How long does the 30psi hold, say compared to the 120psi (leak-down speed)?

[Sterminare]

Thanks guys. I'll try adding oil to the cylinder tomorrow though I think given the blowback, it's almost certainly valve related. But if  adding oil gives no change in compression, then it's definitely not the rings is my understanding.

I'll see if I can check the leak time tomorrow too, before adding oil to the cylinder.

Assuming it's valve related, Im planning on breaking down the cylinder head this weekend. Whether that's in preparation for sending the head to be machined, replacing with another head from eBay, or hopefully it's just carbon buildup that I can scrape off and get it good enough. May try lapping though I've read elsewhere on this forum that the hardening is extremely thin and that's not really advised. Regardless next step for anything valve related is pulling the head off, right?

Thanks everyone for giving advice. I'm glad the mystery is solved, even though it ended up being pretty much worse case scenario  I learned a lot tearing apart and rebuilding all the stuff I tried along the way so it's not a waste.

[Pete in PA]

Let us know what you find.  Very interesting way to do in essence a leak down test without an actual tester.

I did say possible rust and leaking valve way back.

With such low mileage it can't be a burned valve and that's almost always exhaust valve.

Don't have a 650 manual, but 97% sure head comes off in frame.

Might get away with just a cleaning and valve lap if not pitted rust.

In answer to your previous qustion the hydraulic lifters take up clearance. If one is not holding you would have more clearance causing a tapping when running and not opening that valve as far.  But...That valve would still close giving complete seal.

One last step if you haven't pulled the head yet...Does your borescope have side view?
You could check the intake valves, seats, and stems for rust and pitting.  Engine could have sat with those intake valves open then rusted.

On that note... what does the 2 valve stems look like when you look into the intake port with the intakes fully open?

With enough corrosion the valve spring can't pull the valve closed and stick open intermittently. I'd spray some PB Blaster on the stems through the intake port.

[Sterminare]

Don't have a 650 manual, but 97% sure head comes off in frame.

Might get away with just a cleaning and valve lap if not pitted rust.

Yea my manual says the head can be removed in frame. Just a cleaning and a valve lap is the hope for sure.

One last step if you haven't pulled the head yet...Does your borescope have side view?
You could check the intake valves, seats, and stems for rust and pitting.  Engine could have sat with those intake valves open then rusted.

On that note... what does the 2 valve stems look like when you look into the intake port with the intakes fully open?

With enough corrosion the valve spring can't pull the valve closed and stick open intermittently. I'd spray some PB Blaster on the stems through the intake port.

Now that you mention it, my borescope did come with a little mirror I can thread onto the end to get a side view. Ill see if I can find it and see what I can see. But the scope isnt great so im not sure it'd be clear enough to see the rust/severity.

Ill try putting some penetrating oil and seeing what happens thanks for the suggestion

[Bob H]

One last step if you haven't pulled the head yet...Does your borescope have side view?

Good point. 16 minutes into this long video - the teacher explains why modern mechanics use inspection cameras and rarely do leak down tests anymore.

Now that you mention it, my borescope did come with a little mirror I can thread onto the end to get a side view. Ill see if I can find it and see what I can see. But the scope isnt great so im not sure it'd be clear enough to see the rust/severity.

If you don't see much with it, Amazon has some side camera scopes with side lights for under $100.

[mollusc]

Just a thought...
Given the amount of wear on those cams, perhaps one of the valves is slightly misaligned so it's not sealing well.  This is definitely an interesting mystery!  We get to vicariously follow your exploration without having to do the work ourselves.

[Sterminare]

This is definitely an interesting mystery!  We get to vicariously follow your exploration without having to do the work ourselves.

I'm glad you're enjoying getting to follow along! Happy to relay the progress in exchange for all the information and advice you guys are giving me..

Given the amount of wear on those cams, perhaps one of the valves is slightly misaligned so it's not sealing well.

Definitely possible, it seems like the wear on the cams is uneven, some have dark spots, some just look burnished like the photo, some look basically untouched. So in doing this rebuild I have the opportunity to do it the right way (hopefully) and correct any misalignment before it gets worse. Didn't take too many pics of the lobes yesterday but I can add some when I do.