[ddavidv]

I'm posting this hoping someone will have a brilliant idea.
I picked up this Interceptor several months ago. Got it running and took it for it's maiden voyage, which did not end well.
This is the video where it quit. Proof that it actually did run! https://youtu.be/4RGPwivetKM
I got on it with a decent handful of throttle but not wide open. And it just shut off. Note:  it was firing on both cylinders, though I suspect the left bank was weak.
Back home I cannot get it to start. I do a compression test and find the left bank is zero. Pulled the head and found a burnt exhaust valve. This was pre-existing, I'm certain, because the left exhaust never sounded quite right. I suspect the valve burned and the exhaust pipe was discolored from the timing chain tensioner coming loose, which is how the bike came to me. I only found that by accident because I took the mag off to have it rebuilt (preventative measure). After installing the mag, timing it properly and fixing the tensioner it started and ran as shown in the video.

Okay, so it died and had no compression on one side. I replaced the valve, compression rings and pushrod seals and put it all back together.
The rockers/valves all move.
The tensioner has not moved.
The marks on the cam gears are where they should be and line up to each other.
I have compression (tested multiple times).
It pulls vacuum through the intake (tested with the carbs off).
I have spark to both plugs. I've checked multiple times that it is timed correctly, retimed it multiple times, and tried switching the plug wires around even though I knew I had it right. No progress.
I've changed the plugs with another pair of the correct Champions.
I've checked and re-checked the valve clearances.
It seems to be getting fuel. Thinking there may be an issue with the carbs (doubtful, but hey, I'll try anything at this juncture) I've sprayed starting fluid through the carbs with the throttle open as well as removing the carbs and spraying directly into the intake ports. No pop of any kind. This is the thing that troubles me the most.

Previously, I would suffer a loud backfire and a violent kickback on occasion. I believe this only occurred when I had a lot of fuel in the cylinders. This has not repeated in the most recent rounds of testing.

I know...the above sounds like a timing issue. But I have literally checked/reset the already correct timing probably a dozen times by now.

How I set the timing:
Lock the auto advance with a zip tie fully advanced
Find TDC for the cylinder the mag is ready to fire on
With the bike in gear, bump the rear wheel backwards to 3/8" before TDC
Set mag points to just ready to open and lock on the auto advance unit
Remove zip tie

There is no key. This bike only has a kill switch. Which since I have spark can't be the problem as it interrupts the magneto.

Despite the dead cylinder, I believe the reason the bike suddenly quit was the securing bolt on the advance unit (on the magneto) came loose. If the cylinder had crapped out on it's own I still should have fired on the other one. The sudden death seemed electrical-like, which you can hear in the video. I believe whatever that was may still be the reason it is not starting today. Finding the bad valve was just a bonus.

I have literally spent the last 3 weekends screwing around with this thing and have made zero progress. I'm completely baffled. I don't pretend to be a genius mechanic but if I have spark, compression and gas/ether it should try to start. I'm hoping someone will have a fresh idea.

[AzCal Retred]

Sounds like a puzzler. I have no magneto chops, just battery/points/coil.
You say you have spark. So the magneto is firing.
When you determine TDC, you are first watching the intake open at TDC Intake, stay open most of the way until it closes at BDC Intake, then come back up on a compression stroke, right? Verifying actual TDC Compression stroke?
What readings do you get if you leave the mag advance alone, just read the point opening with your VOM for "kick start" speed, rolling it with the rear wheel, verifying again you are actually at TDC Compression? 0.5mm - 1.0 mm BTDC? These are basically a pair of 350 Bullets, so the starting timing should be similar. You said you reversed the spark leads once, so if it was simply 360 out that should have found it.
If you have 70 PSI or more cranking compression, the spark is occurring at or slightly ahead of TDC Compression stroke, and there's some ether in the intake tract it rather has to pop. If the spark is happening maybe 30 degrees ahead or behind that event, maybe not.
I would be curious as to the actual numbers BTDC on the Compression Stroke that the points were opening. The 3/8" full advance may somehow be screwing the pooch here.
There is also the issue of spark voltage. Is your magneto putting out that famous fat blue sizzling spark, or just a pale yellow wisp of a spark? It takes more voltage to overcome a compressed charge in the cylinder than open clean air at one atmosphere. If the volts aren't there...
The fine wire/precious metal electrodes have a lower gap voltage requirement, mostly due to a smaller ( more "needle like") center electrode diameter. Maybe set the gap to 0.010" - 0.015" temporarily as a test?
Good Hunting - ACR -

How I set the timing:
Lock the auto advance with a zip tie fully advanced
Find TDC for the cylinder the mag is ready to fire on
With the bike in gear, bump the rear wheel backwards to 3/8" before TDC
Set mag points to just ready to open and lock on the auto advance unit
Remove zip tie

[Dirge]

try disconnecting the kill switch wire at the magneto end.  They can be a bit tiresome with intermittent earthings.  It's worth eliminating for sure.

[ddavidv]

Having eliminated most everything else I'm now wanting the check the keyways for the cams. However, I have no idea how to remove the cam gears. The puller for the crank sprocket won't work.

It seems like both valves on the left bank want to be open when the timing marks are lined up. Is there some other way to check the timing? Please don't say 'degree wheel'.

[AzCal Retred]

It's a 4-stroke. Suck-squeeze-bang-blow. The center stand, rear tyre, plugs out and trans in 4th gear is an idiot-proof methodology for verifying crank rotation. A stick will find TDC & BDC reliably.
At TDC Compression both valves are closed and have been closed for nearly 180 degrees if it's getting ready to fire. From TDC Compression we go into the Power Stroke, ending at BDC & starting the Exhaust stroke.
From BDC Exhaust the exhaust valve opens, staying open until nearly TDC Intake, where the intake valve opens. At TDC on the Exhaust stroke the exhaust valve will be nearly closed, the intake will be opening, beginning the TDC Intake cycle.
At BDC Intake the intake valve will be closing, the exhaust Valve shut tight. BDC Intake becomes BDC Compression, both valves closed, heading back up to TDC and setting up for the Power Stroke.

Use some kind of graduated measuring instrument for your stick and take careful notes. Actual cam/valve opening is a bit more complex with overlap & duration, but this stone axe method can tell you if you are radically off somewhere. The valve timing on both cylinders should be the same. A degree wheel on the crank and dial indicator on the valve stem is obviously the most accurate & informative way to do this.

Are the points accessible? are there two sets? One set? Could you set up a battery driven coil & condenser and bypass the magneto guts, just using the points? Full retard should be very near BTDC compression, full advance at about 30 degrees or whatever the math tells you the equivalent piston drop should be.

[AzCal Retred]

Ddavidv - any lucky breaks or incremental progress yet? 

[ddavidv]

No progress. Mostly I've been ignoring it, which has been easy as the shop has been in the high 80s lately.

[AzCal Retred]

Copy that - hard to get inspired with sweat dripping into your eyes. 

[oilypuddlefield]

What's the latest on this ? Problem solved,waiting out the heatwave, or still hitting the brick wall ?

[ddavidv]

A combination of all. 
I was sick of it, so ignored it for a good while. I did find out that, after checking the crank sprocket pin, I did not have the intake cam timed properly which is why I got some kickback the last time I tried it. I fixed that this weekend. Timing these is a giant PITA, frankly. Especially so if you don't have it on a lift and can look dead-on at the cam marks.

Anyway, back to where we started with no pop off whatsoever. I've pretty much eliminated anything mechanical and am turning to the rebuilt magneto. I've been in contact with the builder and he gave me a test to try so will do that next.

[oilypuddlefield]

A combination of all. 
I was sick of it, so ignored it for a good while. I did find out that, after checking the crank sprocket pin, I did not have the intake cam timed properly which is why I got some kickback the last time I tried it. I fixed that this weekend. Timing these is a giant PITA, frankly. Especially so if you don't have it on a lift and can look dead-on at the cam marks.

Anyway, back to where we started with no pop off whatsoever. I've pretty much eliminated anything mechanical and am turning to the rebuilt magneto. I've been in contact with the builder and he gave me a test to try so will do that next.

I hear ya on the cam timing being a PITA. The things is, we're out here in the fields and might do this once , or twice if we're lucky. Some factory guy this everyday of the week, so it was second nature for him. Well, I'm glad you found it.

[ddavidv]

For those following along at home...
I've admitted defeat. Everything is timed properly best I can tell. Has spark, fuel, compression but will not fire. Even checked cam timing with a degree wheel.
Shipping it off to a friend who volunteered to give it a go. Failing success there, I will have to find a British bike mechanic somewhere.
Worse than it not running is how stupid I feel not being able to find whatever is amiss.