Kickback problems

[bevanclark]

Hi folks,
Sorry this is not Ariel related, but I'm hoping someone might be able to give me some advice:
I have owned a 2000 500cc Enfield Bullet for 25 years. It has points and coil ignition with a centrifugal advance/retard mechanism behind the points plate.
It had been utterly reliable, starting 1st kick pretty much every time. I've always started it in the same way.
However, over the last year, it has started kicking back, which has now become really violent. This suggests that something has been changing/gradually failing.
The bike hasn't had a lot of use over the last 5 years (because I now also have a Panther, a VB, and a BSA Sloper) - it has only done around 20,000 km in my ownership

This is what I have done so far to investigate things:
I have repeatedly checked the two most obvious causes: too far advanced ignition, and air leaks at the carb rubber manifold.
The tappet clearances are correct.
The advance mechanism did not seem to snap back rapidly if the heel of the points was rubbing on the cam, so I replaced the return springs and verified it is operating correctly.
The ignition coil HT resistance was somewhat below the specs, so I replaced that (as well as the plug lead, plug, capacitor and points, since they were pretty cheap). So, in effect, it now has a new ignition system, correctly gapped and timed.
I've cleaned and blown through the carb (Mikarb VM28) - it wasn't dirty or blocked in any way that I can see. There is definitely some wear in the carb slide.

I'm stumped as to what to check/do next.

Any advice would be most welcome.

Cheers,
Bevan

There is one other observation which is going to sound rather odd: I've always checked the points/timing by putting the bike in top gear and using the rear wheel to turn the engine over, with a timing tool in place. But now, I find that when you get close to tdc, it rolls over it. So I had to remove the primary cover and put a socket on the mainshaft to gradually ease it to the right point before tdc. I have never had to do that before.

[Roger Gwynn]

It sounds like you may be timing on the wrong stroke as your description of 'rolling over' indicates that you are turning the engine whilst the valves are opening or closing. Really the only cause of kicking back that I can think of is advanced timing.

[Chris Sealy]

I'm with Roger. Over advanced ignition timing is the usual cause of kickback.
If you are definitely timing on the compression stroke; then the valve timing has altered somehow.
Cheers, Chris.

[bevanclark]

Thanks for tthe feedback folks,
Just an update: I blew compressed air through all the carb jets and rechecked the intake rubber before making a video of things. It's 8 minutes of your life you won't get back, but if anyone can take a look and tell me what on earth I am doing wrong, or where I go from here, I'd be really grateful.
Cheers, Bevan
https://youtu.be/xc-VqgpnTvA?si=DOnb44xrv8K-ZOMS

[cmfalco]

I blew compressed air through all the carb jets

First, a bet. I'll bet your pilot jet is blocked. Unfortunately, the laws of physics don't permit compressed air to blow objects from small passages, such as the pilot jet. Depending on the diameter of the pilot jet that your bike uses, you'll need something like a #78 (0.016"-dia.) or #79 (0.0145"-dia.) drill bit to clear any blockage. A blocked pilot jet would cause the symptoms you describe.

In no particular order, two random (but hopefully relevant) notes are:

I've found to my dismay that when modern fuel evaporates from the float bowl, a hard, varnish-like, membrane often forms in the pilot jet of Amal carburetors, which are structurally very similar to a Mikuni VM. Once formed, that hard membrane is not dissolved by gasoline. If this is the explanation, I can well imagine a membrane slowly forming over time, each time you've left the bike sit for a few days, which would be consistent with the slow onset of the problem that you describe.

You mention in the video that it only starts and runs with the choke on, but at the end of the video, after stopping the engine, you briefly show the carburetor with the lever up, i.e. the "choke" (enrichener circuit) off.

Anyway, I hope the above helps.

[bevanclark]

Update to the update: I may have just fixed it. I'll let the bike cool down and try another cold start later.
The annoying thing is, I'm not entirely sure what it was. Since the last kickback a few hours ago , all I did was:

- remove the points plate again and double check that the advance mechanism wasn't binding on the back of the plate (it didn't seem to be).
- before refitting it, I used an allen key to make sure the mechanism was tight on it's taper - didn't notice the Allen key move at all, but maybe it did.
- I then refitted the plate and reset the timing and now it seems to be going properly !

I'm off to lie down in a dark room !!!

Cheers,
Bevan

[bevanclark]

Thanks for the reply Charles, all points noted. I did mis-speak on the video - it wouldn't initially run without the choke for the first few minutes or so (for 25 years it has always been happy for the choke to be turned off immediately it starts). As you say, I then demo'd it running poorly when the throttle was opened with the choke off.
The remarkable thing about the bike was that you could leave it for even a month and it would start 1st kick, which is why the recent tribulations have been so confusing.
It could well be, as you suggest, that the pilot jet passageway may have had some blockage that my air line perhaps disturbed but has only just now cleared. So my fiddling with the points plate may have been unconnected.
Frustrating not to be certain of the cause, but unless it is playing games with me, it seems to be OK now. Fingers crossed !