1930 Model B Running Problem

[tony.barnett]

I have a persistent problem with my 1930 sidevalve Ariel. At 40-45 on a constant throttle I can feel it hesitating, then freeing, then hesitating. Its been doing this for about a year now and was still doing it today all be it it seems to be when the bike gets hot, not when its cold. It'll accelerate beyond 45, but back to constant throttle and its hesitating again. Its rideable, but obviously I'd like to find out what's happening. It was fine before.

I've been through the fuel delivery system, as it feels like fuel starvation - I've replaced the solid fuel pipe and old tap with a flexible line and filter, cleaned out the carb completely, core and all, lifted the needle, tried a size up on the main jet, I've also checked for air leaks and that the fuel cap breather isn't blocked - all with no improvement.

So my mind turns to the magneto.

Is the scenario I've described credible for a magneto? Below 40 the bike rides nicely with no hesitation, pulls nicely etc, above that and its messing me about.

I had a new armature (windings and capacitor) in the magneto in 2019. Its a BTH KC1. When it was newly rebuilt the bike would kick back. Now it rarely coughs before it starts, even on full advance, which it seems to do fairly easily most of the time.

Any thoughts? Ideas?

[Roger Gwynn]

Hi tony, I have had a similar sensation with machine that runs fine but on a moderate steady throttle, not accelerating or decelerating, it sort of hesitates. My searches told me that it was probably running a little rich, I must admit that it isn't annoying enough to have done anything about it as that would require a new slide. Worn carb slide/body combo tends to run rich not weak as one might expect. Doesn't sound like a mag problem as they tends to vary with temperature not revs.

[simon.holyfield]

How does the throttle slide feel if you push and pull it through the bell mouth? Any fore & aft movement?

Secondly, and not wishing to hijack your thread but my 1930 A also has a KC1. There's nothing mentioned in the despatch record; is it possible Ariel used BTH for a while during 1930 or are these both aftermarket fitments?

[tony.barnett]

If its slide wear then its come on quickly - its been running perfectly fine for years. And then it wasn't. I'll have a look. I guess I could try a different slide and see if that changes how it runs.

[Roger Gwynn]

In that case, when something has changed suddenly or very quickly it cannot really be wear, usually something has fallen off or broken. I would certainly be fiddling with the carb, try one of another bike.
I don't know of Ariel fitting BTH mags except on early KH models, although they would fit anything that the customer specified, frequently Bosch is seen fitted especially to overseas machines.

[tony.barnett]

I was just thinking try another carb Roger. I'll see what I can find.

Still think its the mag though...

[Roger Gwynn]

Never rule anything out!

[nevhunter]

Sticky bottom feed floats can cause that. BTH are a good magneto. Magnetos usually show up when pulling at low rpm. Nev

[tony.barnett]

Mine is a top feed float chamber Nev. And its probably cleaner than when it left the factory having had it apart several times trying to find a cause.

Now I'm pondering - this problem appeared after I overhauled the top end - valves and guides and a small end bush. I've checked valve clearances, and they were OK. But I probably haven't checked the valve lifter...I guess that could be interfering with the tappets at speed? One to check anyway.

[Roger Gwynn]

Always check the last job you did, 9 times out of 10 it is something to do with that. Best bit of wisdom I received as a young lad.