Help - mysterious misfire

Wondering if anybody might have any bright ideas on a mysterious misfire problem I'm having with my old ex army Series III 109 2.25 petrol.

She runs absolutely normally at tickover and at low revs. Smooth, no problem at all.

Under load though she's misfiring horribly. Not a typical running on 3 type of misfire, but a complete cutting of power, leading to her kangarooing up the road. Keep your foot down and she doesn't pick up, just carries on kangarooing up the road. Lift your foot off the throttle and everything catches up, until you put your foot back down again. I guess it has to be either fuelling or electrics. I've taken the (Weber) carb off and had it cleaned out. That hasn't helped. I've replaced the plugs, leads, points and condensor and that hasn't helped either. I'm now out of my comfort zone. I guess it could be the ignition coil or a fuel pump (although she has two tanks and switching between the two makes no difference).

Any ideas?

Timing ?

cheap quality fuel? mine played up on poor fuel

fooel pump?

timing gets my bet.

Mine does this a little bit when very cold. I haven't set my carb up properly yet as I am on about my 4th one, chopping and changing around! A 1 min warm up sees it ok. Check your idle screw hasn't fallen out and rattled loose or something.

Accelerator pump in the carb may be at fault, check by removing the air inlet and pumping throttle; look for a squirt of fuel.

Jets in the carb may be blocked; you can never properly clean a Weber 34ich as I have found out (I am guessing anyway). Or else its very hard to do so 100%. Try soaking it in carb cleaner overnight.

Ignition coil may be at fault, mine did exactly as yours did, turned out to the coil at fault. It didn't appear to be giving a continuous spark under load.

Incorrect float height in the carb is another potential one.

The other one is vac advance on the dizzy. Check the hose or buy a new one for £4 from a motor factors (a mini one will work fine). Take the dizzy cap off and suck the end of the hose attached to the carb. The advance should move. Put a tiny dab of 3in1 on it if its a bit stuck, but only a little bit.

Good luck!

Have you checked the vacuum advance is working properly -

take the pipe off ans give it a suck the dizzy contact plate should move -

the pipe is known to get brittle and split ...

Sounds like fuel starvation to me. Does it do it all the time, when cold or restarting after a pause when engine hot.

I tried all sorts of arsing around until in the end I've given up and fitted an electric fuel pump and now it's miles better!

D

From your description it sounds as though Dipps has got it right.

If your ignition isn't advancing when you try to accelerate, your engine will splutter and object, so check your vacuum advance is working and that you are creating a vacuum and there are no blockages.

Weber 34 ich can be prone to icing. your symptoms don't sound like that tho'

It turned out to be the ignition coil!

Pleased you've got it sorted.