My 1925 Chummy has always started and never let me down, so the problem i'm having now is puzzling. I'm not that naive but I always look for a reason and in this instance I cant find one. Last summer I went to take her out for a run and the little lady would even fire up, nothing. Checked Spark, Fuel etc...etc ...etc.. As time was getting on I decided to send the Magneto away with a spare I had to be refurbished. It had been running for over 30years so probably needed it. I took the Carb apart and checked and checked for any blockage, all looks fine. The fuel flow was also good.
When I got the Magneto back recently and set the timing, it still would not attempt to fire. Spark, Fuel Flow, Compressions are all fine. I don't have a Spare Carb to check with so if anyone knows of one please let me know. Its a Zenith 24T-2, not a 22FZ or 26VA, but that what is has and has run perfectly for a long time.
If any one has any ideas I would be grateful has i'm out of them.
Fuel, Spark and air should = a big bang, but alas there is nothing.

Location: Yorkshire

Agreed:
(Fuel + air)compressed + spark = bang
Provided fuel is new.
Provided fuel air is being compressed.
Provided Spark is at correct point in ignition cycle.

You have had magneto off so Double check spark is correct for each cylinder.
In case fuel is old, Try "Easy Start" in carb inlet - should fire at least a couple of times.
Is the fuel getting to cylinder, Are the plugs wet/damp?
If plugs dry, Are the valves opening, closing and seating properly?
Check tappet clearances.

Apologies if you have done all this already.
Over to others for more solutions.

Good Luck

David

Location: Hampshire

There is jet on Zenith 24T-2 at the front, directly underneath the air mixture screw.

It is behind a brass plug that you can access easily without removing the carb or taking it apart - it is unusual for it to get blocked but can get overlooked and may be worth checking.

People usually mark the timing of the magneto across the vernier. When replacing the magneto these can then be lined up, but you must make sure you know where the distributor is pointing when you take it off otherwise it is possible even if the marks lined up to find the timing is 180 deg out. Try slipping the magneto back and turn the shaft through 360 deg and re engage.

Many thanks for all the ideas from everyone. Tried everything again from scratch this weekend but no joy.... Totally at a loss where to go from now, other than getting hold of a replacement carburettor and inlet manifold to discount these as one of the reasons for non-starting.

Fuel is new.
Compression on all CYL ok.
Magneto giving spark, correct timing and correct order.
TDC set with Mag firing at Cyl no 1.
Carburettor cleaned, all jets and rebuilt.
Checked that all jets clear
Fuel flow to carb checked.
Tappet clearances rechecked.

Location: Yorkshire

Hello Mike
Really sorry to hear you have no luck. Please excuse us asking the same questions again.
Because No 1 cylinder is at TDC it is not necessarily the firing stroke. For No 1 to be firing the valves of No 4 should be changing over, i.e. exhaust closing and inlet opening.
If this is correct try a little fuel poured down the spark plug hole. If the timing and spark are ok it should at least cough.
Let us know how you get on.
Adrian.

Location: The New Forest

Mike what compression readings do you have? Are the valves opening or have they stuck open due to lack of use? As mentioned earlier try easy start ,horrible stuff but does have it's uses, you should at least get a kick or blow back. Keep a fire extinguisher handy.This would if nothing else rule out ignition failure under compression.How much fuel in the tank? could be low flow if level low.

Location: Piddle Valley

Further to Adrian's post re TDC in No 1. put your thumb over No 1 plug hole until you feel max compression then the piston is at TDC and in firing stroke.

Also the magneto distributor revolves in a contra direction to the vernier drive, check this when getting the firing order.

If the spark is happening at the correct plug at the correct time it will fire if you squirt some Easy Start into the carb inlet. If this occurs you can then concentrate on getting petrol from the carb to the cylinders.

Apologies if I have misread or ignored something elsewhere - but have you confirmed that you are getting a spark with the magneto installed, or merely that a spark is produced when rotated by hand?

If the latter, then consider whether there is a fault in the wiring to the switch panel.

Peter

I had a similar problem with the Cup. When I started it for the first few times it fired up immediately. I then left it for a couple of years while I sorted out all the wiring and various other tasks. The last job on that particular list was to take it round to my local friendly tyre & exhaust place to get the toe-in checked. No problem, I thought, just start it up and off we go.

Would it start? No way. Fresh fuel, checked timing, sparks, towed up & down the road by No.1 son at frightening speeds, but other than the odd splutter, nothing. Nix. Timetable ruined, deadline for a rally gone, so turned to all the jobs that needed doing instead of building a Special.

Fast forward another year or two, more jobs completed (including a frustrating and lengthy hunt for fading oil pressure) and time to start it up again. Same result as before, but didn't bother to faff about with the mag this time. Plugs out, covered in oil/petrol. Cleaned them, still nothing. Why not try another set of plugs? Nothing to lose. Original plugs were a set of new KLG M80s that I had bought back in the 1970s. Tried a set of new Champion D16s. Instant result - yay!!