[09viewlake]

Have acquired one of these in really good nick but I have a problem which I can't solve - it starts from cold but won't run for more than about a minute! The engine is the Suffolk 98G 14 3H with the standard Zenith carb. I turn on the fuel, tickle it, start it from cold and it runs and responds to the throttle. Then it starts to slow and become unresponsive to the throttle, eventually slowing to the point that it stops. Taking the plug out and pulling on the starter shows a really healthy spark and compression is good. Trying to restart after the initial cold start causes the engine to run for a few revs under power then stops again. I have cleaned the points, reset the gap and cleaned and rebuilt the carb several times (and re-set pilot/main) but still have the problem. Any ideas, please?

[grendel]

is the vent in the petrol cap blocked? is the fuel tap filter clean try running it with the petrol cap loose, that will eliminate the cap vent. take the fuel pipe off the carb and check there is good flow from the tank - if not then check the filter on the tap.
Grendel

[Gwiz]

Grendel beat me to it.

[09viewlake]

Have checked that the filler cap has clear breather hole and fuel flows out of the fuel pipe ok - so not those. Have also pulled off the flywheel hoping it might be the condensor (easy/cheap to replace) - but it's encapsulated with the coil so you have to change both if that's the problem - not cheap! Had to disturb the stator/backplate to see the coil which now means I have to re-time the ignition in any case. Anybody got the ignition timing btdc, please - it's not in the handbook?

[Gwiz]

Ought to be fixed timing as the flywheel is keyed onto the shaft.

[09viewlake]

The flywheel is keyed but the stator plate (the ally plate on which the coil and contacts reside) is mounted by two bolts thru elongated holes on the plate. You can swivel the plate on the bolts back and forth by 3/8" each way, thus advancing or retarding the point at which the cam on the engine shaft opens the points. I just forgot my old Villiers days and unbolted the stator without marking it!

[smud6ie]

In the absence of any manual I would start in the 20 to 24 degree BTDC range.
smud6ie

May be a Handy calculator for you?

[Gwiz]

I always set them to the middle, They seemed to run just fine, There is nowhere the same amount of movement as in a car distributer.

[09viewlake]

Many thanks for the calculator (handy for the bikes!) and the suggestion to set the timing up in the middle of the slots on the contact breaker plate - will do that next!

[09viewlake]

I have now run out of (inexpensive) ideas . Set up the stator plate (coil and points) as suggested but no spark when I turned the flywheel by hand as fast as I could - one or two white-ish sparks which looked a bit weak (should I get a decent, regular spark doing this?). Anyway, put everything together (again!) and pulled sharply on the starter cord and got sparks - but still white-ish. Put the plug in, tickled the carb and pulled on the starter cord several times - nothing, not even a cough from the motor. Took out plug and it was wet with petrol and still sparked as before when laid on the head! So I have a clean carb, petrol in the cylinder and sparks when the plug is OUTSIDE the engine - it should start or at least cough UNLESS the coil is not generating a spark under compression - is this likely? While on the subject, a 4stroke fires every two revs of the engine but the points on the Suffolk engine open every rev. How is this ok?

[smud6ie]

Sparking in air is not a reliable test,check out post #13 in this
thread.
Although the plug fires for every revolution of the engine the conditions in the cylinder are only suitable to support combustion   on the compression stroke,alas though,not yet in your case
smud6ie

[09viewlake]

BRILLIANT! Any low-cost addition to my diagnostic capability is really worthwhile. I have been fettling engines for over 50 years (Aaaargh!) and not seen one of these. Have just bought on ebay - presumably you see a spark in the clear plastic sleeve if the plug is sparking - the seller is not expansive on how it works? Much cheaper than a new, suck-it-and-see coil!

[09viewlake]

UPDATE: ignition tester arrived, fitted it and pulled starter. Lamp in the tester multiple flashed everytime I pulled on the starter. Engine would not even cough. Removed plug and laid on cylinder head, pulled starter and lamp in the tester multiple flashed and spark plug sparked.
My conclusion is, therefore, that the coil and points are operating ok. Now I expect the tester will flash even if the plug is not sparking well/consistently under compression so my next step is to buy a new spark plug - sound logical?

[smud6ie]

Changing the plug,yes you can try but logically,You have had it running for short periods before disturbing the ignition system so you really need to return it to that state before going any further.
smud61e

[Fen]

I've always found that any problem with an ignition system is usually constant. ie engine won't start or misfires all the time. An engine which runs well and then stops strongly suggests fuel starvation. Not recommended by all but try some easy start in the air intake as it starts to slow and see if this speeds it up again. Is so definitely short of fuel.