Marquis 4A stops after 20 minutes

 OK, i have either got another 20 minutes out of the machine, or I hve found the problem. After posting and sharing the problem I grabbed my multi meter and went out. I disconnected the starter switch and spark plug and checked the lead from the coil, it shows a decent voltage when cranked. I connected up the sparkplug and switch and ran it again and there was barely a flicker. So I disconnected the switch (one of the few items that had not been changed in the overhaul) and hey presto, volts!!

I connected up the sparkplug to the ht lead and bingo, it started first pull!!

I now have no switch to stop the mower, but I can just let it run to a stop by shutting off the fuel plug...

Good that you have a satisfactory outcome but it seems strange that a possibly faulty switch should have a twenty minute time delay.  I have come across a switch related issue when the back of the switch around the terminals has been contaminated with dirt, particularly in damp conditions, allowing the primary to leak to ground. 

It may be prudent to provide some form of emergency stop, either a new switch or a simple shorting strip for the plug; waiting for the float bowl to empty and stop the engine can seem like a very long time  in an emergency. If it's to be a working machine my choice would be twin cable up the handlebars to a cut out switch.

Sounds as though you may have stumbled upon the cause of your problem but do check that the wiring was not shorting out on the engine casing before replacing the switch as I have seen this happen on a machine where the switch body had rotated in its mounting hole.

If you do need to replace the switch this could be a possible replacement if my memory serves me correctly.

http://www.vehicle-wiring-products.eu/product.php/681/metal-lever-switc…

Thanks both. Wristpin you are right, this isn't a long term solution, but it does keep me running and means I can get the grass under control and prove that the machine is a goer for a while. It buys me time. There is already some cable looped around the handlebar to the failing switch so I'll make use of that with the next switch...

Hillsider, that looks an good switch, but it does look a bit exposed for external operations. Can the terminals be covered over to provide good insulated mounting?

PS Its possible that the 20 minute delay was co-incidence, I don't know. Maybe it was a vibration issue that slowly caused the switch to gap open? 

Can the terminals be covered over to provide good insulated mounting?

Use insulated spade connectors like these:  

https://images-eu.ssl-images-amazon.com/images/I/21xJlryjWuL._AC_UL160_…

The switch that I have suggested will be fine for use in the engine cowling and as suggested you can use shielded terminals - if they are not already fitted.

But you may need to investigated other options if you are fitting a remote switch on the handle bars.

The original switch, very similar to the one posted by Hillsider, would have a bare wire soldered to one terminal and then curled around the threaded spigot so that when the retaining  nut is tightened it is clamped between the switch and the cowling. 

If fitting a handle bar mounted switch my preference is to run a twin cable to that switch  rather than relying on the bars to provide the ground connection via the chassis to the ignition stator. One side of the twin cable goes to a good ground on the engine , such as a valve cover screw , and the other to the coil primary.

Image  of a suitable switch with a  handle bar clamp.

Thanks both for your suggestions. I'll scan around the internet for a suitable switch.

I couldn't resist starting the engine again earlier just to be sure it works... It fired on the second pull with no choke applied and idled just slightly too high and hence was engaging the blades. I'll see if I can get it just right with the throttle adjustment screw, otherwise is it possible to adjust the clutch so that it engages at a higher speed?

Cold starting  with no choke suggests that it is running too rich , so dropping the throttle slide needle a notch would be a start. 

Over the years I have found that Slopers that are reluctant to idle low enough to disengage the centrifugal clutch ( no clutch adjustment ) will usually benefit from reseating and adjusting the valves.

While there is no adjustment for the centrifugal clutch, the springs can weaken ( unlikely to be an issue)  the shoes can partially seize on their pivots or, in the past, someone could have fitted the shoes the wrong way round so that they tend to snatch.

, .

I think I see the problem with the engine running too rich. I spotted it when attempting to adjust the jet needle. I think the clip is missing from the top of the top of the jet needle. Hence there is nothing to hold the jet needle back from extending to its fullest extent. 

I also had a look at the spark plug and its has a fair covering of black sooty deposits which backs up the theory of it running rich.

Could this missing jet needle clip cause the engine to run rich?

The circlip stops the needle from dropping right through the slide ( weak mixture) and under pressure of the spring stops the needle riding up. If your needle is unrestrained I'm surprised that the engine runs at all. I'm sure that George at Villiers Parts can supply a clip.

I'll add an illustration of the carb when I'm back in the office.

https://www.dropbox.com/s/nogmke4i36eok77/Amal%20379%20parts%20list0002…

Perhaps I am wrong about the clip, the needle is certainly restrained and attached to the throttle cable. My suspicion that the clip is missing is because I could find no way to adjust the needle. 

Do I have to remove the clip and reattach it at a different notch on the needle, or is there a different way of adjusting it?

All attempts at using the choke to start the engine fail, or if used when the engine is running the engine dies. I presume this is due to a rich fuel mix.

Thanks for your continued help

Regardless of the correct needle setting applying the choke to a running engine will nearly always stall it.

To adjust the needle setting disconnect the throttle cable at the handlebar lever end to create plenty of slack in the cable . That will enable the slide to be pulled away from the spring allowing enough room for the needle and clip to be withdrawn upward through the slide. It is not necessary to remove the cable from the slide.

Note the position of the clip and then move it one notch upward to lower the needle in relation to the slide and weaken the mixture. When returning the needle to the slide position the open side of the clip toward the cable so that the needle drops in without obstruction and the clip sits level on the floor of the slide.

Then try starting the engine without choke. If it still starts without choke you can try lowering the needle more but I would start to suspect either a partially obstructed air cleaner or raised fuel level in the float bowl.

As I have already mentioned, to get the ultimate smooth running Sloper the valves need to be good. No amount of fiddling with ignition or  the carburtettion will compensate for poor valve seating and sealing.

Thanks, I get it now. I will try that later tonight.

How important is it that the throttle valve cut out section points towards the air intake? I have tried various positions so far but with so much else changing I cannot be sure of the effect.

I note your comment about the valves. Having never removed the valves before, what tools would I need to remove them?

Thanks again for your advice on this.

The throttle valve should only fit one way. There is a slot in its side that engages with a protrusion in the carb body. I'm assuming that you have the usual Amal carburettor fitted to most Slopers.

It's  possible to do the valves with normal hand tools but is lot easier with a valve spring  compressor etc . Suggest that you read up on the process or watch a suitable YouTube vid .