Baffling engine problem

[Vyv Cox]

I am experiencing a baffling intermittent problem and would appreciate any ideas.

My Lomax is standard 602 cc engine, Solex carburettor, 123 ignition, twin 6 volt coils in series. It has been totally reliable for 5000 miles +. Recently the engine has been cutting out, with symptoms of fuel failure, at random intervals, once after about 500 yards from first starting, more recently after 100 miles over two days. The symptoms are a brief misfire, followed a few seconds later by gradual loss of power and ultimately stop. In most cases we wait a couple of minutes upon which it will restart and run normally. During the misfire period pulling out the choke cable will briefly allow it to run correctly. On the occasion of the 500 yard stop we were able to limp back to the workshop and found the carburettor bowl empty.

This has now happened around 10 times, with no discernible pattern of distance, fuel level in tank or anything else. Every time it will restart after a few minutes and run completely normally for miles, with no intervention.

I have done as follows:
Removed the float sender and fuel lift pipe from the tank as far as possible, although not able to remove it completely. Tank is bright and shiny inside, no debris. Filter on the end of fuel pipe is clean.
Changed the tank vent arrangement from a NRV to an open vent alongside the filler cap. (in case NRV was sticking)
Replaced the hose between the lift pipe and nylon fuel line.
Blew air back through fuel hose, no restriction.
Replaced mechanical fuel pump
Ran engine with fuel hose to engine disconnected. Strong fuel flow into a bottle.
Replaced float needle valve
Checked float arm free to move
Checked for debris in bowls and jets. Completely clear.

I have also changed spark plugs, HT leads and verified all electrical connections.

Now running out of ideas I would appreciate any new ones! Thanks!

[Vyv Cox]

Forgot to add. When it stopped yesterday just outside a small Breton town I took off the air filter and could see that there was plenty of fuel at the accelerator pump. It then started almost immediately. Maybe fuel flow was arrested but restarted on starter motor?

[Jack602]

A couple of ideas:-
The (Citroen) nylon fuel pipe on my special grew in length considerably when introduced to Ethanol, leading to kinks in the pipe cutting off the fuel.

Could the fuel filter be being sucked onto the end of sender unit and cutting off the fuel?
I cut the end of the pick up pipe to 45 degrees to prevent the possibility of this this happening.

Hope this helps

Jack
Pembleton Super Sport

[Vyv Cox]

Thanks for these. I have followed the fuel line right through and have not found kinks.
Your second occurred to me also but I cannot withdraw the complete unit without either enlarging the access panel on the floor of the boot or dropping the tank. Something to think of back home.

[Vyv Cox]

Confirmed no kinks.
In-tank filter seemed high on the pipe so I managed to push it down, where it feels fairly secure. Hopefully that will fix it for now but any other thoughts welcome!

[CSC1540]

Hi If it was winter I would suggest carb icing . It might be worth checking the carb gasket.

Regards

[Vyv Cox]

I considered such problems, including vapour lock, but some incidents have occurred so close to home that the engine is not even up to temperature. It would seem that something more mechanical is the cause.

[CSC1540]

hi. I had a cutting out problem which turned out to be a fault with the electronic ignition , I proved it by reinstalling the points . I have read here that the 123 can suffer from water ingress .
Like you I am just looking for what you haven't already checked.

Good luck.

[howardjs]

Hi Vyv.
Does sound like intermittent fuel starvation, but you have checked all the possible causes... so possibly an intermittent ignition issue?

If you look back in this forum there was a spate of 123s with problems apparently a faulty batch, I'm going back 2 or 3 years.

Secondly I would suggest ditching the 2 6 volt coils and going back to a known good standard double ended 12 volt item. At least to eliminate the 2 6 volt units as the cause.

Lastly plug leads, I had an intermittent ignition problem a couple of years ago. I went to the Green Spark Plug company in Cheshire who sold me a length of tinned copper core silicone insulated cable and 2 new plug caps which completely transformed the engine back to smooth running and tickover.
Hope this helps Howard

[Vyv Cox]

Thanks for the additional ideas. I have wondered about the 123 and will bear it in mind. I have substituted the coils for the standard item, which I am carrying just in case. Made no difference. I am using silicone rubber copper cored leads.

The engine failed once yesterday, at full throttle acceleration up hill. It restarted within seconds of stopping. We then did 50 miles relatively gently, trouble free. So it is looking increasingly likely that the in-tank filter is blocking the tube inlet. As I cannot withdraw the unit without a lot of work I will sleeve the tube when I get home next week.