LH Running In Woes

[andy.mcgregor]

A few years ago I posted here about the butchered twin spark head engine I'd found.

Well the good news is that I've now turned a pile of parts into an almost working bike. This isn't one for the purists - most of the original project parts were wrong or knackered, and the bike only been completed with a lot of assistance from single spares, Draganfly, ACME and eBay.

One of the early start ups and first ride is on youtube https://www.youtube.com/watch?v=afy4N64Lwpg

Currently it starts up quite easily and will only run for about a mile before it cuts out. Then when hot it is more difficult to re-start. A dash of fuel down the plug hole helps.

On the return journey it won't run quite so far before it dies again. This has been repeated a few times now and I haven't been able to find the problem. It feels as though the bike has run out of petrol when it pooters to a standstill. It seems to be quite hot, literally smoking hot at times.

I've checked the filters in the fuel tap, cap vent and run with the cap off so I don't think it's a vapour lock and there seems to be fuel in the float chamber. There's spark as my neighbour found to his discomfort and the plug is a light grey colour. When I kick it over after a stop there seems to be compression and if anything the bike is a little tighter than when cold but always turns easily enough.

I've checked timing and clearances but I haven't been able to fix the problem, so I'm obviously missing something. Any suggestions?

[Eero.Korhonen]

Hello,
How much clearance do you have in piston?
It looks very good, great work.
Br, Eero

[Leejm]

Light grey plug means it's running lean, and it will over heat. Check out the carb.

[johnwalsh]

Could possibly be fuel evaporation if getting a bit hot. Have you got a Tufnol insulator btn carb and head, may be worth trying.

[Leejm]

John is right you still always fit a heat insulator. And fit the thickest one you can get. The one on my nh is 5mm thick. When the barrel is hot the carb is still cold! Don't forget the incoming charge is part of the cooling.

[nevhunter]

Condenser and coil in the magneto can give problems like you are experiencing. Has the magneto been rewound and condenser changed? Nev

[wade.edwards]

Such a good job of recreating a fine machine Congratulations! Got to be the capacitor failing. Having spark is not an indication that spark will be there under compression. The mag must be heat soaked tested.

[andy.mcgregor]

Gents, thanks to all of you for your suggestions and kind words.

I didn't realise that insulators got used at the carb - I've only got a paper washer there. I was concerned that the carb body was getting quite hot so that makes sense. I rebuilt it to the settings from the Draganfly site (110 main, slide #3, .107 needle jet and needle at postion 3.) But with the 18mm plug size, I wasn't able to fit my Colortune to check it. So either the carb set up could be too lean, or its getting leaner as it gets hotter eventually leading to the halt.

I've got an Insulator on order now and I'll try that and play with the carb settings first.

The magneto armature was rewound and tested (up to 50 degC) and a Brighspark condenser fitted. So it "should" be fine. Although it was me that reassembled it so very possible that I messed up something in there.

A new over-sized piston was fitted and the barrel bored to suit. At the time I gave the engineers a photo copy from Waller with the clearance specs. I can't remember now if I double checked the clearance when I got it back. I think my gut feel is that its not nipping up, because I've had this run and stop thing happen about half a dozen times. Each time it has kicked over fairly easily afterwards, so it would have to be seizing up and then immediately releasing each time. If the other options don't work out then I'll have a look at the piston soon.

[Leejm]

I never use the paper gaskets that comes with the rebuild kits. I make my own from heat resistant gasket material two of course one for either side of the insulator, my carb stays cold. Even when the engine is at running temperature. Lee

[Leejm]

Excellent work by the way, the bike look great!