If Mctool is on to something here you can take your battery ground off the normal stud and move it to the rear engine ground to accomplish the same thing assuming of course your ground wire is good
The kit he refers to is made by Motolectric.
On my 907 just adding the larger ground wire improved every electrical aspect( problems) the additional solenoid wires are just icing on the cake.

Thank you McTool and higgy - i'm going to try that and get back to you. It makes a lot of sense, i was starting to wonder if previous owner had fitted high comp pistons even. Although not the worst i've had by any means, slow churn speed could produce this very result.

Some progress tonight - I linked up a second battery to ensure a good cranking speed and also added several jump leads to improve earthing. On cranking, engine was still kicking back but not as much so I persevered and manage to get engine running briefly (about 10 seconds) on 3 occasions followed by either no firing at all and/or more kickbacks. I feel certain that the problem is down to a weak/erratic spark now, but it's very subtle.. I can see an almost normal spark at plug if i use a non-resistor type but if I replace it with a resistor plug the spark is almost non existent at times. This is the strangest bit though, for any electrical wizzards to get their teeth into :- I discovered on a later attempt, and quite by accident, that spark at plug was only apparent on alternate starting attempts!! i.e. - (say) first churn, a clearly visible spark, second churn, nothing, third churn, clearly visible spark, fourth churn, nothing etc. Ah well, i'm bound to get there in the end and at least it ran long enough to confirm oil pressure ok and sounded quite nice! Next job will be to power Digiplex seperately as you suggested McTool and after that, I will try and find another unit to try. Thank you all for your help.
P.S. Saw a lovely 907 (?) Paso near Pulborough in Sussex today.

Dude,you need to go over every single wire,ground and termination before you spend dime one. and two batteries is a very bad idea

Haven't tried the headlight thing yet, and thanks for that specialist knowledge which I was hoping for when posting.


It's starting to get exciting now, the poor little thing is trying it's hardest suddenly - a combination of connector checking, adding earths and the bike being bathed in hot sun has resulted in a stronger, regular spark and a good attempt to start every time I try. But still the kicking back and of course, it's more pronounced now. Combustion smoke from exhaust and carb... More and more I am thinking that this is a 'wood for the trees' moment, and i'm still missing something obvious. - All of the above checks still apply and i'm now going to go through again as though I just clapped eyes on it. Clear symptoms now are as though A/ Timing too advanced B/Abnormally high compression. Butterflys are shut, and float contents are not leaking into bores. Belts are timed correctly as per 3 seperate notches on pulleys, and this coincides with Horizontal TDC dot on flywheel. I don't know if this is a clue, but strobe still 'says' that spark is occuring at TDC instead of 6 deg BTDC.

yes I know Tamburinifan, I should have explained that I added it to the post in the hope that it might have rung a bell with someone re: digiplex failure characteristics. Not having opened up a Digiplex before, am wondering if vacuum advance controlling device can become erratic?

I had the same problem on my 750 Sport. It would start then stop.... sometimes backfiring. Plugs were wet indicating fuel was getting to carbs and spark was evident. I took my fuel line apart from the carb and blew through it with the high pressure air hose from my compressor. A loud... pop... then nothing except escaping air in the gas tank. It has run ever since. Apparently the gas line was clogged somewhere (probably the filter) and allowed just enough gas through to wet the plugs but not run the engine....... . Give it a try, it might work for you also.....

Cheers Dewjantim - I will strip carb again as this is so elusive that it's worth checking everything.

I gotta say , Giocast , I admire your tenacity . Over the years I have worked ( professionally ) on household appliances , small motors, electronic and spring ballance scales and weighbridges, outboard motors , bikes, air con and heat pumps . They all have one thing in common in that they can all be bastards to fix when there is more than 1 fault , especially if one fault is intermitant and Im starting to get the feeling thats where you are now. Time to get methodical . Get a good wiring diagram and start going thru all connections ( as Higgy sez )in some sort of order.
My paso came with some interesting problems that no one ( inc the local duke guru ) could fix and this is why the previous owner gave up on the bike (you find these things out later ,I met him at a rally and he was really pissed when I told him what the problem was ), any how it took me a while to nut it out but I fixed it , and then bought a computer , found this web site and discovered that everything I had discovered was already well documented here ...... but thats life
Dont give up, the feeling of satisfaction you will enjoy when you have fixed it will make all the frustration well worth it

A technique which can be useful with intermittent type electrical issues is to add strategically placed led's/lights along the suspect wire/circuit which can provide a visual clue as to what is going on. Even a test light( home made from a bulb) tapped in can provide a clue. Now if you have a dvom with recording capabilities all you need is a backprobe to record all you need to know. You can even use a dvom to measure for poor connections by doing a draw test at the battery and wiggling wiring along the suspect path.

Thank you McTool - it's that kind of encouragement that helps me through, particularly at my age
that's an amazing idea Higgy - I think it might be a bit beyond me, but I love the concept and may have to consider that route. I am tempted to buy/borrow another Digiplex but I need to discover if a car type unit will be close enough. Unit in question is a 403A (Fiat Uno) but have been unable to find an ignition advance graph for it yet.. Best Regards