Non-starting VB

[bob.lowery]

I have rebuilt a 1951 VB which has gone pretty well apart from the fact that the b****y thing refuses to start! I have checked everything time and time again, I have been messing about with old bikes for 40 years or so, so I don't think that I'm a complete numpty! All that happens is that it spits back through the carb. This obviously indicates an ignition timing issue, but I have checked this over and over and over again - setting is 1/4" before tdc fully advanced. As I bought the bike as a loose assembly of parts I am now clutching at straws and wondering if it has the right camshaft. Obviously Ariel used the same bottom end for all of the 4 stroke singles, so there must be a dozen different camshafts that will fit. Mine has base diameters of 1.505", overall height over cam lobes 1.850", giving a lift of 0.345". Approximate angle between lobes is 15 deg (difficult to measure accurately). The Draganfly website doesn't show photographs of the different cams so no help there. Just to give a bit more info I have recently tried a Thorspark electronic ignition with the result that the bike now spits back through the carb with even greater venom!

Any ideas?

[roger.fellows]

Is this one of the models where you can cross over a couple of push rods?

[Ian.Taylor]

Followers might be reversed as suggested by Roger. Inlet first then exhaust ..
The other issues I have seen is when the mag is swapped for another. The advance cable if facing the Motor is Push( Pulling the lever back to the stop Fully advances the timing) ,Most Ariels were facing the rear of the bike,Pull type ( pushing lever open)

[bob.lowery]

Thanks guys. The VB is a sidevalve, so no pushrods to cross. I have tried the followers in both positions, no difference. As you say, the proper way is inlet first, then exhaust. The magdyno is as per original, so slack wire is advanced, tight wire retarded. If there was an issue here I would expect the engine to start or at least try to start with a bit of trial and error on the advance retard lever but run roughly until the issue was resolved. But it's just a complete no-go.

[chris.shearwood]

Bob,
Welcome to the Forum! Why don't you try retarding the Thorspark ignition timing a little bit at a time and see what happens? To me it seems like the timing is presently way too far advanced.
Regards, Chris

[bob.lowery]

Thanks Chris. My first reaction was that the ignition timing is very advanced, so I have tried gradual small changes to the ignition timing, no success.

[paul.jameson]

Bob,

I think you will find you have an A6/664 cam fitted, which is likely to be marked as such on its end. This is VB standard from about 1938. The possible alternative is an F cam, again marked as such on the end and used on the VB up to about 1938. Either way, your cam should suit the VB (the A6/664 giving better performance) so look elsewhere for the problem. Before trying to start the bike, flood the carb until petrol comes out of the hole(s) in the top of the float chamber. Then open the throttle slightly, lift the valve lifter to take the engine over compression then put all your weight on the kickstarter and kick.

[roger.fellows]

Sorry Bob, I'm missing something here. I would expect the inlet valve to start opening just before the exhaust valve closes.

Ignition should be when both are fully closed. - Grandmothers, eggs etc.

I should have remembered the VB is a sidevalve.

Does the electronic ignition have a 'wasted' spark at the top of the exhaust stroke which is complicating things?

[nevhunter]

When people play around with hotted up sidevalve motors they can use some pretty wild cams, sometimes more so than with OHV s as the valve is direct action straight from the cam (no rockers) and also the piston and the other valve never gets in the way. (can't hit them). Usually spiting back is caused by lean mixture low down accentuated by retarded ignition. If the ignition is retarded the exhaust pipe will usually go blue fairly quickly. When the motor is running at high road speed you will need about 38 degrees of advance or more. You "modified" ignition device will probably have "hall effect" providing an automatic advance capability as the revs increase. This also means the thing will retard at slow revs, so the setting is critical of the timing to suit your set up. Nev

[david.anderson]

Bob
Maybe go back to basics. Try a new spark plug before anything else. I have put in a new plug that would not fire. The plug sparked out of the head but would not spark under compression.
Spitting back through the carb can also be valve timing out. Apart from putting the followers in correctly have you correctly aligned the timing dot on the cam with the pinion gear.

David