Commando 750s issues

If your carbs are not flooding then check the fuel taps and the breather in the tank cap. Once you have the carbs flooding correctly with fresh fuel and plugs and it still won't start then :

90% of carb problems are electrical.

After 3 changes of carb I would be doing the full debug on the Boyer.

http://vintagebikemagazine.com/technical-articles/boyer-trouble-shooting/

Also check the earth path from the engine back to the battery, those double sparks need a path, a temporary wire from head to battery earth is a quick test. Also watch for distance from the boyer magnets on the rotor to the stator pickups, too large and the box does not get triggered.

Dear Mike

I would check fuel flow from tank, not unknown for filters to block or taps themselves to fur up, you dont by any chance have original fibreglass tank with sealer in it do you.

Check flow by removing pipe from tap and catching half a pint in a glass jar, then as well as flow you see what colour it is. If flow starts ok and slows, open tank cap and see if that improves it, breather holes in caps are known to block too.

Can't help with ignition as I am dinosaur points man!

Regards Martin

Nb When flooding carbs you should get a steady flow if you hold plunger down, if not you have bango filter and valve in float bowl to check too.

As above, run new ground wire and install a different petrol tank or fuel supply system.

If your carbs are still not flooding then the float is not being depressed by the tickler, I lengthen it by screwing in a self tapping screw into the rolled shaft inside the float chamber, it then becomes easily adjustable.

If the carbs are flooding but the plugs are dry then a blocked pilot circuit is top of the list.

The Amal 25 pilot jet/bush can only be properly cleaned with a #78 or 16 thou drill, no amount of cleaning with fluids will resize them properly.

As you have tried different carbs and seemingly have a good spark, how confident are you that the motor is correctly assembled or in good condition? Cam timing correct? Sticking valves? As it is new to you, there is no history of what fuels or oils it has run on which could cause the latter. They don't have to stay open much or for long to lose compression when starting. 

Another thought, have you tried a/ using the plugs from your N15, or b/ tried starting the N15 with the Commando plugs to check that they work ok? Wouldn't be the first case of no spark under compression reported on here.

Good luck,

George. 

Have you tried starting it with the silencers off?

A good thought re: blocked exhaust. For a quick check, rather than taking the silencer off, a pair of rubber gloves stretched over the silencer ends will let you know if it is blocked or not.  If they inflate or get blown off, it means no blockage. 

Mike, On a similar line, is the air filter clear? Maybe worth trying with it removed. 

Regards, George 

Hi Mike.

You say you changed to Boyer ignition but not whether you did this anyway, or because it wouldn't run properly from the off.  Here are three additional thoughts considering your change of ignition set up anyway.... and considering that issues have a tendency to be related to the last thing that was touched!

1.The degree dial (I forget the correct terminology for this) fitted to the outer primary case that you are using to strobe your new Boyer system to, can often be completely inaccurate.  Mine was 6 degrees out!  Maybe the previous owner knew this and had made allowances for it but you have not. A check with a degree wheel at tdc and 28 degrees before tdc will reveal this. Also make sure the chain case cover always end up in the same position when it's removed and replaced. 

2. Make sure you didn't buy plug caps with built in resistance AND plugs with built in resistance. Only one OR the other is needed with electronic ignition, both causes problems. 

 3. Dont forget it is negative earth. Double check the coils have the correct positives and negatives. Coils will work if it is wrong but they tend to produce a weaker spark and fade. Probably you are sure of this if you have run an external earth wire but its worth double checking as you dont know what has been done to the bike in the past. 

Regards Graham

As well as the degree disk being out, I had a rotor that was out by many degrees for some time, started ok and drove ok but top end a bit flat. I re-timed it by guess and top end acceleration eventually. Lot better acceleration from 70+ on the motorway! Then Police man stopped me! 'do you know your rear light is out?' phew.

If the NGK has an R in the number it has a resistor (or not as the case may be).                              As regards Ammeter behaviour, the Commando is alternator while the AJS was Dynamo, although battery charging is similar as the two systems are different the results on the meter can be different. So in this case the Ammeter results are not anything to worry about. If the lights work well then the battery has enough to get the machine going. Charging the battery is almost incidental.

Sorry Mike. I just noticed I wrote negative earth. Of corse I meant to write "Don't forget its positive earth"  Nobody picked up on it so I suppose my mistake was obvious.

I've still got my money on the wrong ignition timing being the issue. You said the engine seemed to get really hot very quickly. This is a classic sign of the ignition being too far advanced. Hard starting can be either too advanced or too retarded. If you have the timing case off you can make yourself a degree dial with a print out of a downloaded online dial stuck on an old CD and check it in half an hour. If the bike is back together I would put the original carbs back on with the original settings and try moving the Boyer backplate clockwise a good few degrees and see what happens. If its not better try moving it anti clockwise. My most successful attempts at ignition timing were usually achieved by trial and error and not with a strobe light.  It can't do any harm to try (look out for kick backs though ☺)  

Oil pumps can be successfully refurbished by the way. Several companies offer this service. I had mine done by RGM. It will always leak by a bit though even when it's perfect. You just have to ride it regularly!