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Timing and Hot Running

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Timing and Hot Running

Posted by neil_renwick at June 04. 2017

Help please with my 1972 750 combat commando which is now running but with a few problems. The bike takes 5 to 10 kicks to start and then after a few minutes the choke can be turned off and it will idle around 1000 RPM or just below. This seems ok but engine becomes very hot with the primary side being hotter than the timing side based on pipe bluing (see photo). Also the primary side does occasionally pop/bang when revs are decreasing. I have included a picture of the plugs; primary side seems to be ok or is it too light and hence weak?  The timing side plug is showing sights of running rich so I believe?

Also the revs sometimes stick at around 1500-2000 RPM and is reluctant to return back to an idle. To get it back is either further twisting of throttle or waiting a short while or stopping the engine and then restarting.  The revs sticking I believe is attributed to the cable going from the junction box to the primary side carb being damaged. I also found the needle on the timing side carb to be bent so new cable and needle is on order.

So to address the running hot issue I have now started to look at the basics around points/plug gap and timing and my initial questions are

1)      To set the gap on the points I have loosened screw A as shown on the attached photo and then used screw B to increase/decrease gap. Is this correct as it’s different to the way the workshop manual describes based on my reading of the manual.

2)      I have followed the basic timing process in the manual but when I move the engine backwards using the rear wheel to 28 degrees then respective contact breaker is close to the point of opening but if I continue to move the engine backwards to the point that the contact point is about to open then the marker on the rotor is somewhere back before 10 degrees.  I understand that the accuracy of the marker on the rotor cannot be relied on so what is the best way of progressing with the timing?  In the attached picture you will see a photo of the marker at 28 degrees and a photo of the points. The timing was based on the left cylinder (primary side) with the left side contact breaker being the one which is meant to start to open.

3)      I moved the cam to advance it and this took it very close to the point of opening. When I released the cam, should the cam spring back to the original position, this one did not? Do I need to put a new spring on the advance retard unit?

All advice/guidance welcome and if someone is based near Maidstone Kent and willing to help with the timing that would also be most welcome.

As always many thanks in advance for all input given



Re: Timing and Hot Running

Posted by david_evans at June 04. 2017

Hi Neil, The screw you show are correct for adjusting the points gap. The problem with a points system is the amount of slack in the advance retard unit, they weren't good even when new. You should be turning the engine forward to get your 28 degrees BTDC not backward. With the A/R unit locked against the springs and a bulb across to points, ignition on, move the back plate until the light just goes out. turn the engine one revolution to 28 degrees again and repeat for the other set of points. you will probably run out of adjustment and have to reset to whole lot again. Finally after doing this you will find that your timing is actually 40 degrees BTDC when viewed under a strobe.

I think you have an air leak on the hot side of the engine. If you can get your bike to the Suffolk rally I will bring my strobe and do it all for you.

Re: Timing and Hot Running

Posted by Katherine Scott at June 05. 2017

Previously neil_renwick wrote:

Help please with my 1972 750 combat commando which is now running but with a few problems.


I would also  take a look at your valve clearances, before you go much further.



Re: Timing and Hot Running

Posted by john_holmes at June 05. 2017

When I released the cam, should the cam spring back to the original position, this one did not?

If it sticks advanced when the engine is running then your idle will be fast and your pipes will blue up quickly and for a longer length (they all go blue to some extent). That is more likely that a sticking cable going by your description.


Do I need to put a new spring on the advance retard unit?

That would help but the AAU slots and the pins that run in the slots will also be worn so the springs will only partially cure the issue.

Better fix is Electronic Ignition, that brings its own issues eg with the harness being old and the EI needing a proper 12V etc.

Re: Timing and Hot Running

Posted by alan_blackhurst at June 10. 2017

I went the electronic ignition route as soon as I got my bike had Boyer that died, tried Trispark that was totally useless running a Pazon now with no problems

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