Dommie 99 ignition timing

Previously paul_standeven wrote:

Ah yes, you need to make sure you are timing BEFORE tdc, not after.. It is also useful to find out how much advance your ATD is actually giving. Ideally you have correct timing at full advance, and 4 degrees btdc at full retard. Much more than that and you get kick-backs. Another favourite way of getting a bike not to fire is to have the wrong plug firing. If you got it wrong, swap the plug leads over

Paul

Thanks Paul, I learnt that lesson during the first week of flogging the dead horse. It was only after 2 or 3 eruptions from the exhaust ports that the penny dropped, and I recognised that swapping the HT leads made starting more likely. Since that time I have been setting the timing on the offside cylinder.

Previously philip_hannam wrote:

One common mistake with ignition timing is to get confused with the direction the magneto and auto-advance are actually turning.

Looking from above, when the crankshaft is turning forwards both of the mag and A/A actually rotate backwards. Plus you have to move the A/A even further backwards to add in the correct amount of ignition advance. generally by means of a wedge.

Appreciate your input Philip. I fully understand the basic timing procedure but because my engine will only fire up with the wedge still in position,I need to change something and I don't know what.

Previously anna jeannette Dixon wrote:

hello timing on model 99 and 650 are 32 degrees fully advanced and 6 degrees fully retarded and drive side cylinder its on the left and drive side on the magento is the one nearest to the battery box the plug lead farthest away the one nearest the back off the barrel is for the right hand plug timing chest side the ADT should be wedged open with a small bolt and the mags points should be set at 12 thou.if all this helps yours anna j

Many thanks Anna. My problem is that, having followed the setting procedure, the engine only shows signs of life when I leave the wedge in the ADT which according to the numbers, indicates that I have a 6 degree error somewhere.

Previously Mikael Ridderstad wrote:

I have not adjusted timing on a Dommie for many years, but I started a 40M first time a couple of weeks ago. It has some kind of electronic ignition. Started easily, checked timing fully advanced. Was 12.8 degrees BTDC. Now adjusted it to 36.5, which is close to the recommended 38 BTDC. As a Manx is so forgiving on running ( not good but running ) with such a broad span of misadjustment, I believe a Dommie could not be that sensitive to timing. I'm told that some good tuners deliberately set timing at first a bit low to get them running and then make a proper adjustment.

If the bike started without kicking back and run a couple of seconds at full advance indicates that timing is set too low.

A bit unorthodox but I should have tried setting timing not advanced around TDC and tried. Maybe a problem with autoadvance unit or a magneto problem. Could of course be a fuel problem too.

Thanks for your comments Mikael. Today I have re set the timing, yet again, and it has become relatively easy to get the engine running at tickover speed for a few seconds at a time so your suggestion of a fuel supply problem may need investigating further. With regard to the possibilities of a problem with the automatic timing device, I have noticed that there is a tight spot on the chain that makes the advance retard mechanism stick and not return to the fully retarded position, not sure how to tackle that one. I have also found that, whilst I can get the engine to fire and tick over with the exhaust ports open to atmosphere, installing the exhaust pipes renders start up impossible. Am I going mad? Thanks again Ron.

Previously David Cooper wrote:

Its several years since I last set Dommie mag timing but a couple of comments...

Obviously you don't want tight spots in chain. Better a bit slack than damage bearings.

But just because auto advance does not immediately seem to return it might be because it has to pull the mag armature against a magnetic cogging effect so maybe it's OK. Maybe some whose done it more recently might care to comment?

Thanks David. I have had the auto advance off, fitted replacement springs and given it a thourough clean, on the bench it snaps back like a rat trap but once the centre bolt is tightened it requires encouragement to return to fully retarded. Tomorrows schedule is to slacken the chain, remove the exhaust pipes, re-time and pray. For me this is not a good time for cut backs on mental health spending. Wish me luck. Regards Ron.

Previously David Cooper wrote:

If it really is sticking on full advance then it will kick back on attempting to start and could do you a bad ankle injury in extreme. It sounds retarded. You could try timing to spark at TDC with auto advance retarded. It won't be correct but might get it going.

Do you have the correct unit? I have one that operates the opposite way round for a Norton. No use whatsoever.

Right! Since my last plea I have freed up the ATD, replaced the mag drive chain and the cam shaft chain, retimed (exactly by the book). End result is that the old girl will start up and rev. The bad news is that when I put the exhaust pipes on (pipes only, no silencers) there is absolutely no sign of life whatsoever, it's just like trying to start it up with the cut out button depressed. I have even rigged the pipes so that they are not in contact with any other part of the bike, but to no avail. I know that it doesn't make any sense but any comments that would give me a clue as to what to do next would be most welcome. Thanks. Ron.