Boyer timing

Oct 19, 2005
Country flag
I recently read a trouble-shooting guide on another site that by swapping the two wires from the boyer trigger plate you can retard the timing by about 50 degrees. I am somewhat confused by this since my understanding of this system is that both plugs fire simultaneously every 360 degrees of crankshaft rotation (180 degrees of camshaft rotation), with the cylinder on the exhaust stroke being subjected to a lost or wasted spark. Thus, surely if you swap the wires around the plugs will still fire in exactly the same place as they always have? This is more than a thoeretical question since I attempted to time a recently-finished restoration project with the static view-the-dot-through-the-hole method and could not get the beast running until I tried an arbitrary and rather large amount of advance after which it started and ran sweetly. I have yet to try the final timing with a strobe and may swap the wires to see if this indeed changes the timing. has anyone else had this experience?

Wrong two wires it's the alternater wires that if switched cause the timming to jump. And yes it makes a differance try it if you don't belive it.norbsa
Just a couple of suggestions .

1st ) make sure that your timing marks are aligned properly . Crankshaft to intermediate sprocket and intermediate to camshaft sprocket. This will for sure throw you out

2nd ) make sure that the rotor is in fact aligned with the key. This may sound funny but it has been known for the key to push out during installation of the rotor and stick itself to the back of the rotor

3rd ) if your rotor has two timing marks make sure that you are timing to the correct one

Don't ask me how i know all this cause i don't want to admit to anything but i can say for sure that either of these will cause the grief that you are explaining. The alignment of the three timing marks is critical . Too far out of wack and you have pistons meeting valves
Gentlemen, Thank you for your suggestions. The bike in question is a 1970 750 with single phase two-wire alternator, Norbsa your observation that swapping the alternator wires can affect timing is an interesting one. From a brief look at the wiring diagram these two wires simply go to the rectifier and according to Roy bacon's 'restoring motorcycle electrics' they can be connected to either of the outer two spade-connectors on the rectifier. Additionally I could actually run the bike without the alternator even connected and simply rely on battery power until it went flat. However I will certainly try to reverse the alternator wires when I have my strobe connected and see the result. The site where I saw the innitial reference to swapping the two wires from the ignition pickup is 'britcycle,com' (british cycle supply company) in their section on 'Boyer electronic ignition troubleshooting' and this guide is purported to have been supplied by Mr Boyer himself. Ron, your comments about possible timing anomalies are well taken, my first inclination was that I has cocked something up (many years of experience and expertise in this area) however I am lucky enough to have another almost identical bike in my possesion so I do have a reference point. I set the bike to TDC with a dial guage removed the rocker covers and noted that the valve timing was as it should be. I then checked with a dial guage on my reference machine how many mm before TDC equated to a specific position on the primary cover degree indicator - this turns out to be 3mm BTDC = approx 20 degrees and 4mm = 25, my guage has insufficient stroke for further measurement (why is it that in life we could always use just another inch or so?) - So both bikes indicate the same data. I have also run all the actual components of this ignition system on the other bike with no problem at all, so it is a bit of a puzzler. My next step is to strobe the ignition to get a visual reference point from that and then swap wires from the ignition pickup and see what happens, I will then do the same excercise with the two alternator wires. This forum is trully excellent for sharing this kind of information and I promise to post the result of my investigations regardless of how much egg eventually ends up on my face.
Here is a brief rundown on my findings with the errant ignition timing:
I swapped the two wires from the ignition pickup, reset the timing by the static method and the bike fired up first kick. It seems that the connection of these two wires is indeed critical and my suspicion is that it has something to do with the direction of current through the pickup coils. the black box relies on this signal in order to trigger the plugs in the correct position. My mistake was in using a previously repaired unit without the correct colour coding on the wires and my erronious assumption that since both plugs fired together anyway the position of these wires was not critical. One of the other contributors also informed me that although it is possible to get the bike running with the wires the wrong way round the advance curve is apparently disrupted or even non-existant with this wiring. I have certainly learnt something with this whole excercise and hope that it might help others with their timing problems (this is also additional ammunition for the pro-points lobbyists amongst us).

I have been following this discussion and was wondering if this reverse wiring on the alternator just affects single phase alternators (2 wire ) or the 3 phase as well. My bike sstart and runs fine it pulls great till up to 4500 rpm any thing above that it is slow to pickup any more rpm (i have gotten it up to 5500). The timing has been checked and everything else looks fine, battery, soldered contacts on the pickup plates, cleaned contacts on the coils.. I am running an analog Boyer.
James your bike should just fly after hitting 4500 RPM. 4000 RPM is cruseing speed for X-ways. Something is very wrong with your bike. Read your spark plugs for clues. norbsa
The two wires that I swapped were on the electronic ignition pickup trigger plate located under the points cover at the camshaft end of the timing cover. They should be colour coded but mine were not. Since you have managed to check your ignition timing I suspect that this aspect is probably all right. I assume that you do have electronic ignition and I would check for loose or high resistance (ie, corroded) wiring in the ignition system and also for ignition coils shorting out internally at a specific RPM, wrap some insulating tape around them to insulate them from the mounting brackets and see if this helps. If it does replace the coils and especially if using electronic ignition use 6 volt rather than 12 volt coils, even though your electrical system is 12 volts. whether your charging system is single or three phase should not affect the running of the bike.

Thanks for all your feedback. My plugs have been all over the place either lean or rich. The carbs have been resleeved this summer which helped immensely on idle. I have had the bike for 3 years now, bought it at 67k and it now has 88k miles on it. Over the years I have found quite a few thing not right on it, the most serious to date was the the valves were lapped so severly that they were 1/8 of inch below the head surface. I had that repaired and had no choice but to put oversize racing valves. I have also soldered all the bullet connections as they were seriously in bad shape. I am planning a complete tear down this winter and will be closely inspecting the bottom end to see what other things I may find.
James, as norbsa says commandos are normally just getting into their stride at 4500 RPM and should pull cleanly and WILLINGLY all the way up to the red-line. one of my 750s has a 4S cam and it's a struggle not to over-rev it at higher RPM. Have you checked that the cold start choke- slides are not protruding into the venturi even though the lever is fully open? I bought some new cables for my bike and the carb was all over the place until I removed the air filter and checked inside the carb with my fingers. The main slides were not bottoming out on one side and the choke slides were still protruding down into the venturi (this can be felt with the twist grip at full throttle opening. Another thing to check is your valve clearances in view of your mention of head work.
If you are contemplating a complete rebuild I can highly recommend that you dynamically balance the crankshaft, conn-rods and pistons and also the fitting an automatic cam-chain tensioner.

Dave, I don't have any choke slides, but I haven't checked to see if the slides are completely open and full throttle, I did change the cables when I first got the bike, but apart from synchronizing the carbs to open together I never thought of looking to see if they are opening fully. I will look into dynamically balancing the everything as well as the condition of the camshaft. Thanks again to you and Norbsa for the advice.
The Boyer analogue ignition is pretty simple. The pick up arrangement acts like a little alternator. When the black box detects a certain threshhold voltage coming from the pick up coils, it fires the spark.
As the magnets approach the coils a voltage is induced in the coils which increases as the magnets get closer. ( It is also increased according to the speed of approach which is how the advance curve works).
The thing is that the voltage is positive as the magnets approach the pickups , and negative as they go away so the polarity of those two wires is important.
If they are the wrong way round the black box will be detecting postive voltage after the magnets have passed the coils which gives a very retarded ingintion timing.
Sorry I was wrong in the previous post. the polarity of the magnets means the voltage should never go negative. My mistake.
pommie john said:
Sorry I was wrong in the previous post. the polarity of the magnets means the voltage should never go negative. My mistake.

pommie john
Your previous post was correct! The output from the trigger unit is a sine wave which goes positive and negative. The black/white (on the Boyer box) wire is connected to battery -ve (via a small resistance) and the black/yellow wire (on the Boyer box) is used to provide the trigger voltage. The Boyer box uses the negative voltage on the black/yellow wire to provide the spark trigger and it expects to see the negative voltage BEFORE the postive voltage which it ignores, actually it virtually 'shorts' any postive voltages. If the black/yellow and black/white wires are reversed (from the trigger unit) the positive voltage will be produced before the negative voltage and the timing will be altered.

As for the timing advance I thought this was provided just by the Boyer electronics although I guess it may be a combination of the trigger unit and electronics as the trigger unit produces larger negative voltages as the engine speed increases.

>>>As for the timing advance I thought this was provided just by the Boyer electronics although I guess it may be a combination of the trigger unit and electronics as the trigger unit produces larger negative voltages as the engine speed increases. <<<

I was told about the advance curve by Ernie Bransden ( he of Boyer Bransden) maybe I misunderstood him but I thought it was just the threshold voltage.
I just called Boyer when I had a tech question, and they put me onto him. He's very helpful.