Major missing/backfiring/cutting out when hot

A few thoughts :-

1)Which electronic ignition is fitted ? If there is anything suspect on charging/battery (low voltage) Boyer systems don't like these types of faults. If it is Boyer also check the stator-plate wiring (inside the points cover) - over a period of time the wiring/connections on this can get 'tired' due to heat etc. such faults can cause all sorts of mystery ignition problems.

2) It may be worth trying yet another set of plugs - I had a mystery misfire at over 4000 rpm - I had to try 3 sets of plugs to cure the problem.

11,7V is just on the low side. Normally below 12V your battery is suspect. So I would definitely try a new battery, by far the cheapest solution. You should of course check all connections. When heat is involved in misfiring always suspect any coils in the circuitry. Goes for both magnetos and coils, heat will often break a circuit, which temporarily closes when cold, i.e. broken windings and bad soldered connections.

I would suggest the carbs, as they have been changed and the problem occurs at the 5 mile mark which is just about the mark the bike is fully warmed up. Are the insulating spacers fitted, if so re-check this joint as the socket screws can come loose, especially the inner ones (modifiy an Allen key to aid doing them up)

As previous posts, the charging system seems to be controlling at a low voltage so it would be worth checking all associated connections for cleanliness and and fit, including the area where the wire enters the crimp / connector as it may look like it's connected but but may have corroded over the years.

I have what seems to be the exact same problem and have become so frustrated that I haven't used the bike this summer.

It starts off fine and after about 5 miles misfires so badly I have to keep the revs up at lights or it stalls. Embarassing. I have travelled a hundred miles on the motorway and all was OK until slowing down and having to stop at lights. (Andrew is yours OK at speed?)

I have been trying to fix it all last summer to no avail. What I have done is the following:

1. Changed the battery (it was not fully charging anyway)

2. Changed the Boyer - at least the stator and black box but not the rotor or cables

3. Did the twin carb to single carb conversion (I had the manifold) which rules out overheating of the carb because the manifold gives it more thermal separation from the engine

4. Carried out a thorough clean of the carb including the pilot jet, and tried a different main jet

5. Checked and adjusted the tappet gaps

6. Changed the coils and HT leads

After all that it hasn't idled so well in years!

I don't think think its your alternator as if you start off with a fully charged battery it should be capable of serving the ignition system for more than 5 miles even with a duff alternator.

I shall be watching this thread with great interest,

Ian

1972 Fastback

When encountering seemingly unsolvable problems like this, you will have to be more systematic in your approach. Often it will be helpful to return to as close to factory specs as possible. If you have the original points and auto-advance, try fitting these and start from there.

Over the years I have heard about so many inexplicable problems with Boyer ignitions, mind you I haven't had one, I use Pazon Surefire, which works flawlessly, that it must be one of the "usual suspects". Other than that you will have proceed by trial and error, or get a competent electronics workshop to check and measure out your components. Btw. do try a few more sets of spark plugs, I once had to go through two sets, before finding a working set.

Next trick would be to isolate the Blue capacitor out of the cct. In 17 years I have had it fail twice, first time it was blowing the fuse and the second it was shorted enough to drain the battery overnight but not to blow the fuse.

Tape up the capacitor terminals, run the bike and re-check your the voltage level on the battery terminals if it reads higher then suspect the capacitor. With a good modern lead-acid battery you don't really need it in the circuit anyway - I ditched mine years ago.

If the bike was running correctly prior to fitting the new carbs, I would be looking at the carbs. Are the carbs genuine Amal or are they a cloned item. Just because they come in an Amal box means nothing. (unless you bought them from Amal) I have had 2 friends struggle with "new Amals" and then solve their problems with genuine new Amals. Genuine Amals come in a box with the details of settings on the inside of the lid.

In my experience (ha ha) if you get a problem, it's normally the last thing you were messing with.

Are you saying that with your friend's rotor and stator, your problems go away? If so, then you have just delivered the solution yourself! Then your own generator is malfunctioning, in some way or another, and is probably not able to keep the battery topped up, and in a condition to deliver the voltage/watts needed by the Boyer. Classic Boyer condition and symptoms.

Always irritating to resolve a problem but still be none the wiser regarding the cause. But better to be resolved than still stumped, eh? Hope it keeps running well now.

I have just seen the post from Ian Butterss. I have spent six months chasing the same problem but only on the right cylinder. I have systematically replaced one item at a time and still would have the problem. In the workshop the bike would start easily, rev cleanly and idle well but as soon as it was ridden on the road the right cylinder would run for a short while, then cut out then run well again.

I have now finally solved the problem by fitting spark plugs one grade hotter than previous. Now the bike is a dream to ride on the road. I'm told that here in Australia it is necessary to run a hotter plug due to modern petrol.

Worth a try if you are having a problem

regards,

Ian Richtsteig

Hi Andrew

Not sure if you have rectified the problem yet. I suffered from a similar cutting out after about 3 miles. The bike re-started okay after a short pause only to run for about 40 seconds before cutting out. The fault was identified as a short within the handlebar cut out switch. The offending wire is now removed -but I have no cut out switch = and no cutting out!

Steve Brunt

I have solved my problem with overheating. Thanks for the tip about hot spark plugs Ian, but it has turned out to be faulty coil(s). When I swapped the electronic ignition I noticed on the instructions that I should have been using 6v rather than 12v coils, so I changed them as well. Now I have put in the original coils and it works a treat. The brand new ones or one of them must have been faulty. Funny that because they show no signs of bruising on the case from overtightening or anything.

Why did I have to change the ignition system in the first place?? Well in retrospect the misfiring may not have simply been due to overheating at that stage and possibly cleaning the pilot jet sorted that out, but not before I'd introduced a new fault in the coil. The main symptom was not being able to idle, but that could have been with the engine cold as well as too hot. I think I jumped to the conclusion that it was the same problem because I'd already has an overheating episode which involved bending a valve (!), which I put down at the time to sluggish charging leading to low battery voltage leading to misfiring from the Boyer.

So I have had three overlapping problems, one introduced in the course of trying to fix the others, and leaving the bike six months over winter in between times doesn't help with diagnosis. One problem at a time is definitely easier.

It really is going well now though, so all's well....

Ian butterss

750 fastback