Mk3 Commando pops and bangs

Pops and bangs are usually caused by exhaust leaks (air getting in) or intermittent ignition, however it can also be caused by a too rich mixture igniting in the exhaust,I have found that super unleaded has additives that cause my bike to run rich , standard fuel clears up the mixture but contains ethanol which i don't want. A dilemma!.

Assuming you are using the std black box / twin carb arrangement and taking everything you said above intro account, I'm wondering if one of the rubber bellows going into he black box is damaged? To be fair, this would only make a small difference.I have non balanced pipes on my Mk 3 and the only problem is the lack of clearance under the primary chain case.

I would suggest running the bike at below 3000 rpm for a while and then checking the plugs, because clearly there is nothing wrong above 3000 rpm. I dare say you want to keep your bike standard but once sorted out I have found that the single Amal Mk1 carb conversion works really well. (And gives 68mpg)

Popping and banging can sometimes be down to a weak mixture at the lower rev range, such as slide and needle so don't over do it at below 3000 rpm and since you are on unbalanced pipes: is there more blue for example on the RHS?

Do let us know how you get on.

Popping on the overrun is normally associated with an air leak in the intake side. Air does not get drawn in on the exhaust side as there is generally a positive pressure in the exhaust pipes. An overly weak mixture at lower throttle settings will give similar symptoms. Closing the throttle restricts the airflow but the engine is still trying to suck. The pilot circuit comes into play along with the needle at smaller throttle settings. If you have already proved the inlet manifolds, richen the mixture on the offending pilot air screw (screw it in) or raise the needle (again just on the offending side) and see if the problem goes away. Let us know how you get on

Previously andrew_walbridge wrote:

In 1988 my Mk3 Commando stopped working on the way home from work. It was one of the hottest days of that year and I ended pushing it about a mile home uphill. I put it in the garage and with two young children it stayed there until 2 years ago when I had the time and more importantly the money to start restoring it. Six months and a small fortune later it looked as good as new. Unfortunately, it backfired out of the offside exhaust at low revs and on the over-run. Eighteen months later it still does and Iâve run out of ideas. It runs beautifully above 3000 revs but as soon as you have to ride at 30mph it becomes uncomfortable to ride as it pops and bangs out of the exhaust.

Thinking it was ignition related Iâve replaced the condensers and the fitted new points. Iâve systematically changed the coils over, the points and low tension wires and re-checked the timing with a strobe after every change, Iâve changed the condensers over, the HT leads and the plugs and the backfire has stayed resolutely on the offside cylinder. The compression on the misfiring cylinder is good â itâs even higher than the near side one which performs faultlessly.

Thinking it might carburetion I checked the spark plug and even on the misfiring side, it is a perfect biscuit colour and the engine runs fine at high revs! The pilot jet is clear, and on a friends advice I started to look for an air leak on the induction side. Iâve fitted new o-rings and insulated gaskets and checked the flatness of the carburettor flanges. Iâve also checked the exhaust pipes are seated properly and not leaking.

The only modification Iâve made has been to fit non-balanced exhaust pipes and, because when it last ran the bike 4 star petrol was available, Iâve always subsequently used super unleaded petrol.

Any ideas?

I would give your valve clearences a look especially the exhausts to see if they are closing up......Ady

Previously andrew_walbridge wrote:

In 1988 my Mk3 Commando stopped working on the way home from work. It was one of the hottest days of that year and I ended pushing it about a mile home uphill. I put it in the garage and with two young children it stayed there until 2 years ago when I had the time and more importantly the money to start restoring it. Six months and a small fortune later it looked as good as new. Unfortunately, it backfired out of the offside exhaust at low revs and on the over-run. Eighteen months later it still does and Iâve run out of ideas. It runs beautifully above 3000 revs but as soon as you have to ride at 30mph it becomes uncomfortable to ride as it pops and bangs out of the exhaust.

Thinking it was ignition related Iâve replaced the condensers and the fitted new points. Iâve systematically changed the coils over, the points and low tension wires and re-checked the timing with a strobe after every change, Iâve changed the condensers over, the HT leads and the plugs and the backfire has stayed resolutely on the offside cylinder. The compression on the misfiring cylinder is good â itâs even higher than the near side one which performs faultlessly.

Thinking it might carburetion I checked the spark plug and even on the misfiring side, it is a perfect biscuit colour and the engine runs fine at high revs! The pilot jet is clear, and on a friends advice I started to look for an air leak on the induction side. Iâve fitted new o-rings and insulated gaskets and checked the flatness of the carburettor flanges. Iâve also checked the exhaust pipes are seated properly and not leaking.

The only modification Iâve made has been to fit non-balanced exhaust pipes and, because when it last ran the bike 4 star petrol was available, Iâve always subsequently used super unleaded petrol.

Any ideas?

Hi Andrew,are you certain the pilot jets are clear,because they are notorious for becoming clogged (assuming you are running Mk1 Concentrics) and will cause exactly the symptoms you describe

Ron Proctor