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Commando 750 back firing after winter layup

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hi everyone...

Troubles troubles....

after a nasty 3 months without my bike, she developed a cough. 

The first ride was ok, then the next day she started to back fire a little. 

Then today... on maybe 1/4 throttle, she would pop and pop and pop, I can barely ride it safely. But on full throttle she would not pop as much.... so I would have to rev high and slowly let clutch off....

Move taken the floats off to see if it was stuck. Nothing wrong. 

Id like to get a general opinion about it before I start taking things apart... To be honest, I’m a bit lost. It’s not fuel because I have plenty in the tank. Could it be an air leak? 

Thanks everyone. 

Richard.

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I would say almost certainly a blockage / corrosion in the pilot jet system. These are quite difficult to clean properly and simply blowing them through doesn't help much. It's usually the fuel feed to the pilot jet which is blocked hence the low-end mixture is lean.

I can't post links on this message board but 'Google' :- "Bushman's Carb Tuning Secrets" and you'll see some clear cutaway diagrams that show you what you have to deal with.

http://www.jba.bc.ca/Bushmans%20Carb%20Tuning.html

 

 

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Drain the tank and put new fuel in. Make sure the battery is fully charged too.

An Italian tune up is also worth a try, it often cures this syndrome.

Yeah that’s my mission tomorrow. A carb clean.

i have the newer Amal carbs, which has the access screw to the pilot I believe. 

I have read the Bushmans carb tune a few times. I’ll study it. 

Thx.

I would say more than 2/3 of the fuel is fresh. Battery is fully charged on a trickle charger since the last month... I’ll do a battery test tomorrow. Showing fine up to the day before on the Optimate Charger. 

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Intermittent ignition will ignite unburnt fuel in the exhaust system and cause explosions.check the spark on both plugs with them out to make it easy to kick over. plugs need to earth on the head to avoid sparks in other places. Don't get a shock!.

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Think about you may need a new lead and plug cap on the rhs.Could also be flooding on that side,stuck float?  A usefull trick is to turn the fuel off when its misbehaving,if it clears for a while as it leans out ,its flooding.

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Does it make any difference if I now add I have Boyer electronic ignition installed? 

I was really trying to avoid getting this deep into that black box of electronics again... I am really hoping it’s not suddenly a timing issue and I’ll have to go and time this bike again for no good reason...

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So I’ve gone thru with new spark, new fuel, cleaned the float, but didn’t take the carbs off. Checked the timing. It was jumping between 28-32 degrees. 

checked the battery, charge is stable... BUT... the battery is shaking in the tray, and I thought maybe the battery terminals are loose, so tightened them up.... fired it up, now it’s running fine again. It seems the Boyer unit played up because of a loose terminal. 

 

For now, problem solved. 

Thanks everyone! 

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to low voltage and intermittent contacts. On one trip to France I was plagued by what I thought were carb problems. Eventually I realised that the connections to the pickup coils in the timing cover were very poor (horrible pre-insulated crimps as supplied by Boyer). I stripped the wires back and twisted the bare ends together as a get-me-home measure, which worked perfectly, then made proper joints on my return.

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In the end I figured it has something to do with the Boyer because timing was out of whack. Luckily a quick battery fix and not a wire or Boyer unit issue. 

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Vibration and corrosion on wiring or ignition switch. The intermittent break in the connection causes the boyer to fire. Good practice to clean connectors when the opportunity arises.

Chances are if it is carb related it would be one sided, unlikely both would present same symptoms. 

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One connection that is easy to overlook is the kill switch.

If you have the standard switchgear with the kill switch connected, the live feed to the ignition goes through the switch via spring loaded contacts. Pressing the kill button opens the contacts and breaks the feed. The contacts could easily have got poor over the years, as the weather protection is not great.

Fiddly to check, but might be worth it? If you don't feel that a kill switch is necessary, then you could always just hard wire the connection and do away with a considerable length of cable. 

 

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The other one to look for is an internal break in the 2 wires next to the 2 pickup coils on the Boyer stator inside the timing cover, unless you have some foam in there they vibrate and eventually fracture. If most are the strands are broken it will still test ok but be close to failure, not sure if that could give you backfires.

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... that's what I was referring to above. Because the Commando engine vibrates more than others due to the Isolastics, the wires have a hard time.

Battery changed. Seemed ok. Then it started popping again. 

Strapped battery down better. Seemed ok. Then it started again...

looked at all the cables, left the bike running and rev to 3k... pop pop pop... then finger touched the spark led to the coil... loose... pushed it back it a little... no more pop...

need to make a new led to coil... hopefully this is the final fix... or I’ll have to give up for the spring and just ride a modern bike. 

 

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