My sons 1974 Comando 850 will rev up to 4000 RPM and just cts out. Just like it has run out of gas . After holding the throttle open for a few seconds it will start to rev up again. Does blow some black smoke out the exhaust. If he trys riding the bike it coughs and sputters. Would anyone have thoughts as too what is the problem or what to check ? He has had the carbs off twice and cleaned them and new plugs and points with timing done . Dave
850 , cutting out
- Thread starter sand flea
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If it is running points still, I could be the condensers?
To work out if it is electrical or fuel, next time it stops, remove the float bowl drain, and check how much fuel is left, after shutting off the fuel taps. another check is the fuel taps themselves, check if they flow fuel well. It is not unusual for the rubbers to swell in the taps, run a drill through them if they are not flowing, I run the drill in rverse as a safety precaution.
Cheers Richard
To work out if it is electrical or fuel, next time it stops, remove the float bowl drain, and check how much fuel is left, after shutting off the fuel taps. another check is the fuel taps themselves, check if they flow fuel well. It is not unusual for the rubbers to swell in the taps, run a drill through them if they are not flowing, I run the drill in rverse as a safety precaution.
Cheers Richard
If you open both fuel taps, its highly likely its not fuel starvation.
Put a timing light on one of the cylinders, this will tell you whether it is spark or fuel.
Black smoke, sounds like no shortage of fuel, loss of spark more probable.
Which ignition system is on the bike ?.
Put a timing light on one of the cylinders, this will tell you whether it is spark or fuel.
Black smoke, sounds like no shortage of fuel, loss of spark more probable.
Which ignition system is on the bike ?.
check the spark plug wires are not hardened with age and seated really well into the coil. They could be walking out of the coils with vibration. Sometimes the cap can stay located on the coil but the wire itself will work it's way out. Just happened to me.
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Well, lots of worthwhile things to check have been suggested but I'm just curious about the onset of this problem.
1) Was it running well and then suddenly began acting up or was it more gradual?
2) I'm assuming the carb and ignition work you mentioned was done AFTER the problem started and in response to it but was any work done to the bike or any changes made to it just BEFORE the problem surfaced?
1) Was it running well and then suddenly began acting up or was it more gradual?
2) I'm assuming the carb and ignition work you mentioned was done AFTER the problem started and in response to it but was any work done to the bike or any changes made to it just BEFORE the problem surfaced?
Process of elimination.
Pull the wires off the back of the ignition switch and clamp them together to eliminate the switch. On my old 850 the switch would make and break contact at different rpm/vibration levels causing misfires, and loud backfires. I sprayed cleaner in it at one point and the arcing inside simply caught the cleaner on fire, a bad situation for a switch that is right under a fuel tank....
Pull the wires off the back of the ignition switch and clamp them together to eliminate the switch. On my old 850 the switch would make and break contact at different rpm/vibration levels causing misfires, and loud backfires. I sprayed cleaner in it at one point and the arcing inside simply caught the cleaner on fire, a bad situation for a switch that is right under a fuel tank....
When my son bought the bike the previous owner said the bike had just quite on him when he was riding. He never worked on the bike after that. My son got the bike running but now after approx. 3500 to 4000 RPM the bike cuts out for a few seconds then will build RPM. If he trys to ride the bike it coughs and sputters with a load on it. Dave
Pop the points cover off, in a dark shed start the bike and look at the points opening and closing, watch the points with the bike running, is there blue arcing between the points ?, is there blue arcing anywhere else ( i.e. somewhere there should not be arcing ).
Give the points a clean up with a peice of wet and dry sand paper ( or the wifes nail file, don't let her catch you doing that ).
Does the bike have a condensor / condensors ? ( these condensors are little capacitors that stop arcing across the points, these condensors do not work for ever and are a cheap to replace, just about any auto condensor will suffice ).
Either way I'd throw the points in the bin and buy a trispark and new coils, your kick starting leg with thank you.
If your budget is an issue, the Boyer or similar will make the bike a new machine.
Give the points a clean up with a peice of wet and dry sand paper ( or the wifes nail file, don't let her catch you doing that ).
Does the bike have a condensor / condensors ? ( these condensors are little capacitors that stop arcing across the points, these condensors do not work for ever and are a cheap to replace, just about any auto condensor will suffice ).
Either way I'd throw the points in the bin and buy a trispark and new coils, your kick starting leg with thank you.
If your budget is an issue, the Boyer or similar will make the bike a new machine.
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