850 commando ignition/fuel issues?

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Aug 31, 2007
hi, my 73' commando has an issue with misfiring and seemingly running out of fuel after about 3 kms. before then it starts and runs perfect.
the situation is worse if i am going above 3000rpm or 50mph. i can slow down and the problem almost goes away, and can back off then give it throttle and it is good for about 500 metres , then it surges, backfires under throttle, and dies, firing again after about 50 metres.
i have checked the tank vent hole, tap , flow to carbs , bowl filter , all are ok.
cleaned and tested rectifier , zenner diode , fuse connectors , 12.5v at battery , coils ok. plugs are light brown.
the only thing i found was that the fuel in the bowl when drained was hot and 20ml came out, is this the normal capacity?
please help!
Have you got the carb insulation joints fitted.

What ignition do you use.

If you are using the old Lucas coils try insulating the body from the mounting bracket. Failing coil winding can short out at certain engine frequencies.

thanks cash,
the insulators are fitted, im using boyer ignition and have another box on the way to try. i will try isolating the coils from their mounts in the next few days.
How old is your wiring ? This sounds classic Boyer related. I have had similar symptoms when warm caused by an internal break on the wiring from the pickup.

The only theory that I could come up with was that the wire was becoming physically warmer from the engine cases and this was increasing resistance.

The whole thing can be so sensitive that the slightest movement in the harness or even change in wind direction can set it off :shock:
it sounds like fuel starvation and as you've checked the obvious in that area I'd suggest looking at your fuel level in the float bowls.
Mine had a similar problem after I changed it's single carb at around 70mph. The float level was too low and it was sorted by raising the level so that the float comes to rest 2mm(0.080") below the bowl rim before the fuel is shut off by the float valve.
Sounds like a float level problem. Adjust float level, also check for fuel leaking into the floats.
850 commando fuel / ignition problem

Hi nathan, my Commado had a fuel starvation problem,it turned out to be bad gas taps on the tank,the seals on the tap body were breaking up and partially blocking the fuel flow,ok at low speeds, fuel starved at higher speeds -- may be worth a quick check to see if you have good flow. Good luck. James.
open up your reserve petcock and run it. that would proove your main being partially blocked and get a positive fuel flow, unless your reserve is failing also. easy to check before you drop the carbs.
thanks for all your ideas! hopefully i can get it sorted over the next week or so. i will let you all know the outcome.
Re: 850 commando fuel / ignition problem

james said:
Hi nathan, my Commado had a fuel starvation problem,it turned out to be bad gas taps on the tank,the seals on the tap body were breaking up and partially blocking the fuel flow,ok at low speeds, fuel starved at higher speeds -- may be worth a quick check to see if you have good flow. Good luck. James.

You may even find pieces of black rubber in the fuel. Some of the Chinese repro fuel taps deteriorate internally and the rubber seal will swell and shed hunks of rubber.
hello commrades,
i just put the new boyer box on, isolated the coils, ran the reserve tap as well as the main (both have good flow), and still the same dying backfiring problem after about 2kms.
i have ruled fuel out as when it dies it has no electrical power at all(indicators and lights go out for about 4 sec), then she fires again and all is good.
battery is perfect, test shows 12.5- 13 v.
suely i can rule ignition out as all power is lost, not just spark.
any new ideas???
regards nathan

I would start with the keyswitch. Make sure you have voltage at the brown/blue wire at the back of the switch. If not then check the wiring from the battery paying close attention to the fuse holder. These tend to fray and cause intermittent problems.

If there is consistent voltage at the brown/blue wire, turn the key to "on" and check the voltage at the white wire. Jiggle the key and see if the voltage drops. The keyswitch is another source of poor connections. It is possible to carefully take the switch apart and clean the contacts, but replacement is usually a better option. You can buy a replacement Lucas or reproduction switch that maintains your keyset, or a generic replacement that might result in losing the parking light function, but appears to be better sealed from the weather.
I agree with Ron.
I used to live in an area with a very high water table condensation was a real problem. The battery terminals, ignition switch, fuse and fuse holder contacts would oxide up ( You know that green stuff) and give similar symtoms. The battery terminals, fuse and holder are easy to clean. If you are carefull the switch can be uncrimped and the internals accessed. However, switching the ignition off and on a few times can have the same effect for a short time.
I have heard of another, vibration can suspend battery silt causing the battery plates to short. When the vibration stops the silt settles and the battery works again. Long shot I know.
Check the easy ones first.

If you still have the original Norton kill switch, check it. They have long been a source of intermittent electrical faults. Next time the bike stalls, toggle the kill switch a couple of time and see it it picks up.
I had a similar situation with my 71 Fastback. Has the single Mikuni conversion. Turns out there were two problems occuring simultaneously and this was making it difficult to isolate either. I stripped the carbs and cleaned all jets, however I neglected to throughly clean out the needle jet. I also removed all electrical connections and cleaned them. Turned out, the needle jet remained slightly clogged, and the ignition switch backing had some loose from the steel cylinder. I crimped the backing back on to the switch securely, and blew out the needle jet with carb cleaner (If you place plastic tube of aerosol cleaner into hole in needle jet, it wil blow out of jet, and there should be a nice fan shaped spray coming from jet). This solved backfiring/missing from running too lean, hard starting, and also cutting out of ignition.
I have the same issue with the Mikunis on my cafe racer. Basically because it sits too much. However, from Nathan's description of the problem, I believe it's electrical. At least I would start there and get that straightened out.

By the way, Spub, nice selection of bikes. How does the 900 bevel Duck compare with the Laverda? Always wanted a Jota. A friend has a couple twins, an Eagle and a SFC 750, and another buddy had an RGS, but the Jota with the 180 degree crank is the one I always lusted for, but never found one for sale.

You're probably right re: electrical, however it is maddening when two things conspire making it difficult to diagnose either one separately...

Thanks re: the bikes. I love my 900SS, it is has a lot of torque and is very light and nimble. Of all 70s superbikes, it is the one that has the most chance of keeping up with a modern sportbike. The JOTA, well, that's a different beast altogether. It weighs over 100lbs more than the Duc, so while the Duc carves corners, the JOTA requires the rider to get it in a headlock and force it to submit in order to change course. That being said, and while I absolutely love the Bevel engine (I just bought a 1980 Darmah Supersport), I am consumed by the Laverda 180 Triple. It sounds life V8 truck at idle, an AC Cobra under 4K, a Lamborghini from 4 to 5, and a Ferrari from 5 up. Much different from later 120s, such as the RGS (I had a 120 Jota and sold it). There is a 180 Jota in pretty good condition at the Garagecompany.com, here in LA. Nice bike, and he wants 9K. Apologies to all for putting up with my off-Norton ramblings....back to our regularly scheduled Norton programing....and to make this somewhat pertinent: I love my Fastback....
By the way Ron, nice collection of yours also...we have similar tastes I see....

I had dramas with the multiple wiring block under the fuel tank ..
Replace it.. a bloody birds nest of dodgy connections..

Never had a problem with that after replacing with a 'modern' block ..
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