The ten mile Norton Commando...

Help!

Just rebuilt the cylinder head and hone/new rings. Bike was burning oil before and plugs very sooty with new Mikuni VM34. Bike kept cutting out and backfiring before I stripped it...

Have just put about 70 miles on the bike since the top end job. Went to work on it today. At ten miles it backfired and popped , I slowed and got to work the last two miles. Took plugs out at dinnertime (or lunch if your down south) and quite dark/sooty. Drained float bowl in case any dirt in there. On the way home the same happened at ten miles. The right hand plug is darker then the left for sure.

Came home tonight and took tank off. Retoeued the head again, set tappets and dropped needle (raised clip one notch). Just went out for a ride. Tem miles misfire again! Came home took plugs out. Left one perfect, right one a bit darker.

Any ideas. I'd like to go moe than ten miles. Boyer bransden and new Nippon Denso plugs.

I haven't strobed the timing yet but it starts first/second kick and accelerates fine (although I am running the rings in).

It's probably a mal tuned Mikuni. I have helped dozens of people who have run too fat a cutaway slide.
3 is often to fat and 3-1/4 or 3-1/2 has transformed many bikes. While Mics are a good carb, I don't run any of my bikes with them.
A local club guy just picked up a combat where the guy could not tune amals.....he put a mikuni on ....and then sold the bike cause he couldn't tune it either........If you know carbs, then tuning either one, mikuni/amal, is a snap.
The cutaway is important because when you're running around town and sub highway speed you are mostly running on the cutaway and main and idle jets have little effect on the fuel mixture and the plugs then tell the story if its running cold/black.

Fred - what ignition? I ask because I had similar symptoms, where the bike would go a specific distance and then quit running, with (what I thought were) carb-related indicators. Turned out the prob was a loose connector in the Boyer fittings in the timing chest.

-OG

It's probably a weak battery. I've helped many people with these symptoms of lower than 12 V. power. Single carb should show plugs running similar colours so take a volt reading please and report back.

Boyer Bransden. Will take a volt reading. My old comaando ran great on a VM36, no tunig required. thsi VM34 is more common but certainly doesn't look like a fit and forget..

What about a coil breaking down when it gets hot?

I'd be suspect of the coil(s) I'm not familiar with your ignition system but coil was mentioned.
Had that problem with an old Chevy. Run fine for a handful of miles and then the best it would do is idle. let it cool and all was dandy for a few more miles. Pulled a used one off the shelf and replaced it; that was probably 15 years ago; all is still good.

Tom

Worth checking your coils and plug wires.

Battery voltage 12.48V ignition off
then 12.53V on tickover
and 13.3V when revved around 3000 rpm

Going to buy new battery as it's the wromg one but it didn't get much above 13V when revved? proabbly not enough volts for boyer? When lights were on it struggled to hold 12.5V across battery

Sufficient voltage to ignore as source of trouble. Ugh.

Sorry Hobot, didn't understand that.You think it is enough volts? I read a post on here other day stating you should show 13.5 to 14.5Volts when revs raised?

Bike has 6V colis which are crimped by clamps by previous owner. Going to renew those

I you've got them handy try a pair of hotter plugs

Cash

Its not top notch Ideal voltage but still rather better than plenty of other fine running Cdo's, look some where else is all. I've been caught out enough times but new or seemingly good looking plugs, till really nasty rusty old one put back ugh.
Should buy in V8 packs just to put that issue out of mind while scratching ends to get going again.

These were those denso iridiums that cost an arm and a leg. Tried a few sets of NGK and chanpions before and RH was always darker than left side

I am going to change the coils then will try hotter plugs

still betting on coils..

Fred, if you're still using the zener diode, I doubt if you will get much more than 13.5V charging, depends on the diode. I'm sure the modern boxes will allow higher charging rates, up past 14V, but if you're getting 13.3 at 3K it probably won't go up much more than that. 12.4V on battery should be perfectly sufficient. Brand new battery is usually around 12.8V after full charge and sitting for a few hours. I'd check the coils or replace.

Dave
69S

Thanks. It's got a boyer power box. Yes I reckon coils.

Hi FastFred

FastFred said:

Sorry Hobot, didn't understand that.You think it is enough volts? I read a post on here other day stating you should show 13.5 to 14.5Volts when revs raised?

Click to expand...

Only if you want to charge the battery!
But revving to 3000 rpm is only of use when the battery is fully charged. Which your battery was not.
I check my electrical system by connecting my volt meter to the battery and putting the volt meter in the map pouch of my tank bag and taking a ride.
TomC in Ohio

Latest update.

Just came back from holiday and fitted two new 6V coils, HT leads plus caps. Went for a run.

The ten mile commando is now a three mile Commando... Did a total of ten miles and it spat/backfired three times before finally conking out. I then let is stand two minutes and turned the reserve petrol tap on (it shouldn't have needed it as I recently put a new main tap on the bike and know the fuel was above the tap inner pipe) and it started. As I was 400 yards from home I didn't test it further but rode it home to ponder. The bike feels weak at the bottom end as if it's weak on fuel yet the plugs have been sooty as reported. Perhaps that's just when it misfires that is causing the sooty plugs??

One thing that I'm not happy with is the fuel pipe routing of the new Mikuni I fitted as I have the tee piece below the carb body requiring the pipe to the carb to then go back upwards to the carb befre going back down again into the carb inlet. Air lock perhaps? I guess as the fuel lowers in the tank the hydrostatic head decreases which won't help? Does anyone have a picture of their fuel line set up to a single Mikuni as the carb entry is at the same level as the base of the fuel taps hence why I routed it the way i did?

You have nothing to lose by taking your main switch apart to clean it along with your kill switch. But do one at a time our you wont kmow witch one it was. Id do the kill switch first.

Phil

Stopped up fuel tank vent in the filler cap?

I kept opening the fuel cap when riding to rule that out. It misfired a few hundred yards later so I don't think it's that. Will check kill switch and ignition but latter is 200 miles from new.