Britbike forum Forums British Motorcycle specific Brands Norton bulletin board 1975 850 Commando Mk3 No Power

Well, my 75 850 Commando Mk3 has been running great - until it isn't. I was on a ~25 mile ride and was on the way back home when suddenly, I lost at least 50% of the power. The bike starts right up,and idles perfectly. Accelerating, it has very little power, although the engine is running smoothly. I finally got it home running in second gear. It has very hot spark and I changed the plugs to no avail. I noticed that the left plug was very black and wet, which leads me to believe I have a problem with the left side AMAL, although there is no visible smoke from the exhaust. The right-side plug looked great. When I changed the plugs, I went ahead and checked the compression while the engine was still hot. The left side was 151 PSI and the right side was 152 PSI, almost identical. I checked the compression with wide open throttle and both plugs removed and used the starter to spin the engine. For both sides, after ~5 revolutions the compression stabilized and the pressure didn't leak down. I haven't taken either carburetor apart since I bought the Mk3. With the wet plug, I think it's time to dismantle the left AMAL and see if I can find a problem. Since it idles so well, any problem has to be with the main jet and/or needle. I'm glad the problem happened so close to home because I was planning a ~200 mile trip in a few days.

Compression, ignition, exhaust - you're halfway there. Hot spark, you say, and that's good, but is it on time? Points could have slipped or something ...

If it's an EI, probably not. Fuel, then .... Sudden change like that could be a main jet falling out of its holder or some other internal falling apart. OR (and this has happened to me twice, once on a BSA and once on a Mazda 121) a baffle has fractured inside an exhaust pipe and blocked it.

Lannis

Not very scientific, but holding hands in front of both exhausts it feels the same, regardless of throttle position. When it happened, it was quite sudden and not a gradual loss of power. I'm pretty sure that with the sooty and wet plug on the left side that I have a carb problem, although that may not be my entire problem. I'm going to check the timing since I have never checked it since I got the bike. It ran so good that I have really only changed oil, etc.

So would you say that when this happened it was only running on one cylinder ?

Even though your compression test looks good I wouldn’t discount a sticking valve on the left. Valves when hot do different things to valves when cold.

Perhaps whip the rocker cover off and check the exhaust valve isn’t sticking.

Good suggestion on a sticky valve.

Can be tricky to spot when its not hot though.
My old Enfield (single) did this when I first got it - hot day, no compression !
Good as gold when it cooled down ...

Turns out it had a new valve guide in it.
Bit tight in the clearances.
Some engine oils are more ashless than others, it seems.
Nothing on the side of the can about this either ....

New bronze guides, which are available for Commandos these days can be
supplied quite tight in the clearances ?
The originals were cast iron, and not so tight ..

This could also be something as simple as a bit of gunge or water in the fuel.
Once swallowed, back to its usual self ?
Drained the floatbowls/tank ever to see what thungs look like ??

Check the needle in the carb, could have dropped out of its clip.

GEE a lot happening in 3-1/2 hours.!

1. mechanical 150 both sides

2. ignition spark amplitude and correct timing ?........new or cleaned/tested plugs or change sides

3. fuel. low speed circuit modes ?
bleed=idle
feed=slide raised above idle controlled by slide& cutaway
NB: Explained in Mikuni manual but AMAL tuning guide is VERY deficient in this distinction.
Turning idle adjustment, each side up exactly one turn then return to original setting. repeat for other side.
Does each respond the same?

4. main jet fell out? The engine will run up to 3/4 throttle WITH NO MAIN JET AT ALL Usually detected at significant highway speed

ADDED:
5. yes possible! but would make a lean condition and then blackening the plug? , however should not affect step #3

Yes, a lot happening and most not good! Due to the cold weather I haven't looked into the problem further. However, I plan on pulling the float bowls on both carbs today and check on the jets. The bowls both likely need to be cleaned anyway. I also plan on confirming correct timing as well. However, I'm almost positive it is a fuel problem with the left carb due to the carboned-up and wet spark plug. Like any new acquisition it is good to confirm the basics. I'm alternating working on the Mk3 and my BSA 441 Victor Special which still has a speedometer issue.

It's running on one cylinder. It will idle well enough on one. Check the left carb over thoroughly. Needle clip loose on needle. Good spark on both sides.

Problem solved. Jet came loose. The problem was evident when I dropped the float bowls. Easy enough to fix. The bike now runs great! I should have checked this earlier. Now I'm back on the speedometer problem with my 69 BSA 441 Victor Special.

Gary, these guys are pretty amazing in this group, with several of them diagnosing the issue without even looking at the bike.Good job in sussing it out.
-Dave

I absolutely agree. I have had several problems sorted with help from everyone. I have four bikes, three of them vintage British, and it seems I always have some issue that the cause isn't obvious. This is a very good forum.