Boyer Black Box Failure? - Left side misfiring badly

Gents, Finished spring upgrade of new Amals and headers (1974 850). Left cylinder is sporatically firing, poping, banging and if reved over 3k, occasionaly kicks in and runs smooth for a few seconds, then back. Blows out of pipe strongly, black smoke or white vapour out of peashooter, feels cold compared to right side. Had some similar issues last season on a few occasions. Changed plug to apparently fix. Right side set up nicely with left grounded out. Left side has base 1.5 turns out and same stop as the right. Read numerous postings on similar problems. Pulled plugs and checked spark, left seemed weaker and sporadic compared to right. Boyer has wasted spark so began testing. Swapped coils, HT wires, plugs, cleaned up Boyer ground. Problem stayed put. Wires from stator to black box (has the screw down crimp mod) test good with ohmeter. Valve clearances set properly

My brain is telling me the the black box has failed.

Question 1 : Can the wire from the black box to coil be swapped (black, red) to test black box? Black wire side with problem. I am thinking not but...

Question 2: Found used Boyer MkIII black box in "boxes o parts" and plan to install and see what happens. PO has soldered or crimped and heat shrinked all connections so this will take a while. Is the MkIII for Norton? Looks the same as one installed on the bike.



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Boyers run a wasted spark, which means the black box doesn't have separate outputs for each side, so I don't see that it can be the black box. Something else must be the problem, checked everything including valve clearances, compression, carb balance ( throttle cables are adjusted correctly)

Check your timing again......... then the carbs,,,,,,,, my guess.

Tim_S

Your left misfiring side has a plugged idle jet passage. Boyers either live or die. They love a good solid ground. Put in new plugs. It is spring so start driving.

Niagara850 said:

Can the wire from the black box to coil be swapped (black, red) to test black box?

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No, do not reverse the black and red wires. If the black wire is connected to the coil(+) terminal when the coil ground wire still connected, then the box is likely to be damaged.

Niagara850 said:

Black wire side with problem. I am thinking not but...

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There aren't two "sides" to the system. The (positive ground) black wire circuit is: Box>>>black wire>>>>(-)coil(+)>>jumper wire>>>(-)coil(+)>>>>>>ground.
The red box wire is the box ground.

Niagara850 said:

Is the MkIII for Norton? Looks the same as one installed on the bike.

Click to expand...

"Boyer Bransden Micro MkIII" is the name of the ignition system (now superseded by B-B Micro MkIV), so "MkIII" doesn't identify which type of motorcycle that particular box is for, however if the wire colours are the same as the box that's already fitted then it's probably OK to try it.

http://www.boyerbransden.com/micromarkiv.html

After a good nights sleep and a cup of coffee went back at it. I'm in agreement that theoretically it shouldn't be the Boyer

- timing set last summer at BTDC 31 degrees with strobe, ran well for a while then
- Coils, Plug wire and plugs swapped between sides, problem remains on Left
- Wires from Stator Plate to Boyer checked with ohmeter: Test Ok
- Boyer grounded to bracket with ballast resistor - cleaned
- Battery 11.99 V
- Carb pilot circuts checked are clean and open (Carbs are new)
- Put in another new plug in left side, sparks when ignition is turned on and off. Installed, started bike, left side still not firing. Pipe cold, gas apparent in exhaust
- checked compression L= 135, R= 95 (the side that runs). Strong pulses from both exhaust pipes R= normal, L= cold with pops of black smoke and mostly gas smelling white vapour

Stumped!!

Am I looking at timing issue between cylinders R: Ok L: Bad?
Worn valve, guides, cam or ? Bike has 30,000 miles and I have no engine rebuild history

CHECK YOUR BATTERY.

Deleted

Niagara850 said:

After a good nights sleep and a cup of coffee went back at it. I'm in agreement that theoretically it shouldn't be the Boyer

- timing set last summer at BTDC 31 degrees with strobe, ran well for a while then
- Coils, Plug wire and plugs swapped between sides, problem remains on Left
- Wires from Stator Plate to Boyer checked with ohmeter: Test Ok
- Boyer grounded to bracket with ballast resistor - cleaned
- Battery 11.99 V
- Carb pilot circuts checked are clean and open (Carbs are new)
- Put in another new plug in left side, sparks when ignition is turned on and off. Installed, started bike, left side still not firing. Pipe cold, gas apparent in exhaust
- checked compression L= 135, R= 95 (the side that runs). Strong pulses from both exhaust pipes R= normal, L= cold with pops of black smoke and mostly gas smelling white vapour

Stumped!!

Am I looking at timing issue between cylinders R: Ok L: Bad?
Worn valve, guides, cam or ? Bike has 30,000 miles and I have no engine rebuild history

Click to expand...

If your plugs fire when you switch the ignition switch on and off, your boyer is working. It doesn't discriminate - it either fires, or it doesn't. But it definitely seems as though your LH side is not firing properly when the bike is running, based on cold pipe and black (rich) mixture. Can you kick the bike with the plug grounded to the head and see what happens? Does it spark properly? This will test the stator and pickups, as they will be what is triggering the spark as opposed to the on/off switch.

When you put in a new plug and try to run the bike, is the plug soaked when you remove it or is it dry?

And Grandpaul is correct, check your battery. Just because it says 11.9v sitting still, you need to check it under load. This is easy to do by putting your battery meter leads onto the battery terminals, then hitting the kill switch in rapid succession, thus triggering the ignition. If your voltage drops appreciably (more than 1v) as you do this you probably have a duff battery. Boyers start to act really weird at low voltage.

GIven everything you have said, I would next swap the carbs left to right.
That way if the right side is now the weak sister you know for a fact the problem is with the carb and nothing else.

Really, this takes just a few minutes.

Niagara850 said:

- Wires from Stator Plate to Boyer checked with ohmeter: Test Ok

Click to expand...

Wiggle or static test?

Lsat time the valves were adjusted?

Your battery will settle in between 12.5 and 12.75 if good. Misfiring occurs below 12 V. on a Boyer.

Put jumper cables from Jeep battery to Norton battery- 12.35 V. Plug sparks good when out, turning ignition on and off. Put into cylinder and started , same result no left side firing. Plug is wet with gas still mostly white with a bit of soot. Gas/soot mixture leaked out of Peashooter/ Header joint.

- Pulling left carb, looks like it may be flooding it out

Checked valve clearances 2 days ago before putting on Carb check and they were on speck

Also gave the Stator to Boyer wires the pull test with meter connected. Was a Bell Canada Cable Repair Tech in my youth. Used to find faults over miles of cable. For intermitant opens, I would pull on wires hard. If the copper was broken or almost coroded through, it would snap and stretch the insulation. I can usually find electrical probs... Will take a look at the left carb. Didn't take the bowls off to inspect, may end up kicking self

Niagara850 said:

(Carbs are new)

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does not mean thy are good. been known to have machining swarf in passages and not set up properly.

As the say in Newfoundland

Whale Oil Beef Hooked!

Problem solved. The needle in the left carb had slipped out of its clip and was hung up in spring up in the slide. Put it back, runs well now. This one is on me, apologies to those whose names (PO, Suppliers, Boyer et al) I took in vain over the last 24hrs and thanks to forum. I will be replacing the battery as its vintage is unknown and certainly isn't nearly new.

Try the newer gel type battery as it resists vibration.

Any battery manufacturer recommendations?

Odyessy mat gel type.