MK3 quits when warm

Hello all,
I've got a recent issue with my 75 850.
It started about 3 months ago. The Bike died on me after about a 40 mile ride.
I left it set overnight and it fired right off.
I figured the stock coils were tired so I replaced them.
The bike starts fine (like it always has) but will only run for about 10 minutes before quitting.

I'm now thinking that maybe my 17 year old Boyer is giving up the Ghost.

What do you think?
Does this sound like a dying Boyer black box?

Hi Mark, I had this same problem long ago when I had a Boyer.

It is obviously an electrical current cut off issue.

Honestly, what I would do if I were you now would be to remove all the Boyer wiring, all of it, and then rewire with all new wiring.

In my case, although I selectively tried to replace individual wires, it turned out that one of the wires that came down the frame tube and entered the back of the timing cover was actually fractured from time and vibration.

It would work for a while and then complete ignition cut off when that wire somehow moved in such a way inside the timing cover to cause the current flow to interrupt.

Since you have no way of knowing just which wire or connection could be the culprit, I would do a complete rewire.

On a another Boyer I had the problem was the soldier itself had cracked on the Boyer plate inside the points cover and that caused intermittent current flow and consequent maddening cut off.

Time maybe for a new electronic ignition anyway with that old of a boyer?

wild shot, blocked tank vent?...someone here will put their finger on it.

Take a look at the pick up plate (where the points used to be)where the wires attach. I had an issue where the wires got brittle and broke.

I just finished pulling the pickup plate and doing a continuity test on the wires from the box to the plate,
all is A-OK.

Yes - all is OK now, but what about when it is hot? I agree with the "wiring" boffins, above - my 72 with Boyer would consistently run for about ten minutes, then quit until cool. My particular ogre was one of the Boyer wires inside the timing chest - the wire would lose continuity when hot, but worked fine when cool. You will have to chase the electrons when the wires are hot to be sure you have continuity. THEN, if all is well, try other stuff. However, this sounds so identical to my previous problem, I am betting on bad wires as the source of your aggravation.

I am unfamiliar with the boyer ignition however this sounds to me like you are losing your fuel flow due to a blocked breather in the fuel cap. Next time this happens immediatly and slowly open the fuel cap. If you hear a hissng noise the fuel vent is blocked. I hope this helps.

Its an ole motrocycler's prank to drop rag in tank so starts fine but 10 min later rag shuts er down, ditto ditto etc, so peek in just to relieve us all. Leave cap loose instead of trying to catch it before stalls and if it don't bingo!

I've also had water in bowl do that, start ok but then misfire when warm and stall. Of course there should be no ballast resistor in power to Boyers or other electro brains or lowers V when warm so Boyer losses it timing brain function. Its also possible broken copper inside installation that flogs and separates once warm but checks fine cold and static.
Trigger wires most infamous for this as mentioned.

Mark said:

I just finished pulling the pickup plate and doing a continuity test on the wires from the box to the plate,
all is A-OK.

Click to expand...

Unless the insulation is cracked 360 deg. at the entry , and goes silly occasionally .Theres a wire under VH Holdens does this at a clip to a tube , at the wreckers THAT wire was Cracked on all of them .Easy one to spot . :lol: :shock:

HENCE the ' Replace Every accessory wire ' advice , or at least inspect thproughly . Whadabouthe Condensors . :mrgreen:

This sounds like a classic case of the Boyer stator wiring. Where they are attached to the plate and are encapsulated in resin they tend to break. It's usually intermittent like you're describing. I don't think I've ever seen one last 17 years without failing this way. An elegant repair is to scrape away the resin de-solder the wires and solder screws on as posts to which to attach the wires. Someone on the forum (Norbsa I believe) sells a small kit with everything you need to do this. It's a quick and easy repair and something Boyer should have done many years ago.

I also had this problem, and found it was to do with the key switch. If it happens just try wiggling the key. I replaced the switch now no probs.

Nelson said:

This sounds like a classic case of the Boyer stator wiring. Where they are attached to the plate and are encapsulated in resin they tend to break. It's usually intermittent like you're describing. I don't think I've ever seen one last 17 years without failing this way. An elegant repair is to scrape away the resin de-solder the wires and solder screws on as posts to which to attach the wires. Someone on the forum (Norbsa I believe) sells a small kit with everything you need to do this. It's a quick and easy repair and something Boyer should have done many years ago.

Click to expand...

With the continuity test, wiggle the wires and you might find a break inside the insulation just before they are soldered to the stator plate. Install new stator plate and drill it, bolt the wires on and use nyloc nuts to ensure vibration proof. There is a nice info sheet somewhere on the Internet explaining the process nicely, and it worked for me. I did find the break in t he wire eventually, but it would show continuity and randomly no continuity as it was wriggled.

My problem was that it would start but suddenly start backfiring and running on mostly 1 cylinder and fowling the plugs. Further oil analysis showed 1% fuel in the oil (caused from running home on 1.2cylinders more than once with bursts of 2 cylinders along the way ( as if trying to convince me she had just cleared a fuel blockage or something).

Once I confirmed the broken wire, I resoldered the stator plate real good and thought the problem was fixed. It cam back 100k latter exactly the same wire, broken inside insulation again. Second time I replaced stator and drilled and bolted connections. I also replaced the black box while I was at it.

Now she is running and starting like a rocket. Just fixing other stuff like broken throttle cable, etc etc....

The wires on mine broke under the zip-ties. Using the rubber sleeve 061093 might help, but in the end I just made up the posts. I believe Boyers were originally set up this way.

Gosh its been a long time since I last saw the back side of a Boyer, ugh.
ReGardless of what combo of things you find preventing constant joy those Boyer trigger wires Will break soon enough so while its out and on you mind do your self and bike innards a big favor and DIY upgrade them or ping here for the easy way out if following instructions to a Tee. Greg Fauth Norton Bsa <norbsa@hotmail.com>

I don't think that I need instructions.
After seeing rpatton's Pic, I see what the mod is and I can do it no problem.
I'm gonna re-wire the whole thing like 1up3down suggested and see if that fixes the issues that I have been experiencing.

If not, then maybe I'll just have to spring for one of those Pazon units.
.

Sorry no offensive remark intended, just reflecting on my own miss understanding in placing a back up nut in Greg's kit on correct side mechanically but wrong side magnetically conductivity is all - for a famous night long noise event at a rally. Greg's kit instructions are correct. Many ways to skin the Boyer trigger wire cat- just be aware not to foul things behind plate is all. Hope yours makes lasting sweet fires.

If my Pazion failure was like another one with clearance issues d/t Norton or Pazion manufacture, then twice I've been foiled by things hidden behind trigger plates.
No make that 3x's now, last month a mechanical advance weight pin came off - hidden till popped the trigger plate out. Fixed by NOS adv. unit I could make sense of.

It woudnt hert to clean the kill switch. What ever you do do them one at a time so you know what the problum was. Unless you dont care

Phil

Last time i adjusted Trixies valves she wouldn't start easy nor idle smooth
til I realized, again, fixed one thing but knocked a wire off or pinched something else. I can't claim much competence as last 3 attempts failed to start Trixie and don't know if just too cold for her lean tune or something broken or just over looked again. One time Peel wouldn't make it to drive way across grass w/o missfires then died but she started fine, tuned out to be tail light ground inside lens area bouncing to short out -intermittently and steal Boyer voltage.

I have a MK111 and a Trident, and fwiw in my experience the cause is almost always the simplest root cause. I've experienced a blocked tank vent as has been suggested by several others and it behaved exactly as you have described. the thing that threw me off was the motor would die at varying times. That turned out to be caused by the amount of gas in the tank. Just pop the tank top and start her up. If she doesn't die in 10 minutes, you're healed. I also agree with some electrical contact cleanr in the stop switch. In any even, don't go pulling ignition systems till the easy stuff is eliminated. If you'll indulge me, I'll tell you a true story that illustrates how we can make things more complicated than they are.
About 15 years ago I was walking down Monument Ave in Richmond, VA, when I came upon a young man with a new honda. He had the tank off, the seat, timing covers, plugs, valve covers, carbs half apart and battery out all off and spread out all over the place. He had a work shop manual out and half the bike spread out on the sidewalk. I stopped and mentioned that I knew a little something about motorcycles and asked what the problem was. He said "the bike won't fire. she turns over but no fire". "I traced the electrical system and I have spark , but the plugs are dry". It was then that I noticed that the bike had 30 day temporary tags (brand new bike, brand new rider). I looked down at my feet and saw the gas tank. I bent down and picked it up and gave the tank a shake. Bone freaking dry, I mean not even a drop of fuel. The young riders face became red as realization hit home. I smiled and told him I was happy to be of service and to not be too hard on himself.
Cameron
'75 mk111 850 roadster
'74 T150V Trident

Old Brit sells one that replaces everything from pickup to spark plugs kinda pricey but works good and has a selectable advance curve just don't try to start it on the "aggresive" curve if you don't want to look down on your bike from way up.