Author Topic: Can a duff magneto cause engine seizure?  (Read 930 times)

Offline owain

  • A's Best Friend
  • ***
  • Join Date: Mar 2017
  • Posts: 135
  • Karma: 2
Can a duff magneto cause engine seizure?
« on: 09.11. 2017 22:28 »
I've posted this issue on the engine thread with some good replies and fingers are beginning to point an ignition system problem. The problem I'm having is that the engine is shutting off (possibly a partial seizure) after 3-5 mins of riding on the motorway i.e. riding in 4th gear at maximum speed of 55-60mph. Once I've stopped and wait a few seconds, the bike starts up again.

This has only happened during motorway riding and has left me in some pretty hairy situations. I recently had the engine and gearbox renovated as the bike has not been on the since 1966. Anyway, so now the motorcycle is in my garage until I fix this issue. The oil pump seems to be working (hot oil in the oil tank after use), Oil pressure relief valve 'should' be working ok (I took it apart and gave a good clean during the restore). A carb/fuel problem shouldn't result such a sudden and immediate engine stoppage. I suspect that engine is just getting too damn hot and I'm wondering whether it could be due to either a hot spark plugs (NGK B6HS) or a duff magneto?

As far as I know this magneto hasn't been renovated since the 1960's (if ever) but it was giving me a spark, so I decided not to touch it. Can a duff magneto cause overheating and engine seizure?
Sweden & North Wales
'50 BSA A10
'69 BSA A75R

Online Klaus

  • Valued Contributor
  • ****
  • Join Date: Jun 2015
  • Posts: 376
  • Karma: 10
Re: Can a duff magneto cause engine seizure?
« Reply #1 on: 09.11. 2017 23:23 »
Hi Owain,
I  can say its not the magneto. The signs your show is a seizure with a few points you had to loock at.
First is the ignition timing, without a strobe you can say nothing about the ignition point. Do you have manual or automatic advance?
If you have a manual timing you can play with. In case of the automatic one ist sometimes realy hard to find the right timing, because itsdepend on wear of camring and and the automatic advance unit. Is the camring worn out there should be a a timing offset. So one pot is running hot.
You on the way with this combination ?  Its a hard work for the engine and on the motorway you are full throttle at most time. The fuel consumption is much higher as with a solo, so check your main jet. May be its to weak.

I had the same problem with one of my bikes normal crousing no problem, but  at long hills or  against wind at top gear, it takes one or two miles and the engine suddenly going harder  with a little seizure. This motor had nearly ten soft seizures, an open ear is golden, pull the clutch and the engine stops before the rear wheel blocked. Rolling out and befor standing still release the clutch and the bike run as nothing happend.

Thats my experience.

cheers Klaus


If you think, everything is under control, you are not fast enought.

Online Black Sheep

  • Resident Legend
  • *****
  • Join Date: Apr 2017
  • Posts: 670
  • Karma: 6
    • Where black sheep live
Re: Can a duff magneto cause engine seizure?
« Reply #2 on: 10.11. 2017 06:34 »
If your ignition timing is over advanced, you could get pinking/detonation which is damaging to the pistons. The first thing I would do is fit a larger main jet. Have you checked the plugs immediately after a seizure? They should be brown. If white, the mixture is too weak and that can result in running hot and seizures. Lastly, what grade of petrol are you using? Use premium grade only.   
2 twins, 2 singles, lots of sheep

Online Triton Thrasher

  • Scotland
  • Wise & Enlightened
  • *
  • Join Date: Feb 2009
  • Posts: 1504
  • Karma: 20
Re: Can a duff magneto cause engine seizure?
« Reply #3 on: 10.11. 2017 06:47 »
Too much Sherlock Holmes.

Loss of spark stops the engine, as does lack of fuel.

So does seizure.

You need to find out which it is.  No need to mix them up.
Don’t look for causes of seizure until you know it’s seizing.


Offline owain

  • A's Best Friend
  • ***
  • Join Date: Mar 2017
  • Posts: 135
  • Karma: 2
Re: Can a duff magneto cause engine seizure?
« Reply #4 on: 10.11. 2017 10:55 »
It's running the auto-advance mechanism. I've checked the spark plugs and they don't show any white deposits, just a caramel brown colour.

Good point triton thrasher. The issue is how to determine whether it is seizing or not. Seems abit dangerous to take the bike on the motorway and wait for the engine to stop again? The engine does stop very abruptly and rear wheel did lock yesterday (although it could have been because I slammed on the rear brake in a panic..). The engine seems to kick over easily after stopping. Is there any other way ti determine whether the engine is seizing or not?
Sweden & North Wales
'50 BSA A10
'69 BSA A75R

Online Greybeard

  • Jack of all trades; master of none.
  • Wise & Enlightened
  • *
  • Join Date: Feb 2011
  • Posts: 6172
  • Karma: 35
Re: Can a duff magneto cause engine seizure?
« Reply #5 on: 10.11. 2017 11:05 »
I've been waiting for someone to say that dodgy magnetos have a habit of conking out when hot.

Online Rex

  • Resident Legend
  • *****
  • Join Date: Apr 2017
  • Posts: 558
  • Karma: 2
Re: Can a duff magneto cause engine seizure?
« Reply #6 on: 10.11. 2017 11:07 »
Restarts after five seconds, and it's a partial seizure?
Gedartofit.
More like petrol starvation...!

Online Greybeard

  • Jack of all trades; master of none.
  • Wise & Enlightened
  • *
  • Join Date: Feb 2011
  • Posts: 6172
  • Karma: 35
Re: Can a duff magneto cause engine seizure?
« Reply #7 on: 10.11. 2017 11:16 »
We don't really know that it is a seizure; Owain says he may have locked the back wheel when braking in panic. The plug colour is good; no mention of hot oil smells and smoke coming off the engine.

Are the exhaust headers rainbowed? How hot is the oil tank? After a hard ride my bikes oil tank is warm, not hot.

Offline owain

  • A's Best Friend
  • ***
  • Join Date: Mar 2017
  • Posts: 135
  • Karma: 2
Re: Can a duff magneto cause engine seizure?
« Reply #8 on: 10.11. 2017 11:34 »
There was smoke but I thought it may have been due to oil soaked exhaust bandages from a recent oil change...Although come to think about there was no smoke when I turned the bike off normally about 15 mins before the engine stoppage. I can't tell if the exhaust  headers have rainbowed as they are wrapped in exhaust bandages (there are lots of holes in the very old exhaust headers, so I sealed with coke cans and exhaust bandages.)

I'm not too sure how hot the oil was but when I opened the inspection cap off the oil box there was steam/vapor coming off the oil. The engine also took several hours to cool down to completely i.e. the crankcase and cylinder head were hot but not hot enough to burn you after sitting for 3ish hours
Sweden & North Wales
'50 BSA A10
'69 BSA A75R

Offline RayC10

  • A's Best Friend
  • ***
  • Join Date: Oct 2017
  • Posts: 51
  • Karma: 0
Re: Can a duff magneto cause engine seizure?
« Reply #9 on: 21.11. 2017 18:39 »
No one has mentioned tight tappets yet....cetainly not a cause for a seizure, more like fluff to a halt but in case it aint seizin' yet another thing to easily eliminate. Somtimes those pattern ally ones need more than 8, 10, 12 thou or whatever. Too many suggestions?

Offline Rocket Racer

  • Wise & Enlightened
  • *
  • Join Date: Jun 2009
  • Posts: 1431
  • Karma: 16
  • A kiwi with a racing A10 rig and too many projects
    • NZ Classic Sidecar Racing
Re: Can a duff magneto cause engine seizure?
« Reply #10 on: 22.11. 2017 03:25 »
Getting a bit off topic: But I have a good mate who's race engine mag failed in competition resulting in snapping his crankpin at the big end. I recall the mag armature broke, so it wasnt simply a faulty coil or condensor problem which will simply have not run reliably.
So mags can cause big expensive problems! but so can many other things.

If I had suspicions of a piston seizure (as distinct from a gearbox seizure - well documented in longhaul roadtests of the day) I would definitely be stripping the top end and rectifying any seizure related marking found.

But I think Klaus is also right, check the timing.
A good rider periodically checks all nuts and bolts with a spanner to see that they are tight - Instruction Manual for BSA B series, p46, para 2.
New Zealand

Online RichardL

  • Outside Chicago, IL
  • Wise & Enlightened
  • *
  • Join Date: Nov 2007
  • Posts: 5060
  • Karma: 48
Re: Can a duff magneto cause engine seizure?
« Reply #11 on: 22.11. 2017 05:19 »
I agree wth TT regarding jumping the gun on a diagnosis, but can't resist speculating on seizure, since I've experienced one in the last year.  I think my seizure was likely caused by running too lean, but that's not where I'm going with this. Owain said he just had the engine built. This makes me wonder if the mechanic was very optimistic over how much piston clearance to allow. I think if you allow clearance according to the needs of old-school slit-skirt pistons, then install modern cast pistons, a seizure may be a'comin'. Others here can cite the min clearance from memory, but the exact number is not my point.

Richard L.
Plan on signing up for the world-wide 2020 DISTINGUISHED GENTLEMAN'S RIDEon September 27, 2020 (if it's not cancelled and we are free to move about by then). Watch website at https://www.gentlemansride.com for details.

Offline owain

  • A's Best Friend
  • ***
  • Join Date: Mar 2017
  • Posts: 135
  • Karma: 2
Re: Can a duff magneto cause engine seizure?
« Reply #12 on: 27.11. 2017 22:06 »
I'm going to remove the header (seeing as it's winter anyway) and have a look for any signs of engine seizure. I am a little concerned about whether the newly bored engine is out of spec for the new +.60" hepolite pistons fitted. I assume that I'm looking for scoring of some kind on the cylinder?

Also is possible to remove the rockers/cylinder head with the engine in frame? (plunger model) I haven't tried before as I went straight for a total restore last time.
Sweden & North Wales
'50 BSA A10
'69 BSA A75R

Offline duTch

  • Ricketty Rocketty Golden Flashback
  • Wise & Enlightened
  • *
  • Join Date: Oct 2011
  • Posts: 4540
  • Karma: 41
Re: Can a duff magneto cause engine seizure?
« Reply #13 on: 28.11. 2017 03:16 »

 
Quote
.....Also is possible to remove the rockers/cylinder head with the engine in frame? (plunger model) I haven't tried before as I went straight for a total restore last time.....

 Yes, but probably need to remove at least the top rear rocker cover stud, and two rear studs through the head (I say 'probably' as I have an alloy head on mine )...back on you just need to deal with the pushrod placement; can't wait *smile*

 Didn't you do a trip to Wales or somewhere, by which time I'da thunk it'd be past the critical run-in phase ? *dunno*
Started building in about 1977/8 a on average '52 A10 -built from bits 'n pieces never resto intended -maybe 'personalised'
Have a '74 850T Moto Guzzi since '92-best thing I ever bought doesn't need a kickstart 'cos it bump starts sooooooooo(mostly) easy
Australia

Online Triton Thrasher

  • Scotland
  • Wise & Enlightened
  • *
  • Join Date: Feb 2009
  • Posts: 1504
  • Karma: 20
Re: Can a duff magneto cause engine seizure?
« Reply #14 on: 28.11. 2017 06:42 »
Any seizure will have left marks on the piston flanks.