Author Topic: Misfire under load again  (Read 1381 times)

Offline bsapete

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Misfire under load again
« on: 15.12. 2018 06:26 »
Hi gents,
This misfire and lack of power occurred previously (on my first ride of the bike) when the advance/retard outer cable had come out of its sleeve on the lever therefore retarding the timing. Once this was put right it seemed to run well. I’ve just fitted new handlebars and when I refitted the A/R lever and checked it’s operation it became stuck in the retard position and the lever and cable wouldn’t return to the full advance position. I removed the magneto end cover (venturing into new territory here) to see what was happening. I removed the cam ring and put it back and the lever was operational again with the cam ring rotating freely.
I then took it out for a quick ride to check the new handlebars and it’s running poorly again - missing under load and lacking power. It starts easily, usually first kick and idles well once warm. I replaced the plugs with NGK BP7ES gapped at 18 thou but no difference. Odd that I had a moment of smooth running earlier and now the problem has returned. What should I be looking at next - points?
Bloody difficult magneto access too with the high level exhausts.
PS when blipping the throttle I noticed a puff of smoke coming from between the head and barrel on the timing side. I’ve also noticed the exhaust ring on that side isn’t hard up against the head as it is on the left side so maybe it’s not sealing the pipe properly. Hopefully the smoke is coming from there and not the head gasket? Don’t really fancy pulling the head after a month of ownership!
Any suggestions welcomed.

Cheers,
Pete
62 A10S Spitfire Scrambler
54 A7 Star Twin (sold on)
61 Jaguar E Type FHC Flat Floor
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Offline Servodyne

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Re: Misfire under load again
« Reply #1 on: 15.12. 2018 09:53 »
Hi there.
It may seem a daft question, but do you have a spring fitted to the A/R plunger at the mag end of the cable. This ensures that the slip ring returns to it's fully advanced position and stop it 'fluttering'. This may give the misfiring and lack of power symptoms that you are experiencing.
I leave my A/R lever at the full advance position at all times and just give it a hefty kick when starting.
Cheers
Jim
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Online groily

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Re: Misfire under load again
« Reply #2 on: 15.12. 2018 10:48 »
You obviousy need to verify the head gasket hasn't gone walkabout, and then maybe look at the plunger and the camring to see if the two stay in engagement through repeated applications of A & R at the lever. Wear on either or both can cause slipping out of engagement as you discovered initially. A poorly-cut or slightly off-position notch in the camring can also be the culprit.
Sometimes it's possible to limit the cable movement to prevent disengagement using the cable adjuster - but you do want to be quite sure you're getting full advance.

As Jim implies, you can always leave (or jam) the camring at full advance and it will be fine if you are blessed with a good right foot for starting. I've been running two of my other bikes (with much higher compression ratios than my A) on fixed advance for years now, without any problems.
Bill

Offline mugwump

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Re: Misfire under load again
« Reply #3 on: 15.12. 2018 13:43 »
Ht tracking  can also  be a problem under load too.

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Offline berger

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Re: Misfire under load again
« Reply #4 on: 15.12. 2018 14:35 »
bsapete this is a long shot but I once found that a cam ring was working itself out a bit and losing its plungy thingy contact with the ring even with the outer cover on---- just a thought

Online Klaus

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Re: Misfire under load again
« Reply #5 on: 15.12. 2018 20:03 »
Hi Pete,

the hight level pipes are strait throu, or do you fit a silencer?
My first test was with only the pipes, it was horryble loud, but great performance and ascelleration.
than I fit the silencers I have ordered from Amours, no way teh bike did nt run popper. No Power and stall by full throttle.
She drive me crazy...with silencers off, great again. I ordered a cheap taiwan pair with no baffels, easy running bike.

Cheers Klaus





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Online duTch

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Re: Misfire under load again
« Reply #6 on: 15.12. 2018 22:16 »

 Something makes me think this problem may be symptomatic of a condenser starting to fail....but I'm sure groily would have mentioned that... *dunno*  Just I had similar happen in my Tilly a while ago

 
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Offline bsapete

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Re: Misfire under load again
« Reply #7 on: 16.12. 2018 03:13 »
Thanks as usual gents for the suggestions, I will do some more investigation.
I do leave the A/R lever in the fully advanced position and give it a good kick and never a problem starting although it will kit back if you’re too gentle!
I’ve disconnected the cable from the lever to make sure it was getting full advance (ie if the cable wasn’t allowing the camring to fully rotate. Klaus, I do have a pair of baffles and it makes no difference whether they are in or out apart from the noise of course! As suggested I’ll try jamming the camring fully advanced. Should it be lubricated at all and what with?

Cheers,
Pete
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Re: Misfire under load again
« Reply #8 on: 16.12. 2018 08:30 »
DuTch could well be right of course, depending on the state of the thing, when last looked at, etc.
If the camring is wedged at full advance, lube isn't really an issue, but a smear of grease wouldn't hurt so it couldn't get corroded in position over a long winter! Generally, I use a light lithium grease, but I don't think it matters hugely as long as it doesn't get all over the points. I've used marine anti-corrosion grease on one bike for years, as it's out in the shite more than most and the camring has got stuck after coming back from wet days out on salty winter roads.
But I'm sure you wouldn't abuse a pretty machine like that!
Bill

Offline bsapete

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Re: Misfire under load again
« Reply #9 on: 16.12. 2018 10:33 »
Took the end cover off the mag and operated the lever back and forth a few times and I guess it’s operating properly. The cam ring doesn’t jam at all and doesn’t rotate any further even with the cable disconnected from the lever so I suspect it’s as it should be.
I’ve just remembered that when I got it running properly after the first issue with the A/R cable I adjusted the carb when I got home as it was idling too fast. Now, looking at my new manual I’m sure I’ve inadvertently adjusted the air screw to lower the idle. Presumably I should’ve instead adjusted the throttle stop screw?
Lesson learned: don’t screw with things (literally) when you’re not sure what you’re doing. Could this be causing my running issue?
62 A10S Spitfire Scrambler
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Online groily

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Re: Misfire under load again
« Reply #10 on: 16.12. 2018 11:58 »
Hmm  . . . pilot air setting affects mixture across the whole range, but if you richened it to slow the idle down (?) by screwing it IN, then I'd have thought it would burble at you at low revs, or even '8 stroke',  and make the plugs a bit black up the range. But not maybe cause a misfire. If you UNdid it to weaken the mixture until the thing slowed down - tho usually revs will rise a bit, not drop off, as you start to weaken - then it could have affected the thing I s'pose? But it would probably have been spitting and coughing on the throttle at all settings if you'd unscrewed it that far, and maybe become a pig to start as well?
Too many variables! I'd start with going back to maker's on the air screw - turn and half out from fully in maybe, depending on what carb? - set the idle by the other screw like you said, and then see if things are better, worse, no change.
Thinking more about what DuTch suggested, if you have a dark corner in the shed, you might just be able to see any stray arcing from an HT pick-up, if there is any. Usually it'll be the one on the front, cylinder side, of the mag, and any sparks will show up quite well in the dark! But then again, if it needs a heavy load to make it misbehave, it's harder!
Bill

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Re: Misfire under load again
« Reply #11 on: 16.12. 2018 13:01 »
If a rich idle mixture soots up the plugs, it can miss under load.

Offline bsapete

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Re: Misfire under load again
« Reply #12 on: 16.12. 2018 19:24 »
After I fitted new plugs hoping it would cure things it was running just the same immediately so wouldn’t have had a chance to foul the plugs presumably. Won’t hurt to re-adjust carb. I’m fairly sure I turned the air screw ant-clockwise and only a little to slow the revs down. Perhaps a ride in the dark would show up any arcing from pick-ups or HT leads.
PO said petrol was new but new from petrol station or old can in the shed? I’ll drain the tank and add fresh petrol too.

Cheers
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54 A7 Star Twin (sold on)
61 Jaguar E Type FHC Flat Floor
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Re: Misfire under load again
« Reply #13 on: 16.12. 2018 21:36 »
Maybe a shot in the dark but fairly easy to check.
My A10SR never did run right ever since I bought it and I suspected a dodgy mag so had it rewound and a new condensor and it seemed to improve things but slowly got worse again - difficult to start, running on one, misfires under load.
Following a post a while back regarding dodgy brushes with the carbon too soft, I cleaned up the slip ring and replaced the brushes and the brush holders with quality items from Brightspark. (The old brushes could write on paper and the brush holders were the "universal" type with the hexagon ends). I also needed new acorn nuts as they were a different thread to the ones fitted.
This totally transformed the bike and it runs perfectly now and starts first or second kick so I assume that the spark was tracking around the slipring and taking the easier path to the other plug which was not under compression, or to earth through the holders, or perhaps both.

Hope you get it sorted

Jim

You may find this topic interesting https://www.a7a10.net/forum/index.php?topic=13258.msg106664#msg106664
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Offline bsapete

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Re: Misfire under load again
« Reply #14 on: 16.12. 2018 23:08 »
Thanks Jim, I read recently a post somewhere regarding cleaning the slipring using a cloth wrapped blunt end of a pencil if I remember correctly.
Any tips here would be useful as this is all new territory for me as I’ve never played with a magneto before.

Cheers,
Pete
62 A10S Spitfire Scrambler
54 A7 Star Twin (sold on)
61 Jaguar E Type FHC Flat Floor
99 Jaguar XKR Coupe