Author Topic: Is this my magneto?  (Read 11616 times)

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Offline aggettd

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Is this my magneto?
« on: 04 Jul 2006 at 12:03 »
I've been slowly increasing the length of my rides in order to determine the reliability of my T35. Yesterday did about 40 km in 25 degree Celsius weather, when the bike, which had been running flawlessly, started to miss on one cylinder and after about 15 seconds, died. It wouldn't restart. After checking fuel flow, plugs, magneto connections, cut-out etc. for about a half hour, she started again. Ran for about 5 minutes and died again, first losing one cylinder then the other. Repeat previous sequence, started again then quit after another 5 minutes. Repeat again. and finally my wife arrived with the trailer (complete with smug, "I-told-you-so" look). Once home, she (the bike) started again enough to run the bike up the drive and empty the float bowls.

Is this my magneto? Any other suggestions (besides the ones I've already heard from my wife)? I run without a head gasket, two petcocks and separate fuel filters (clean).

Thanks

David Aggett

Offline trevorp

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Re: Is this my magneto?
« Reply #1 on: 04 Jul 2006 at 13:31 »
i have read an article which for the life of me i cant find that describes what u are having as a magneto failure from memory it was the condensor and or the earth brush but the article basically said that the shellac used on magnetos doesnt last that long even in ideal conditions
contact peter scott magnetos by email and tell him the problem he will soon set u straight
oh there is another way to tell if u have a spark but invloves a wife with a smug look holding on to lead and then a possible divorce so i would use the previous method

Offline alwyn

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Re: Is this my magneto?
« Reply #2 on: 05 Jul 2006 at 02:07 »
Once home, she (the bike) started again enough to run the bike up the drive and empty the float bowls. David Aggett

Hi all,
David - this may sound simplistic but the type of death you describe suggests fuel starvation to me - a magneto failure is more likely to provoke instant death to both cylinders - on that final run up the drive did you start with the pets on or off? The fact that she runs again after a delay of some minutes suggests a fuel flow problem - it could be that there's not sufficient fuel flowing past your filter systems to keep up with consumption - check the flow by disconnecting the pipes at the carby bowls - this should reveal any restriction of flow caused by the filters or even through the petcocks.

Alwyn

PS I do not recommend you follow Trevor's suggestion to test your spark :roll:
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Offline Doug

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Re: Is this my magneto?
« Reply #3 on: 05 Jul 2006 at 02:14 »
David,

Does the bike lean out before quiting? You could be drawing a vacuum on the petrol tank with a clogged vent hole in the petrol cap. Depress the tickler on the float bowl to see if you have fuel immediately after it quits. Or open the cap and listen for a inrush of air.

Second, before suspecting the magneto, you really ought to see if it has spark. Once it quits again, take out both plugs and lay them aside on the cylinder heads connected up to the ignition leads. Kick over and you will be able to observe if you have a spark without electrocuting yourself or spouse. (Never try this with fuel injected vehicles, they will blow combustible mixture out the plug hole. Quite spectacular.) If it sparks but looks a bit weak, it might not have enough potential to jump the gap under compression. Try running the sparkplug gap at the minimum 0.015", or even a little less, this will lower the voltage required in the magneto armature.

Condensers have been known to go open circut when hot. If so, you should also see your points burning up and needing dressing frequently.

Softening of the varnish, or shellactitis, will sling the varnish off the armature, and create a tremendous drag between the armature and the field poles. It has been known to stall an engine giving the symptoms of a piston seizure, then when cold lock the magneto up solid so that the engine can not be turned over. But it does not cause a the cylinders to cut out.

-Doug
« Last Edit: 05 Jul 2006 at 02:19 by Doug »

Offline trevorp

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Re: Is this my magneto?
« Reply #4 on: 05 Jul 2006 at 07:58 »
Have a read of this page dave and interesting point he makes in if u raise your compression ie no head gaskets this puts a greater strain on the spark as it has to jump a gap with a higher pressure higher pressure more resistance
See Here
« Last Edit: 05 Jul 2006 at 08:01 by trevorp »

Offline aggettd

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Re: Is this my magneto?
« Reply #5 on: 05 Jul 2006 at 12:12 »
All good suggestions. I tickled the carbs immediately upon stopping and both flowed strongly. I pulled the plugs, one at a time and got clearly visible sparks when kicking over the kickstart lever by hand ( although the motor had cooled a bit by then). I started the bike back at the house with the petcocks open and carbs tickled (no choke), but turned them off as soon as I got to the garage. Both cylinders died almost simultaneously after the bike reved up a bit for a couple of seconds (suggesting that they aren't set terribly lean).

By the way, the last time it cut out on the road, it actually ran on one cylinder for a couple of hundred metres, only finally stalling on an incline.

I will try  a couple of other things. I have an old inline timing light which connects in series with the high tension lead and plug. It will tell me if the spark is stopping, causing the stall. I will also try draining the tank through the fuel filters to gauge flow rate in case there is a (two) blockage(s). Will also pressure test the cap vent and inspect the points for burning.

Is there any magneto fault which would see one circuit go before the other? They are new plugs, caps and wires. Finally, are modern, rewound armatures any more reliable than the old shellac insulated ones with paper condensors (capacitors)?

Thanks for all the help, although I'm beginning to think Alwyn is a killjoy. I thought Trevor's suggestion was clever.

David A

Offline graeme

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Re: Is this my magneto?
« Reply #6 on: 05 Jul 2006 at 23:11 »
David,
If you have fitted fuel filters, make sure that they are suitable for a gravity fed system. Many fuel filters are designed for use in a fuel pump system and won't flow adequately for fuel flow when the engine is revving. I have suffered from this in the past with symptoms very similar to what you describe. The bike ran fine in the driveway or after standing for a couple of minutes, but a few minutes on the road and it died. Replacement of the filters with ones made for a gravity fed system cured the problem instantly.
Cheers, Graeme

Offline Ian

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Re: Is this my magneto?
« Reply #7 on: 05 Jul 2006 at 23:11 »
The new armatures are very reliable and so are modern condensors !!

One problem I have had over time is that modern spark plus can't handle the heat on some machines. I don't know anything about the later Douglas machines but do they run particularly hot ? Is the plug metal turning blue ? Have you tried carrying a new set of plugs and changing them when the problem occurs ?

Offline Doug

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Re: Is this my magneto?
« Reply #8 on: 06 Jul 2006 at 04:06 »
There was a batch of repro dodgy slip rings manufactured a number of years ago. These were made of the wrong molded plastic, that was hygroscopic (some sort of nylon.) They would absorb moisture from the atmosphere, which once the armature warmed up, would be driven out to the surface, causing arcing to ground, killing the ignition. As the bike cooled, the surface would dry off, but until all the moisture was driven out, the problem would persist when hot. Sometimes it was not severe, the engine would continue to run, but if it stopped you played hell getting it started again. I nearly gave myself a corenary trying to restart a borrowed Goldstar with this affliction. Perhaps it might start shorting out one cylinder first. Has the magneto been redone and the slipring replaced?

If the condenser was going bad, both cylinders would cut out.

-Doug

Offline trevorp

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Re: Is this my magneto?
« Reply #9 on: 06 Jul 2006 at 05:32 »
Another thing to check dave is the carbon bruses on the pickups and the earth carbon pick up to make sure they are not worn and i hope u are using wire leads as carbon ones are no good for magnetos
has your wife suggested pedals to be welded to your new crankshaft lol
dont worry dave when the t35 is going properly i will know who has the biggest smug look

Offline aggettd

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Re: Is this my magneto?
« Reply #10 on: 06 Jul 2006 at 19:32 »
More excellent suggestions. I had assumed that buying the filters from a motorcycle shop would guarantee they were good for gravity flow (or high flow). Must try that one too!! Trevor, I found the article you were thinking of on a Vincent site. Useful. I was also thinking that the ground brush might be poor. A Lucas Service bulletin suggests that a twin magneto fires one plug positive electrode and the other, negative. Also said that one or the other was stronger. If the ground bush was failing due to heat, dirt, etc. possibly it would mean that the weaker would stop first. I hadn't heard of the hydrophilic or hygroscopic (water-absorbing) slip rings. That will be a last resort. I'll get to the diagnosis early next week and post results.

Thanks so much for the great response. My riding season is far too short to spend it scratching my head. Actually my wife is very understanding. It's just that the breakdown interrupted her ride too. Gotta find a way to make that up soon.

Cheers,

David A

Offline eddie

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Re: Is this my magneto?
« Reply #11 on: 06 Jul 2006 at 21:18 »
Dave,
I had similar symptoms on my Comp. It turned out to be dirt clogging the hole in the bottom of the float chamber - the hole for the float needle, thus preventing the float from dropping and giving full flow of fuel. The dirt can be removed with a drill bit held in a pair of pliers.

Eddie
« Last Edit: 06 Jul 2006 at 23:02 by alwyn »

Offline aggettd

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Re: Is this my magneto?
« Reply #12 on: 10 Jul 2006 at 10:52 »
Eddie, my T35 has the twin carb set-up. Is it possible that if one of the carbs was blocked as you suggest, that the other would become sufficiently overloaded to run dry, trying to keep the engine running on one pot? Ordinarily, when I am balancing the carbs under virtually no load, the motor will run quite happily on one cylinder only. Could the fuel draw needed to keep the engine running under load be enough to starve the operating carb?

I haven't had a chance to do a full diagnostic yet, but I have been puttering. I noticed when I pulled out the grounding brush it was rather wet with oil. In fact the oil was sufficient to leave a visible impression on the brass ring seen through the brush mounting hole. I have not yet modified the crankcase breathing system. Is it possible that oil blowing past the magneto seal is entering the magneto, and that this could be causing my symptoms?

A very gracious friend has agreed to come on an extended ride, following me in his pickup truck, to help diagnose the problem on the road, if I can't find the problem in the garage. Thanks to your response so far, I have lots of test options. Stay tuned.

David A

Offline trevorp

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Re: Is this my magneto?
« Reply #13 on: 10 Jul 2006 at 11:54 »
No if your magneto has oil in it then solvents and shellac dont mix let alone burning oil turns it to carbon
dave undo banjo at bottom of carby with a container underneath and just loosen with fuel on if fuel comes pouring out and continues to do so fuel system should be fine
im only leaning the way of the magneto because a mk3 i have been working on suffered from the same symptons except that fired on left or right side whenever it felt like it drove me nuts as i cleaned the carbies ultrasonically 3 times
« Last Edit: 10 Jul 2006 at 12:01 by trevorp »