Magneto problem?

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MalcW
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Magneto problem?

Post by MalcW »

I've been working my way through issues with my recently acquired Model 31, and I wonder if I may have a magneto problem. I've cleaned out the fuel tank and carburettor, and added fresh fuel. Now, instead of a stony silence, I get signs of life. However, with the A/R set anywhere from just under half way from Retard to Advance onward towards Advance, I get a fairly reliable kick back. At least I guess that proves that the fuel is getting through. If I go any further towards Retard, however, it's back to silence. I seem to recall reading that the spark is progressive weaker toward the Retard end, so I wonder if that's the problem. The plugs fitted are currently NGK B8ES, and they seen to spark OK when resting on the engine, but that's no guarantee of what's happening under compression. Does all of that sound feasible? Any other ideas? What plugs do others use?

Thanks,

Malc
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dave16mct
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Re: Magneto problem?

Post by dave16mct »

I've just done exactly the same, NGK plugs sparking nicely lied on the head. Put back in and...nothing. 2 new plugs and it fired right up straight away. It was a Kawasaki twin but same idea. I'd get a pair of Champion N5C (set to 22 thou) and I bet it starts.
Dave.
MalcW
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Re: Magneto problem?

Post by MalcW »

Hi Dave

I's certainly worth a try. I'm suspicious though that it is firing when sufficiently advanced (hence the kickback), but not when further retarded. But then, thinking about it, if that's when the spark is weaker, new plugs night overcome that.

Cheers,

Malc.
g5wqian
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Re: Magneto problem?

Post by g5wqian »

hi dave ;
i set the retard so the lever shows about 1/4" of the cable slot , ie about 20% of the total movement of the lever .
the lever only moves a small amount anyway and not all way round like the air lever .
fully advanced is when the lever is closed .

the air lever is closed when its closed and i start it with the air lever fully closed and then after it runs i open it half way for a bit until warm and then fully round and fully open once its warm ..

before i start it i allways need to screw the idle speed screw in on the carb by about 3/4 turn when its cold so i have a fast idle when i let the throttle go and then gradually reduce it back as engine warms up .

if bike is on centre stand be aware that if the idle gets too fast the bike will move backwards , which might be ok on a concrete drive but not if it rests on a small bit of flat material , because the bike may walk backwards on its own and fall off in other words .
so you need to keep an eye on the idle speed as it warms up .

i usually flood the carb first and then lightly turn engine over once and then flood it again and it will go on the first kick .

you will need to open the throttle slightly in conjunction with kicking it over and it should come to life easily .

with the magneto , if it has an earlier brass points backplate i think there should be a small carbon brush in the back of it , there is a small hole in the rear of the plate and on my spare mag if it wasnt in there i didnt get a very good spark , but when i put a brish in there it had a lovely spark.

the later backplate and points are different and i dont think need the brush .

make sure you dont have suppressed plug caps on or the spark will be weak as well .

be carefull if checking TDC for timing , try to use a plastic stick or something non metallic to check the piston coming up etc , because as the engine turns over and the valves open and shut you can get caught up with one of the valves , you will see the edge of the valve through the plug hole if you look closely .

those TDC tools they sell on the internet are nasty , i just use a trip of plastic and a pen mark .

points gap is 0.12"

regards
ian
MalcW
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Re: Magneto problem?

Post by MalcW »

Thanks Ian, that's really useful information.

The bike is currently sitting in the garage with the timing cover off and the magneto pinion resting in it, ready to re-time the ignition from scratch. I use a vernier gauge - the end of the gauge sits across the plug hole with the 'tail' of vernier reaching down to the piston (I now know that 11/32in is 8.7mm). With the tank off you can get the vernier vertical. My magneto has the later points plate, so there's no brush to worry about. I don't know about the plug caps, but i think I'll get new ones just to be certain. One other question; one of the HT leads seemed a bit sloppy where it goes into the magneto, so I've packed it with a washer for now. Do you think this is OK?

Then I can put the tank and seat back on (again!) and see how I get on.

Cheers,

Malcolm
Trefor
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Re: Magneto problem?

Post by Trefor »

Try a hotter plug I run my G80 on a NGK 6. 8 is quite cold. I find the old bikes seem happier with a hotter plug, I'm not sure if the fuel has changed or I'm riding a lot slower these days.
MalcW
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Re: Magneto problem?

Post by MalcW »

Thanks Trefor, I'll give that a go. I don't ride as fast as I used to. I once saw it referred to as 'closet slowness'.

So, an update on the magneto saga. I had convinced myself that it is at fault, so I removed it to be sent away. The first person I contacted said they are booked up until May. Not what I wanted to hear. The second said they were not taking on any work at all, as they have such a backlog, but was happy to give advice. His opinion is that since the mag is causing kick backs anywhere between half and fully advanced, and nothing between half way and fully retarded, it is probably OK. He said a weaker spark towards the retarded end of the scale is normal. So I popped the gear back on temporarily and spun the magneto by hand, and I do get decent sparks, so hopefully he's right. He also suggested ignoring the standard timing advice and not advancing it so much, modern fuel being different etc., but I'm not sure where to start. The standard figure is 11/32", so I thought I might start at 1/4". Any thoughts or advice? Just as a reminder, it's a Model 31.

Thanks,

Malcolm
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ajscomboman
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Re: Magneto problem?

Post by ajscomboman »

Leave the timing as 11/32 BTDC . I run my 650 outfit on this setting and never had an issue. My CSR which has racing pistons I set at just before 1/4 BTDC but that's because of the higher compression.
Groily
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Re: Magneto problem?

Post by Groily »

Think you shoud do some basic tests while it's off the bike Malcolm. It might be OK, indeed it might, but then again . . .
If you take a pick-up off, and put one probe of a meter on the brass segment of the slipring that is visible (in certain positions of the armature) and the other probe to the mag body, what resistance do you see? You want to see about 5000 ohms if the HT coil is continuous. If you see Open Line (infinity) then there is a problem. The spark may be jumping a break and trying to work (they do that) but OL is bad.
With the mag on the bench, see if when you flick it by hand you can get a decent 5 or 6mm spark in the advanced position. It should do that easily, and for that matter retarded as well. If it doesn't, but will maybe spark a plug with its small gap all the same, then it's telling you stuff. A good wheeze is to take an old plug, grind off the earth electrode completely, and use it as a test piece. The gap will be about 4mm or 160 thou typically, which is more useful than a 20 thou one for this purpose. (4mm says you need about 5000 VOLTS to get a spark - and that's the minimum you need under compression on most engines.) It should make a satisfying 'thwack' sound as it sparks, and it really should do it easily with said flick of wrist.
Then, if you feel up for it, heat the thing up with a hair dryer or paint stripper - carefully - until it's hot to the touch and the heat is well into the thing. Lucas said '50°C' for test pruposes (well soaked through). See if it sparks as above? If it doesn't, you're into the usual 'won't work hot' situation, which speaks to condenser, coil or slipring/HT pick-up leakage, in any combination.
Whether you set things spot-on at 11/32 or whatever, the beast should run - even if badly - if the sparks are any good (and other things are right, like the carburation, top end stuff, etc). When retarded, the mag is about 18° off its advanced position, or 36° on the crank - but it should still work with a decent flick on the bench. Less powerfully, yes - but it shouldn't die. although the engine would be flat as a pancake at anything off 'idle'.
Any serious doubts, and there's no point putting it back on the machine - unless you like taking the darn things on and off!
MalcW
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Re: Magneto problem?

Post by MalcW »

Thanks AJSComboman and Groily. Groily, I'll check out the mag as per your suggestions. One thing I've only just noticed - the manual says time the offside cylinder. I've been timing the nearside, but surely that wouldn't make a difference would it?

Update: I've checked the magneto this lunchtime, and as per Groily's tests I am getting a resistance of around 4900 ohms, and it's throwing a decent spark with the plug's earth electrode removed. Then the actuator for the slip ring fell out, so I've learnt how to do that as well.
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