HUGE magneto breakthrough

jseng1

jseng1

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I've had a recurring magneto miss thats been bugging me for years - driving me crazy. I tried everything. Replaced everything etc etc. No one could help me. So out of desperation I decided to do something crazy. I wired up two condensers and now it runs perfectly clean. I just went blasting up and down the highway through the lower gears at 7000+ RPM. Its never run this good. Arching at the points seems greatly reduced. Next I'll be replacing the two bulky condensers with two tiny little capacitors.

HUGE magneto breakthrough
 
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Just a wild shot in the dark .....
You might try connecting a diode, ex: 1N002 , across the points, as is usually done across the coil of a DC relay.

Slick
 
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I always thought that a magneto had a low voltage spark across the points? Maybe I am thinking of the Lucas mags or the old BMW units for the /2s.
 
There’s an old story about the AJS race team in the late 1940s, testing and tuning their Grand Prix bike out in the country on a quiet road. They were troubled by a high rpm misfire.

A yokel leaning on a gate, casually observing, called over: “Your magneto is under-condensed.” He wandered off and came back after some time, with a condenser from some engine.

This greatly delayed them because the factory team, being experts, had to try everything else first, to avoid proving the straw-chewing rural character right.
 
jseng1 said:
I've had a recurring magneto miss thats been bugging me for years - driving me crazy. I tried everything. Replaced everything etc etc. No one could help me. So out of desperation I decided to do something crazy. I wired up two condensers and now it runs perfectly clean. I just went blasting up and down the highway through the lower gears at 7000+ RPM. Its never run this good. Arching at the points seems greatly reduced. Next I'll be replacing the two bulky condensers with two tiny little capacitors.

HUGE magneto breakthrough
Click to expand...
I bet you feel on top of the world (-:
 
Its funny how one Joe Hunt magneto can have problem with the condenser and others have never had a problem, having had 2 JH mags on both Triumph and Norton (23 years for both), older and newer versions and never had no problems with misfire or breaking down of the condensers, yes its wise to replace every so often why new point, condenser and plug leads are in the tune up kits, I have also had mates run older JH magneto's and none never had problems with high RPMs misfire or had to add another condenser to the JH, whether my mates and myself have been lucky or Jim you been unlucky with your misfiring JH.
But I am glad Jim you sorted your misfire with an easy fix just by adding an extra condenser as a little fault like that can be frustrating to sort out.
Everyone knows my opinion about Joe Hunt magneto's, its the best ignition I have ever ran on my Norton no matter what anyone says about it sticking out the side.

Ashley
 
When you parallel capacitors/condensers, you are adding the capacitance together. Since two work properly and one doesn't, it simply means that you needed more capacitance than you had.

One time took a bunch of new Lucas condensers and bunch of used Lucas condensers and tested their charge speeds and discharge speeds (indication of capacitance). They varied wildly. Even in quality electronics, 20% tolerance is common in capacitors - automotive condensers are very crude.

The job of a condenser is to momentarily hold the points closed electronically. When closed, they are discharged. When the points start to open the condenser charges but while charging there is no spark across the points. If you get the capacitance right for a given frequency of opening, there will be no spark across the points. Of course that frequency changes as the points open faster with higher RPM. So, you need the capacitance that gives the least points spark across the range. Also, in magnetos, the higher the RPMs, the higher the voltage across the points.

Although slight, the condenser affects the timing as there is the delay in the points electronically opening.

All this does not mean to put a big capacitor across the points. For instance, put the Lucas blue capacitor or larger across the points and it's unlikely that it will run at all as the points will never electronically open.

This is a more detailed description, and the magneto says it better than I can: https://wrcoutboards.org/wp-content/uploads/2020/04/Part3_Sizing_Condensers_Correctly.pdf
 
marshg246 said:
The job of a condenser is to momentarily hold the points closed electronically. When closed, they are discharged. When the points start to open the condenser charges but while charging there is no spark across the points. If you get the capacitance right for a given frequency of opening, there will be no spark across the points. Of course that frequency changes as the points open faster with higher RPM. So, you need the capacitance that gives the least points spark across the range. Also, in magnetos, the higher the RPMs, the higher the voltage across the points.
Although slight, the condenser affects the timing as there is the delay in the points electronically opening.
All this does not mean to put a big capacitor across the points. For instance, put the Lucas blue capacitor or larger across the points and it's unlikely that it will run at all as the points will never electronically open.
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Interesting. I don't know that I've ever heard that mechanical/electrical explanation of the capacitor's job explained like that, and I've read plenty of stuff and heard the basics (and some advanced) in high school and in the NAVY. Either that, or I just forgot...
 
I'll be experimenting with larger size condensers and hopefully settle on a tiny high capacity film capacitor. The orginal condenser as supplied by JH was only .136uF, I doubled them up and now it runs perfect and still starts easy.

Quotes below are from the link that marshg246 shared https://wrcoutboards.org/wp-content/uploads/2020/04/Part3_Sizing_Condensers_Correctly.pdf

"Here’s the “sizing problem” in a nutshell – If the condenser is TOO SMALL, it won’t have much effect. You will have a VERY hot spark, but will also have lots of arcing across yourpoints, dramatically shortening their life. If your condenser is TOO BIG, it will lower thevoltage in both the primary and the secondary by too much. You won’t have any arcing acrossyour points (so they will have a long lifetime)…..but you won’t get enough voltage in yoursecondary to actually arc across your spark plug, either. So, our challenge here is to find the“sweet spot” that is a compromise between contact / points life, and still getting an acceptablyintense spark across your plug."


"SO………for a given engine, you can DECREASE the condenser size by half without mucheffect (but not much more than that!)……and you can INCREASE the condenser size by slightlymore than 10x too large before completely killing your engine’s ignition. So, the conclusionhere is that the condenser size is FAR from critical; anything large enough to quench the arcingacross your points is all you need. Putting in a condenser that’s slightly too big is the safer wayto go!"
 
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A couple things to point out here that is being missed in the discussion. A magneto is operating in AC. The role of a capacitor (condenser) in AC is different than in DC. A DC capacitor has polarity where an AC capacitor has no polarity. Capacitors store DC voltage, not AC voltage. This is because AC voltage reverses direction periodically in a capacitor. When AC reverses direction, the capacitor alternates between charging, discharging, and charging in the opposite direction. This helps AC capacitors manage ripple and regulate the voltage being sent to the coil. Think of a capacitor in AC as more of a regulator than a storage device.

The voltage rating on the capacitor has nothing to do with the ~36,000v output of the hunt magneto but more to do with the back feeding from the coil.
 
jseng1 said:
I'll be experimenting with larger size condensers and hopefully settle on a tiny high capacity film capacitor. The orginal condenser as supplied by JH was only .136uF, I doubled them up and now it runs perfect and still starts easy.

Quotes below are from the link that marshg246 shared https://wrcoutboards.org/wp-content/uploads/2020/04/Part3_Sizing_Condensers_Correctly.pdf

"Here’s the “sizing problem” in a nutshell – If the condenser is TOO SMALL, it won’t have much effect. You will have a VERY hot spark, but will also have lots of arcing across yourpoints, dramatically shortening their life. If your condenser is TOO BIG, it will lower thevoltage in both the primary and the secondary by too much. You won’t have any arcing acrossyour points (so they will have a long lifetime)…..but you won’t get enough voltage in yoursecondary to actually arc across your spark plug, either. So, our challenge here is to find the“sweet spot” that is a compromise between contact / points life, and still getting an acceptablyintense spark across your plug."


"SO………for a given engine, you can DECREASE the condenser size by half without mucheffect (but not much more than that!)……and you can INCREASE the condenser size by slightlymore than 10x too large before completely killing your engine’s ignition. So, the conclusionhere is that the condenser size is FAR from critical; anything large enough to quench the arcingacross your points is all you need. Putting in a condenser that’s slightly too big is the safer wayto go!"
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Hi Jim,
Is 0.136 uF the nominal JH condenser value or did you measure yours. Does JH give a nominal vale?

Ed
 
maaseyracer said:
A couple things to point out here that is being missed in the discussion. A magneto is operating in AC. The role of a capacitor (condenser) in AC is different than in DC. A DC capacitor has polarity where an AC capacitor has no polarity. Capacitors store DC voltage, not AC voltage. This is because AC voltage reverses direction periodically in a capacitor. When AC reverses direction, the capacitor alternates between charging, discharging, and charging in the opposite direction. This helps AC capacitors manage ripple and regulate the voltage being sent to the coil. Think of a capacitor in AC as more of a regulator than a storage device.

The voltage rating on the capacitor has nothing to do with the ~36,000v output of the hunt magneto but more to do with the back feeding from the coil.
Click to expand...
It only matters when the points open - the condenser is shorted out when the points are closed.

Easier to understand with battery ignition and much lower voltages, but everything I said holds true for a magneto.
 
The Lucas K2F competition magnetos had the capacitor inside the armature. The racing magneto which was used on the Manx was similar to the Lucas SR industrial magneto. It had an external capacitor which is easily replaced. The SR type is good for racing, and can still be found. It can handle 1, 2 or 4 cylinders. I used a 4 cylinder version and ran two HT leads to each plug on a Triumph twin motor. These days my bike has a Boyer with a double ended coil and a battery. It is much more reliable.
 
Al you talk about old Lucas magneto's but the new Joe Hunts are so much more reliable than the Lucas, they produce a bigger, hotter spark and are very reliable, I ran a Boyer EI for over 30 years and I can tell you the JH will out shine the Boyer by far and the best thing about the JH magneto's they are so easy to work on, point, condenser and coil are just under the front cover, but after 12+ years on my Norton and over 35k + road miles I haven't touched the JH at all.
I have a Lucas competition magneto on my 1960 Manxman project bike and if the Lucas magneto ever fails I be replacing it with a new Joe Hunt.
 
Good news. I doubled up two small .220uF 630 volt AC capacitors and it runs perfectly clean to 7000+ RPM - same as it did with two original stock S6-136 condensers. These tiny capasitors will fit easily under the cap. One single .220uF 630 volt AC capacitor would miss the same as one single original .136uF JH condenser. Next I may try a single 440uF or higher capacitor (or just leave it as is).

Ed -I didn't measure the original stock JH condenser. I just took down the number. S6-136 and I assume its .136uF

This is the .220uF 630 volt AC capacitor I'm using. Using just one (as in photo below) misses but using two stacked on top of each other runs perfect.

HUGE magneto breakthrough
 
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The Joe Hunt high capacity conderser #ASC PR7A .36MFD GMV 600VDC 0218

Seems to run clean but maybe a crack or two above 7000RPM on one run.
 
I do not like historic racing, however my bike is a 60s bike. I have seen Joe Hunt magnetos on historic racing motorcycles. I think those guys have lost the plot. The best historic meeting each year, is cars and bikes at Winton in May. The car guys are not even allowed to run replicas. The problem is, young guys have no memory.
One of my mates said 'at least your bike looks right'. What he does not understand is that I could not have built it in any other way using old parts, and have it function properly.
When I look at it, I can see what is wrong - but he cannot.
 
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jseng1 said:
The Joe Hunt high capacity conderser #ASC PR7A .36MFD GMV 600VDC 0218

Seems to run clean but maybe a crack or two above 7000RPM on one run.
Click to expand...
Jim,
The two 220 uF run better than the JH high capacity condenser?

Ed
 
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