HELP ! condensers(technical/practical)for two strokes.

[jmp]

A friend is having trouble with his seagul O/B,like starts but dies after 20 secs or so,sometimes restarts repetitively,but sometimes won't restart after first burst. All normal procedures :- fuel,plug,points,jets etc checked,and now suspect something sneaky like condenser!
Does anyone know of any way to test the component? I have not yet met anyone who really knows much about them ,ie exactly what they do and how their function/spec is measured or defined.I know about the issue with points pitting or turning blue ,but can't help thinking someone out there knows much more!
Spoke to a vintage motorbike specialist today who immediately stated he was not too sure about them.He produced one for an old two stroke M/B which had:- .25 MKFD and +25% printed on the body,and said it might do! Didn't buy as can't help thinking there must be some sort of spec guide for different applications. ANY HELP or ADVICE PLEASE?

 

[rosssavage]

There is a condenser or capacitor connected across the points when they are open and this component is vital to the ignition system. Its main purpose is not as many people think to protect the points from burning (although it does this as well) but to cause the coil to generate a good strong spark at a known time. Because the coil is a transformer it will only generate a voltage in the HT (and hence a spark) when the current through the primary is changing not when it is steady. The faster the current change and the greater the voltage swing in the primary the higher the output voltage generated. When the points are opened instead of the current immediately ceasing to flow through the coil as you might think it continues for a very short time while it 'charges up' the condenser with the voltage spike that would otherwise arc across the points. It is only when the condenser is charged that the current ceases to flow. Furthermore the condenser and coil when the points open are interconnected in such a way as to form a tuned L/C circuit (L = inductor or coil C = capacitor or condenser) and this causes the current in the coil primary to oscillate rapidly (about 15 thousand times per second) back and fore with a peak-to-peak voltage swing of about 400v. The effect of this is to generate an output pulse and hence a spark of about 10 thousand volts that lasts for about 2 thousandths of a second (i.e. 2 milli-seconds or 2mS). Not very long you might think but at 3600 rpm any one cylinder is firing 30 times a second i.e. every 33mS so at that speed the spark lasts for 22 distributor degrees which is 44 crankshaft degrees! By comparison the spark duration without a condenser is only about 0.2mS i.e. one-tenth as long.

The secondary effect of the condenser is to cause the spark to occur at the correct time. With the condenser in circuit the high-frequency oscillation that occurs immediately the points open means that the output voltage and hence the spark commences just 0.02ms or 20 millionths of a second after the points open. Even at 5500 rpm the effect of this delay is less than 1 crankshaft degree something that is easily compensated for by the centrifugal advance of the distributor. This high-frequency oscillation also protects the points because the voltage spike that occurs the instant the points open decays to zero again (as part of its first cycle of high-frequency operation) in about 20 millionths of a second and this nips the spark off. Without the capacitor the spark only ceases when either the voltage drops sufficiently or the points open sufficiently and this takes about 2mS. During this time the points are arcing which as well as eroding them and causing spikes and pits means that some current is still flowing through the coil for the duration of the arcing which delays the main collapse of the flux and hence delays the output voltage pulse and therefore the spark. This delay is again about 2mS and does not vary much with rpm. This 2mS delay effectively retards the spark during cranking by about 1 crankshaft degree i.e. not very much. But the delay increases to about 24 crankshaft degrees at 1000 rpm 48 at 2000 rpm etc which means that as well as only having a very short duration spark it is also very retarded even at quite low speeds.

The capacitor has a value of about 0.2uF and this value is critical for a good HT spark. Experimentally varying the value by quite small amounts shows little variation in voltage waveforms on the LT or HT or visually in the spark but a there is a definite reduction in the strength of the audible 'crack' heard at the spark plug.

 

[pbeard]

I do wish people would give more in-depth replies to other people requesting information, this really doesn't give you any idea does it???

 

[VicS]

Hi

It does not sound like the condenser, if that was going it would become progessivvely more difficult to start until it won't at all. You can't test them other than by replacement.

You do do not say what model year it is

Are you absolutely ceratin it is not a fuel system problem. Tank vent? sticking float/needle valve? I think you should double check.

Early ones had a Villiers ignition system with a separate condenser that can be replaced (with any car type one that can be fitted in if necessary) The Villiers syetm was superceded by a Wipac system which was sealed. (Later ones still had electronic ignition systems) The Villiers ones had a silver flywheel the Wipac ones were bronze. If it is a Villiers one inspect the state of the coil for signs of deterioration. The two systems can be interchanged as a whole. Baseplate components and flywheel.


The best advice I can give you is to post the details of you problem on The saving old seagulls forum. Click the link in my signature or SOS forums The regulars on there are real experts and fanatics. They even collect Seagulls!

BUT QUOTE THE FULL MODEL DETAILS that are stamped on the boss below the crankcase.

Spares new and second hand are available from John Williams at Saving old Seagulls You'll find every thing you could possibly want to know about Seagulls on there (and loads you will never want to know.

DO NOT ATTEMPT to remove the flywheel before reading the notes on John Williams' site. If you use a puller you will bust it and there are virtually no spare ones now.

Are you always getting a good fat spark? You are not using stale fuel are you?

Are you using the correct fuel oil mix. Later ones use a 25:1 mix while earlier ones used 10:1 some of which will have been converted to 25:1. Very early ones cannot or at least should not be converted. Some reqired no changes others needed the needle to be changed. John Williams' site has all the details that will enable you to check what needle is fitted.

 

[Clyde_Wanderer]

Best way to test the condenser is hold the body of it to the negative and the wire to the pos of a 12volt batt for a couple seconds, then hand it to your mate and ask him to try to pull the wire from the condenser body, if he jumps about 6ft off the ground and swears at you, it is working, be prepared to administer cpr.
All joking aside, check the carb float for petrol inside it, give it a shake and listen for liquid sloshing about in it, if petrol gets into it, it will stop it from regulating the fuel flow in the float chamber causing the motor to flood, or not get enough fuel, cant just remember which at this moment without seeing a SG carb.

 

[VicS]

[ QUOTE ]
check the carb float for petrol inside it, give it a shake and listen for liquid sloshing about in it, if petrol gets into it, it will stop it from regulating the fuel flow in the float chamber causing the motor to flood

[/ QUOTE ] Thats a good point which I forgot. (Different carbs were used at different times on different models)

Very old ones had cork floats as well I think which deteriorate.

 

[MrCramp]

Either take the condenser off that motor and try it on a motor that works or take a condenser off a motor that works and put it on your friends seagul. That way you don't need to know irrelavent complicated stuff about how a condenser works but you should be able to work out if it is that causing your engine not to work.

 

[William_H]

Hi Clyde of course a condenser (capacitor) charged to 12 volts from a battery will not usually give any sort of shock unless you get a contact into a cut. It can be no more than that from the 12v battery in the first palce. It is only if the condenser is connected to a coil in some way that either form transformer effect or back EMF there will be an increase in voltage to enable your mate to feel it.
A condenser can be tested however by connecting it to a 12v battery when you have a voltmeter connected also to the condenser. You should of course measure 12 volts when it is connected to the battery but when you remove it from the battery the voltage should remain across the condenser and slowly decay to zero. This decay rate depending on the capacitance and the resistance of the voltmeter.
An easier way to test the condenser is to use an ohm meter on a multimeter. On intial connection you get low resistance which then reverts after a few seconds to high resistance. it should remain high then until it is disconnected from the ohmmeter and the condenser is shorted out to discharge where upon you can check it again. Or you can reverse the connections to discharge then recharge the other direction.
I have just checked a .22ufd on digital mm 2 megohm max range took about 2 secs on one cheap meter and about 6 seconds on a more elaborate meter to get to 2 megohm indicated. A good meter may even have a capacitance measuring facility. However of course Capacitorss can fail from high voltage which of course a MM does not test.
Any way I agree the condenser (cap) is not likely to be the problem rather a fuel problem. good luck olewill

 

[jmp]

Many thanks Will and everyone else ! I certainly know more about condensers,but still not sure about specifications for different applications and the range of possibilities.Have tried another carb,new fuel etc, before this post .Also mention that the engine has been used in the same hands for years before suddenly developing the problem, so fuel mix known etc.Already tried seagul man and awaiting response! Thanks again all . Will try your suggestions and will report back if revealing! Regards
Michael.

 

[castaway]

I case you hav'nt done this yet check the in tank filter and also the feed pipe from the tank to the carb...my old Seagul would slowly let fuel through a 'gum' blockage and allow it to start but not sustain sufficient fuel to run for more than a few seconds.

Nick

 

[VicS]

[ QUOTE ]
Already tried seagul man and awaiting response

[/ QUOTE ] I cant see your post on the SOS forum!
Put it there with the model details and you will get sound advice, but also tell them exactly what you have already tried.

If you have replaced the carb with a known good one and have verified a good fuel flow then you eliminated the fuel and carburation problems.

You have fitted a new plug haven't you, and checked the HT lead including the plug cap and connector at the engine end for both continuity and insulation integrity

It could be the insulation on the coil breaking down, but you still have not told us what type of ignition system it is! Villiers or Wipac or even CD

It could be the condenser as you first suspected. You have read a couple of lengthy contributions about condensers but the bottom line is at the end of Will H's post. They can fail at high voltages so cannot be tested with a multimeter, although a 500v insulation tester might be worth a try if you have access to one.

If it is the old Villiers system then you can at least get at both components but if it is a Wipac system it is all encapsulated and you can't. Check the encapsulation though. (BTW the condenser in the Villiers system is in the back of the points box)

You are sure you have set the points gap correctly after cleaning them (20 thou when fully open, but check with the owners manual or the "Frequent Queries" on the SOS website for your model).

A final possibility is that it as a leaking head gasket (see the bit in John Wiiliams' FAQs about that)