The miscreant: my venerable 1990 FJ1200 3CV(approx 170k miles on the clock but full strip-down and rebuild at 75k miles following chain breakage cracking crankcase).

All was running ostensibly smoothly following oil change, carb balancing etc. The problem: on a wet ride it suddenly went permanently onto 3 cylinders. Dried it out, carried out grounded plug test on all 4 pots and no.2 was found to have a weak and erratic spark. Replaced all 4 plugs. Still no good. Nipped a few mm off the end of no.2's HT lead to get a better contact. Some slight improvement. Replaced no.2's plug cap. Again, possibly some slight improvement but still misfiring drastically. Swapped over leads of no.2 and no.3 (they fire together). No apparent difference but hard to tell which was misbehaving. Put the problem in the "too hard" tray and used other bike for several weeks.

Puncture in other bike so forced to use FJ12 again for a few days. Surprisingly it started up immediately so I've been riding it into London for the past couple of days. The present symptoms are that on drawing away it fires on three at best and often only on two but, mysteriously, if I grit my teeth and thrash it hard it eventually reverts to firing on all four but when the throttle is eased it's back to 3 or 2. Under load on the motorway it will pull well to 6500 rpm or beyond and is running on all four, although not as smoothly as usual and down on power. The fuel economy, needless to say is very poor.

I am mystified. Sometimes it seems like a fuelling problem but I don't think it can be because the behaviour doesn't seem consistent with that. At other times it seems like an electrical problem but it's odd that it sometimes affects only one cylinder and sometimes two. Since 2 and 3 have the same spark source it presumably cannot be the igniter box. Could it be dodgy HT leads at this age/mileage?

Any wisdom or suggestions would be much appreciated (but not "bin it", please - I'm very attached to the old girl having had her from new) .
Neil.

Hi there,

Sounds like a dodgy coil to me. From what I know of FJ's, there are two coils, one of which fires 2 & 3 cylinders. Swapping the leads of 2 & 3 over would still give the same symtoms if the coil was faulty.

You're right but my reasoning was that if it were the coil then both 2 and 3 would respond in exactly the same way at any one time i.e. both firing or neither - or is that not how it works?

no... that's why mechanics HATE "intermittent" problems - tracing can be VERY difficult!

e.g. bad coil scenario where one coil does two plugs...

coil is weak, so produces poor spark. in order to spark at all, it MUSt fire both plugs - BUT - bike is obviously now old, and so the two cylinders aren't identical - in fact one is burning a tad more oil than the other, or the carb is a little more out of whack, so a bit rich - and it quenches the spark so it doesn't DO anything useful on that cylinder, causing a misfire.

A combination of the two weaknesses is the symptom, but the REAL PROBLEM is the coil.

so my guesses in some resemblance of order...

dodgy coil, bad ht leads, plug caps, coil low tension wire ground(s) or power wires, plugs, dirty carbs/fuel supply problems

Change spark plugs, checking caps for cracks, pull the cap and cut a 1/4" off the wire and reinstall plug caps, try it, check/replace leads (they're old anyway ) check all undertank wiring, grounds etc.

If that hasn't got it, clean the carbs and fuel taps (again, probably due anyway). Failing that, you're into deep electrical trouble and in need of an expert.

good luck!

------------------
Grant Johnson

Seek, and ye shall find.
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One world, Two wheels.
www.HorizonsUnlimited.com

__________________
Grant Johnson
Seek, and ye shall find.
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Inspiring, Informing and Connecting travellers since 1997!
www.HorizonsUnlimited.com

To confirm it's the coil, swap the coils & leads round so that the current coil firing 2 & 3 fires 1 & 4 and vice versa. If the coil is the problem, the poor firing issue should be transferred to 1 & 4. Swap the coils & leads over as a complete set but this may not be possible due to each cylinder requiring a specific lead length.

Have you checked for cracks in the coil body?

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Many thanks for all the helpful suggestions. As you can see from my post, Grant, I've taken most of the steps you suggest, but there are clearly some other possibilities to pursue so I'll have another go, in a more systematic manner. Grant's explanation of why two pots served by the same coil may not necessarily behave in the same fashion is convincing.
Neil.

Have you tried removing the low tension spade terminalled leads that go into the coil and replacing the female terminals with new and scraping the males on the coil itself. Had a very similar problem to yours on my 3CV FJ and an old GPz750. It was the low tension side causing the problem through corrosion. It was affecting one plug lead more than another which I know shouldn't happen but it did.

Hi, Bayonet. No I haven't tried that. In fact I have not examined the coils at all so am not sure what you mean by scraping the male spade connectors on the coil. If you could please clarify I'll give it a shot when I have the hardware in front of me. If corrosion is a factor then my FJ can certainly offer plenty - 15 English winters on salted roads have seen to that.
Neil.

He's referring to terminals on the coil where the wiring loom attaches. They can corrode & cause a misfire.

Remove coils one at a time, check body for cracks (they'll let moisture in) & clean the terminals until shiny. Sandpaper or similar will do. Also clean the matching female terminals on the loom itself. Smear terminals with grease before reassembly to limit further corrosion, preferably grease of the type that does not conduct electricity but anything is better than nothing.

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My photos: www.possu.smugmug.com

OK, thanks Steve, I think I get it now - I misread the previous reply.
N.

The problem is you can't see those pesky electrons. Buy or borrow a simple ohm meter. set it to K ohms and stick one lead up one plug cap and stick the other lead up the other plug cap. You should get a reading and then compare it to your other coil. Then take off the plug caps and take a readingthru the wires.Test each cap they should have about 5K ohms resistance each. Look for a difference. As far as carburation, the FJ's in the states have a small port on the header pipe with a 10mm bolt plugging it. That port is for hooking up a exhaust gas analizer. Yamaha shops here in the states at one time were required to have a EGA to meet emission standards. If you don't have access to a EGA machine and the bike misses at idle to low speed you can uncover the idle mixture screws on the carbs and then as the bike idles badly screw each mixture screw in till it bottoms (gently). As you go thru each carb if you have a cylinder that isn't firing well it will not make a difference but the cylinder that is running will cause the engine to die when when you close it off. Aso to locate the weak cylinder on a multi cylinder bike do the spit test... spit on your fingers and then touch each head pipe, the running cylinders will burn you and the missing ones will be cooler.