David Jenkins
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| posted on 23/10/07 at 03:39 PM |
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Strange fault - backfires and odd timing light results.
(Also posted this on the MJ forum...)
Just been out to take my car for its very first MOT (is it really 3 years since I got on the road?) and it's started to behave strangely.
Story goes back to a few days ago - started the car from cold, and got a backfire through the carb before it fired up properly... didn't think
anything of it and the car drove OK.
Today, when I stopped the car after an enthusiastic run (warming the car up ready for the MOT, officer!) the engine ran on, coughed, and fumes came
out of the air filter after it stopped. This has now happened a couple of times.
My first thought was that the megajolt had packed up and I was running in limp-home mode - fixed 10 degrees BTDC. So I got the timing light out... and
got some very odd results.
Put the sensor clip on plug lead 1 and the timing's where it should be, at 12 degrees. Revved the engine, the ignition advanced - and the timing
light stopped flashing after about 2000 rpm. Tried the other plug leads and found that nos 2 and 4 worked as expected with the strobe flashing all
through the rev range. Nos 1 and 3 will only go up to 1500 - 2000 rpm before the strobe blanks out.
My last test was to disconnect the MJ box - and I got sensible results on all leads at all rpm, albeit at a fixed advance (limp-home 10 degrees).
The engine's running OK, if a little flat & unresponsive when pushed.
I'm really puzzled... can anyone suggest what may be going on? Or are my test methods suspect?
Background info:
Ford Kent engine (crossflow), 1660cc.
VR sensor firmly mounted at the front crank pulley.
Latest version of MJ firmware on a V3 board.
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BenB
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| posted on 23/10/07 at 03:52 PM |
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Are 1 and 3 sharing a coil on the EDIS system? If so it could be that packing up at higher RPM or the driver collapsing....
I'd also try swapping some leads- although two leads simultaneously going Pete Tong sounds strange......
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pewe
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| posted on 23/10/07 at 03:53 PM |
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Having been laid up that long could be a number of things.
My first would be to check the plugs or replace them just in case.
Second (and most likely) sticking and/or slow valve return on one of the inlet valves (hence the fumes coming back through the carb and the
back-fire). Cheers, Pewe 
Sorry Dave, didn't mean to imply it was under-used - just the way I read your post. P.
[Edited on 23/10/07 by pewe]
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David Jenkins
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| posted on 23/10/07 at 03:59 PM |
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It hasn't been laid up - I take it on the road several times a week, weather permitting.
The coils are 1 - 4 and 2 - 3. The thing is, in limp-home mode all work OK.
Sticky valve - I haven't checked the valve clearances for a while, although it's only done a couple of thousand miles since I last did it.
I could do that, and check that the valves are free at the same time.
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matt_claydon
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| posted on 23/10/07 at 04:25 PM |
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What confuses me is that a wasted spark system does not know the difference between cylinders 1/4 and between 2/3, so it seems really odd to get a
problem with 1 and not 4, and a problem with 3 and not 2.
Sorry I can't help 
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DarrenW
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| posted on 23/10/07 at 04:29 PM |
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Do you have another Edis you can try to rule that out?
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MkIndy7
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| posted on 23/10/07 at 06:03 PM |
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We've had the interior cores of the plugs break on our NGK plugs.
They all appear good looking at the plug end etc and even sparking on the manifold, but under load and higher rev's they break down.
The way we spotted it was the Nipple on the HT lead end of the plug could be turned about 5 Deg clockwise and anti-clockwise even when the nipple was
tight.
Might be worth checking, as said with the cylinders twinned up as far as the coil packs, its unlikely to be that.
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David Jenkins
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| posted on 23/10/07 at 06:37 PM |
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I keep getting the nagging doubts about that strobe lamp result, and wondering if it's a red herring. After all, the car still revs freely up
to the 6300 rpm limit and pulls hard all the way - I'm tempted to ignore that and go back to basics.
In other words, what would make it cough back through the carbs on start-up occasionally, and to run-on and cough through the carbs after some brisk
driving?
For now, I've checked the plugs and they look OK - no cracks or loose bits, and the gap is fine. I've also checked all the tappets and
they're all up to spec - didn't need to adjust any.
DJ
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MkIndy7
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| posted on 23/10/07 at 07:41 PM |
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A work collegue who has done lots of work on classic bikes says a weak mixture can make it cough back through the carb... would maybe ring true at
this time of year now its colder?
Have you checked the chokes operation (if it has one)?
Also our megasquirt had been coughing back after switching off as the coil pack has been discharging about 5-10 secs after the ignition is turned off
(guess who had hold of a plug at the time!) to find that one out!.
But that has been tempremental of late
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David Jenkins
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| posted on 14/1/08 at 09:15 AM |
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Might have found an answer to the timing light issue - why 2 plug leads give a good flash, 2 don't.
Apparently, According to this link, the coil packs used with EDIS and
wasted spark send negative high voltage to one terminal, and positive to the other. My timing light has a clip-on sensor lead with an arrow showing
which way to attach it to the plug lead. If this ends up on a lead with an opposite polarity to what it's expecting, then there's a fair
chance that it won't fire properly.
next time I get the car running I shall be experimenting... 
David
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02GF74
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| posted on 14/1/08 at 11:58 AM |
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what carbs you got?
check for air leaks between carbs and manifold. (i had bad enging running and whistling - turned out one of the O-rings slippped between insulator
and manifold).
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David Jenkins
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| posted on 14/1/08 at 01:01 PM |
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The timing light was my main worry at the time - I couldn't work out why the engine was running well but the strobe was suggesting that the
ignition system was rubbish.
The backfiring etc. went away later on. I suspected a bad batch of petrol as it didn'thappen again after the next refill!
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