Nickp
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| posted on 4/4/15 at 07:18 AM |
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Wasted spark + long duration cams = inlet popping
I've just got the newly rebuilt twin cam in my Lancia Montecarlo running. It's got a Canems ignition ECU (wasted spark Ford coil-pack),
twin Weber DCNFs and some fairly lairy (295deg/11.4mm) Alquati cams.
It starts first time and revs up well. I wasn't expecting a great tickover tbh but I'm getting random popping through the carbs at
tickover and just above. All valve clearances are spot on, ign advance is 10deg at idle. I went to great lengths to ensure that the cams were timed as
per Alquati specs (100deg).
I'm thinking the problem is due to the wasted spark system (1+4 fire together, then 2+3) and the large overlap of these cams (about 5mm lift at
TDC). So when no1 is firing at TDC with valves shut no4 also fires but both valves are nearly half open. Any mixture in the inlet then ignites.
I'm hoping that leaning the idle mixture right off will help matters, otherwise I might have to look at the cam timing.
Anyone else experienced similar with wasted spark systems?
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Jenko
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| posted on 4/4/15 at 07:36 AM |
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I doubt it's the wasted spark, the additionl sparking cylinder really should have no waste fuel in it.
But, with a 295 cam, in all honestly I would expect a rough idle if using carbs, is your tick over at reasonable rpm (i.e. around 1.1k), to low and
there wil be overlap, poping and banging.
MY BLOG - http://westfieldv8.blogspot.co.uk/
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Nickp
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| posted on 4/4/15 at 07:45 AM |
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I was expecting a rough idle as you say just not this random popping which continues even if I increase the idle speed. The popping can only be excess
fuel and the wasted spark surely? If I can lean off each cylinder then hopefully this will help. A quick check showed them to be pretty well balanced
so that doesn't seem to be the issue.
I'm about to build a sealed airbox for the carbs which will hopefully make it much less noticeable.
[Edited on 4/4/15 by Nickp]
[Edited on 4/4/15 by Nickp]
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Shooter63
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| posted on 4/4/15 at 08:24 AM |
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I'm not sure if this will help at all but here goes,
I've got a harley that I re-built a few years ago in an attempt to give it more power and better ride ability etc these engines used wasted
spark from the factory and used to tick over like a pig, if you fit bigger cams into them ( which I did along with doing ridiculous amounts of other
works) the situation gets worse, the trick to sort this problem out is to fit what is known as a single fire ignition ie only fires the cylinder that
requires it. It does smooth them out at low end but in your case with 290 deg cams I'm not sure if it would cure a lot.
By the way, your engine is not suffering from stand off or inlet resonance is it? as this can also cause your problems.
I apologise in advance if I have wasted 2 mins of any bodies time in reading this.
Shooter
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Nickp
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| posted on 4/4/15 at 08:44 AM |
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quote:
Originally posted by Shooter63
I'm not sure if this will help at all but here goes,
I've got a harley that I re-built a few years ago in an attempt to give it more power and better ride ability etc these engines used wasted
spark from the factory and used to tick over like a pig, if you fit bigger cams into them ( which I did along with doing ridiculous amounts of other
works) the situation gets worse, the trick to sort this problem out is to fit what is known as a single fire ignition ie only fires the cylinder that
requires it. It does smooth them out at low end but in your case with 290 deg cams I'm not sure if it would cure a lot.
By the way, your engine is not suffering from stand off or inlet resonance is it? as this can also cause your problems.
I apologise in advance if I have wasted 2 mins of any bodies time in reading this.
Shooter
Not wasting time at all, in fact I'd just been reading about the Harleys wasted spark system and their issues.
I am getting noticeable fuel stand off before the revs increase and it comes 'on cam'.
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Nickp
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| posted on 4/4/15 at 09:56 AM |
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Ok, quick update. It's stopped doing it this morning!! I'm hoping it's OK but not sure yet tbh.
I have a possible theory though. I was messing about with the throttle linkage all yesterday morning so the carbs kept getting pumped to check the
travel and hence were pumping fuel in everytime. I'm wondering if initially there was excess fuel in the runners causing it then I messed with
the mixture screws making it worse?
Anyway, I evened up all the mixture screws at 1 turn out and once warmed up it's ticking over happily at 1krpm, bit lumpy but pretty steady tbh
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MikeRJ
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| posted on 4/4/15 at 10:26 AM |
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A friend of mine has even longer duration cams in his rally car (an Mi16 engine making over 270bhp on engine dyno) along with a super light flywheel
and multiplate clutch and wasted spark. The idle is amazingly smooth for the spec. with no popping back, though it does use fuel injection with
throttle bodies.
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Oddified
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| posted on 6/4/15 at 08:42 PM |
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As long as the carbs are balanced, it's normally down to it being little to lean.
Wasted spark and cams isn't a problem, when mine was na it was on wasted and L1 cams which are 306 deg with no issues at all.
Ian
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