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Damn car!

BACKGROUND

1982 S3 NA - starts ever time but misfires,, particularly under load.

This is what is definitely NOT causing it.

> Coil, ballast resistor, plug leads, distributor cap, rotor arm, plugs - because they are all new.

> Compression - because I've checked it; pots 1-4, 155,  150, 140, 150 psi respectively.

> Ignition timing - because I have checked it.

> Vacuum takes-offs for the heater box and brake servo - because I have checked them too.

> Inlet manifold, carbs/ mixture, etc. - fully rebuilt by Max 500 a few months ago.

I doubt that it's the alternator because the gauge on the dashboard shows it's output is OK.

Can't be valve clearances because that would have showing up on the compression test.

What this leaves as far as I can see is the fuel pump, ignition module (Accuspark), valve timing (could the belt have slipped by one tooht - tension is OK though)?

QUESTION

What have I missed; any ideas?

Bob

 

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Have you done a careful static timing check? The accuspark won't trigger by hand turning(mine doesn't) but check that rotor is actually pointing at correct position to fire #1 lead when in position. And yes, check carefully that nothing has slipped on timing belt.

 

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When an engine is under load the compression ratio increases making it more difficult for the spark to jump the plug gap so it finds an easier route to earth.

So, I'd be double checking all of your ignition connections & components, just because they're new doesn't mean there's nothing wrong with them. One thing you might try is to reduce the plug gaps, if that reduces the misfiring under load then that's pointing to an ignition problem. Good luck 😀

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Cheers,

John W

http://jonwatkins.co.uk

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Did the misfire happen before the parts were replaced? If not then I'd suggest revert bits at a time because as JonWat says, new doesn't mean it's fine. I was caught out by a new rotor arm that was faulty due to its design.

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I know of an owner with a carb'd turbo, that replaced the fuel pump, then a few months later had the symptoms you describe.  Turned out to be the 'new' fuel pump.


Atwell Haines

'88 Esprit

Succasunna, NJ USA

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Well, it's definitely not,

> The ignition timing because I have checked it again.

> The new parts because it was mis-firing before I replaced them.

> The  valve timing, because I have just checked that too and it's fine - the cambelt has not slipped.

.... this just leaves the Accuspark unit or the fuel pump, I think.

If it's neither of them, then I all out of ideas!

Bob

 

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I just replaced a fuel pump that worked sometimes not and so occasionally made a misfire and under load it gets worse. I could see that because it has a fuel gauge.

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I've had misfire problems and found a timing light to be invaluable. I put it on all four leads one after the other and rev through the range. I found intermittent and momentary spark loss on plugs 3 and 4. I replaced the new cap and rotor arm back to the original pair which I swapped out to simply refresh when the engine was out and it's now running fine. At least you can confirm consistent spark that way. 

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22 hours ago, 82s3 said:

.... this just leaves the Accuspark unit or the fuel pump, I think.

I'd lean towards the Accuspark as it sounds like an ignition problem to me as it's only happening when under load, unless, of course, it's the carb float levels. If they're too low that could be giving you too weak a mixture which can cause misfiring under load.😀


Cheers,

John W

http://jonwatkins.co.uk

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It won't the float levels. Max of Max 500 sorted the carbs for he earlier this year when I was having a hot running problem.

Is it possible to check an Accuspark unit with a multimeter?

Bob

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You'd be better to check the spark voltage using a meter which is a pair of sharp points where the gap can be altered, the shortest distance at which the spark fails to cross, gives you the voltage of the spark. Then just compare to Accuspark's specification (you may need to contact them to get that).

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What Jon mention regards a lean mixture might a point check. 
I installed a lambda sensor and found that I consistently is running lean under boost. 
it is happening about 3500-3700 rpm.

Havent solved it yet, thoug being going thrue the carbs numeral times.
 

gave up and purchased EFI throttle bodies!

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All an engine needs to run is a correct air/fuel mixture & a spark to ignite it.

So, if the mixture is correct & all the ignition components are faultless then that only leaves the Accuspark then. 😀


Cheers,

John W

http://jonwatkins.co.uk

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And it's not the Accuspark unit either because I replaced it this afternoon and it's still misfiring.

And yes, the timing is spot-on.

...am completely stumped!

B

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On 10/11/2019 at 10:24, 82s3 said:

Damn car!

BACKGROUND

1982 S3 NA - starts ever time but misfires,, particularly under load.

This is what is definitely NOT causing it.

> Coil, ballast resistor, plug leads, distributor cap, rotor arm, plugs - because they are all new.

> Compression - because I've checked it; pots 1-4, 155,  150, 140, 150 psi respectively.

> Ignition timing - because I have checked it.

> Vacuum takes-offs for the heater box and brake servo - because I have checked them too.

> Inlet manifold, carbs/ mixture, etc. - fully rebuilt by Max 500 a few months ago.

I doubt that it's the alternator because the gauge on the dashboard shows it's output is OK.

Can't be valve clearances because that would have showing up on the compression test.

What this leaves as far as I can see is the fuel pump, ignition module (Accuspark), valve timing (could the belt have slipped by one tooht - tension is OK though)?

QUESTION

What have I missed; any ideas?

Bob

 

The new coil, is it the correct 6 volt coil? A 12 volt coil being supplied only 6 volts would give problems.

Yes, it's possible for the belt to jump a tooth, the earlier trapezoidal belts were prone to this on cold mornings if the tension is loose but it's easy enough to check.

Sounds like ignition rather than carbs but how's the fuel filter & have you checked the float levels? Just because they've been rebuilt doesn't mean they're correct. ☺


Cheers,

John W

http://jonwatkins.co.uk

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I'd suggest you either borrow or buy another distributor cap and rotor arm. They are not infallible under acceleration load and a real bitch to track down. I've seen it time & time again with other makes of cars and there is no reason not to suspect that on an Esprit.

Edited by ekwan

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Mistimed cams, in cases where valve-piston interference has not occurred, typically results in a drastic change to the power curve. What one may expect in such a case is the engine will start and run clean but shut down hard at a premature rev level. Misfiring in a case such as this where the engine is mechanically sound is either fuel or spark. Tell us more about the nature and range of misfire please.

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14 hours ago, drdoom said:

Tell us more about the nature and range of misfire please.

If you read the first post he's provided that information. :thumbup:


Cheers,

John W

http://jonwatkins.co.uk

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8 hours ago, jonwat said:

If you read the first post he's provided that information. :thumbup:

Thanks, John, I have read the lot. I ask for more details in hope of lending help.

Cheers

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