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1500 misfire mid range unless choke


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Good evening ladies and gents, apologies I've not been around lately. I've been working in Prague and I'm still half way between there and Leeds.

Last week or so I've taken some time in Leeds to take care of the Spit that has just received a 1500 (was running) and gearbox with overdrive. I put the inlet and carbs from the 1300 on it because they'd been reconditioned a few hundred miles ago and the 1300 was sold to me as a 1500 so all parts were bought with the assumption it was a 1500. I anticipated some tuning of the carbs, but after setting them and getting MOT, it turned out it just wouldn't run well unless on choke.

The idle is lovely, and over 4000 revs works. Various needles have been tried, and the carbs and inlet that came on the 1500 have been refitted.

While doing all this we found a leak at the timing cover, so we took it off and replaced the chain and sprockets at the same time.

We've tried various needles, air filters, checked for leaks including spraying easy start about the place, ran with and without vac. connected to the dizzy, mechanical and electronic (CSI) dizzies, electric and mechanical fuel pumps, swapped plugs, swapped coils (and connected coil to battery once started), countless combinations of all the above, but between around 1500 and 4000 revs it just isn't firing on all cylinders.

Plugs 3 and 4 are black, plugs 1 and 2 biscuit colour.

Cam timing is off by 8 degrees (not enough I would think to cause this and I will correct this) as per the equal gap at valve 1 and 2 on the rock/balance point method of testing. As far as I know cam is standard.

Dizzy is pointing approximately at cylinder #4 when the piston in #4 which is filthy is all the way up and valves #1 and #2 are at balance point.

Two things I really wanted to ask because I'm still new to all this stuff, this is my first car!

1. Is the dizzy meant to be pointing at cylinder #4 when valve #1 and #2 are on the rock?

2. What is the correct wiring order on the dizzy?

I've struggled to find answers for these two questions.

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Engine is 1500 assumed all standard.

Inlet manifold, tried two, both standard.

Carbs, tried two pairs, both standard. The set on her now is the set that came on the engine (which brought her back from Wales to Leeds and around town a few times without issue)

Needles, tried ABT, AAQ, AAC and AAU - this is what I have to hand hence what I've tried.

Exhaust and manifold, standard 1500.

Dashpot oil, started with engine oil - we have two spits, midget and sprint and they've all been "okay" with this or a blend of engine and 3-in-1 shed oil.

Plugs are whatever was in each spit when we got them plus a set of those gimmick 4 point bosch plugs. These have done a few thousand miles no problem.

Dizzies, two, one mechanical that was on the 1500 before the transplant, and one CSI electronic for a 1500 that ran the 1300 sweet.

I've got access to a CO and a O2 meter so I'm not going for rolling road if I can avoid it, yet.

What I'm most intrigued by is that foot to the floor, once she catches up, she screams no problem past 3500/4000 revs. It's under load mid range where things are iffy. One half of the engine just isn't doing any work.

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Black plugs suggests that rear carb is running too rich. Had a similar problem with my S some time ago. Actually black smoke from exhaust and running badly.
I got NEW needles and jets. (S U's) This cured the problem. I notice you had had replaced the timing chain, no offence, but are you sure you got valve timing right ? Valves on No 4 cylinder should be rocking not No1. I would think 8 degrees out in timing wouldn't help things.

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When we swapped chains we turned the crank to top dead and marked everything before making sure it was still in the same place before place the new set on. If it did move throughout this it could only be a degree here or there. The 8 degrees it is out now is 8 degrees per the valve #1 and #2 on the rock (exhaust closing, inlet opening)

Can anybody second what nang says about cylinder #4 on the rock at TDC?

For clarification:

Pulley is 8 degrees ahead/behind (it's 8 degrees passed the marker in the clockwise direction).

Valve #1 and #2 on the rock.

Cylinder #4 at the top.

Dizzy pointing approx. #4.

Cylinder #3 and #4 black.

I haven't checked compression yet.

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Was puzzled by Nags remarks......

At the TDC mark, cylinder 1 & 4 will at the top but only one on the firing stroke.

With valves 1 & 2 on the rock the rotor arm should pointing at #4 and with valve 11 & 12 on the rock it should be pointing at 1.

As regards cam timing, first thing to check is that the TDC mark is correct.  To do this you really need to make a piston stop and use it with a degree wheel (can be printed at home and stuck to a bit of card)

Once you've done that you can use the valve rock method to find the cross over point between the inlet and exhaust valve and thus the cam timing point.  If yours is really 8º out, that could be at least part of your problem.


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Hey Ben! How's things?

Sort your cam timing out mate and rule that out of the equation. Check your TDC marker, print off a giant protractor and then check that the valves on no. 1 are opening/closing at the right points.

You don't mention (that I can see) changing plug leads?
Also - carbs in balance and (roughly) even in mixture terms?

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Hey Sam! As they say in Prague, jde to. How're you?

I'll do the giant protractor thing as soon as I get chance. And the leads, I haven't tried changing those. There was a spark on every plug so I assumed they worked. Besides that, it's only mid range where it misfires. It idles and if you can get it in the high revs it runs. It's only apparent under load. No load, no problem. Carbs were balanced with a dial gauge, emissions tuned with a CO analyzer, listened with a little pipe and so on. Rinse and repeat.

It just occurred to me that I've not tried different gasket on the inlet, and I've got one of those heat shields in, that I might try removing. It's quite old and tatty. I've done what I could to test leaks, listening and spraying things, but you can't get everywhere can you?

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I've tried a few dizzies Sam. I replaced the dizzy in this car when it was a 1300 with one of those too expensive CSI electronic versions - when I thought it was a 1500. So I have tried that one, and the mechanical it used to have. I'm going to take the cover off and do the timing, and swap the leads because I do have a new set of leads and a new dizzy cap. Then we'll see where we are.

A side note, and a bit of a conspiracy theory. I'm sure I've old you the story already. But when I got the second Spit, whose engine this is, I drove back from Wales to Leeds in it and had all kinds of problem owing to general neglect. Bad electrics, lights catching fire, floppy gear stick and the destroyed reverse idler. The engine itself was fine except for the story behind it. The seller had "set the carbs up because the last person had them all wrong". This is the taxed and tested Spitfire that had neither been taxed or tested for over a year - I must have a guardian angel.

Anyway, when I got back to Leeds and had a look at her the carbs were full of red crap, whatever it is, and I had to soak one piston in carb cleaner for some time to get any movement at all because it wouldn't move by hand. It was locked at the bottom of it's movement.

The point I am getting to. What would have happened to the cylinders 3 and 4 if that carb was setup to run nicely when needle and piston were inhibited? Imagine the majority of fuel/air came from one carb and the poor crossover pipe. Is it possible a cylinder could get so mistreated that the remnant muck on the valves could create the sort of conditions you could never recover from through normal tuning?

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I had a 1500 engine that had sat for a while, having previously run rather rich. When we put it in the Herald and got it started again, it ran for only a very short while before packing up completely. The huge quantity of soot build-up on the backs of the valves had come dislodged and jammed all the valves open. Head off, valves out, clean up and re-bed, and it ran beautifully.

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