Old misfire - again

[johnnydog]

If you want to go down the secondhand route, which would be the cheaper option primarily to establish if the distributor issues are the cause, be mindful that there are several variations which visibly look the same, but are identified by the number stamped on the lug on the body, and the vacuum advance unit needs to be the correct one to provide the correct degree of advance which is identified by the faint stamped in numbers on the trumpet part.
I have a list of the correct distributors and vacuum advance units for the Mk1. I obtained this info after having replaced the distributor in my 1969 Mk1 2000, when it continually pinked even under light load, before I realised the replacement vacuum advance unit wasn't suitable for my engine. With a correct replacement fitted - problem solved!

[Clifford Pope]

Chris Witor lists only

RKC1814SH Distributor 2500TC & S = Lucas 41624 45D used. From MM20,001 SUHS6 carbs, 307621 cam, 1975 onward.

for the later 2500TC and S engine. Mine is MM 45,985.

I'm tempted by the complete Accuspark version they list as correct for that range.

[johnnydog]

I remember now your Mk1 has a S engine fitted with a 45D distributor, not a Mk1 engine with the 25D distributor ......
So just ignore my last post about distributor and vacuum advance numbers..... !!

[Clifford Pope]

I've fitted the Accuspark, and touch wood, that has cured the problem. it appeared that sometime in the past someone had fitted a rogue distributor. It had the correct number on the casing, but on comparison with the new one the contact base plate was clearly slightly too small. The gap around the edge of the casing was about 1mm larger, the slot for the condenser was not quite right for the condenser, which in consequence sat badly and was liable to rub on the lobes of the cam.
Combined with the screw/clip or whatever was meant to hold the cam assembly down on the spindle being missing or damaged, the whole thing was a bodge.
It's just sod's law that it's the last thing anyone would think of.

A few days after fitting and timing, I started up from cold and it fired instantly with no choke, and ran and pulled splendidly. So the distributor seems to have been the cause of the reluctant cold-starting too - see earlier thread.

Timing the accuspark was interesting. The instructions warned that it wasn't possible to do a static timing, but I hoped nonetheless to do it approximately to get it going.
Turning the ignition on and backing the distributor round up to TDC on no. 1 it would spark a spare plug on the coil centre lead, but on starting that position proved to be hopelessly advanced and the engine turned over with a jerky rotation as if it were jamming - obviously wantingto fire backwards.
Getting it going by trial and error and then using a strobe put the correct timing quite a long way retarded from the estimated point. It also, interestingly, aligned the rotor arm squarely at the cap contact segment, which had been another doubtful feature of the old distributor. (I recall a long conversation here about the geometry of the cam lobes in relation to the rotor arm)

[tony]

Clifford,
I've used Pertronix, which I believe is the same as what you're using, on a number of vehicles. I've always static timed them with a lamp as I would have done with a normal points set up with no difficulty.
Tony.

[Clifford Pope]

Clifford,
I've used Pertronix, which I believe is the same as what you're using, on a number of vehicles. I've always static timed them with a lamp as I would have done with a normal points set up with no difficulty.
Tony.

Yes, I'd have thought it ought to work. But the fact was the point at which it triggered when turned very slowly by hand was quite a long way round from the trigger-point when running. The leaflet does say the latest version has an automatic dwell-angle feature, and adjusts itself according to speed. So perhaps that's why - at 5 rpm it can't work out what dwell angle to use.

[Clifford Pope]

Well, here we are a year on, and guess what, it's gone wrong again.

I've hardly used the car in the last year, for various reasons but now of course coronavirus.
I've started it up every few months, warmed it up, driven a few hundred yards up and down the track, and all seemed well.

Until today. Not a brilliant start, but it did start, and appeared to run well. It idles nicely, I drove out of the garage, and immediately found it has no power whatsoever. It revs, but on engaging the clutch it just fades out. It has virtually no ability to get up the gentle slope of the drive, and it was a real stuggle to get it back into the garage. Normally it will do it at hardly more than idling speed, but now I have to rev it like mad and keep the speed up and slip the clutch.

There's nothing binding - it will roll gently downhill in neutral, so not brakes binding.
It has the HS6 carbs, so no diaphrams to rot.

Any ideas anyone on what can have gone wrong in 2 months of standing? Last time I started up it felt raring to go. I could accelerate hard up the track, spin the wheels, ease off and it would trikle smoothly along at idling speed.
Fuel gone off - why suddenly and not in the previous 2 years?
Something gummed up?

Now there is a definite misfire when reving - not apparently ignition, as pulling each lead off in turn has the same effect of running.

[Charles H]

I would go back to the ignition circuit first starting with the low tension side , coil, wiring, timing, vacuum advance pipe, then the high tension side, leads, rotor arm and plugs before blaming the fuel/carbs.

[Mike Stevens]

I agree with Charles. I've had a similar problem before that ended up as the points gap closing right up. The car would start but driving it was not easy.

Cheers,
Mike.

[Tinsmith_Skippy]

To chop and change parts is folly, surely testing items is more sensible than condemning them.
Buy a decent auto multi meter. Condensers and coils breakdown inside by humidity within them. Your coil maybe new 2 years ago but how long has it been on the shelf prior to purchasing.
Carburettors tend not to detune themselves by sitting still.

I have a little lock down story of a similar problem.

One of my experiences whilst driving to the national we're running fine until 20 miles from the venue and then running very much like your symptoms. So I'm on the field I bought a new condenser from Chris Witor (handy him being there) I fitted the condenser and reset the points and it still wasn't right. Whilst doing this the dizzy cap wasn't put on correctly so that got smashed up new rotor and cap still the same. By now a crowd has assemble and I'm being told you have bubbles in number 6 injector hose (if had black injector leads it would never have been mentioned) so now I'm changing bung seals in the metering unit by this time. I Now have 4 cars running Pi, they all leak back when not running. It Might not be correct but I have changed numerous no return valves and suffer the same problem they all have a go at leaking back randomly. So I effectively I bled no.6 injector at the national and the car still ran like Pony. So I drove home with it. I did some of the checks below with multi meter like in the you tube videos below all good. Then dad noticed the cheap points that are being sold the rivet that holds the contact in place was loose, enough to erratically close the gap causing the piss poor running.
New points fitted all good relief! At the national I managed to get round the field and see the cars but would have liked it to be more relaxed.

Not to feel sorry for myself I think I remember Charles telling me he had to change a clutch in the field at this national, now that's impressive, for me that would have been a wipeout and a relay ride home.

Some testing you tubes here to get back to the point. I'm sure I could have condensed this post but these are unusual times indeed.
https://www.youtube.com/watch?v=DRD94arK964&t=200s
https://youtu.be/FIx4dxew_UU