Misfire - please help!

[tyasman]

I have an occasional misfire/cut out on my 1970 S4. Took her out yesterday and the engine was cutting out/missing very badly for 3 miles. I stopped the car and after re-starting it was much better, but still occasionally present when accelerating. Have had the car on a proper engine diagnostic kit, with a skilled engine tuner. We changed the fuel pump, coil and HT lead. The carbs are in tune. At tickover the car runs perfectly, but when I drove home the misfire was still occasionally present under load.A fuse has blown (found to be 8 amp) and replaced with 35 amp. Everything functioning again, but the rev counter is bouncing all over the place, which it didn't before the fuse blew. Could the misfire and rev counter issues be linked?
Also, the tuner thought the fuel pressure at 1.5/2.0 psi was a bit low, but the new pump gives the same reading- does anyone know what this figure should be? Thanks for any answers
Simon

[neilsjuke]

I would hot wire the coil so removing the cars ignition Low voltage side ( wire from the solenoid to the coil )remove the feed to coil . Have you changed the capacitor in the distributor as if it is going faulty the spikes may make the revcounter give faulty readings but this should have been found when on the tuners scope.

[martinbrowning]

Tyasman,

I have almost exactly the same problem with my S130 running on dellorto's. The symptons as you described gradually got worse over 6-9 months; eventually the car would start and idle but cut out after a few miles. Once it was left for about an hour it would start again which leads me to believe that it is heat related. In addition to your replacement parts I have replaced the carbs with new, new fuel tank, new distributor. Still can't solve the problem and now have the head off to check valves/head gasket.
If anybody can solve this problem we would both be very happy/impressed!

regards

Martin B

[richardcox_lotus]

Do you have any sort of electronic ignition (Aldon, etc) in place of contact points ? Failure in these can affect the operation of the rev counter - & the engine (!)

[andrewdmoore]

Take off the Rotor Arm and check inside for discolouration. I had a barely noticable crack in mine which caused a short and the symptoms you describe. I suppose you may aswell replace it whilst its off.

Regards, Andrew

[elansprint]

I am with andrew inspect the rotor arm very carefully for traces of carbon where it flashes over also check inside where the arm sits on to the shaft i have recently had one flashing over to the distributor shaft through a pinhole in the new rotor arm
Ian

[ianf]

I'm with the dodgy rotor arm as well, but if you have the steering column lock the back (with wires attached) can come loose from the body and give intermittent contacts for the ignition. - Took me a few months to locate the problem which I solved by returning to an ignition switch in the dash.

Ian

[cliveyboy]

My Sprint used to have the same symptoms.
Run for a bit then missfire especially under acceleration.
Everything is new:- engine, ignition, rebuilt carbs etc
The plugs tended to foul up so I reduced the problem a great deal by changing to hotter plugs NGK BP6ES and also not venting the engine breather into the airbox.
The plugs still foul a bit now and again which then makes the rev counter bounce. (I have Lumenition optronic ignition).
The current plug fouling is most likely down to the fact that on my Dellortos the main jet of 120 size is still abit too rich. I initially increased the air jet size which helped a lot.
I cant speak about Webers but the Dellorto jet values in the Lotus manual are very suspect.
Clive

[hatman]

Has your car still got the anti-theft switch in the glove compartment? If so, suspect that - in fact, better, get rid of it.

[seaandmoor]

Hi yes I think you have to suspect all of the items raised by the other posts as you may have one or a combination of them.
My tacho was a bit bouncy with a similar problem of misfire and the lumenition suddenly gave up. Fitted new lumenition and sent tacho away to be checked over and set up for use with lumenition as bouncing is a known problem. Both fixed and tacho VERY stable now but still misfire. This could be on start up after which it would cough into life and you think all will be well but like you this tended to onset when hot but ok again after cool down which suggests increased resistance somewhere and/or spark going somewhere it shouldn't. You might try looking under the bonnet with the engine running in the pitch dark just incase something untoward can bee seen.
I have checked that the compression is what it should be on all cylinders to avoid pressure in the crankcase, removed the crankcase breather pipe from the back of the airbox and changed my plugs for one that runs a little hotter (someone has used denson iridium IW20/22)
All of this has much reduced the tendency to misfire under load when hot but occasionlly there is still just a hint of it.
One final thing I haven't yet tried - my original dellortos have been fully overhauled and set up on an analyser/rolling road and the fuel enters in a downward inclination... the more recent dellortos I've discovered fuel in a more horizontal inclination and a post on another forum reported an improvement for this.
Keep going and let us all know how you get and what you find to be your problem as it could be very helpful to the rest of us!
Best regards.

[worzel]

Hi

You don't by any chance have an immobiliser/alarm fitted do you?. I only mention this because some time back the Excel I have would cut out/misfire after about 20 minutes running. Didn't matter how far I travelled it would still give problems after 20 minutes or so.

Finally traced it to the immobiliser which had been wired such that the immobilised circuit was continually being fed current (it was the ignition circuit). Switching to the starting circuit cured the problem.

Otherwise- apart from the already posted suggestions Lumenition doesn't seem capable of firing a gap greater than 25 thou- (at least on my car a sprint)- I originally set them to around 30 thou- it would idle but would misfire under load.

Hope this (might) help.

Regards

John

[oldokie]

I keep a spare cap with solid core wires for trouble shooting misfires.

[del@ace]

your misfire is probably the rotor arm , revconter bouncing about classic sign everything you discribed points to rotor arm cheers del