20 replies to this topic

[bob540]

Driving the car earlier today and once or twice it jerked when accelerating. Went out in it a few minutes ago and it's now jerking pretty violently when I accelerate. Seems to only happen when in gear and doesn't seem to happen in reverse. If I can get it up to speed it stops or at least jerks a lot less. Car is not driveable, any ideas?

[Cooperman]

Very difficult to diagnose without more details. Jerking can mean several things and could be the result of a variety of faults.

Sorry not to be more help.

[bob540]

What other details would help?

[peterjh]

is it carb or injection ? points or electronic ignition ?

[bob540]

It's a carb, HIF38 I think and it has points.

[Cooperman]

What model/engine is it? What type of ignition does it have?

When you say 'jerking' what are the actual symptoms? Does it mis-fire, run rough or simply cut-out completely then fire up again?

You need to check out all the electrical and ignition connections first before going any further. Check the points gap, if it has points and consider changing the condenser (if it has one) and the coil. Check the rotor arm.

Check the ignition switch as these can fail.

To run a classic car you need to be able to check the basics yourself. That is not meant as a criticism, but as an observation with regard to driving old cars as opposed to modern ones which just plug into a diagnostic computer.

The Haynes Manual gives all the basic settings

[Richie83]

Is it an SPi? Mine did this exact thing because one of the sensors was loose (green one on air box). Basically when moving forward it pulled it out a tiny bit sending it jumping; this was compounded by worn steady mounts. In reverse the engine wouldn't move much, so the sensor didn't pull itself out. I fixed mine by securing the sensor and then replacing the steady mounts. Doubt yours is the same problem, but the symptoms sounds similar to me.

[bob540]

Sorry I should've given all these details to begin with but I was just back in from the car and a bit pissed off. It's a '94 1275 saloon with carb and points. The jerking doesnt seem to be from the engine, it seems to be running fine, doesn't cough or anything, feels more like it's a slip in the drivetrain somewhere. Like as if it looses drive for a second and then grabs again and the whole car jerks forward.

Edited by bob540, 27 July 2015 - 09:05 PM.

[bob540]

I've been having a think about it and I could be completely wrong here but if the engine runs completely fine when in neutral does that not rule out electrical/ignition?
Also I noticed tonight that the rev counter was jumping up and down erratically when the car was jerking, last week I noticed the same thing a few times with the rev counter but there was no other symptoms, could this be related?

[Cooperman]

Just do a thorough check of the ignition system - coil, condenser, points, plugs, 12 volt feed wires, HT leads, rotor arm, ignition switch (key operated) and any other ignition circuit.

[Mini Manannán]

Like Peter says above, you really need to have the fuel and ignition systems in top order before you can come to any meaningful diagnosis. It's pointless trying to guess what it might be if the lifeblood systems aren't 'sorted'.

[bob540]

Grand so, I'll check everything you mentioned tomorrow and see what I can find. Thanks for the help lads :)

[Cooperman]

With old classic cars there are so many small things which an owner will need to be able to diagnose and fix without having to pay a professional organisation to do the work.
They are not like modern cars which just plug into a computer to get a fault-code and thus identify and cure the problem.
The OP's car has a clear ignition fault and he needs to work through the ignition system to identify & rectify it. The Haynes manual will be a help, but to own and work on classics does require a good basic knowledge of ignition, fuel and hydraulic systems, plus a good understanding of how things work. That is what classic car owning is all about really.

[bob540]

With old classic cars there are so many small things which an owner will need to be able to diagnose and fix without having to pay a professional organisation to do the work.
They are not like modern cars which just plug into a computer to get a fault-code and thus identify and cure the problem.
The OP's car has a clear ignition fault and he needs to work through the ignition system to identify & rectify it. The Haynes manual will be a help, but to own and work on classics does require a good basic knowledge of ignition, fuel and hydraulic systems, plus a good understanding of how things work. That is what classic car owning is all about really.

That's at least part of the reason I bought a mini, I want to build up a good knowledge and be able to work on it myself and not have to pay a garage. I'm still very much at the novice stage but every time something goes wrong I get to learn something new, just hadn't had any ignition problems before now so I didn't have to learn. Gonna spend every free minute in work tomorrow reading the ignition chapter in the Haynes and hopefully by tomorrow evening I'll be ready to give the ignition system a thorough check.

[cal844]

I've been having a think about it and I could be completely wrong here but if the engine runs completely fine when in neutral does that not rule out electrical/ignition?
Also I noticed tonight that the rev counter was jumping up and down erratically when the car was jerking, last week I noticed the same thing a few times with the rev counter but there was no other symptoms, could this be related?

Coil LT( small wire) circuit failure, replace the coil

EDIT: dont buy a lucas branded coil!

Edited by cal844, 28 July 2015 - 09:39 AM.