jerky ride

[richard1705]

hi just come back from a run from work slow moving traffic 20 30 the car is jerking at this speed. it pulls really well with no hesitation otherwise and no missing etc its just at this speed
any ideas would be appreciated could it the leads need changing
richard

[richardthestag]

does it feel like the back end of the car is jiggling up and down?

[richard1705]

hi no not that I noticed to be honest just seems engine missing slightly

[richardthestag]

ok so definitely engine and not drive.

Is yours a mk1 or mk2 car, is their a white block with two wires that sits near the coil or is there a pink/slate coloured wire that goes to the coil +ive

Easy bits first, thin wires to the top of the ballast resistor and/or coil, are they all tight and in good condition, any fraying or loose connections are easy to fix and would cause this problem.

what sort of rotor and distributor cap do you have fitted? there is a world of cheapo rubbish out there, distributordoctor provides me with all of mine for various vehicles now. Rotors with rivets right through them are known to cause misfire.

With the cap fitted but not clipped down is it a snug fit or does it wobble about? when the engine is running can you see the dissy cap wobbling about?

HT leads next. start up the car, bonnet open when it is dark, can you see sparks and/or glow around HT leads? yes = replace them with quality silicon versions. no = HT leads probably ok

Check also that each HT lead is properly fitted to plug and cap. A squirt of wd40 allows the boot to slide up the ht lead, you need to feel the end physically click into the dissy cap before sliding the boot back over the top, if it doesnt you can widen the contact a little with a flat blade screwdriver.

Sparkplugs, what are they, should have maker and part number on the white insulated part. Not unheard of for a sparkplug to be faulty and leak the HT direct to the body of the plug and earth rather than via the electrode and contact.

I have a strobe that clips over the HT lead, looking at the strobe while the engine is running will quickly identify an erratic or non existent spark



On my mk2 engine I had a chronic misfire that was caused by the ballast resistor wire breaking down. only happened under load and once warmed up. sit idling in the drive for 20 mins but after 4 mins on the road it was undriveable.

I diagnosed it by running a clean 12v from the battery direct to the coil +ive and the fault went away. beware though that running a 12v supply constantly to a ballast supply coil will cause it to overheat and also fry the contacts in the distributor cap.

If the above fixes the issue you will need to find a way to bypass the ballast wire or replace the physical ballast resistor. I ran a clean wire from ignition control (white wire) direct to a 12v coil and did away with the ballast supply. ask if you need details.

[richard1705]

cheers rich. good advice and plenty to get on with. after new year
thanks
happy new year

[DJT]

I would remove the air filter and inlet elbows, then check to see if one of the carb piston valves is sticking. If one is not dropping down smoothly it can cause the symptoms you describe.

[garyf]

I would remove the air filter and inlet elbows, then check to see if one of the carb piston valves is sticking. If one is not dropping down smoothly it can cause the symptoms you describe.

Come to think of it,Ihave had this and it was dashpot oil leaking past o ring in carb and making the piston gum up and stick