Jerky pick up from idle

[WokingWedger]

I know its been discussed before ( I think without conclusion).

My Chim has developed a jerking/ snatching action at just above idle. You know when you are just trickling along in a queue at traffic lights etc. It does it in any gear.

I have noticed that the distributor cap is not a good fit. You can rotate it maybe a couple of degrees or more with it clamped down. Is this normal ? Will it affect timing? Could timing be my problem ?

Any suggestions ?

[thegman]

TADTS.

probably a mixture of cam profile and ecu. you may see an improvement if you upgrade either. I swapped my cam for a MC1 and it virtually removed the problem entirely. i have heard people say mark adams can work similar magic without the need to change the cam

[WokingWedger]

It hasnt always done it though !

Its just started

[blitzracing]

Its not reached a conclusion because its caused by multiple reasons. Basically a performance cam does not draw as much air as the Range Rover one at low RPM. This means the airflow meter output is a bit too low, so the ECU puts in less fuel. I dont think this is helped by the way the ECU cycles the mixture for catalyst use either. The engine then runs a bit lean, and to make matters worse if the ignition is not 100% it cant properly ignite the fuel that is there. Net result poor combustion, and the car starts to snatch. This gets worse as slack develops in the transmition, and the whole lot gets worse. This is way you will get multiple "it worked for me answers", as anything that improves combustion or puts in more fuel will help.

A rechip to add a bit more fuel at low RPM.
A change of cam may help increase the airflow at lower RPM.
Improved ignition. Leads, coil amp etc.
An aftermarket ECU for improved fuel delivery.

Mark

[thegman]

Mark when are we going to be able to buy an UBER AFM off you that fixes all these issues?!

[blitzracing]

Lifting the AFM output is only partly succesfull so far. It does improve things but not 100% fix. Current project is to try and make an enlarged AFM provide the same voltage outputs as a standard, with the ability to tweek the output within limits to alter the fueling. I have had a prototype running on my car, (6mm bigger) but not managed to get it to fuel evenly across the whole range as yet. It worked perfectly on the bench, strapped to a 2kw garden vac' but not on the car, as I guess the aiflow is more pulsed. Ill keep tweeking.

Mark