Plugs, injectors and misfire

[jemgee]

At the risk of yet another thread on the old proverbial I am about to start on tracking down the elusive misfire.

 

Symptom - Warm engine - rythmic misfire at tick over and continues upto about 2200 revs. Above that it is impossible to tell whether the misfire is there. Occurs under load and just when steadily increasing revs. Above 2000 rpm engine pulls strongly but with a very infrequent hiccup. Running on V Max petrol for last 5 years.

 

Checks so far

All injectors pulled and seem to be similar discharge pattern. However pulse on pipes (not braided) strong except for distinctly feeble on no 5.

 

Plugs 1-4 look a shade rich but 5 and 6 look more sooty. I realise however that I am running BP5ES in 1- 4 and BP6ES in 5 and 6.. I think I meant to run the hotter plugs in 5 and 6 but got it back to front!

Have just ordered 6 new BP5ES from ASC on ebay - £8.66 delivered which seems a very good price

 

The MU and injectors were replaced by Malcolm about 3 years ago and the ignition leads, distributor cover and Lucas coil about the same time. I will next change the condenser and points and also clean the large single K&N air filter element. The rotor arm is a red one bought from the TSSC at Le Mans a couple of years ago.

 

Apart from checking the static timing have I missed any likely culprits? I am assuming that potentially more serious isssues eg low compression on one cylinder is unlikely

Edited May 7, 2013 by jemgee

[Peter Cobbold]

Identifying the misfiring cylinder at tick over is where I'd start.

[tjs]

Are you having troubles of excessive fuel consumption? If you are i would suggest checking the MU diaphragm assembly holds depression. I doubt this is the culprit as likelihood is this would effect cold running too.

 

Do a quick compression test, best not assume things at this stage or you could be wandering down the wrong path for a while. Even if this is not the culprit its good peace of mind. Even better and if you have the kit, a leak down pressure test although for this you will have to remove the timing gear.

 

You mention weak pulse on no.5 hose, I would suggest checking the opening pressure of this injector. If you are running Bosch type fuel gear then you can set the opening pressure a bit higher, say 65psi. It's really easy to do and you don't need any fancy gear, just a good stand up bike pump with a gauge, some test fluid (petrol or paraffin).

 

Immediately from cold start, are any plugs wet with fuel? This is another easy way to rule out a dribbling injector.

[John L]

Make sure your not using 95 petrol! It gives the same symptoms.

John

[ianfraser]

I have a very similar sounding misfire, worse under load but rest of the time engine sounds pretty lumpy and plugs in 5 and 6 very sooty. Could this be a timing problem?

 

Ian

[Denis]

I'm just in the middle of changing the two Banjo Bolts on the MU of No2 and No5 for the very same reason...what I found was the 'O' Rings had perished and the inside of the Banjo's were littered with little bits and pieces of black rubber...which obviously travel down to clogg up the Injectors!

[Peter Cobbold]

I have a very similar sounding misfire, worse under load but rest of the time engine sounds pretty lumpy and plugs in 5 and 6 very sooty. Could this be a timing problem?

 

Ian

I found PI timing by the book is not so important as to make 5,6 run rich- more likely the rear butterflies aren't opening properly .