Morris 8 Misfire

[SanRod]

Hi as a newbie please forgive  me for the following rant about our misfire but I love a challenge and hate having to give in to a mechanic !! 

Our 1936 Morris 8 has a serious misfire. When we purchased it a few months ago and took for a test drive all was fine but soon after we got it home it suddenly lost a lot of power (and there wasn't a lot to start with!!) during a wee outing and barely made it back to base.

We were told by the seller and noted at purchase that it had new spark plugs, coil and coil to distributor HT suppression lead with a "spitfire multi spark"  some sort of inline voltage booster in this lead(see image) Think there might have been a clue in all that but we missed it. 

Investigation of the misfire revealed that it was not firing on 3 & 4. After a bit of trial and error found that by swapping out plugs 3 & 4 for a couple of spares the problem disappeared. Armed with that knowledge we elected to try a  complete new set of plugs figuring that perhaps the relatively new ones that came with the car were faulty. I had nagging doubts that this was the problem but it seemed a relatively cheap fix.

The original NGK B6HS were replaced with Champion L82Cs and off we went runnin pretty good. A couple of short runs around local roads went fine but then Lo and behold suddenly the misfire was back and investigation revealed number 4 plug  misfiring. Refitted an original NGKs to 4 and it came right again but soon after the problem was back. While # 4 was misfiring I tried moving the plug to other cylinders and the misfire moved with the plug which made me think it's not the distributor cap leads or rotor or mechanical or fuel related ( compression is fine) 

Replaced the condenser ...  no change. 

Swapped out coil HT lead and booster for an old school wire one ... No change. 

Plug and contact breaker gaps are set to hand book specifications 

Distributor to plug HT leads are wire type and appear sound 

The car at some point in the past has been converted from 6 to 12 Volts and I note from research on the interweb that the NGK B6HS plugs are correct for the model/year which would have  been 6 volts. Is some one able to tell me if the 12volt coil/condenser combination plus multi spark thingy is simply too much for plugs perhaps designed for a 6 volt system and somehow frying them prematurely?

I'm reluctant to purchase different plugs of any sort  without knowing  if this is the problem and how to address it. 

Look forward to some feed back cheers Rod.

 

 

 

[piazzanoob]

After running the engine with the new spark plugs and there is no miss.

Then removing the plugs when the miss appears what condition are all the plugs in? particularly no 4   

This also might help identify what the plugs are doing.

[SanRod]

Plugs are new and after only a few Kms look only slightly blackened an wet with unburnt fuel cos they're not firing 

[tortron]

Throw out the spark booster.

I have bh6s plugs in my sidevalve minor as standard so 12v is fine.

Inspect the distributor cap for cracks. The distributor in these is right between the exhaust ports and gets pretty hot.

I imagine it still has copper leads, potentially they are cracked too, or the insulation is passed its best.

It's hard to get the screw in style distributor cap, but you can get reproduction ones that take the more modern press in leads, so you can run modern/new leads

 

I also had an issue where the distributor body had come loose on its shaft so it could rock back and forth (but no play in shaft) it only became apparent when I had the head shaved and started getting wierd missfires

[azzurro]

I suspect that multispark thing provides multi choice variation of between 0 and 1 sparks, and after sufficient number of 0 sparks the plug gets is too wet to fire.

Bin that silly nonsense  and put another coil lead on.

also test the spark with the plugs out and engine cranking - should be a nice fat blue/white spark - a yellow spark is weak.

I wouldnt worry too much about the specific type of sparkplug, its a fairly low compression engine, so if they fit in the hole they should fire if given the juice - i suspect the issue lies in them not getting sufficient voltage rather than the plug itself.

It may be

- that the coil isnt getting enough voltage to push enough out the hot end- check that against the voltage the modern coil thinks it should be getting (beware the difference in required voltage between ballast resistor coils and non-ballast)

- hows the inside and outside of the distributor cap and rotor looking?

- spark leads good including good connections in the cap and to the plug? 

[Kiwibirdman]

What is the general condition of the engine, I would do a compression test on all cylinders. Less compression will make the fuel air mix harder to fire. 

[tortron]

It's a 1930s flathead. It has approx 6:1 compression ratio. If there's something remotely combustible in it, it will run.

 

Have had one of these engines for over 10 years. My bets are 1. The leads are passed it (brittle plastic over copper core) and 2. The bakelite distributor cap is passed it.

 

Why 3 & 4 cylinder goes? It's a thermosyohon system. #4 runs pretty hot so the plugs and leads are hearing up before the rest

[SanRod]

Thanks for the comments everyone, good feed back. I have been working on a process of elimination in order of cost and based on your comments I will upgrade the leads as is being suggested and go from there. They are old and would look better even if it doesn't run haha.  I will post again after that and advise how it went. Dizy cap could be next after that if no success with leads

[SanRod]

Hi again,

replaced plug and coil leads and all connectors.. Multi spark is gone. Discovered coil to distributor low tension wire nearly broken, only a few strands left holding at the distributor end so has been replaced. Spark at the plugs while cranking as suggested above is yellow white but not sure it is of the "fat" variety were looking for. Also discovered the bolt holding the distributor clamp bracket to the cylinder head was loose and tightened up, clamp bolt is secure. Still no joy and can't even get it to run

How does one determine that the coil is getting the voltage it requires as mentioned above ? Bit of a grey area for me I've never been able to grasp the intracacies of electrical goings on very well just not my forte!! 

The distributor cap is an old original type with side exit leads and I have suspected this from the beginning but it's one of the more expensive options to replace. Earlier when I could get it to run I observed it in the dark and could see no signs of tracking on the outside at least..

As a side check there is fuel to the carburettor and it was running ok previously. The  plugs appear to be slightly wet when removed so am assuming fuel is getting thru to the cylinders. 

In respect to Tortrons comment above re the distributor body being loose on the shaft I'm not sure what to look for there, do I need to remove distributor to check that out ?

Frustrating !!!!

[ajg193]

I wasn't paying enough attention earlier in this thread. How are your points? They might be a bit burnt so not making/breaking the connection properly. You can clean them up with a bit of fine sand paper. And you've set your ignition timing correctly after all of the changes?

[tortron]

Yeah set the points. My distributor could be clamped down tight and then you could move the body a few mm but the shaft would remain in place. It tore through the centre carbon in the cap pretty quick.

You should get a good cracking spark (despite it being Lucas electrics). Does your engine have an earth wire? 

Is the coil actually new? Or is it a swap meet special with a date code of 1963 on it?

 

I don't think you will need to replace the distributor, but you can put in a later blmc one if you want. A 25d or 45d will fit and work pretty well. Just need to plumb in the vac line.