Firing on three cylinders

[grandpascot]

I am new to the Morris Minor having only recently bought Doris although I have had an Austin Seven for 10 years.
I have fitted electronic ignition and a new coil, replaced HT leads and replaced the vacuum pipe with a rubber one. Engine fires up on three cylinders only. I was at my wits end this morning then, for some unknown reason I removed the Vacuum pipe from the distributer end and the car now runs fine
My question is why?

Mike

[bmcecosse]

I doubt then it was running on 3 - just running poorly due to the timing not being correct. Presumably the vacuum is pulling the timing too far advanced - try retarding slightly till you get a nice smooth idle - then road test to make sure there is no pinking.

[grandpascot]

I have done as suggested but to no avail. I get a smooth idle with no vacuum pipe connected but as soon as I connect it the engine goes rough.

I have checked the vacuum and there may be a problem there. How much movement would one expect of the base plate as I could only make it move 2 to 3 mm. If indeed this is wrong will I do any harm to remove the cover of the diaphram to see if its leaking or faulty in any way?

[bmcecosse]

You just suck the pipe and watch to see if the dizzy baseplate moves. Also - take the pipe off at the carb and put your finger over the port on the carb where you removed the pipe. How does it run now??

[grandpascot]

There is no difference when I put my finger over the port on the carb either in idle or in revving the engine but as soon as I replace the pipe the engine gets very unhappy!

I ought to mention that there is a leak in the exhaust pipe at the join with the manifold and I wonder if this is having any effect (The car is going into my local garage next monday to have the exhaust refitted.) although I have to agree that it all points to the timing but I have not had any luck here.

I will run a compression test as soon as I can to see if anything shows up there but when on idel the engine sounds as sweet as anything!

[bmcecosse]

So - it's not the little air leak that's causing the poor running - it must then be the vacuum pipe pulling the timing too far advanced - OR - perhaps when it moves the backplate - it disturbs the little 'low volts' wire that snakes across the dizzy internally. Maybe worth a look at that.

[grandpascot]

No joy at all. Car goes into garage and I'll ask them to sort it. I'll let you know what they find. Thanks for your help.

[bmcecosse]

They won't have a clue - unless they are specialist A series people ? Better to try another dizzy.

[SteveClem]

I hesitate to suggest anything in the light of expert knowledge but a while back I had a similar issue and it just turned out to the be a dodgy lead connection into the distributor. Sent me mad at the time.

[grandpascot]

I wondered that so I replaced the leads. Nothing no change. I get the best result with vacuum tube disconnected. As I said earlier it seems like one cylinder is not firing!?

[bmcecosse]

Well - have you pulled off the leads in turn to determine which cylinder that is ? I just cannot see any connection between the vac tube and rough running - other than basic wrong timing or the little low volts wire being affected by the dizzy baseplate moving when you unplug the vac. And certainly no direct reason why it would (could!) affect just one cylinder. A dizzy swap is the easy answer to check all these possibilities.

[grandpascot]

It's a new replacement dizzy because I thought it might be the old one. Could it possibly be something at the carb end?

[bmcecosse]

But you said it made no difference if you blocked the hole when you removed the vac tube ?

[grandpascot]

I can't do anything now until Tuesday but if the garage can't resolve the problem I'll take some sound and put it up for you to hear. The garage is a good old fashioned garage with skills with tools and knowledge rather than computers. The owner has had various classic cars so I'm putting my trust in him.

[grandpascot]

You are right. Silly question sorry.

[bmcecosse]

Well - it's either upsetting the timing - or the mixture , by letting air in. We've covered both possibilities with the questions.... Good luck with it!

[grandpascot]

I'm planning on bring it down to the Trossachs for Drive it Day so it has to be resolved before then.

[grandpascot]

It turns out that the distributer was faulty! The company say " it sounds like the module is too early and when the vacuum is attached it fires to the cylinder before."

Received a replacement distributer next day and the engine is fine.

Thanks for your help.

[bmcecosse]

Well - that's a new one - and I hadn't picked up that all this happened AFTER fitting a NEW dizzy. Still - as long as it's fixed now - happy days!

[The vast minority]

So more "new" parts fit only for the junk pile then!

I advise you to get a good, known working original dizzy and keep it in the car with tools to change it.

Al