Still won't run properly, getting despondant now!

The engine starts straight away and drives ok around the yard but on the road it's down on power and sounds like it's running on 3 cylinders. If I remove the leads one at a time it's apparant that Cylinder 1 makes no difference at all whereas removing any of the others causes it to die.

The timing is correct
The leads are in the correct order
The rotor arm points to No.1 when it's firing (both valves closed)
There is a spark at the plug
I've fitted a brand new plug
I've changed the lead
I've changed the distributor cap
The valves are opening and closing properly and I've set the clearances
The compression is 95 psi

Any thoughts?

 

Why is the compression so low, what was the static compression ratio?

With a C25 cam the cranking compression should be around 135psi but it needs around 8.5:1 static CR. But that’s not the reason for a cylinder not to fire; if it’s getting a spark it has to be missing fuel or air. What does the plug look like in the dead cylinder, is it wet or dry?

I can’t remember what carbs you have, are they single or twin choke, or it is fuel injected?

 

Your compression is really low!

 

I think the @Fruitcake bus does have a progressive on it.

Is the compression a lot different on that one cylinder ?
I am sure it should be more like 130-150 but its that cam..

Mine idles on 3 cylinders from cold with a progressive but drives on 4 for the first 50 metres. Always has done, even three different engines with the same carb., stock cam. But once done with that, perfect.
Even Scott at SGS said its wierd.

With a non stock cam its probably got even worse meaning only 2 cylinders are doing any work until its screaming.. then you cant get it there because of a lack of power.

 

All cylinders or just that one?

 

I don't know the static CR I'm afraid. As Mike said it has the progressive carb, I'll run it at a higher RPM and then remove the lead and see what happens. I know the compressionos low but still in spec, according to my Bentley 85 is the low limit with 71 being unserviceable.

 

The others are all around 115, all 4 compressions have been the same since I bought it.

@mikedjames is spot on though, at high revs it runs on all 4 and removing the lead makes a big difference, I just need to drive it a lot harder than I used to. Now that I know what the issue is I'll fit the twin carbs and take it from there.

 

It'll always be lumpy with that one cylinder having such low compression compared to the others. You need to grind the valves, replace the rings and do something to up the compression ratio at some point, which might be as simple as removing gaskets from the head or spacers from under the barrels. Time consuming but at least it should be cheap.

 

The cam is a Scat C25, it’s mild and will make torque from just off idle, pull strong from 1500 revs and tail off above 4500 revs – it won’t scream and doesn’t need to. I ran it in my engine and it fired on all cylinders but I do have two twin choke carbs. Cranking compression was 135psi on all cylinders.

 

If it's not the lower compression and it was fine before running with one at 85, what's causing it not to fire on that one cylinder? Is it now too rich and flooding?

 

I dont think it has ever run properly. Much worse and more mysterious with an extra sealing ring, I seem to remember. It was the TE mystery machine.. Actually that time it would rev up make a hell of a racket and idle badly or not start.

 

Where was the ‘extra’ sealing ring?

 

I don't know if it was on the cylinder that's causing the problem, but there was a ring on #3 and there wasn't a ring on #4. I think that was right @Fruitcake ?

 

Did anyone check if the cylinders were at the same height?

 

The issue was a head/barrel spacer left under No.3 resulting in the head not sitting squarely across both barrells. The cylinders are all the same height.

 

i did that once. Slightly annoying isnt it!

 

Really? Eyes shut, fingers crossed engine assembly!

 

I think it's moved into the realms of being justifiable as therapy now

 

Well it was therapy until @Zed had another poke