Engine backfire

[StagNL]

My engine rebuild has come to an end so now it is time to get it running properly.
After fitting it back in the car it started up perfectly first time but did run a little rough (tractor sounding), otherwise smooth as I could have hoped for.  My initial thought was the fuel delivery (carbs) needed a fettle seeing last year I had a similarly rough engine after a similar time standing still.  
Next day I try to address some other things related to the engine such as the cooling system but did start it up to move inside to work on it.  Still a tad rough.
Again today I started it up let it run for a while to warm up a little, leaned the mixture a tad and drove it inside to bleed the brakes (see other thread).  After this I was wanting to address the carbs by checking the fuel mixture etc.   To do this the engine ideally needs to be running, so outside it goes again in order not to gas myself and collegues.  Only thing is that the engine is now backfiring (loud popping in the exhaust) a fair bit.   I understand this isn’t a good thing but I could not get it to stop.
I did attach a vacuum meter which showed a healthy 18-19” at 1000+ rpm but at idle it was near 1-2”.  Not good and the chart I have says blocked exhaust or leaky manifold.
As far as I can tell the exhaust is clear and the manifold is OK.  
As said, before it wasn’t backfiring, but after bleeding the brakes half an hour later it is.

If anyone has a tip on where to look I’m all ears.  I was intending on taking carbs off again and giving them another once-over to check they are indeed OK but the cold (-7 degrees C) isn’t a fun temperature to work in so I put that off for Thursday.

Julian

[DJT]

Check your plug leads are in the correct sequence. Easy to get one or two mixed up and the engine will still fire.
Have you timed it against #2 plug? The natural thing to do is use #1, but people forget that the engine is essentially a 'double-dolomite' engine and that #2 on the V8 is the equivalent to #1 on the 4-cylinder.
Have you checked the carb diaphragms? A bad split could have the same effect that you describe.

[neilnaz]

Does your Stag have a brake servo? If so is there any possibility that the diaphragm is leaking and you are losing vacuum and getting brake fluid into the inlet manifold. You might find that this may answer the issues with the engine running rough and also your spongey brakes.

This may be way off the mark and I am not knowledgeable about Stags.

Cheers, Neil

[stag_ste]

What are you using to check that mixture is correct?

[StagNL]

taylormoran wrote:

Check your plug leads are in the correct sequence. Easy to get one or two mixed up and the engine will still fire.
Have you timed it against #2 plug? The natural thing to do is use #1, but people forget that the engine is essentially a 'double-dolomite' engine and that #2 on the V8 is the equivalent to #1 on the 4-cylinder.
Have you checked the carb diaphragms? A bad split could have the same effect that you describe.

All OK as far as I could tell - dark and hand held light doesn't help.  The carb diaphragms I had not yet checked but I don't think they are stuffed.  I'll be checking the whole carb unit Thursday.

neilnaz wrote:

Does your Stag have a brake servo?

They all do.  Maybe....

stag_ste wrote:

What are you using to check that mixture is correct?

Well, the only decent thing I have at hand is a Colourtune plug.  It has done well for me in the past.   Mixture was on the lean side but turning needles rich had little effect.
The whole carb pedestal unit was removed about 3 months ago and put back on just this weekend.  Nothing done to it in that time as it was faultless prior to engine rebuild.
Backfire is also mainly when at idle.  If the rpm is taken up past 1000 the backfires are almost gone but not completely.
Choke has no noticeable effect on the matter.

[StagNL]

Right, been at it tonight with some headway but still not running right:

I inspected the carbs and found the diaphragms were OK.  I reset the needle height as I had been turning a fair bit on them the other day.  

Took off the brake servo connection and plugged the hole in manifold.

Did a compression test as I had the gauge sitting nearby and had the plugs out anyway.  All very consistant at a smidge over 125lbs/in2.

Put the HT leads back on taking care to ensure good connections.  

Checked all pipework for leaks and even plugged holes to make sure there were none missed.

Result is no backfire anymore so I guess something wasn't right in the HT leads despite checking.  
The engine still doesn't perform like it should when under load, i.e. driving.  Very much tractor-ish and lower than normal power, but not very low.

When stationary the engine revs a lot better but still a tad rough.  Mixture is good enough to allow rapid rpm increases without bogging down.

Stuck on the vacuum gauge which showed a healthy 20" at 2000rpm.  However below that is just drops off to zero - idle is zero and the vacuum slowly increases to 20" with increasing throttle opening.  Past 2000rpm it remains at 20".

The only thing I could find that this type of reading would indicate is an induction leak.  Thing is, I haven't found one yet.  I have managed to get an ultrasonic leak tester to try on Saturday, so I may still find something.  

The other thing that disturbs me is a worrying vibration from the clutch/gearbox area.   When sitting in the car and revving the engine it is audiably very prominent.  Depressing the clutch reveals a vibration through the pedal that is at its worst at full travel.

Just to clarify: Engine was put together carefully last week and the gearbox I had already rebuilt just prior to the engine rebuild.  The complete crank, conrod, piston, front pulley, flywheel and clutch assembly have been balanced.  The clutch needed material removing from one side and added to the other in order to be in balance.

Is there a possible problem with the clutch/gearbox that causes this vibration?
What in the world is going on with the engine and the zero vacuum reading?

Julian