VH 253 dual fuel running VERY poorly..ideas?

[Rzeractor]

I have an VH 253 V8 on dual fuel with a Holley 600 Vac secondary. Yes, I realise the carb is a bit big for the engine however this issue never used to happen so please dont tell me 'get a smaller carb' etc...
The carb is recent, got it November last year and is still pretty much clean..

On petrol the car accelerates fine, until about 3,000+ RPM where it runs VERY rough. Even if sitting at 60 km/h with a light throttle you can feel the car still shudder. Also if you put your foot flat you can sort of hear a rattle..

On gas, it runs VERY poorly now - Can't accelerate smoothly at all without it shuddering all over the place..putting foot flat results in the car backfiring..Also at 3,000rpm it becomes VERY rough.

Now I originally thought it was the spark, which I changed the spark plugs to no effect..
Could it be a timing issue? or the leads even? I am really stumped at to whats doing this as it never used to happen until recently..

Tried adjusting the A/F screws to no success..
Could it be the fuel floats? I've set the fuel to just dribble out of the sight holes when the plugs are removed & the car is rocked..

[Tree cutter]

The rattle you can hear when you put your foot flat, would be pinging caused from the ignition timing being too advanced or a lean fuel mixture. The backfiring on gas could also be from a lean mixture. It looks like you could have a vaccum leak so try checking all vaccum hoses to see if any have come off or are perished. It could also be a leaking manifold gasket or carby gasket.

[JD]

im thinking leads.

[Rzeractor]

The rattle you can hear when you put your foot flat, would be pinging caused from the ignition timing being too advanced or a lean fuel mixture. The backfiring on gas could also be from a lean mixture. It looks like you could have a vaccum leak so try checking all vaccum hoses to see if any have come off or are perished. It could also be a leaking manifold gasket or carby gasket.

The rattle is an odd sort of rattle, like I could be doing 20 km/h in 4th and put my foot flat and still be able to hear it..I usually thought pinging happens at high rpm??
I don't think it's cause of a lean mixture cause I adjusted the mixture a bit richer and it still did the same problem..

Don't think its the gaskets as they are new..I'll have to check the hoses.

Most people are saying its leads so I got some new ones, haven't put them on yet though.

Also it has new dizzy points and condensers & spark plugs too..

[Bilbs]

An engine will ping at idle revs if put under enough load, kinda sounds like you have a nut rattling round in ur aircleaner, but i tend to agree with tree cutter there on the vacuum leak, to test get some aerostart and spray around all vacuum lines, manifold, base of carbie while engine is idling, if it speeds up, there is a vac leak in the area that you are spraying, the vac leak will cause gas to run like shyte, but ok on pet at low revs and load, hope this helps