Hi, I put my 2.3l petrol LPG transit Rear wheel drive in for a new clutch and dmf as it was shaking vilontly when the engine is under load. I thought that the dmf was to blame as there have been a few folk on here with the same symptoms as I had. Got the van back from the garage and it still vibrates in the same way. When accelerating it vibrates badly, dash, steering wheel, gearnob and crusing only a little like a wheel needs balanced but when I put my foot down on the clutch the ride is smooth and doesn't vibrate, same in neutral and slowing down.
When I got it back the mechanic said it sounds like it missind slightly on one cylinder and thought it might be a burnt valve. My questions are,
Would a burnt valve cause all this massive vibration?

Would the helicoil holding cylinder 3 spark plug have anything to do with it?

Could the gearbox be the problem as the gearnob vibrates like hell?

I started it up from cold and put my hand over exhuast and there was no suction.