I'm ssuming your 706 is a gasser. Is so and if the engine otherwise runs fine, a bit of flame out of the exhaust under heavy load is normal. It's there all the time but may only be noticeable in low light.
FWIW our Oliver 1850 would shoot a yellow-tinged blue flame a good foot or more out the muffler when plowing. The muffler would become cherry red; the manifold a reddish-orange. The tractor plowed many thousands of acres like this.