Keeping Temp UP in my OTG

Greeting Brethren and Sistren,

A quick question for the masters of meat. How do you keep the temps UP on your OTG or similar cooker. I was doing a brisket last weekend and trying to go hot and fast. The OTG seemed to settle in at 340 or so, and I felt that was good. I added fuel every hour or so, but after about 4 hours, even with the vents open all the way the temp wouldn't come above 250 and was heading down pretty fast.

Some details on the set up. I fill two Weber char-baskets with unlit KB and added a starter cube to each with the lid off. After the started cubes were gone and the coals were going, I put the lid on with the vents open. I wasn't paying attention and didn't have the lid all the way on. So, my temps got away from me. I closed the vents to a sliver and waited for it to come down. Once it settled in around 340, I put the brisket on. I checked the temps each hour and added a few briquets to each side.

Sorry this is so long, I need to learn to take pictures.

Thanks,
Chris

I was afraid I was getting ash build. I tried banging on the legs of the kettle with the plastic handle of my grill brush. I read somewhere that you can do that to refresh your fire without getting ash on the food.

Aside from lump charcoal, are there any good methods for preventing/clearing ash build up.