Cooking lamb roast on Weber kettle questions

I plan on cooking a boneless 4lb lamb leg/roast from Costco tomorrow. I'm wondering how long it's going to take...so I can time it right to be done for supper.

I will be using a HeaterMeter, so I can cook it nice and slow, if that's the way it's suggested to cook it. I was thinking in the 200-225 range. Cooking to an internal temp of 135.

The instructions on the package say
for the grill: meat side down. Indirect heat. Turn as required. cook to 145 internal temp. about 11 minutes per pound.
or
bake/roast in oven at 400 degrees until 145 internal temp. about 20 minutes per pound.

I'm guessing the instructions must assume over 400 degrees for the grill then.

Any suggestions? Should I plan on 40min per pound? Something else?

Sorry I didn't see this sooner!
I would not roast it for more than an hour, TOPS!! I'd maybe go 45 min, indirect, I like it more medium rare.
Forty minutes per will give you fuel for another cook, IMHO.
I love lamb, would hate to see you lose a nice piece.
I generally remove most of the fat cap, it can be too much for some people.

I'm late to this one too, but I'd shoot for 125, not 135. Lightly tent it and rest for 30 minutes. I'll come up another 5 or 10 degrees on it's own by the time you cut it.

And just to pile on the late train... If you have a rotisserie, I've found that to be the best way to grill lamb. I go indirect, on the rotisserie, with a herb/ garlic/ red wine marinade that I will then also mop on the meat while cooking.
Think of lamb more like steak rather than bbq. It will need the higher temp for proper cooking.
145 is the FDA requirement. I like to pull the meat off the heat at 125-130, tent with foil and rest for about 1/2 hour.

It didn't turn out very well.

RKHanso said:

It didn't turn out very well.

Click to expand...

What happened?

It cooked for well over 4 hours. I did eat some but it wasn't very pleasant.
I'm not sure how to really explain the taste.

Burned sand?

I'm with Andrew, treat a leg of lamb more like a piece of steak, that makes good sense. Four hours was about three too long for my taste but, that's me. "Eleven minutes per pound/four pounds" sorry, sometimes the package recipe is a real good place to start.
I have a feeling the "indescribable" bad taste was from not trimming the fatcap, it can make the flavor very...."unpleasant" to many.
I did a boned out leg in the depths of winter when a lamb loving friend came to visit, my wife isn't a big fan, it was glorious! It started out as an eight pound bone in leg.
Boned, lovingly treated with some Rosemary, red wine, balsamic vinegar and garlic. Overnight, full chimney, indirect, fifty minutes, 20 min. rest, sliced and enjoyed with three beautiful blondes (out of town guest, another friend and my wife) and a few bottles of wonderful reds, my basic rice and green beans. The cook for lamb is so often more complicated than it needs to be, I see recipes all the time which are so involved that you lose the character of the lamb. For my taste, don't try too hard, it's where most first try lamb fiascos happen.
Do not be discouraged!
I often do a recipe called "Scotta Dita" with lamb rib chops which is pretty amazing! When you find them for a good price, stock up and make twice as many as you think you might need, they will NOT go begging! I use the Union Square Cafe recipe. They are generous enough to give exactly what they do at the restaurant in their cookbook, thanks Danny Meyer!
Who told you to go low and slow for that small a piece of lamb?