I had the same problem too in a way. My left bicep was smaller than my right. Then, I started concentrating more on my left bicep and incorporated more dumbells and now they are just about dead equal. So what I advise you to do is concentrate more on your form with your left arm. Actually feel each rep and feel your muscle contract and expand. Use dumbells for your arms will each use equal force when lifting. Say you were doing lying tricep extensions, your stronger arm tends to take over more than your weaker one, which would be your right arm in your case. With dumbells though, each muscle uses the same amount of force as the other one. So stick with dumbells mostly and concentrate more. Good luck!
