If you can't find something obvious. I would try to find someone who has a set of scales. JMO

Its abird
 

hey its a bird ur reasoning seem to fit best so i added more tension to the rt rear spring added a pound heavier battery and went to the track the truck puulls 90% less than it did before , as luck would have I left my spanner wrench home so i couldn't make anymore adjusment .....yep i'm definitly getting close. I will be back at the track sat and give you more input. thx bird