Can anyone tell me if it would be ok to use an 8.4 batt with the traxxas EZ start,or would I burn up the electric motor?

Motors are usually just stock 540s.Shouldnt hurt.I wouldnt run too long though.Maybe a few starts.

Out of curiosity, why?
6 cell not enough starting power?

You might blow the glow plug... but it should be fine.

The ez-start motor is a 380 isn't it?

I think I read somewhere that the motor is a 370 size brushed motor.

Good luck getting an 8.4v battery to fit in the wand though. They're only made to accept a 6 cell NIMH, unless you have the Old style EZ start in which the button case just zip ties to the battery. Either way I'd just take that EZ start off and throw it in the garbage and switch to a pull start. Much easier than having to wait an hour for a NIMH to charge. All you need is a glow ignitor and you're good to go.

this is an instruction set that modifies the ez-start hand wand to use the battery in your street truck to start your r.c. truck. it will put 12 volts to your starter on your r.c. truck. I can not be sure that it will not harm your ez-start system. however I have been using this mod for 9-10 mo. with no trouble. my r.c. starter has never overheated, and I have cranked it a lot with no trouble. do this mod at your own risk. by using this instruction set you are taking on all risk. now a list of things you need.
1.radio shack. wire 278-567. 18 gauge speaker wire.
2.radio shack. cigarette lighter plug. 270-070.
3.dura trax. battery connector and wire. dtxc2260. radio shack has this connector with out the wire. but its easier to use the dura trax
4.for reference ez-start hand wand traxxas #5280. about $14
round up all the parts. first cut a hole in the wand cover. between the belt clip and end of the cover, NOTE: this mod may NOT be for every one. if you cant get you your street truck close to where you bash/race it may not be a good mod for you.