just got car back from mechanic and it runs worse
 

Just got my car back from my local mechanic (very good rep) and it is running kinda crappy now.

I sent it in to get the timing and carb set, gm 350 crate motor, and when I got it back it drove home fine. After it cooled down I took it for another spin and It started backfiring through the carb. it did that before but only once in a while. They said that it was advanced 20 degrees when I brought it in and backed it down to 10 btdc. does that sound right? and if it does do you think fouled plugs may be the issue?

ican u turn your distibutor? mark its position and try to turn it, if it moves they forgot to tighten your distributor retainer and your ignition timing has deviated.

one thing I forgot to add is that on my paperwork they wrote down that my engine was a 305 on the info section and 350 for the comments. Maybe its possible they got it mixed

Could be its mixed up.. also, turn the ignition off before messing with the dizzy.
20 degrees is way too much, ten is more like it... however, its a starting point. depending on the year and actual displacement it will be lower, closer to 8. A backfire through the carb is a good indicator of timing issues.

I would take it back and raise cain.

Hopefully it just slipped timing,did you get on it hard at all on the way home?
My old 427 would get out of time after a few hard runs and I'd have to turn the dizzy back,always kept a mark on the intake and dizzy to line it back up to.
A worse case might be a bad timing chain,slack in the chain can cause torque pulses on the dizzy and make it move,soon after it will jump a few teeth and you'll be bending valves.

I dont think it has anything to to do with the chain, My engine has less than 10 grand on it and I dont drive it too hard. I talked to them this morning and they said that they would take it back in. What I am worried about now is that the carb was backfiring on the 20 degree advance setting, if I drove it before it was warm.

I advanced a little and it is better. I am still going to have them look at it

all motors I looked up 305 and 350 had same timing sets so don't think thats a issue but did notice all of them saying for iroc form of timing to be 6b timing and popping out the intake in general points to to early ignition...

And theres 2 ways of moving timing advancin or retarding advance is earlier retard is closer to tdc if your timing is way off the marks u might wanna look at ur balancer if 2 pieced to be off or not do u have hei in cap or coil seperate?

Might be a bad vacuum advance unit.