Wont start, Loud exhaust Backfire! Help

[Guest]

Today i tried to start for the very first, i couldn't get it to start, with the choke open it will backfire from the exhaust (did it from both pipes) it was very scary and loud like a rifle shot with actual fire coming out from the pipe. with choke close it just revved but didn't start.
her's the bike data
its a 84 GSX400, electronic ignition, no mechanical advance.
I stripped and cleaned the carbs, replaced all o-rings and the jet rubber plugs.
using uni filter pods,stock exhaust but no mufflers yet-pipes are cut after the link pipe
i believe i got the timing and valve clearance right when i put it back together, i did my best to verify that i'm doing it right.
new spark plugs as well.
i didn't use the fuel tank, but i hanged a plastic bottle with the pipe connected to the carb and plugged the vacum hose.
pilot screw were turned all the way in and then opened 1 1/2 turns.
i did not sync the carb on the bench, and didn't do anything with the idle screw under the carbs.
I need to say that this is my first project, i did restore a two stroke moped before and it runs fine, but never a 4 stroke motorcycle.
I need some advise on where to start looking for the problem
thanks
Eran

[gustovh]

Revisit your timing, it is not correct.
V

[pete]

I'm not sure if the ignition on the GSX400 is similar to the GS450 or not, but it is possible to put the ignition rotor 180 degrees out when you put it back on to the advance mechanism.

That will likely give you those symptoms as it is timed 180 degrees incorrectly.

[eran_k]

Pete, i think it's different, there's no visible advancing mechanism. i found a photo i took

I will take pictures of my timing marks tomorrow.
if it backfires from both pipes dose it at least means that my ignition is working?

[pete]

Bugger, yeah that's way different... electronic advance.

The fact that it's backfiring is a good sign you're at least getting spark though.

Is it possible for the rotor to go on back to front though? Ie. the tab facing right in your photo... should it be facing left? I'm only guessing here... mostly they can only be assembled one way, the 450 is a little different in that regard.

[eran_k]

it has a locating pin, so i can't be installed backward.
how should the idle screw be set? honestly i didn't even noticed it's position (lack of experience + eagerness to get it working i guess)

[eran_k]

How should the idle knob be set initially? can it cause my problem?

[eran_k]

another question, does it matter which coil is connected to which plug?
to my understanding both coils fire each time so it shouldn't matter but i might be wrong

[eil]

If you have two coils with one spark plug wire coming out of each, then they fire at different times and it will matter which one is connected to which plug. (Unless your igniter works differently than the other two-cylinders I've looked at.)

[eran_k]

If you have two coils with one spark plug wire coming out of each, then they fire at different times and it will matter which one is connected to which plug. (Unless your igniter works differently than the other two-cylinders I've looked at.)

that's my setup (2 coils,one spark plug each) so how can i tell which one should connect to which plug?

[tatu]

that's my setup (2 coils,one spark plug each) so how can i tell which one should connect to which plug?

Can you not just switch the leads to the opposite plug? For testing

[eran_k]

Can you not just switch the leads to the opposite plug? For testing

I will do that, just wanted to know if there's a way to tell?

[jeff.saunders]

If you want to check, take the plugs out of the head, insert them in the plug caps and lay them on the cylinder head.

Turn the crank slowly and when a plug fires, the cylinder that goes with will have the piston at the top of the bore.

[eran_k]

It Started!! i switched the coils and it fired right away.
works rough and dies without the chock but it's a start
Thanks for all the good advice

[Badooka]

Plug wires inverted?