Gs750 Back fires and wont start

[Guest]

So i baught a none running 81 gs 750 thinking it was something simple, so i rebuilt the carbs checked compression checked spark and hooked up a temp gas tank i put a set of new plugs in and baught a new battery and put some oil in it. so the first time i tried to start it all i got was a single loud back fire and thats all i could get messed with the wiring a little more and now all i get is intermitten backfires i checked the coils they were good and seems like the preveus owner might have tried to rebuild the motor any ideas on what is going on? Thanks

[Lorenzo]

Can you check the cam timing?

[tom203]

As you sit on bike, the left ignition coil fires #1 and #4 spark plug (1 being the cylinder under the clutch lever). Real easy to misconnect plug wires.
Previous owner probably mucked maintenance things up, but unlikely that he rebuilt engine. Describe how you rebulit carbs.

[Tysonhatfield]

Took everything apart cleaned the jets put the needle and all small parts in carb cleaner to soak cleaned all rust and crap out of the bowls blew out with air hose to dry put everything back together and replaced all gaskets set the float hight and put back on bike

[Tysonhatfield]

How exactly do i do that?

[tom203]

Did you replace all the o-rings? This guy (member here) sells kits

http://www.cycleorings.com/

Once gunked up ,these carbs need serious soaking, have a look here...

http://www.mtsac.edu/%7Ecliff/storage/gs/Mikuni_BS-CV_Carburetor_Rebuild_Tutorial.pdf

[Agemax]

as stated before, check your plug leads are on correctly, left coil = 1+4, right coil = 2+3

[Tysonhatfield]

Worth Mentioning when i took the carbs out and apart it had looked like the pevuis owner had taken them and cleaned them and put back on but then just sat for a while so the carbs them selfs werent that bad but did need a little love and yes i replaced the o ring and another the plug leads are on the right coils. i took generator plate cover off and found where you adjust the ignition timing the screws were all loose.

Also i have a video on youtube of what the bike is doing if you could check it out.

https://www.youtube.com/watch?v=apHqFXFlLXE

[Brendan W]

If that plate is loose the sparks won't be happening at the right time. Unburnt fuel will be pumped into the exhaust and a late spark will pop it. There is a service manual here http://members.dslextreme.com/users/bikecliff/ to help get the timing set right.

[Tysonhatfield]

Alright well i went of that and timing for spark seems to be right on now, i tried it again and same thing all i get is backfires. would it be possible for old owner to put the cams and timing chain wrong or out of adjustment any way to check that without pulling the head off?

[tom203]

Sure it's possible , but probably unlikely. Remove valve cover and check how marks line up to TDC spot on crank. Something like this sketch

[Tysonhatfield]

So i pulled the valve cover off and put the motor at top dead center and going off that sketch it looks like the cam with numbers 3 is almost 90 deg off its pointing directly at where the 454 numbers are in the sketch how exactly do i adjust the cam back to were it should be? Thanks For all the help

[Brendan W]

. would it be possible for old owner to put the cams and timing chain wrong or out of adjustment any way to check that without pulling the head off?

There is no limit to previous owners ingenuity.

[Agemax]

So i pulled the valve cover off and put the motor at top dead center and going off that sketch it looks like the cam with numbers 3 is almost 90 deg off its pointing directly at where the 454 numbers are in the sketch how exactly do i adjust the cam back to were it should be? Thanks For all the help

is it TDC on 1+4? if it is on 2+3 then they wont line up like the diagram shows.

[Tysonhatfield]

Its Top dead center on 1 and 4