Backfires, won't start

[wingsfan98]

So here is the background to this post. I have a 1984 gl1200a. When I first picked it up, it needed a starter switch (the key switch on handle bars) and a timing belt (left side as if sitting on bike facing forward). Replaced both and everything started as it should. Had fairings and body pieces off to repaint. finished painting and put all together. Was idling in drive and ramping up and down. at 4600 rpm's, it shattered a valve in the number 2 cylinder (front left). Destroyed cylinder head and piston. Took engine out of frame and replaced piston, cylinder head (replacement came with valves, springs and all the internals) and gaskets. Put everything back into frame, set the timing and hooked everything up.
First thing to happen was the starter relay (switch between battery and starter) stopped working and had to bypass with a dead switch. Tried to start engine and all it does is Backfire, alternating between exhaust and carbs as to where it is back firing. Sounds like it is the left side only doing it. But will not start. Took apart front timing covers and reset timing from scratch, reassembled and still same problem.
So, what is the most like cause of this? I keep reading several different things on this forum, but where do I start to trouble shoot? What is most likely to least likely? I am dumbfounded as I can see nothing obvious that is causing this. Like I said, it ran fine before the valve went.

 

[joedrum]

hmmmm...this is something ....im guessing this is a case when there was a valve problem of hanging some while running ....im not sure why it would keep doing this with new head on

 

[joedrum]

need to try explain more ...its very easy to get belts off a few theeth ...but it seems to be more than that ....id recheck the belts ...maybe pic of location and post so we can see where its at on both sides ...if you can

 

[slabghost]

Check your ignition wiring. You may have the coils or plug wires backwards. Be certain the belt timing is absolutely right.

 

[mcgovern61]

Check your ignition wiring. You may have the coils or plug wires backwards. Be certain the belt timing is absolutely right.

I agree to start here first!

Also, if you had the cams off, be sure you are lining up the cam to the correct mark on the flywheel on the correct rotation. Remember, the cam does two turns for every flywheel rotation.

 

[wingsfan98]

Well...spark plug wires correct, cams and gears correct. So far, found two gaskets on intake ports that were questionable...replaced both and a secondary air supply line going into the first cylinder with a crack in it, replaced it. Continued searching and found the PC Port vacuum line broken (how that got by me I don't know). Just getting ready to put back together and see what happens. If not, next step is taking off heads and inspecting valve and springs for anything...broken inner spring or not seating valves right/sticking open...hope the vacuum line and seals is it, though...will know shortly.

 

[slabghost]

Bad vac leak can cause your issue for sure. I hope you have it. You can check most of the valve train by removing the valve covers. Springs and such. If any valves are bad that will show with a compression test.

 

[joedrum]

me too good luck i think you got it maybe ...but maybe backfires may have cauded that too ...fingers crossed

 

[dan filipi]

Nothing much to add here other than to do a compression test. The numbers will tell if a head needs to come off.

 

[slabghost]

Now that I've thought on it some you might want to check your valve clearances too. If too tight the valve won't close entirely and backfire through the carbs.