Topic: Help: Bike backfires and louder now

[phengv18]

Hello all,

I have a 1975 honda cb550f and just a couple days ago my bike was running fine until yesterday.  The bike turns, but does not start. If it does start, I have to hold the throttle to keep it going but if I let it go it dies.  Its also really extremely louder and somewhat backfires from the exhaust.  I have adjust the tappet valve clearance, changed spark plugs, and oil prior to my problem.  I do not know what else to do or what went wrong and I'm wondering if any of you guys can suggest whats wrong with it.
 
Since I haven't yet  adjusted the ignition timing and read through several posts regarding to problems similar to mine,  I figured I'll check the ignition timing following the manual to figure if this is the problem why my bike runs rough/barely starts.  I first check the contact breaker gaps and readjustment them according to what is recommended by the manual.  After doing that I went onto trying to adjust my ignition timing.  Using a timing light, I see that the light does not flash when the index mark is at the "F" mark but flashes only at the 1.4 marks.   Do I just move the base plate until the timing light finds the "F" mark?

Though it seems like the timing is off? I don't believe the ignition timing is the cause to why my bike is not running or has become much louder.  Do you guys have any other suggestions as to what is the cause? Should I readjust the tappet clearance again?  I know this is also a stupid question, but is cylinder 1 on the points cover side or the opposite side?

[flybox1]

When was the last time you did a complete tune-up?
do a carb clean yet? vacuum sync?
be sure your valves, timing and cam chain adjustment are proper before you do the vacuum sync.
#1 is on your left (when you are sitting on your bike)

[phengv18]

Well I just bought this bike about 3 months ago and it was a running bike.  Since I bought it, I have did the usual maintenance, however haven't yet cleaned the carbs or vacuum synced them. The bike had idled fine and ran really well before the problem that I am having now.  Could it be my tappet clearance? Should I readjust them again? This is frustrating, my first bike and hoping to ride it for the summer.   

[flybox1]

valve clearances? dont know as i cant speak to your level of ability to set them correctly. 
if you think you done good...then move on...
start by opening your shop manual or clymer, and follow procedure for the 3000mi tuneup.
dont get frustrated.  shouldnt take you more than a weekend if its your first go (well, unless you find a sht-ton of PO mods).  make it fun, not a chore. drink many frosty beverages! 
Take your time tho...learn your bike, and check in here for specific questions.
Garbage in = garbage out.

Valves, timing, air, oil, cam chain, carbs.
Clean tank, clean gas, strong battery.
..polish it of w a vacuum sync.

[Bankerdanny]

I had a similar problem. Check and make sure that the wires that run from the coils to the points are tight. Mine had vibrated loose on the 1-4 coil. it backfired like mad and refused to rev.