Bike won't start

[nyraised]

Hi Guys, went for a ride ..came off turned off bike..now no start..is getting gas, turns over but no start..what do you think? Coils? Ignighters?  Where should I start? This bike has 2 coils correct? Left and right? Or am I overlooking something?

[LukeM]

assuming we're talking about the cb750, yes there are 2 coils: one for cylinders 1 and 4 and one for 2 and 3.
One coil is fired by a blue wire, and the other is fired by a yellow wire.  The coils share a common connection (black wire, if memory serves).

One test is to let the bike cool down all the way (overnight) and try again the next morning. If you have a Battery Tender or similar device, connect it to your battery in case you have to do lots of cranking on the starter. If it starts and runs, then it's possible it's temperature related.   If it doesn't start the next morning, then something else has gone sour.  I'd recommend getting the manual from this site, and starting at one end of the ignition system and checking everything from the pulsers or points (depends on the year of your bike) through all the electrics (the DOHC pulsers have igniter systems, the SOHC points act directly with the coils), all the way down to the spark plugs.

When it's running, it's running strong with no lack of power or missing in the engine?
Did you change something recently? New plugs, carb adjustments, coils, battery?
Is the timing properly set?  The procedure is also in the manual. Pretty easy for DOHC, a little more involved for SOHC.

That should get you started (no pun intended).  Let us know what you find, and if we can help further. Good luck.

Luke M

[nyraised]

Sorry I didn't listed the bike. It's a 1980 CB750F supersport (DOHC). Had changed out the entire charging system a month ago, new battery, new rotor, new stator, regulator/reciever and new igniters. Nothing has been done since then, the bike ran fine with no loss of power or hesitation whatsoever, I already have the manual, both from online and hard cover. Going out to try it again this morning and check to make sure I got spark. Will post with my findings later. Thanks for the pointers. Dave L.

[nyraised]

OKay tried again this morning still won't start. Tested each plug wire with spark tester and the light flickers indicating I am getting spark to each of the plugs. However Im not sure if it is enogh spark, the test light flickers but no terribly bright. Any ideas??? Can it be a bad IC Ignigter? Or are the coil packs suspect?

[Re-run]

Well I can't see them all going bad but do you know how old the spark plugs are? Or what their gap is set to? When you look at the spark itself, it should be a fairly strong blue spark.
Try this, use jumper cables and hook up to a car battery. DO NOT start the car, just leave it off.
Now check your spark. If it is much stronger looking, then you may want to check your wiring and clean up the connections and one may have corroded enough to be causing an issue or one may have come loose.
Also, since you have removable wires, check all 4 at the coils and make sure it is all tight. I would also test the plug caps to make sure the ohms are around 5K.

[nyraised]

Well, Im about to take it off road for winter. I just got in new IC igniters, 2 new coils, new plugs. Gonna just redo the starting system. I already redone the entire charging system. Will check out the test like you suggested and check the wires, might as well replace them too. Where is the advance and points located on this bike and how do I test them. How about timing? Could it have jumped timing?

Re-run wrote

Well I can't see them all going bad but do you know how old the spark plugs are? Or what their gap is set to? When you look at the spark itself, it should be a fairly strong blue spark.
Try this, use jumper cables and hook up to a car battery. DO NOT start the car, just leave it off.
Now check your spark. If it is much stronger looking, then you may want to check your wiring and clean up the connections and one may have corroded enough to be causing an issue or one may have come loose.
Also, since you have removable wires, check all 4 at the coils and make sure it is all tight. I would also test the plug caps to make sure the ohms are around 5K.

[nyraised]

BTW what are the gaps on the plugs supposed to be?

[Re-run]

Gap should be about .030 or so. Mine are .028 but your ignition is more powerful. If your timing jumped, your chain is heavily stretched or teeth are missing on the gears which would be very bad but I don't think that is an issue.

[jamman18]

How did you go about testing the plug wires?

[nyraised]

I used a spark plug tester to ground. Finally took the bike to garage..turns out the carbs were full of rust..still waiting on getting it back...had them order me some new handlebars and install new carb boots while they have it there..they better hurry..its awfully cold here in NE PA...Brrrrrrr