Need help...rebuilt carbs, now doesn't want to start

I need some help here guys.

Last night I was riding my bike home...maybe 2 miles from my house, I got on the throttle to go around a car and the bike started running like total dog doo. It was running very rough and had no power (it's happened once or twice before but it went away...this time it didn't)

I managed to get it home (slowly) and decided to just go ahead and rebuild the carbs.

I took the carbs apart last night, found a couple problems that might explain some of the issues I was having (the throttle stop screw was missing, and one of the starter plunger collars on the choke was very loose). Anyway I rebuilt the carbs, didn't separate them, just followed some instructions I have for rebuilding and replaced the jets, needles, etc.

I finished late so I didn't want to start the bike. Got up today, went to start it, and...nothing. I crank it, it sounds like it fires a couple times, then just keeps cranking. Certainly getting fuel, I have a spark tester but I'm not sure where it is (I just moved over the weekend so everything isn't put away yet).

Anyway what do I need to check? I've tried turning up the idle with the idle screw but that doesn't seem to be helping. I guess I'll try adjusting the pilot jets until I hear from somebody here that knows more about this than I do. *shrugs*

Might be problem due to insufficient spark.

Could remove plugs, attach caps, hold bases against engine, and spin over with ignition switched to "on" position. Look for good fat spark.

Imo, best disgnostic procedure is to first assure adequate properly timed spark, and compression (test psi and check valve clearances), ample clean fuel supply to carbs, and clean clear unobstructed air filter, all before addressing carbs.

If problem defaults to carbs, might be a problem with the enrichener circuit, and/or pilot circuit. But thinking neither of these problems would have arisen suddenly when attempting to pass a car.

Good Luck!

ghunt81 wrote:

... got on the throttle to go around a car and the bike started running like total dog doo. It was running very rough and had no power (it's happened once or twice before but it went away...this time it didn't)...managed to get it home (slowly)...sounds like it fires a couple times, then just keeps cranking. Certainly getting fuel...

Certainly getting fuel --- Hmmmmm

What are plugs reading (sooty? wet? dry?)?
Would install brand new correct plugs anyhow, because the existing plugs may be fouled beyond recovery.

The plugs are wet and I can smell raw gas from the exhaust, so I know it's getting fuel.

Also the plugs that were in the bike look fairly new, and I checked them a couple days ago, both were pretty clean and not fouled. They don't appear to have all that much gas on them right now but I will spray them off with carb cleaner and reinstall (and check spark as well).

I almost kinda have a hunch that it could be spark as well because it's running very, very similarly to how one of my old cars did when the ignition module was on the fritz...

I should add that it started doing this at the end of a 30+ mile trip. Ran fine the whole rest of the way.

BTW what is the stock spark plug on these (part number)?

Some possibilites underlying excess fuel diagnosis ---

Leaking float needle;

Leaking float;

Obstructed air filter;

Float bowl fuel level too high
(would do clear plastic tube test);

Obstructed air passage in enrichener
and/or pilot circuit;

Am dubious as to both carbs
crapping out simultaneously.

Also -- even new plugs and new looking plugs
can be quickly fuel-fouled beyond recovery
(at least on Z1 and KZ900).
Important to inspect spark quality
with plugs removed from head.

Am a little out of my league with the twins,
and am basing comments on experience with
Z1 and KZ900, believed also applicable to KZ twins.

Am uncertain about oem NGK plugs for 1978 KZ750.
Am thinking B6ES, but please await better advice.

The spark quality inspection is an
important first step in the diagnosis.

Good Luck!

I got it started!

Sort of...well, I got it to fire up at least, it's idling pretty rough. I think I must have flooded it out earlier. I'm probably gonna have more questions because it doesn't seem to be running all that well.

I pulled the right side plug to test the spark and it was igniting the fuel vapor coming out of the cylinder.

BTW what's the clear tube test?

ghunt81 wrote:

I got it started! Sort of...well, I got it to fire up at least, it's idling pretty rough. I think I must have flooded it out earlier. I'm probably gonna have more questions because it doesn't seem to be running all that well. I pulled the right side plug to test the spark and it was igniting the fuel vapor coming out of the cylinder.

BTW what's the clear tube test?

it's idling pretty rough --- perhaps from goofing around with the pilot circuit air adjustment screws. :laugh: :laugh:

What about firing on the left-side plug?

clear tube test -- connecting a clear tube to the float bowl drain in order to actually see the fuel level inside the float bowl as governed by the float level. Could do a forum search for several excellent threads detailing the procedure.

Good Luck!

I put the pilot screws back to the original position that the maintenance manual I have says to use...turn in until snug, then back off 1 1/2 turns.

I just checked the spark on the left cylinder...it's there, but looks a little weak compared to the right side.

Maybe I'll look into getting a Dyna coil...

Hey there, sounds a lot like what my issue was, read here:
kzrider.com/index.php?option=com_kunena&...ew&id=234744&catid=3

The correct plug for a 78 750 twin is NGK B6ES. Also, the one coil fires both plugs at the same time, every time (wasted spark). If the right side looks good, but the left side does not, check the left side plug wire (not removable on an OEM coil) or the plug cap. The plug cap is a 5K ohm cap that is replaceable.

On my original coil I did replace the wires by pulling them out of the coil (were glued in) and then I cut the side of the coil where the wires go in, put new wires in and epoxied the coil pieces back on. It worked, but I didn't like to leave it that way. I eventually bought a Harley Orange Screamin Eagle coil at a good price ($45-50 at a Harley dealer) and am very pleased with it. It does not mount in the original location, but in the tool kit area by the battery, I than put on spark plug wires from an old Jeep that I had.

Perhaps the battery is failing?

Could also measure voltage reaching coil primary terminal.

Possible failure of points and/or condenser?

Old worn out leaking plug wires?

Alright, I decided to just rule out everything...found a good deal on a used Dyna 3.0 ohm coil on ebay, so I bought that, and ordered new plugs, wires, and points & condenser from z1 enterprises.

I figure that should help. Who knows how old the stuff on the bike is anyway.

BTW I did the coil rewire mod awhile back so I'm assuming THAT isn't the problem.

dont get me wrong here, but you seem to be guessing alot,
I am not sure about your bike, but what about the points and timing, have you checked them?
Also, I would double check the wire mod, you could have a relay that is trying to go bad, just me but check first then replace