KZ 750 TWIN Spark/Throttle Issue

I have been working on 82 KZ 750 TWIN for months and finally have it idling correctly and it will run all the way to redline in Neutral. (carbs were cleaned and a new fuel tank/petcock)

My problem come about when I get on the bike and try to run it down the road. Once in gear, it won't go above 2500 RPM and it seems to bog down and not want to run correctly. After I get to a stop sign (100 feet away from my driveway) it will die.

Again, it will idle ALL DAY and run to red line in NEUTRAL, but once in gear and rolling it will not pull any RPMS above about 2500-3000 and then it will die. Also, odd that by then the battery is drained enough that the electric start won't get the bike going again.

Is this a timing issue, charging issue or what?

Any help is GREATLY appreicated. I want to get this thing on the road. It's gettin' COLD outside!

It can be a faulty kickstand switch cutting out the ignition, but that wouldn't drain the battery.

You need to strap a voltmeter onto the bike and ride it while measuring the battery voltage. If the voltage drops while you're riding, there could be an intermittent short, or a charging system problem. You should see about 14v on a sustained cruise of 30 mph.

Check or bypass (connect the wires together) the kickstand switch first, though.

It shouldn't be timing, because the 82 750 twin has an electronic ignition. Are you running the stock airbox or do you have pods? No kickstand switch, either.

Right!! No kickstand switch on this bike!

I am running a stock airbox. Could this be something to do with the IC Ignitor or the Rectifier/Regulator??

I just don't understand this problem!!

If you can get a Factory Shop Manuel, they show up on e-bay fairly often, there are diagnostics for the reg/rec and igniter as well as coils. Does the bike just die @2500 or just start wanting to die, no power, and you can keep it going by dropping down on the rpm's or clutching and reving? The electric starts on these bikes are notorious for their crappieness. Even if your charging system is junk, you should still be able to run the bike, for awhile, off the battery. Steell, is the twin guru, and should be poking his head in @ some point. I just know a bit about the 82 twin because I just got mine running right.
BTW, welcome, it helps if you put your bike and location, in your sigline. Never know if one of the more experienced members, is local to you.

You mentioned you cleaned the carbs. Did you re-balance the carbs after cleaning? Did you set the float height?
These two things could trick you that way... let you run ok, but not under load....

But remember, that around 2500 RPM is right where two important things are happening (or starting to happen): in the carb, you're starting to leave the pilot circuit and enter the main circuit more fully, and also, your auto-advancer is approaching full advance.

So, to combat the first angle, you might want to clean your carbs again. Or run Seafoam through it. Pour hald a can in the tank and run it for about 100 miles... if you can...

To combat the second angle, make sure that your advancer is clean, rust-free, and able to easily move from full advance and back.

...and set the timing... and check your coil... and find that charging problem.. and so on...

Post edited by: Biquetoast, at: 2007/11/08 19:37

Biquetoast wrote:

You mentioned you cleaned the carbs. Did you re-balance the carbs after cleaning? Did you set the float height?
These two things could trick you that way... let you run ok, but not under load....

But remember, that around 2500 RPM is right where two important things are happening (or starting to happen): in the carb, you're starting to leave the pilot circuit and enter the main circuit more fully, and also, your auto-advancer is approaching full advance.

So, to combat the first angle, you might want to clean your carbs again. Or run Seafoam through it. Pour hald a can in the tank and run it for about 100 miles... if you can...

To combat the second angle, make sure that your advancer is clean, rust-free, and able to easily move from full advance and back.

...and set the timing... and check your coil... and find that charging problem.. and so on...<br><br>Post edited by: Biquetoast, at: 2007/11/08 19:37

This is what I'm thinking, also, Toast. Since he was asking about the electric system, I wanted to make sure that the elec wasn't cutting out @ that rpm. If it's just dying under load, I think he should be looking at the carbs.

Thanks for the help so far. I do have a shop manual. I paid a hefty price on Ebay for it, but it has helped me more every day.

When I say I got the carbs cleaned, I mean that I paid a guy $50 to wash them. Sounds like these carbs are finecky!! Maybe I need to take them APART and see what I can find. It would be the first time I have worked on motorcycle carbs, though. The Motorcraft in my 1972 Ford Truck is little different!! I also noticed that parts for a "rebuild" kit are not easy to come buy. Does anyone know a place to find rebuild kits for these twin carbs?

I think the next step is to check the advancer and ensure it isn't sticking at around 2500RPM...(Although, it would be sticking in Neutral as well as in gear, if that was the case, right?) Then if it is still busted, I guess taking the carbs apart is the next logical step. I didn't realize that carbs can begin to give you problems under load and not at idle. Wierd!

I will update once I fiddle with the advancer tomorrow!!

Thanks to all so far for the guidence.

Evan Terhar

Cincinnati, OH

1982 KZ TWIN (First Bike)

I'm glad you have the manual. That means you can start by checking the things that are able to be set at fixed values first - I mean things like the float levels, valve clearances, and timing.. things that can be set correctly no matter how the bike is running.

Then you set the things that are variable... pilot screws (though they have a reasonable starting point), carb balance, etc.

As for the carb cleaning... I'll tell you I thought I had mine clean, too, but there are tiny little passages that get clogged easily that are nearly impossible to clean except by the best experts (or a chem dip). Barring that, that is why I have fallen in love with Seafoam... it just goes where the gas goes while running, and cleans it all out... awesome stuff...

As for under load... the engine as a whole suffers under load when the 2 cyls are out-of-balance for any reason, especially carb balance, but not only that. I mean a weak coil will run the bike fine, but may be wasting alot of (or starving for) fuel in a bad mixture, that doesn't reveal itself until you really ride it. I'm not saying that's one of your problems, it's just an example...

To put a fine point on it, just adjust everything you can, and clean or replace everything else. How's that for advice..

Cleaning and fixing everything is GREAT advice! LOL

I was going to start by checking the balance/synch of the carbs. In the manual is shows a special tool you need to use to check the vacuum balance. Does anyone know a) where I can find this tool or 2) what I can use instead.

Thanks!

Evan Terhar

Cincinnati, OH

EvanXU1 wrote:

...idling correctly and it will run all the way to redline in Neutral...Once in gear, it won't go above 2500 RPM and it seems to bog down and not want to run correctly....

On 4 cylinder bikes having a centrifugal force type advancer unit, springs hold the advancer unit "closed" in the idle (retarded ignition timing) position until rpm's increase.

When the advancer unit is "stuck" in the closed position (staying in the retarded ignition idle position), the engine may rev easily to redline in neutral, but have almost no pulling power in gear when trying to accelerate the bike.

EvanXU1 wrote:

......In the manual is shows a special tool you need to use to check the vacuum balance. Does anyone know a) where I can find this tool or 2) what I can use instead...

Oh boy. This is a voluminous topic if you do some searching. I'll cut it down to a few pointers/options:

1.) Most manuals will teach you how to do a "bench sync". This means, you take the carbs off, and adjust them with a feeler gauge so they are both equally open. This is a good FIRST step.

2a.) Cheap tool option: Lots of guys get a piece of tubing, a yardstick, and some liquid (another point of debate as to which liquid), and you have a sync tool.

...or...

2b.) Non-cheap tool option: I bought a Morgan Carbtune. Motion pro makes a mercury-based one, and maybe others make them too.

Hope that gets you started...