Runs fine for about 10 minutes then DIes

I have a 1979 kz 400 ltd,
I bought the bike from dead and I have been working on it for about 6 months now picking away and have almost got it going.
I can ride it or keep it going for about 10 minutes and when I slow down or let off the throttle it sputters and dies and its really hard to get going again. Ive restored the gas tank so no rust in the fuel, ive put on new intake boots, cleaned the carbs and put on a new coil, points and condenser seem fine. They are clean and have volt metered them to make sure .

Im really stuck right now and dont know what to do next.

Has the fuel level been checked using the clear tube method as described in the factory service manual?
Is the fuel filter for a gravity feed (no pump) system?
Valve lash within specifications?
Was that a brand new coil or a new to you used coil? A weak coil will crap out when it gets warm.

Take a look at the vent in ths gas tank cap

I would try too replace the points and condenser. They may test OK but still be faulty. Cheap insurance. Maybe even a good clean may do if the lobes are fine.


How through did you clean the carbs? ?

Make sure there are no kinks or blockages in the fuel hoses. Ed

These guys have covered all the things to check that first jumped into my mind:

1. Fuel cap vent. When it starts to die try opening up the fuel cap? If that fixes it then you need to clear out the vent or buy a new cap. Typical issues when the fuel cap vent is not working is that it will run fine for a few minutes and then start stuttering and die. It will be hard to start. After waiting for several minutes it will start up and run again but then repeat the process.

2. Old coils start breaking up after the coil gets warm. Once it cools down it will start and run fine again until it gets hot again. New coil will fix it - if you can find one of the Emgo coils it is a cheap but good replacement. And new plug wires are always good too.

3. Check the fuel levels in the bowls using the clear tube method as already stated. If the float bowl level is severely low the bike might idle fine but once you give it more throttle for an extended period it may not be able to keep up.

4. Check that adequate fuel is flowing in. Check fuel lines for kinks. Is the petcock flowing well - you may need to remove it and clean the screen of any debris that is hampering flow. Are you using fuel filters? If so make sure they are not hampering fuel flow from either being clogged or being too restrictive.

5. "Hard to start" is usually an indication of clogged pilot jets, but if it starts okay at first and is only hard to start after dying then I'd look somewhere else first. The KZ400s are notorious for being cold-blooded but we used to have a saying on the old KZ400 forums, that was grounded in experience, that you have to clean the carbs 3 times before they are really clean.

6. All the other maintenance items suggested: valves set, points set, etc.

7. Are the plugs getting fuel or carbon fouled? When I'm trying to get a bike running or dialing in carbs I've learned to put in new plugs any time I run into any kind of trouble. That seems to cure a lot of ills. I've found the KZ400s to be rather finicky with plugs. Buy a big box (10 or 12 per?) from Amazon. If you suspect that you've fuel fouled a set then put them back into boxes and label them as such, you may be able to use them again later after they have dried out.

Looks like you've come a long way. Best of luck to you.

I agree with 650Ed and diggerdanh's #1 and 4. You have restricted fuel flow somewher. The float bowls trickle fill, but the flow is not enough to keep up when the bike is running. Float bowls empty and the bike quits. The bike will be hard to start until it sits long enough for the float bowls to refill. Check petcock, lines, filter and tank vent.

undo the carb float bowl drains, put fuel tap on 'prime' if you have a vacuum operated fuel tap and check you have fuel flow. If not your issue is the fuel tap, probably blocked prefilter or vacuum diapragm.

Lift the cap and see if problem persists, if not then fuel cap vent as above.

Coils and ignitions can be erratic when warmed - and it doesn't always show up on a meter check. You would be more likely to have missing as the engine warms up rather than the engine dying.

thanks for the replys, ill start at the simple fixes! the coil is stock, I rebuilt the carbs, new needles and bowl gaskets. also they are vacume.

SOLVED, I was missing the pilot jet PLUG, it was getting way to much gas. I just made some out of some rubber that wouldnt dissolve in gas,
works like a champ now! DO NOT forget your PILOT IDLE JET PLUG.

Ah, I had the same issue with my 76 KZ400 when I first got it but it had much different symptoms. First, it was really, really rich, The whole garage smelled of gas when I fired it up. Second, it would not rev past about 3500 RPM - just started gagging from too much fuel.