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KZ750E starting problems 29 Mar 2019 14:13 #801250

  • RupertBear25
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Hi all,

Can I ask for some help on my bike:

Problem is it won't start.

Fuel tank is newly de-rusted and treated profressionally.

Fuel inside is 4 months old.

Fuel petcock runs freely in 'prime' position and in 'on' when vaccuum applied.

Carbs were professionally ultrasonic cleaned and serviced, garage claims to have identified one wrong jet and replaced it. But, I had bad running and starting when I first put it back together. This was due to blocked pilot jets, all four. I cleaned them out with carb cleaner and very small copper wire, till I could see them clear. Took out all the jets that are accessible from the float chambers, squirted them out wth carb cleaner, checked could see through them. Starting was only a little bit better, running was lots better. Took it for a run to the MOT (government safety inspection, 5 miles away) and back. But now starting has gone again.

Attaching clear hose the float bowl outlets and opening the drains, they fill to the right level as in the manual (floats and needle valves working ok).

Valve timing shims recently set, as well as I can do it, and engine has good compression, equal on all cylinders.

Ignition has new coils and HT leads, new spark plugs, timing is set and moves the right way on the centrifugal weights when speed rises. Spark tester (with opening jaws to measure spark length) shows good spark on all cylinders, will jump 1/2" or more.

So let's fire her up…

Full choke, starter spins OK, traces of a engine cough, but only at very slight throttle. Any more throttle instantly kills it. Squirt of Engine Start causes momentary firing but won't sustain. At any other choke setting no firing at all. Leave to clear any flooding for half an hour, try at zero choke, still no firing at all, but still fires with engine start spray.

I'm thinking I need to take the carbs apart again and see whether the pilot jets are blocked again… but need some more ideas.

Anyone got any?

Thanks,

Rupert
Rupert

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KZ750E starting problems 29 Mar 2019 14:26 #801251

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I assume that the carb air cleaner is attached and that there are not any other things that are not hooked up; otherwise starting the engine can be problematic. You should not open the throttle when trying to start the bike; try the choke in different positions. When my bike is cold it starts easiest with the choke on but not all the way on. Ensure that the battery is fully charged. Do not try to start the bike with jumper cables attached to a running vehicle. Four month old fuel should not be a problem, but if the bike has had fuel sitting in the carbs for that long (or longer) without the engine running there may be a buildup of varnish in the carbs. Also, I would suggest you clean and dry or replace the spark plugs. Ed
1977 KZ650-C1 Original Owner - Stock (with additional invisible FIAMM horn)

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KZ750E starting problems 29 Mar 2019 14:30 #801252

  • SWest
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Could be clogged pilots again.
Steve
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KZ750E starting problems 29 Mar 2019 14:38 #801254

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Thanks Ed,

Battery fully charged and new air filter fully connected, in standard filter box. Float bowls looked pretty clean when I took apart last weekend...

R
Rupert

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KZ750E starting problems 29 Mar 2019 15:48 #801257

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First off get rid of the old gas. If it only starts and runs on choke then your carbs still need cleaning, the pilot circuits are still gummed up.
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KZ750E starting problems 29 Mar 2019 15:56 #801258

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THAT'S WHY I POSTED THE FILTER

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KZ750E starting problems 30 Mar 2019 15:24 #801302

  • RupertBear25
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Thanks all. Strange thing, today with warmer weather and battery charged overnight on trickle charger, just a few mm off full choke, just a few mm off zero throttle, there is a sweet spot where it starts! After it warmed it up I could tell the idle speed was set too high, maybe that was preventing it starting before. Anyway, it runs and will get new fresh gas. And another cleanout in the carbs if it misbehaves again.
Rupert

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KZ750E starting problems 30 Mar 2019 17:06 #801303

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If the idle seems to hang and not settle down after the engine is warm you have a vacuum leak. Most common source is the rubber carb boots that attach to the head. A new set is highly recommended.
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