backfiring

[Richard Q]

I don't get to ride my KZ900 often enough and last year was so busy, I am sure I didn't ride it at all.

Charged the battery on Friday, bit of fresh go-go juice and prodded the button, and he started straight away. Choke full on, ticking over at 2,500 to 3,000 rpm. I got on and rode it out of the factory and up and down the car park, and it ran perfectly, but then started backfiring like a teenagers corsa.

I took the plugs out and they're black, which is no surprise as it was just started on full choke. I do have some iridium plugs so put them in, and no difference at all.

It seems to tickover ok, but any revs, especially under load, and all power gone and lots if misfiring. It did do this in the past but after a few minutes, it would clear be ok again.

After a great deal of thought and looking at coils and ignition, I have come to conclusion that the buckets may be sticking and holding the exhaust valves open. Is this a common thing with these engines?

I am going to check the valve lash now as I haven't done it yet

Any suggestions? or advice on the sticky buckets/valves?

Richard

[warren3200gt]

Possibly sticky fly weights on the advancer? Maybe?

[Mrg]

A failing condenser will do this type of behaviour if you still have them?.

[gray]

Backfiring sometimes is caused by a hole in the exhaust. Have a quick check there while engine idling.
What kind of exhaust have you on it? If its a 4 into 4 you can put a hand on each pipe end while it backfires and see which cylinder is doing it.That might localise it to 1 cylinder. ( suggesting spark plug/ HT lead or that cylinder's carb) If its 2 pipes ( cylinder1 and 4, or 2 and 3) then that suggests a coil issue, or a condenser issue if you're still running points.
May be worth draining the carbs too, to make sure there's no debris or water in them. That would be quick and easy too.

[Richard Q]

Thanks for all the ideas.

It's running Dyan S electronic ignition so no condenser.

I have lifted the top off, and all of the valve gaps are between 0.07 and 0.1mm. All of the buckets spin easily with just the prod of a finger tip, so they don't appear to be sticking. I tried spinning the engine on the starter to check there wasn't a bucket which was slow to return, and it fired up and just sat there burbling away happily. I watched all of the buckets individually and none seemed slow to return, not realising the timing chain was throwing oil everywhere

The exhaust is a standard 4 into 4.

I've removed the timing cover and moved the advancer by hand, it's all fine in there everything is smooth with no tightness or slack anywhere.

Exhaust is nearly new, so no holes in there, and it only backfires under load, barely pulling up a slight slope outside of work. I am sure it did this once before, a year or so ago and after coaxing it up and down the road for 10 minutes, it cleared and took off like it should.

I put it all back together, now it won't start at all so I have left it for now. I'll drain the carbs tomorrow.

[zed1015]

Thanks for all the ideas.

I have lifted the top off, and all of the valve gaps are between 0.07 and 0.1mm.

Valve clearances are on the tight side.
The should be 0.10mm minimum and ideally 0.15mm.
Whenever they get down to 0.10mm they need the next thinnest shim..
Backfiring and loss of power under load may be plug leads arcing under the tank or poor contact where they enter the coils amongst other things.

[Richard Q]

Thank you Zed1015. I followed my manual which says 0.5 ~ 0.1mm.

I left the plugs out overnight, put them back in this morning and it started up, but not running well. It wouldn't rev above 3,000 rpm and the left hand exhaust didn't get hot. I changed the plugs back to the NKG B8ES and it started and ran better actually allowing me to ride it up and down the carpark, but revs still limited to 5,000.

After 5 minutes of struggling up and down the carpark it started to clear and pulled like a train but only momentarily, then back to coughing and spluttering.

I am going to get it insured, then get a new set of plugs and take it around the industrial estate, and hopefully that should sort it. It would be interesting to find out exactly what is happening though, as everything seems ok under inspection. I am under the impression that it doesn't like to be sat there and not running.

[zed1015]

The 0.05mm to 0.10mm clearances are the old specs which were revised to 0.10mm to 0.15mm due to instances of valve seat burning, especially on the exhausts
They should be kept as close as possible to the upper figure of 0.15mm and never let to get below 0.10mm.

[Richard Q]

The 0.05mm to 0.10mm clearances are the old specs which were revised to 0.10mm to 0.15mm due to instances of valve seat burning, especially on the exhausts
They should be kept as close as possible to the upper figure of 0.15mm and never let to get below 0.10mm.

If it wasn't for forums like this one, I would never of known that Thank you for that, I need to get some shims ordered.

[gray]

A leak in the exhaust would cause a backfire on the over-run , not a backfire under load. Burnt valve seats would give low compression - worth checking that.

[kz900B1]

Have you checked the pick ups in the points housing ? My dyna packed in last year after 18 years of working without an issue .If there are signs of it over heating, the plastic cover expanding then it could be fried .Measure the voltage at the coils it should be at least 12v.

[Richard Q]

New set of B8ES plugs, started first time but still struggling so up and down the car park a few times. It seemed to rev better on full choke, even when warmed up but wouldn't tick over for long.

So, got it insured, and then down to the bottom of the estate and back - about a mile. First trip it coughed and spluttered the whole way, second I found it would pull hard over 5,000rpm, then after 6 trips (so 6 miles) it cleared fully and would pull well right from 2,000rpm to the red line with choke off. Everything nice and smooth again but I am still wondering what caused it, so will be giving the bike a full service over the next week or so, including checking all of the wiring, the Dyna S, shims, filters, oil, and every adjustment I can find.

[KWACKERZ1]

I got a couple of bikes up and running last year both of which had ran fine years ago.
Both ran like pigs until you got them on the main jet.
Turned out that to get them to run on modern fuel i had to put bigger pilot jets in the carbs . #20/25s i think. Yet previously they had been set up properly.

Admittedly they are both running 33 smoothbores, but your symptoms sound the same. Both of them would run better with choke.


Might be worth trying bigger pilots.

[Gray17]

Good evening Guys, i am sorry you are having these issues, me too on a newly rebuilt Z900 motor, like yours it refuses to run right at low revs but get it on to main jet it pulls very well, trouble is you cant ride it flat out all time. Whilst mine doesnt back fire , it does miss and cough, and holds back, like you i have bought new carb kits, brand new Denso plugs and had choke plungers replaced, but still it runs crap.
The last post said try bigger pilot jets, just wondered if these worked and has your problem been now sorted, if so how?
Thanks

[Ultim8pc]

Good evening Guys, i am sorry you are having these issues, me too on a newly rebuilt Z900 motor, like yours it refuses to run right at low revs but get it on to main jet it pulls very well, trouble is you cant ride it flat out all time. Whilst mine doesnt back fire , it does miss and cough, and holds back, like you i have bought new carb kits, brand new Denso plugs and had choke plungers replaced, but still it runs crap.
The last post said try bigger pilot jets, just wondered if these worked and has your problem been now sorted, if so how?
Thanks

Going up to the next pilot jet size is worth a try.