Misfire/lack of power

[Oggers]

Gents

78 KZ650 has performed wonderfully since restoration a year or so ago. I took it out on Friday and it ran beautifully. Saturday another spin, but this time filled it up with super unleaded as normal. After that, bike runs fine until you give it some throttle at @ 4500 rpm and it hesitates - somewhat alarmingly lurching as it does so. It tries to go, but hestitates again. Very erratic, very hesistant, will not go above 4500. Below 4500 it seems OK - ish....

Clearly I have though about rubbish in the tank being disturbed when filling up, and finding its way to the carbs. I took the tank filter off - very clean, and the extra in-line filter between petcock and carbs looks fine and clean. Cleaned plugs - all OK, checked gap - OK, cleaned air screw needles - all OK, fuel line blown through, vacuum take off to petcock is OK.

Taking the carbs off and cleaning jets etc. is a royal pain. Has anyone got any other suggestions that I could easily do, and I am on the right track with the root cause? Deeply annoying after it was running so well!

[warren3200gt]

Oggers responded to your pm

[Mr Bump]

Some thoughts:

Take all the fuel out of the tank and drain the carbs. Leave the tank out in the sun to dry right out.

You may have water or diesel contaminating yr fuel. If this doesn't cure the problem it'll at least eliminate it.

Could also be that the tank is pressing on an electrical component and this is worse when it's full of fuel and therefore heavier, also possibly the the coils are on their way out and got soaked in heat from the engine while the bike was standing the filling station which showed up the issue.

Olly

[Oggers]

Olly

Yup -the only thing that has changed is the fuel fill up. I will indeed drain the tank/carbs and refill. All a bit odd. I have never experienced bad or water contaminated fuel which has had such an effect. I thought I had filled it with diesel at first!

[z2.john]

I'd put a new set of plugs in. Old plugs can look OK but can deceive you

[Oggers]

I am very pleased to report I have just ridden the bike very quickly up my very convenient, quiet test hill just outside the village, and I can report all seems well. In fact far better than before! It pulls like a train. The problem did indeed appear to be condensors, and in all probability, one of them was on the way out creating less than adequate sparking before it finally gave up.

Couple of points - pun intended....

Intially I must have inserted the fork connectors of the condensors to the small bolts incorrectly. I do realize there are insulating washers and that the contact fork terminal sits in there also. Initially the bike would not start after replacing the condensors. I then fiddled about further, and inserted both fork terminals under the washer which sits under the head of the bolt. The insulating washer follows this, which would seem to insulate the bolts/forks from a bracket and thus the contact breaker plate? When it would not start, I feel the fork connector(s) may have been in contact with bracket/contact breaker plate. I am no sparkie so lesson learned perhaps....which is carefully note how things are connected/attached before removing them!

Everything pointed to fuel as the problem. I guess the condensor died when I turned the ignition back on at the petrol station. One for the brain's hard drive - not to be wiped. Thinking about it, the bike now performs so well that it may be worth replacing condensors on other bikes as a matter of course. I have no idea how these things behave and if condensors degrade over the years, but for £10 from Fleabay, it may be a worthwhile thing to do on the 400/4 and the GS1000.

Very many thanks for all your suggestions and help. I find these forums are a truly beacon of kindness in this increasingly disagreeable world.

[warren3200gt]

Oggers, just a bit of a heads up. If those condensers are Chinese they only seem to last about 6 months before crapping out.
Try to find some Japanese denso ones and get them for future stock.

[Oggers]

Hi Warren

Thanks for that - they were the below - which "seem" to be made in Japan. Where would one buy the Denso make? I looked on various sites before Fleabay and found nothing...

https://www.ebay.co.uk/itm/115284721079 ... 0290.m3507

[warren3200gt]

Finding them can be difficult. Problem is points/condensers are old tech. As such they get bought from the cheapest supplier and as the market is decreasing decent manufacturers can no longer justify manufacturering them. Consequently China jumps in and makes cheapo copies.
Original denso ones have a bronze/gold sheen to the metal case.
The crappy Chinese ones are silver metal case.

[Oggers]

I suppose if it does the same thing again, at least I'll know where to look. A tenner won't break the bank. The ones I have seem to be Japanese in origin but could be fake of course.....I'll search some more for the real thing.....

[zed1015]

Personally i would take the points off , stick them on a shelf and fit a Dyna 'S'.
You won't have any more mucking about after that and it won't ever need any adjustment once set.
Fit and forget, easier starting and more power.

[gray]

I had a similar symptom of refusal to rev out beyond midrange. It was a cleaning cloth under the seat sucked into the airbox opening. Nice cheap quick fix.