K75S ..first start in 23 years.. will not idle above 600rpm Ok above 2000rpm.

[Ray20]

 Hi all,

I have a beautiful K75s which is back running after many years. It has a real rough spot between 600 and 2000rpm.
I have only ridden it about 40 miles at this point.

 The bike had been very well stored in a very dry environment and was in excellent condition when I got it, apart from the usual fuel problems. Pump and suspension unit was replaced, all fuel lines and filters replaced, injectors cleaned.. they were gummed shut... the tank had three corrosion holes the size of a match head in the lower left side.  The tank was cleaned with acetone, and an epoxy sealer used locally on the holes. All good

All visible rubber hoses around the engine look good. The vacuum plug tubes have had the caps replaced..the originals were split.  The rubber manifold boots all look really good, same with crankcase breather hose.

The K75S has 7,000 miles on the clock, and was last used in 1990  I bought it via an online bid from the friday motorcycle auction in Japan, which is where an amazing collection of older, very low mileage bikes are disposed..no private sales in Japan.

OK... it idles strongly at 500 to 600rpm.  The brass air screws have been adjusted to optimise the idle at those revs.. basically they are all about quarter turn back from fully seated..seems that's where it likes them.

Any attempt to open the throttle butterflys via the stop screw..results in a very broken erratic running... this coincides exactly at the point the TP switch clicks...  I don't think it anything to do with injectors

So.... it will tick over strongly at a very low idle speed of 500 - 600rpm below the "click"    ...above the click..it practically dies...  but once above 1000rpm it is starting to smooth out, but its not really normal till 2000rpm plus.. so it has poor bottom end and is prone to "flame out" when pulling up.

It looks like it is running lean...the exhaust pipes are very blue.. but use of the choke doesn't make any difference .. except it likes the choke when cold starting.

Japanese market K's were fiddled with to meet tough emission laws... I know they often have 2 row radiators instead of the usual 3 rows..so they run hot.. who knows what else is different?
 If it is a mixture strength thing, is there any way to adjust mixture strength ? what else could it be, pump and filters are brand spankers ?

Thank you in anticipation

Cheers

ROY

[Scott_]

I was just reading another K75 thread this last week that ended up having issues with the air flow measurement baffle being stuck/broken. You might give that a check if you haven't already. Maybe a critter nest......

[Ray20]

         Thanks Scott,

Forgot to mention this is a January 1986 bike.

 That's one thing I'll look at.......   I'm hoping that the flat spot may just get better all by itself if I put more miles on the bike... but I got a feeling it's here to stay.
Whatever the problem is, it MIGHT have something to do with the Throttle Position Switch.  It runs fine when I ask it to work..getting plenty of power and running strongly...just a real pain in the arse at low revs up to the point the TPS clicks...  which is at about 500rpm..unless that's just a coincidence and its getting enough fuel to idle on 3 cylinders at that point.  It does feel like something is happening when that TPS clicks though.. it goes from an even idle .."click"...  and instantly is running like a complete dog... only just barely running...running really rough and not for long...that's 600 -800 rpm..... if you can pull it through that big hole, it gets progressively better..and at 1200 -1500 rpm its sort of OK... and running sweet at 2000 and up.

There is another recent post on a K75 running rough, and the culprit was water in the tank from incorrectly fitted tank cap... I'm glad I read that  as I looked at mine and the plastic drain hose has rotted away and drains straight into the tank .. ha ha
It's dry here and I haven't washed the bike since I filled it so it hasn't any water in it YET.. but another potential problem to fix before it become one.
ROY

[Chaos]

just a thought, try running it with the tps unplugged.  It's only there to prevent backfiring on de-accelleration, which can be fun if you like that sort of thing.

[mac]

Whatever the problem is, it MIGHT have something to do with the Throttle Position Switch.  It runs fine when I ask it to work..getting plenty of power and running strongly...just a real pain in the arse at low revs up to the point the TPS clicks...  which is at about 500rpm..unless that's just a coincidence and its getting enough fuel to idle on 3 cylinders at that point.  It does feel like something is happening when that TPS clicks though.. it goes from an even idle .."click"...  and instantly is running like a complete dog... only just barely running...running really rough and not for long...that's 600 -800 rpm..... if you can pull it through that big hole, it gets progressively better..and at 1200 -1500 rpm its sort of OK... and running sweet at 2000 and up.

So it runs OK at 500 RPM when it's sitting on the TPS, but when you open the throttle and it moves off the TPS you get rough running until 2kRPM?

This might be more evidence for a stuck/faulty airflow sensor, as the TPS (simply an idle switch) switches the FI to a different fuel map I believe. If this is the case, then you're OK on the idle fuel map, and OK above 2kRPM, but no good at low RPM when not on the idle map. Call it an (mildly) educated wild-ass-guess on my part. Maybe someone with real knowledge can set me straight.

[Ray20]

    Problem Solved !

     The engine had all the signs of an air leak, and that's exactly what it was.
      I finally got around to removing the air box and have a look at the airflow metering
 
      What I found when I had it all out was the hose from the air box to the Plenum had a half circumference split in it.

       The crank case ventilation hose also has a couple of cracks in it, but I reckon they get that way very early in the life of a K, and don't  plan to replace it just yet....

  I hope this helps others with a lean running/non idling K bike

[Westone]

The crank case ventilation hose also has a couple of cracks in it, but I reckon they get that way very early in the life of a K, and don't  plan to replace it just yet....

Why would you not replace a cracked crankcase ventilation hose? The engine is just going to suck air through it, same as the air box to plenum hose.

[JamesInCA]

The crankcase breather hose fails frequently. If it's cracking, it'll become an air leak soon enough.