Why won't it idle??

G0FHM · Post by **G0FHM** » Mon Mar 10, 2014 12:56 pm

This 650 is starting to get on my nerves a little bit now......

Have had the carbs off, all cleaned, checked and reassembled. Butterflies have been bench set.

Airscrews have been set at the default position of 1.5 turns. Inlet stubs had new O rings as part of the rebuild.

The misfire has been sorted.

It will start on choke, and will run at 2k rpm on the throttle without choke and rev cleanly to 4k rpm (I'm not prepared to rev it any higher, as the top end has just been rebuilt).

But the moment I bring the throttle down to circa 1500rpm, it starts to struggle and if I let it off, it just dies.

I've tried turning the airscrews out in 1/4 turn increments on each carb, but still nothing. If I turn the idle screw any which way, it still won't play.

One thing I have noticed, on cranking with the throttles closed is that the cranking sounds "uneven" (like a whump wer whump wer whump noise), but hold the throttles wide open and it kicks over cleanly.

Exhaust is on, and air filter also.

Can anyone throw any light on it? I'm about ready to take the contents of the petrol can to it......

Cheers,

J

Kryten · Post by **Kryten** » Mon Mar 10, 2014 1:24 pm

Hi,

I would be tempted to check the ignition timing, and action of the of the advance/retard unit first.

Then confirm that the valve clearences are in tolerance, and the float heights are set correctly.

Re-set the pilot screws to factory

Then if you have the kit do a carb balance. Idealy using a set of vacuum gauges.

When I fired up Skruffy Kat for the first time after it's slumbers/rebuild. I had no end of trouble getting it to idle. It would rev up,down on it's own spit through the carbs pop and bang out of the exhaust and generally behave like a stroppy teenager. But I was expecting that, so was ready to balance up the carbs and it settled down to a steady tick over and good throttle response.

But first off you need the timing and valve clearances good as a balance wont compensate for any faults there.

My 550 was a pig to start when hot and after advice, I checked the pilot settings and found they were all over the place. Once set up to factory reccomended it was fine.

Good luck and persevere with it, the Sun is shining

callyboy · Post by **callyboy** » Mon Mar 10, 2014 2:52 pm

Carb balance I reckon would be a good start.

Pete

Kryten · Post by **Kryten** » Mon Mar 10, 2014 3:13 pm

You can add "Check for air leaks" as well before doing a balance, especially around the carb/cylinder head adaptors

Uncle Bob · Post by **Uncle Bob** » Mon Mar 10, 2014 8:53 pm

Although for the Mikuni VM series this is a decent read to get the theory:
http://www.mikuni.com/pdf/vmmanual.pdf

I would check the pilot circuit again (jets and passages) and also the starter circuit (though probably not that). Check the starter jet that is in the carb bowl (think that is the case on the GS - been a while and my last carb strip down was VM carbs on my GT200X5 so my head is full off those) which is labelled on page 3 of that PDF linked above.

Here's a guide someone did for the BS carbs on a GS1150 (US) - the bits I was referring to are shown on page 50 and 51 of this:
http://www.mtsac.edu/~cliff/storage/gs/ ... torial.pdf

Did you clean with an Ultrasonic cleaner?

If you do get it running I have some balance gauges here in Norwich and also the long screw driver / box spanner thing for adjusting the stops easily. I also have an ultrasonic cleaner (small 2 litre that will just about do carbs 1 or 2 at a time).

Rob

G0FHM · Post by **G0FHM** » Mon Mar 10, 2014 9:04 pm

Hi Jim & Pete,

I've made a bit of headway since my original post, and have managed to get it ticking over. Lumpy as hell, and audibly pinking, but ticking over.

The valve clearances are good. I'll check the ignition in the morning.

My money is on the carbs not being set right.

They have had a complete strip down and ultrasonic clean (the pilot jets were severely bunged up with crud), and the 3 balance adjuster screws/pins were seized solid and had to be freed off, so I'm guessing it's a settings problem.

One thing that does interest me though, is the definition of "factory" as far as the airscrews are concerned. The factory manual says "pre set" on that subject, and says no more! I'm assuming that the correct position for them would be fully screwed in, then 1.5 turns out?

With them set at that position and cranking it over, it will "try", but won't quite catch. Look upon it as the engine assisting the starter motor if you will! Obviously with the choke rod pulled out, it will fire up and rev at circa 3k rpm as there are no jets involved.

I really need a set of guages to be able to see what it's doing when I can get it running, as at the moment it's all guesswork and I'm not much good at that kind of thing! I know that adjusting the 3 balance screws will make it run, but then there comes the knocking as they are obviously out of sync and over fuelling causing detonation.

I've tried to beg/borrow/steal a set locally, but nobody seems to have any. Hell, I'd even make a set with some coke bottles and plastic hose but I will still have the problem of the rigid tubes that screw into the inlet stubs!

Oh, and there are no air leaks on the inlet stubs - I've been round with the rubber pipe "stethoscope"

I'll reset everything to zero in the morning, warm it up on the choke and then see what gives.

Probably not much!

J

Uncle Bob · Post by **Uncle Bob** » Mon Mar 10, 2014 9:06 pm

J - I modified my post above with another link to a BS carb stripdown and rebuild that might be useful, after you posted this so you might not have seen it,

Rob

G0FHM · Post by **G0FHM** » Mon Mar 10, 2014 9:20 pm

Hi Rob,

You posted your previous post while I was typing out war and peace

Not near PC at the moment, will have a look when I get back upstairs!

Thanks,

J

G0FHM · Post by **G0FHM** » Mon Mar 10, 2014 9:54 pm

Rob,

That BS tutorial was very helpful!

One thing that does stick out like a sore thumb, is the lack of springs on our pilot screws! No O rings either.....

Maybe I have stumbled upon our problem already?

It surely can't be that simple?

Thanks,

J

G0FHM · Post by **G0FHM** » Tue Mar 11, 2014 3:37 pm

Well,

I've whipped the carbs off (again!) and stripped them and shot the best part of a 500ml can of carb cleaner through them.

The pilot screws do have springs and O rings, I just wasn't looking hard enough!!

Have bench set the pilot screws at 2.5 turns (as written in the "how to") and aligned the butterflies on the bench, so that they are all at the same opening.

Partial success!!

It will now tick over at 1000rpm. A bit lumpy, but no audible knocking - So things aren't far out, but that's as far as I can go for now. I bit the bullet and bought a set of guages from Busters earlier, so when they arrive I'll do the balancing act.

One problem still remains though - it seems very slow to drop the revs after a blipping. I can bring it up to 2k rpm and roll off, and it takes a good 5+ seconds to drop down again. Likewise when I lowered the idle setting, it took a while to respond.

I know that's indicative of air leaks... But I can't identify any!

J

Katana Owners Club U.K.

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