Hanging Idle and poor throttle response.

[Sesman]

Just thought I’d share my experience with an issue on my recently restored 500 four.

Although I’ve owned the bike for a year or so, I’d not really ridden it seriously….just a quick trip round the block. I’m preparing the machine for sale and was intent on ensuring the bike ran perfectly. I’d noticed that the bike was fine on choke, but wouldn’t idle correctly and responded poorly to blipping the throttle, noticeably hanging and failing to settle properly. Naturally I suspected the idle circuits. I’d previously checked all other influencing items such as leaks, timing valve gaps etc, etc.

I removed the idle jets (#40) soaked them in STP carb cleaner and blew them out with an air line. Interestingly I also aerosoled the jets and it was clear they were passing cleaner under pressure. I reassembled, but the situation was the same. I dismantled once more and this time viewed the jet through a bright back light. I could see clearly what appeared to be a translucent, spiders web type structure that would not allow a .4mm wire probe to pass through .

In an attempt to clear the hole, I heated the jets then plunged them in cold water. I prepared the wire probe by cutting it with pliers to create a small cutting edge, inserted the wire and rotated until the wire passed through, then blasted again with aerosol cleaner.

Result: impeccable tickover and throttle response….

I was nervous about passing wire through the jet, but it clearly had the desired effect and removed the obstruction. All four idle jets were affected identically in the same way.

Just thought my experience might help others on this forum. If you use this method, ensure you use a wire gauge slightly smaller than the jet drilling size. I used .4mm on a #40 jet and it worked just fine. But it might pay to break up the obstruction with a .38mm in. #40 jet….who knows.

[Trigger]

The spider creates a vortex so droplets don't form ( the spider creates a mist) . It should be cleaned but not removed 
 

[Sesman]

Interesting that Trigger.  The bike didn’t run properly at all until I’d cleared it with the wire!

Or are you winding me up…..😉

P

[Trigger]

Interesting that Trigger.  The bike didn’t run properly at all until I’d cleared it with the wire!

Or are you winding me up…..😉

P

No, i am not pulling your leg. If it was not running right then, the hole or spider was dirty. Destroying will make droplets and it will run rich. From memory the bike will miss behave at higher revs. The spider is there for a reason   

[Sesman]

That’s really interesting Trig. I thought the pilot jet was all but redundant on  rising throttle, being superseded by the slide cutaway, needle then main jet. Another school day it seems.

Strangely the bike runs absolutely fine all the way through the rev range. Just returned from a 30 mile trip and the plugs are perfect. Looks like I’ve done a wrong to achieve a right!

I’ve no doubt there is a reason for the design and I’d love to know how they create that finite bit of engineering during manufacturing.

Thanks for the info.

Regards

P

[Sesman]

Incidentally, I’ll replace the pilots with new replacement just for peace of mind. Would you know where I might acquire 4No new ones?

I see Allan’s have some, but I’m a little unsure on the specification. Are the cb 500 4 the 28mm type?

[JezzaPeach]

Interesting. I wonder if anyone has a close up photo or diagram of these structures? It never occurred to me when I cleaned mine. Some were blocked solid, so an ultrasonic cleaner would be needed?
Also wondering if this could cause a stall at tickover when hot.

[Sesman]

I’ll attempt that when my replacements arrive, but to be fair the bike starts and  runs like a new one. I’d love to know how they manufacture them.

I suspect that by clearing the ‘web’ the engine will run slightly rich at tickover and this may contribute at high revs to a slightly richer mixture …but I’m absolutely no authority on the topic.

I’ll know soon as I’m anticipating that with new pilot jets, more choke may be required for cold starting….we shall see.

[Sesman]

It’s possible that if the webs were creating a mist, fuel droplets and an over rich mixture would create a hot stall….just guessing. The air screws can only do so much in terms of vernier mixture adjustment? I’m guessing that it’s the combination of fuel delivery and air screw adjustment that’s important and by clearing the webs ( if they really exist) it’s possible to wander off scale too far.

But again I stress, if  I’ve mistakenly removed the webs thinking they were some kind of blockage….my eyes are not good….i’ve not suffered any noticeable detrimental effects…so far.

[deltarider]

Interesting. I wonder if anyone has a close up photo or diagram of these structures? It never occurred to me when I cleaned mine. Some were blocked solid, so an ultrasonic cleaner would be needed?
Also wondering if this could cause a stall at tickover when hot.

[Sesman]

Thanks deltarider.

Where would that spider structure exist. Or is it simply an optical illusion brought about by reflections at the drilling junction?

[Sesman]

Wonder if I’ve just spent £28:on replacement pilot jets for no good reason.😭

[Oddjob]

Both 500 and 550 use the same pilot jets.

Even when my eyesight was perfect (and I had exceptionally good eyes when younger, still not bad now) I never saw this spider web on the inside of those jets, I've even done what you did and used a piece of wire to clean them out and never noticed any difference in how the engine ran.

[Sesman]

Mmm, me thinks Trigger is taking the piss……

[deltarider]

There is no spider web structure. It's an optical illusion. http://www.sohc.co.uk/index.php/topic,22417.msg204301.html#msg204301