My bike randomly cuts out?

[Riknos]

I can't work out why! It's a 99 Hornet 600. It doesn't always do it, but only seems to do it when it gets warm, I.e 15-20 minutes after riding. But then another day I can ride it for an hour and it's fine? (I think that was a rather cool day, so this might be a contributing factor.)

The bike will start to run on 3 cylinders, I'll have to rev it to keep it alive and just a slight twist of the throttle sees it revving it's nuts off. It will then die if I either go into neutral, leave off the throttle for a second etc, and then wont restart until I've left it for 10 minutes. If it does restart, I'll have to run it with the choke on just to do a few feet before any deceleration causes it to cut out, rinse and repeat.

The last time I did it I got recovered home and the driver said he used to be a bike mechanic and said it must be electric if it's only doing it when warm, he recommended it's either the coils or water in the carbs. So I drained the carbs, completely drained and cleaned out the fuel tank, and then bought new coils and got an electrician to fit them but he tested my existing ones and said they tested fine, so I sent the coils back.

I then spoke to a local bike mechanic who said he thinks it's the carbs that need cleaning and will be £400+ to do. I'd rather not pay that just for him to be guessing and get it wrong.

Anyone got an ideas? I want to sell the bike so don't want to spend too much, but I want to get it fixed before I sell it so it's working properly for the new owner - everything else works perfectly on it!

Thanks for any help!

[orangefzr]

Certainly sounds like carbs to me ... Buy a workshop manual and do it yourself.... Only really need basic tools

Oh and £400 to clean a set of carbs is a fking ridiculous price

[Moulder]

How did you test an intermittent fault that happens when hot on your old set of coils?

Coils would also have been my first suggestion, or a loose connection/broken wire. Hard to see how this would be carb. related.

[cmaguire]

Does sound spark related. Whilst it is revving higher the spark is frequent enough to keep it going, but when revs drop the irregular spark makes it want to stall. And when stopped, restarting is really difficult as the starter can't turn it quick enough.
Bit like an intermittent CDI unit on an old bike, when it stopped it was sometimes possible to bump-start the bike but using the kick-start you were on a hiding to nothing. Next day and it would start again no problem.

[MajorMajor76]

Apart from the revving bit, it sounds like it could be fuel related based on what the OP has said, do the carbs ice on these? Sounds very similar symptoms to a problem I had on an old Passat once (80's), took ages to work out.

Quick Google suggests fuel tap/pipe, this helpful?
https://www.bikechatforums.com/viewtopic.php?t=155...

[orangefzr]

Moulder said:

How did you test an intermittent fault that happens when hot on your old set of coils?

Coils would also have been my first suggestion, or a loose connection/broken wire. Hard to see how this would be carb. related.

The reason I say that it sounds carb related is from experience, you see what happens is you get a build up of sediment / rust / ste in the float bowls which opening the drain screws will not shift because the ste / petrol has to leak out past a threaded screw .... The petrol does but the ste can't get past the threads and even if you remove the drain screw the ste sticks inside the float bowls.... The main problem starts when new fuel enters the carbs and stirs up all the ste sat in the bottom of the carbs intermittently blocking the jets causing it to run like a bag of st .... Park the bike, let the sediment settle to the bottom again and the bike will start and run for a while before running like st .... I have seen this several times in the past ..... The only way to rectify is to clean the carbs out off the bike.
I may be wrong but the op gas already replaced ignition coils (probably thought the insulation was breaking down when the coils warmed up) this can happen also

I bet at least one of those pilot jets is blocked as well.
I have a cheap ultrasonic cleaner and use diluted sugar soap with hot water, plonk the jets in and run the cleaner, clean out the bowls / carb bodies with carb cleaner spray then give the jets a spray out to get any remaining water off em.
Note to the op, drain all the petrol out of the carbs while the carbs are the right way up otherwise if you turn them upside down with fuel in you may damage the diaphragms

Edited by orangefzr on Thursday 12th April 13:33

[Moulder]

orangefzr said:

Moulder said:

How did you test an intermittent fault that happens when hot on your old set of coils?

Coils would also have been my first suggestion, or a loose connection/broken wire. Hard to see how this would be carb. related.

The reason I say that it sounds carb related is from experience, you see what happens is you get a build up of sediment / rust / ste in the float bowls which opening the drain screws will not shift because the ste / petrol has to leak out past a threaded screw .... The petrol does but the ste can't get past the threads and even if you remove the drain screw the ste sticks inside the float bowls.... The main problem starts when new fuel enters the carbs and stirs up all the ste sat in the bottom of the carbs intermittently blocking the jets causing it to run like a bag of st .... Park the bike, let the sediment settle to the bottom again and the bike will start and run for a while before running like st .... I have seen this several times in the past ..... The only way to rectify is to clean the carbs out off the bike.
I may be wrong but the op gas already replaced ignition coils (probably thought the insulation was breaking down when the coils warmed up) this can happen also

I bet at least one of those pilot jets is blocked as well.
I have a cheap ultrasonic cleaner and use diluted sugar soap with hot water, plonk the jets in and run the cleaner, clean out the bowls / carb bodies with carb cleaner spray then give the jets a spray out to get any remaining water off em.
Note to the op, drain all the petrol out of the carbs while the carbs are the right way up otherwise if you turn them upside down with fuel in you may damage the diaphragms

Edited by orangefzr on Thursday 12th April 13:33

Okaaay, I have never had that with any of my bikes with carb's. OP, it will be interesting to see what it turns out to be.

[gareth_r]

Heat-related usually points to the coils or the pickups that trigger them.

However, it looks like there is only one pickup:
11 on this diagram -- https://www.fowlersparts.co.uk/parts/4778919/cb600...

and each coil fires two cylinders (wasted spark).
https://www.fowlersparts.co.uk/parts/4778964/cb600...

Therefore, if it was the pickup, it wouldn't run at all, and if it was a coil, two cylinders would be down (unless the coil could fail in such a way that one HT output worked and one didn't).

I'd start (because it would be easy and quick ) by finding out which cylinder is not firing (if it's two, 1&4 or 2&3, that points to a coil), then swap coils, then HT leads, then plugs, to see if that moves the problem to a different cylinder.

Then I'd look for a youtube video on stripping and cleaning Hornet 600 carburettors.

Edited by gareth_r on Monday 16th April 00:35

[Jazoli]

gareth_r said:

I'd start (because it would be easy and quick ) by finding out which cylinder is not firing

There an easy way to tell, point an IR thermometer at the downpipes and see which one is cool, or just use your hand, for £9.99 its easier to use the thermometer and less painful.

https://www.amazon.co.uk/Thermometer-Tacklife-58%E...

A carb clean at a garage should really be no more than 3 hours labour at most, they are not difficult bikes to work on, but alternatively if you say the problem doesn't appear till after the bike is warm and has been ridden a while just sell it and keep schtum

Edited by Jazoli on Monday 16th April 07:51

[myvision]

I was having the same issues on my 2002 600 Hornet.
I found a couple of issues carbs did need cleaning this cost me £80 not an eye watering £400. I also found my fuel pipe had perished and the vacuum was sucking it flat starving the carbs of fuel. You could see the pipe was weakened I think it was the original pipe.

I replaced the fuel pipe first this made it a bit better but it ran spot on again after cleaning the carbs. Now I've seen inside them I'll tackle the carbs myself next time.