600 Bandit Carbs

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600 Bandit Carbs


My '99 600 Bandit has been running rough at around 6000 to 8000 rpm for a while and it's getting steadily worse. The bike is high mileage (69,000) and is completely stock. I've had it from new so I know its provenance.

At lower speeds the bike runs very well, but whack the throttle open at 80 and the front end dives for a second before the engine catches its breath and picks up again.

The exhaust has had to be repaired at the joint under the sump and at idle there's a slight metallic rattle coming from the silencer suggesting that maybe some internal baffling might have come slightly adrift. But I don't think either minor niggles are enough to cause this problem.

I've checked the resistances on all the ignition parts and they're all fine and the spark is really healthy. Which leads us to carbs.

I've always maintained the carb balance with a Morgan carbtune so it isn't that. But there is a faint fuel type smell around the bike when its running and the plugs were slightly blackened last time I looked suggesting rich mixture.

My plan was to buy carb rebuild kits and replace all the jets and other bits and pieces in all the carbs, but I cannot find anywhere that does them. My Suzuki dealer says that all the individual parts are available separately, but to be honest I imagine that a worn jet or emulsion tube will look much like a new one, and if I have to buy each part individually for each carb it'll cost a fortune.

I guess I'll just have to open them up and have a look, but thought I'd better sound out the Visordown wise before I take the plunge.

Which internal carb parts should I replace first? Main needle jet, emulsion tube?
Do the large round diaphragms get tired with age and should I replace them as a matter of course?
Is there anywhere that does complete carb service kits off the shelf?
How have other high mileage Bandit owners sorted this problem?

Any thoughts or advice gratefully received.

Whip the carb tops off and look down the emulsion tubes to see if they've gone oval .... From the Factory Pro site -      This is enough wear to make a bike impossible to tune - Poor mileage and rough, rich running is the resultAlso check the needles for wear

Thanks Fasty (wow, I remember your handle from when I was quite a VD regular some years ago).
That gives me a good starting point.
Will let you know how I get on.

You can also try NRP for carb spares - they seem to do pretty much everything.

I finally got round to pulling my carbs apart today. Unfortunately I can't see a single thing wrong with them.

The insides are clean, no blocked jets, and not an oval to be seen. The needles look sound with no signs of scoring or wear and the emulsion tubes, although now a matt finish instead of shiny brass (presumably due to oxidation from long exposure to the air bubbles that they're pushing into the fuel) also look sound.

The only thing I did find was one diaphragm which had a couple of nicks on the edge, but I think they would have been outside the actual operating area and under the bit that holds the diaphragm in place (if you see what I mean). The other diaphragms looked sound, if possibly very slightly wrinkled.

I'm at a bit of a loss as to what to replace. My instinct is to replace the diaphragms (in case age has made them a bit saggy) and the small pilot jets (just to the side of the main jet through which the needle runs) as they seem to have odd little dark stains on them suggesting they perhaps have the hardest life.

Other than that I'm at a bit of a loss. The bike was beginning to be on the verge of unrideable with the engine hunting from around around 4000rpm. As I mentioned before there is a fuely smell to the bike when it's running and it's using way more fuel than it used to. My theory is that rich running has steadily fouled the plugs making the poor running steadily worse, but with no visible faults in the carbs I'm at a loss.

If anyone has any miracle suggestions, I'd be most grateful for them.

Well it sounds like it's generally running rich so my guess wouldn't be the diaphragms.I'd start by doing a float bowl fuel level check with the carbs on the bike and the bike level. Easy to do and will tell you if there's something wrong with your floats even if they're supposedly set right.   I'd have a look at the air filter incase something's been sucked in there or it's badly clogged. Is it full of oil emulsion after a cold winter for example? Or indeed a mouse I'd also be looking at the carbs' cold start enricheners and maybe the coasting enricheners if it's got them... I think it might well do.

Thanks Tappy.

The cold start enricheners crossed my mind, but again they all look sound. Also I tend not to use the choke very often so I'd be surprised if it's a wear issue (there's certainly no wear showing on the choke plungers). I live on a long hill out in the sticks and generally start the engine then simply coast down my lane in neutral holding the throttle manually at a fast idle for the first quarter of a mile or so while it warms up.

It's true to say that the air filter is grubby, but it doesn't look any worse than previous air filters have got. But yes, it's due to be changed which will undoubtedly help and there may be something more sinister blocking its pores. No sign of oil emulsion as far as I can see, but I will look in more detail when I get back out to the garage.

I confess I haven't checked the float heights yet and was planning to do it when I rebuild the carbs. I may replace all the little float needles as they have a tiny plunger and spring in them which, as you say, may be worn or behaving strangely even though the measurements might suggest that the float height is right. Unfortunately it's rather hard to check if they're seating properly (thus preventing flooding) as the float chamber is at the bottom of the carb. The needles seem like a very light alloy and the brass seats into which they fit are pressed into the carb body so can't easily be replaced. But there's no sign of visible wear or of crud holding a float valve open (I have had that problem before - it filled the air filter box with petrol and was dead obvious when I opened the carbs up that time).

Could it be possible that the float needles are worn or 'dried up' so that they don't slide up and down as smoothly as they might when the carbs are in situ? (You would think that fuel would lubricate it, but I did once have a fuel tap on a trail bike that was really stiff until I opened it up and put a little grease on the internal sliding gate-valve type bit. I assumed that fuel would wash the grease away in minutes, but many months on it's still as smooth as silk).

Many thanks again for your thoughts Taps. I think the search is narrowing, but it's still a bit of a puzzler.

Sorry, forgot to add, I'm not sure quite what coasting enricheners are, but I've found no mention of them in the Haynes manual. There's idle jets (adjustable), main jets, emulsion tubes, pilot jets (next to the main jets), main jet needles and cold start enricheners but that's about it.

Sorry to bump this 7 month old post but any news on what was wrong my 99 b600 is doing the same thing but the engine completely dies at between 6-8k rpm now

It turned out to be a sort of accumulated error.

I started by replacing the needle jets and jet needles. This improved things and I think the old ones were worn, but it didn't completely fix the problem.

Next I went up one size on the pilot jets (from 035 to 038 if memory serves). To be honest I think this may have been a red herring and I needn't have done this, but since the bike is now running pretty well I plan to leave well alone.

However at this stage the problem still existed so it was off to PDQ and their Dyno.

The guy ran it up and found that for the most part it was running massively rich. The power curve graph also clearly showed the dead spot at 6000rpm. Initially he looked at the air filter and thought, like me, that it looked ok. However as soon as we replaced it, to my deepest embarrassment, lo and behold, problem completely sorted!

The text books tell you to replace the air filter first of all. However anyone who does their own servicing knows that when you do replace it it makes no difference at all to how the bike feels. As a result you get sloppy and don't bother. Clearly my air filter was far more clogged than it looked.

The moral of the story, change your air filter regularly like the book says.

Many thanks to those who helped me with this.
Hope it's useful SwearBlocked.

How do I get the diaphragm back in on my 600 bandit

A light smear of vaseline around the groove on the carb body will usually hold the rim of the diaphragm in place while you get the carb top on. Not generally a problem though.

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