XJ600 backfire at speed

[andym]

Was doing a little test run after replacing the oil and filter, cleaning the plugs, replacing the brake fluid and a couple of other little bits and pieces. I went down the dual carriageway and decided to see how it handled, I got up to about 80mph and it started backfiring and chugging a lot, so I slowed down and it ran rough the rest of the way home (about half a mile).

When I took one of the plugs out it was a bit sooty, but cleaned off with a light wipe.

Any ideas on where to start? I was going to strip the carbs off and give them a clean, but don't want to do too much work that doesn't need done.

I should point out that the bike had been stored in a shed for 5 years, but it looked like someone had put petrol and oil in quite recently as they seemed quite fresh, although the petrol had been either drained or dried up from the petrol filter and pipes.

[finpos]

It's most likely started as a small misfire which has fouled the plug so it's only running on 3 now which is why it's gone all rough.

Trying to clean plugs is a bit of a mugs game because you only clean the outside of it and that's not the part that matters - put a new set in.

f.

[temeluchus]

If it repeatedly fouls the plug, check the condition of the corresponding ht cap and lead. Unscrew the cap from the lead and inspect for corrosion.

If it checks out, you will need to have a look at the carburettor on that cylinder-check for dirt and clean thoroughly. Remove the float and inspect the float needle for wear. A worn needle will have an obvious shoulder which will allow the cylinder to flood.

If you have a diversion xj600 the carbs aren't hard to work on. Can't say for the old xj.
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Some shite cruiser. Now with guns and FREEDOM!

[andym]

It seems easy enough in the book, is there a way to clean them out without removing them though?

I've replaced the petrol and put a good splash of redex in there, as a short term thing though. I know I'm going to have to take the carbs off at some point, just dreading the thought of doing 4 when I've only ever done one from a 50cc scooter in the past.

[temeluchus]

Not really. Redex won't do much for any real contamination.

You'll have to take em off to inspect the float needle in any case.
____________________
Some shite cruiser. Now with guns and FREEDOM!