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Hi, had some great help here recently ... hopeful for more!
GPZ900R, 4 cylinder, carbs. Have had a fuelling issue so replaced the vacuum petcock which has sorted it... I think. Had the carbs off, cleaned (they were pretty clean), tank is sealed, fuel cap stripped and cleaned. The carbs didn't flood when off the bike after cleaning so assume the floats are all good (and tried blowing reverse through them and they capped the air off nicely).
Put it back together and it turns over really well, but won't fire. V occasional backfire.
Taken the plugs out - 1 is very sooty and a bit wet (fuel), others looked tired but nice tan colour. The carbs pilot screws are set to the specified 2.5 turns. Only 1 spark plug sooty tho and it has a v v weak spark (unsurprisingly). Going to replace the 4 plugs tomorrow, but with the sooty plug, do I just adjust the air/fuel mix to make it leaner?
Pre taking the carbs off, it started on the button!!!
with the sooty plug, do I just adjust the air/fuel mix to make it leaner?
Try one thing at a time, don't start messing with other things while you wait for the plugs otherwise you'll have no idea what's working and what's making it worse. Not starting at all hints at a big problem, a few turns on the pilot mixture or the needle in the wrong position won't stop it starting. Most engines will run on 1 cylinder if the mixtures rich enough.
Is it back firing or just popping unburnt fuel out the exhaust, different things entirely, backfiring is what breaks your legs on a kickstart when the ignition isn't retarded over TDC for starting and the engine fires the wrong way before shooting flames out the intake (often with fairly catastrophic consequences for various parts of the intake system).
Popping from the exhaust - normal for a rich mixture not igniting properly.
Popping from the intake - you've got the HT leads leads in the wrong order and one (or more) cylinder is firing while the intake valve is open.
my guess would be plug leads in the wrong order. Easy to do.
Thanks, good advice on 1 thing at a time!
From your description, it's popping from the exhaust, not backfiring.
I'll check leads but I did remove plugs 1 at a time, and then replace before removing next one, so they should be fine as it started well previously.
Are you getting good spark on all 4?
Long shot but the stator might have given up, you never know.
Put it back together and it turns over really well, but won’t fire.
Did you turn the fuel tap to 'prime' to fill the carb bowls up when you put it back together?
(Are they vacuum carbs or just slides?)
Yes to priming the carb once refitted.
Didn't initially check spark as it started perfectly pre carb removal. But now checking it, spark at cylinder 4 (the sooty one) is poor.
Will hopefully try new plugs tomorrow.
Just odd it started fine before...
as it started perfectly pre carb removal.
occams razer would say you did something to the carbs when removing and cleaning them. Perhaps disturbed some dirt? How did you clean the jets? being soft brass they can be damaged by poking wire thru them, Did you remove the jets? Are they seated properly? Did you bend the float tabs? That can cause incorrect fuel levels leading to funny fueling
If you did the HT leads one at a time its unlikely to be that
Start with basics
Spark on all plugs? Leads on right
Fuel? Taps open and a flow present?
Chokes working?
Will it go with a sniffof brake cleaner?
Start with the basics ... oh so true!
So went back to basics ... is there fuel?
I had being having fuelling issues and one suggestion was that the vacuum petcock could play up, and indeed, it was. So I bought a service kit to replace the innards - quick bench test and it seemed to work fine (fuel flowed on prime, and when I applied a vacuum, fuel flowed on 'on'). Job done. So assumed it was fine.
However the current non starting issue was resolved by ... checking is there fuel. Nope. No fuel. Put the old petcock on, which flowed happily on prime, and bingo, it starts. So happy!
Inspecting the 2 diaphragms, the o-ring on the replacement kit looks a tad bigger so wonder if it could get stuck, so worked sometimes, but not always.
Just had a 30min blast. The back end wheel replacement and shock is sooooo lovely, engine was running beautifully through the range, idling v happily. The sun was out. So happy.
Really really hope I'm moving into fine tweaking now!
Thank you for sage advice.
