GPz900r fouls plugs?

[lotii10396]

After a good hills run (2-3hr) I'm often coming back into Adelaide on mainly trailing/light throttle for 10min or so. You just get caught behind traffic so it happens regularly. Unfortunately what often happens is that she starts running really crap, won't rev, no power, so I have to pull over. Whole thing goes pear-shaped real quick - only takes a couple of minutes to go from perfectly fine 2 absolute crap. After only another couple of minutes twiddling our thumbs she fires up again and away I go. Temperatures are perfectly fine, nothing else seems awry and there doesn't seem to be any running problems either before or after. As bike still pulls like a train everywhere else I'm thinking symptoms could be plug fouling rather than fuel or worn valves (now done 52thou), but as theres no signs of smoke nor does it happen even with low revving city running it's got me a bit mystified. Ticking off Phillip Island bucket list in October, so as it's a pretty long run from here I'd appreciate any thoughts before her next service.

[pkay]

I'd probably start by tuning it back to stock settings and going from there. Problem being is that you can chase your tail with an old bike and unless you start from a sound basis you shit yourself to tears. I generally try to get the tune settings - timing, carb settings, balance and stuff per the book then go from there. Correct plugs and so on. How stock is the bike? eg original air box, no leaks around the air pipes creating a lean condition when hot or leak in the vacum system for the fuel tap (strange one I know but had that as aproblem before). Its a bitch when nothing is obvious but these things are an arvo's work and you'll be able to narrow it down pretty quick. Also on my GPz900 I noticed the fuel gauge doesn't go down slowly but in steps - so don't rule out fuel feed if it only happens occasionally esepcialy at the low end of the tank but the stop then restart doesn't sound right for that. Another one is something I had a problem with on my Z1300 where someone stuck an aftermarket fuel filter in the fuel line that sat behind the block - of course when things got hot I got vapour lock, 5 minutes on the side of the road and off again.

I would have thought dud valves would show up straight away as they did on my FZ750 which would start but had no pull.

Good luck and let us know when you find the problem

[bonester]

My GPz900r had kinda similar problems- worn needles (but not seats) in carbs and diaphram in tap was the causes

[lotii10396]

Apart from 4-1, rear Hagon shock + manual fan over-ride bike is completely stock as far as I know. Owner (last 10 years) didn't mention anything else and after full mechanics check & service last year (my first with bike) they didn't mention anything odd or unusual either. It does occur at the end of rides so the tank level would be lower, but as you mention stop-start shouldn't make any difference to fuel starvation. I might try running back home on reserve though just to see if it makes a difference. Symptoms do sound like vapour lock - but it doesn't happen even whilst getting hot sitting in traffic , even when I forget to turn on fan...... Normal running temps never get much beyond the warm-up mark, nowhere near even 1/2 way. My GPz fuel gauge also reads rather erratically. Hmmmm, I wonder if it's an issue with poor fuel tank venting? A mate mentioned the other day that he thought the small ding in the side of the tank (not me) was bigger & I over summer I often manually opened the fuel filler to speed up the equalisation process. I can hear whistling so I know it is working, but maybe it's just not balancing the pressures quickly enough creating a partial vacuum in the tank. Hence fuel starvation & increased "ding" size...I'll experiment and update.

[lotii10396]

UPDATE:
I pulled the fuel cap apart and everything seemed OK. I cleaned everything I could and since putting it back on the whistling is reduced (but still there) but now the bike doesn't cut out when low on fuel.

[Gosling1]

sounds like it could have simply been a bit of crap in the fuel cap venting system. Not that unusual in older bikes, especially those with a tricky venting system compared to old-school caps.......there are a few places that dust/crap can get caught and eventually block the vent - leading to exactly the symptoms you described....

One of the first things I do to older bikes which come into the workshop for maintenance etc - is take the fuel cap off and give it a really good hit with compressed air to clean out the vents.

[goanna_38]

Mine used to cutout when the fuel level was low or flood on a hot day because of the tank cap vent not working properly. A bit of careful file work rectified it.

[lotii10396]

Mine used to cutout when the fuel level was low or flood on a hot day because of the tank cap vent not working properly. A bit of careful file work rectified it.

Just got back from MotoGP (2.5K ride) and basically ended up having to ride with the fuel cap open to prevent the "vacuum locking" effect. Can you describe your "careful file work" in more detail? Cheers.

[goanna_38]

I just dismantled the whole cap and worked out where it should vent and made sure it did. Just put a couple of very small V-shaped grooves in the cap internals. When you think about it the grooves don't have to be very big. Be careful pulling the fuel cap apart (I usually do something like that inside and over the top of a newspaper) , mark anything relevant with a sharpie so it all goes back together the right way and don't lose anything.

[lotii10396]

Thanks will do. It can't be very safe riding with the cap open, even if it works!