Hi Everyone, I could do with some ideas to track down an issue with my 900ss... I have spent the last 2 weeks tearing down the bike and have done a lot of (needed) maintenance. Perhaps to help fault find, I have: Replaced belts Checked Valve clearances (all ok) installed new valve seals (old ones utterly knackered - one was in two pieces!) installed new battery installed new up-rated starter/earth/battery cables replaced Spark Plugs replaced Ht leads and caps removed and polished exhaust I have not touched the carbs at all, but have obviously had the airbox off. Coils not touched. So bike is all back together and is definitely running better, pulling cleaner through the rev range, except: If i give it the beans and then back off and then feather the throttle it misfires, it also occasionally backfires through the airbox, and pops on over-run through the exhaust. I figured that the clamps between the exhaust and head were not tight enough, and have tightened them further, (no change to symptoms) but daren't go further or the studs will shear... So would an exhaust leak give these symptoms? The bike ran ok before the maintenance was done... could the carb have been set that would have accounted for worn seals...and now is off as seals are new?!...seems not possible to me, but I have not fiddled with the carbs as I know it has a stage 2/3 dynojet fitted and well set up from new (baines) Any pointers? ideas? I appreciate any comments. Thanks Grant
Hi Grant, first off it’s nothing to do with your valve seals, it sounds as though you may have an air leak , I would check the carb rubbers for leaks at the joints and also for cracks in the rubbers themselves, also how long was the petrol left in the carbs?, it only takes a few weeks for modern fuel to start causing problems especially in warm temperatures, if I were you I would take the carbs off, take the bowl off, take the jets out and give everything a real good clean with carb cleaner, also pop the slides out and ensure it’s all clean in there, be careful with the diaphragms use only warm soapy water to clean them, order a new gasket set for the carbs from NRP and replace all the gaskets/O rings, you can then check out the carb rubbers when they are off, also check for air leaks into the exhaust at the gaskets and joints , but I think it will be intake air leaks or carb blockages, if you put it all back together and it still does it wind the mixture screws out half a turn and go for another ride, if it still does it try another half turn out, hope you get it sorted quickly. Note the mixture screws are fuel screws so turning out richens the mixture.
Plus one for air leaks at the carb rubbers. I found it very difficult to get them to seat and seal properly
Thinking about it, there is chance that while the throttle is feathered off; which would mean the main needle isn't lifted and the carbs would be supplying fuel via the primary jet circuits; there is a possibility the primary circuits / jets are partially blocked or maybe even, the fuel level isn't quite high enough. AL
Nothing wrong with rubbers (carbs to manifolds), I'm going to do a full carb strip down and clean, also have new exhaust gaskets coming, hopefully that will sort it. It's annoying that there is no symptoms while reving at standstill...hence difficult to troubleshoot. G
I think you’ll find it worthwhile, I did mine recently and I’m still astonished how easily it starts now, it never was a bad starter but took about 3revolutions to start, now all I have to do is open the garage door and the bloody thing fires up!
Stripping the carbs is a good place to start then. My 750 backfired like mad when on the over-run with a closed throttle. Turned out to be blocked pilot circuits. However when I first reassembled them, I adjusted the float levels to the so called magical figure of 14mm and it was backfiring through the zorsts and popping through the airbox. That's when I discovered the best setting was 12mm which raised the fuel level in the float bowls, this making the pilot circuit work properly.
I agree with Al there on the float heights need to be 12mm, I followed his advice (actually found a previous owner had set them at 12mm) runs great, thanks again Al.
Just to report back, in-case anyone has similar issue... Full carb stripdown and clean has sorted this issue. There was no problem with any carb rubbers, just lots of oily gunk throughout the carbs. Job sorted!!
Great news, and good that you reported back, it’s really frustrating when following a thread and the OP doesn’t bother to say if the fix worked.
