Hello everyone, Last October just weeks ahead of the end of the autumn riding season in the northern hemisphere I picked up a 1996 900 SS/SP with just under 6000 miles on the clock. I changed the belts and fluids as well as the rectifier/regulator which was dead and the battery too. A couple of weeks after the riding season came to an abrupt meteorological end. I don't recall if I changed the spark plugs. I did notice a tendency for the bike to back fire at idle a lot at start-up and with a cold engine, and somewhat less when the engine was warm. The choke is not functioning well as the lever has suffered some disintegration of some internal plastic. The choke lever won't hold it's position save for the operator physical holding the lever. I am in the process of fixing that. Is it normal for this model to backfire at cold idle? If not is there a way I can tell which cylinder is to blame or if both are to blame? I would expect carburetor wear to not be an issue given the low mileage, but I imagine there could be mild blockage in fuel or air passages from long intervals of storage with fuel at least in the bowls given the motorcycle's age and low mileage. Should I look to simply clean the carbs thoroughly? Or maybe opt for installing the next size smaller idle jets? Or perhaps just open the idle air adjustment a touch? The bike uses CV Mikunis if I am not mistaken. I am unfamiliar with CV carbs. Could this be a symptom of a worn or torn diaphragm? I appreciate any advice given. Jim
You definitely don't need smaller jets - you actually need more fuel at idle. You don't have idle air adjustment as such, it is more idle fuel adjustment - the more they are turned out, the more fuel. Try turning the pilot screws out 1/2 a turn or even more. Screw them in first and count how many turns out they were (don't overtighten them). Should be between 2.5 to 3.5 turns out depending whether a Dynojet kit is fitted or not. Make a note so you can re-set them if necessary. If you get to 4 turns out, then the pilot circuits are probably blocked a bit. FYI, mine would do it at cold start up for a few seconds then it would stop once the engine was hotter. This was because the fuel level could have been slightly higher or the pilot screws could have been turned to about 4.5 turns out (leaving them a bit loose against their springs). Mine were set at 3.75 turns out IIRC. It is easy to enough to check for a split diaphragm - fours screws and lift the lids off (don't loose the little O-Ring).
Those four screws can be extremely tight though. Make sure your screwdriver bit is a perfect fit and you may need to grip the head of the screw with pliars at the same time to get some extra purchase. Iirc, dynajet kits come with replacement screws because so many get ruined in the attempts To get them undone.
That's the first I have heard of it..............which screws? (and actually they only come with needles, jets, springs and shim washers) You are talking about float bowl screws, not the diaphragm cover screws, which is what I was talking about.
Thanks to you fellows for replying and for the good advice. In retrospect I should have mentioned that the backfire is coming out of the exhaust but then you guys gleaned that even though I probably didn't make it clear. More that I should have mentioned is that the bike seems completely stock as the air box and filter are stock, the mufflers are too and the foam inserts are still inside the fairing panels. Nothing appears to be touched on the bike which is why I nursed it home and immediately changed the belts as I had to assume they were 20-year old OEM items. So I am thinking the carb jetting is probably stock too. I did put Termingnonis on the bike last week and tucked away the original cans for the next owner should they want to return it to stock. The backfire was there last fall with the original cans, and it seems no worse or better now with the Termingnonis, only much louder. At startup the bike makes sounds much like I am being shot at from very close range. I'll be changing the plugs. That's good advice, with very little cost and effort involved just to rule them out or perhaps fix the problem. As for the idle air/fuel adjustment, I was thinking there was an excess of un-burned fuel in the cylinder after detonation which was subsequently getting expelled into the headers/mufflers and exploding within from the heat. For that reason I thought I should try to go leaner. However you (Arquebus) are telling me that I actually need more fuel at idle. That's opposite to my reasoning process but then if I knew how to diagnose what was happening I wouldn't be here asking for help. While I don't understand the logic it's worthy of a try. Thank you all for your advice. Jim (BTW, I recognize Guy Fawkes' likeness in Arquebus' avatar. Not bad for a Yank to know who Fawkes was.)
Uprated coils were a popular mod on these bikes, as they enabled a better 'burn'. I did this to mine, as it was very prone to plug fouling. The coils completely cured that problem and gave a better idle, throttle response and low speed running too. You can get coil kits from California Cycleworks. These bikes struggled to meet emission and noise regs and the standard set up was severely compromised as a result. So rejetting is a sensible move all round. I found Ducatisuite a very useful resource. The site was set up by a Monster owner but the two bikes are very closely related.
There is a chance it could be unburned fuel in the exhaust if your choke mechanism is faulty - but it would most likely be the enrichment devices in the carbs rather than the metal slide lever at the back of the carbs. They are the things that the slide hooks over. They have plastic needles which can wear quite badly. A good thing to do is get the right Dynojet kit - Stage 1 for the stock bike; or Stage 2 if you take the trumpets off the airbox (try that before cutting it about) and fit straight through cans.
Any fuel that hasn't been burned will end up in the hot exhaust piping and normally pass right through. But if you add more air to the fuel from an exhaust pipe fitting or crack, you now have all the ingredients for an explosion, the hot pipe is the ignition source and you get a backfire. Without the additional combustion air from the exhaust leak you are unlikely to get a backfire. First, you need to inspect the piping for cracks and leaks at or near the exit from the engine — then check for leaks at junctions. Aftermarket pipes or catalytic converter eliminator piping segments are not a precision fit at junctions so you need disassemble and reassemble the joints using a high temperature silicone sealant. I recommend Permatex Ultra-Copper high temp RTV silicone gasket maker. http://www.permatex.com/products-2/...-temperature-rtv-silicone-gasket-maker-detail
But the OP has stock exhaust and airbox, plus there won't be a Cat in the system (although I suppose there may be one fitted in California but I would be surprised if a 1996 model does). Do you work for Permatex by any chance?
You mention the backfire is when cold, and that you have a sketchy choke lever. I think I'd try fixing the choke and running some seafoam through it before throwing new parts at the problem. If that has no effect then try turning the mixture screws out a half turn at a time and see if there is any improvement.
I had a very similar situation after cleaning/rebuilding my carbs, fired it up and every 10 seconds or so ‘Bang’, then I realised I had forgotten to reset the mixture screws, they were at approx 2 turns out, I turned them out another turn each and the backfire disappeared, I ended up after the usual trial and error at just under 4 turns out each, starts and runs great now, so start it up and turn your screws out , if the backfire disappears you’ll know your on the right track, I would recommend stripping cleaning and rebuilding your carbs, set the float heights at 12.5mm (ignore what Ducati say), mine starts instantly and pulls like a train since doing this.
Ducati don't specify what the fuel level should be. This is because they consider it necessary to set up the bike properly on a rolling road etc etc.......in fact they are right. However, one or two websites say the float height measurement (not the fuel height) should be 14mm. IMO and experience that is entirely wrong. I found out by trial and error (and reported it on this forum) and set them at 12.5mm but I reckon they should have been around 12mm. In addition, you can't expect both carbs to have exactly the same settings hence Ducati's stance of setting the carbs up properly. What we do with our SSs is just get them roughly right.
