Ever since I bought the Darma SSD new in 1980, the idle has been rough. Tickover speed is correct, but then it gently slows down and will stop. Both plug colours are ok, there no heavy sutting, but the L/H exhaust does have a slight blackness (a sure sign of over-rich?). I've tried several times to adjust mixtures and I've stripped the carbs down and cleaned them out several times. The rubber inlet gaskets are not split. Carb float heights has been checked and neither carb is dripping fuel. Many years ago, I stuck an exhaust gas monitor up each exhaust and the readings were well matched (though the balance pipe was still attached so may have given a false reading).
I have never checked the (Bosh) ignition timing.
Any ideas anybody ?
Rough Idle
rough idle
Hows compression? Leaky valves due to worn seats or incorrect valve clearance can cause a rough idle. As can wrong idle jet size. Are your mixture screws around about 1 1/2 turns out?? If you pull a plug lead off do both pots sound like they are doing the same job?? If ignition timing has`nt been tampered with it`s hard to imagine it would change. Having said that the wiring from the stator up to the magic boxes is prone to failure as they did on mine and are a worth while fix as history shows the insulation does fail sooner or later. In my experience however it does`nt show itself by poor idle. More by poor mid-upper range RPM.
Thanks for the replies.
40mm carbs with ticklers, so no cold start pistons.
It has been like this since new and the engine has gone through one complete rebuild (my own) since. This included a big valve conversion at some point. Both mixture screws are out by the same amount (can't remember the setting right now). Running on each cylinder alternatively shows they are more-or-less doing the same job, but the engine stops much faster when running on the rear cylinder only. Non-idle running seems fine. No backfires, no carb pop-backs (very, very occasionally perhaps). The only visible sign of something odd is the slight black smoke from the L/H exhaust on tickover. I was thinking of trying a new carb if I can get hold of one.
40mm carbs with ticklers, so no cold start pistons.
It has been like this since new and the engine has gone through one complete rebuild (my own) since. This included a big valve conversion at some point. Both mixture screws are out by the same amount (can't remember the setting right now). Running on each cylinder alternatively shows they are more-or-less doing the same job, but the engine stops much faster when running on the rear cylinder only. Non-idle running seems fine. No backfires, no carb pop-backs (very, very occasionally perhaps). The only visible sign of something odd is the slight black smoke from the L/H exhaust on tickover. I was thinking of trying a new carb if I can get hold of one.
rough idle
Black smoke can point to partially burnt fuel. Trying out another carb might be a quick and simple test. If it`s been like that since new I would maybe imagine it is a little more complex. There used to be a line of thought around that bevedrive Duc`s suffered from poor cam timing. Claimed to be due to an accumlative error built up over all the bevel train and that slight discreptancies at time of manufacture, all within spec when added together at the cam equalled less than spot on cam timing.
I fitted some desmo heads to a GTS after the heads cracked behind the valve seats and while the motor was out went through the rigamaroll of checking the cam timing and guess what I found.................bugger all!!!! Spot on to the endth degree. Guess what I`m trying to say is I`ve heard about it. But never experienced it. Checking it is fairly complex and is best done with the motor out. But go all the simple things first. Nothing worse than going through all the hassle and finding a cracked insulator on one spark plug.
I fitted some desmo heads to a GTS after the heads cracked behind the valve seats and while the motor was out went through the rigamaroll of checking the cam timing and guess what I found.................bugger all!!!! Spot on to the endth degree. Guess what I`m trying to say is I`ve heard about it. But never experienced it. Checking it is fairly complex and is best done with the motor out. But go all the simple things first. Nothing worse than going through all the hassle and finding a cracked insulator on one spark plug.
cam timing
http://www.ducati-upnorth.com/tech/camtiming.php
Here`s some stuff on it, interesting read anyway
Here`s some stuff on it, interesting read anyway
It's true! And you can read our roundcase experiences here...There used to be a line of thought around that bevedrive Duc`s suffered from poor cam timing.
http://www.bevelheaven.com/~BB/phpBB2/v ... php?t=3321
My information is that this problem continued into squarecases (and that's what Peter Shearman found - as referred to in the previous post). We haven't done any of our S/C's yet, so can't confirm.
Everyone takes a great deal of effort to get ignition timing right, but cam timing could be anywhere! (Which is why I question people using vacuum gauges to synch carbs with that situation unknown. If your valves are out of time, what does the supposed objective of equal vacuum (at a particular RPM - because it will change due to swirl effect with different valve timing) actually mean for performance?
You don't need your engine out to check cam timing, but yes, it is a PITA to do, so your approach to eliminate everything else first is the right one. Cam timing is unlikely to cause different sooting effects on it's own, for example. If you're convinced it's fuel soot and not oil, I'd still be concentrating on your carbs, if I was you.
Kev
I only just noticed this bit...
Just guessing!!!
Kev
That could influence slide, slide needle size and height, main jet and idle jet matching. It'd be worth trying dropping your slide needle down one slot (i.e lean your main circuit delivery) on the cylinder that may be running rich and see what happens, but it's just a guess. I've never changed a valve size, but I'd imagine the effect on your idle and main carb circuits could be different (so my next test would actually be reducing the idle jet size cause the increase in vacuum might deliver too much fuel at idle)This included a big valve conversion at some point.
Just guessing!!!
Kev
Thanks machten, but I had this problem before and after the valve job 
For information, both carbs have same jets and valves (but of course, the front and rear carbs have different model numbers).
I will try another 40mm carb - now, are ones from a 79-81 900SS, MHR the same ? I seem to recall different inlet (angle?) manifolds.
For information, both carbs have same jets and valves (but of course, the front and rear carbs have different model numbers).
I will try another 40mm carb - now, are ones from a 79-81 900SS, MHR the same ? I seem to recall different inlet (angle?) manifolds.
Just to confirm I was interpreting you right, you have increased the diameter of the valves in the head? I ask because I would think any change to valve diameter will require changes to carb fuel mixture delivery at all stages of throttle.
Also you can always swap the carbs temporarily with some temp fuel hose and see if the problem follows - a tad awkward, but can be done. If you do that, I'd transfer the slides, carb caps and slide needles with them. That would mean your carb sync will be out, but if you have the "cutting out" at idle as a symptom, carb synch is irrelevant anyway.
If the problem follows to the other cylinder at idle, then you have a carb problem. If not...think some more!!!
Kev
Also you can always swap the carbs temporarily with some temp fuel hose and see if the problem follows - a tad awkward, but can be done. If you do that, I'd transfer the slides, carb caps and slide needles with them. That would mean your carb sync will be out, but if you have the "cutting out" at idle as a symptom, carb synch is irrelevant anyway.
If the problem follows to the other cylinder at idle, then you have a carb problem. If not...think some more!!!
Kev
If its any consolation, my SS has never had an even tickover either. I have to set the idle speed around 1800 revs on the tach otherwise it gradually slows and dies. Even at the best of times, the tickover (if you could call it that) is ragged and uneven. Once off idle, the bike pulls fine with excellent throttle response although it doesn't particularly like running below 3000 rpm.
I've always blamed it on fairly radical cam timing and too big carb chokes.
Bruce
I've always blamed it on fairly radical cam timing and too big carb chokes.
Bruce
1979 900SS
Fredericton, NB
Canada
Fredericton, NB
Canada
Andy,
Just a thought, but have you checked your throttle cable set-up to make to make sure the carb slide is seating all the way down once the throttle is backed off ?
And on that point, do you have a dual cable set-up ?
may not be it, but it's a quick easy one to check and eliminate.
Cheers
Rick
Just a thought, but have you checked your throttle cable set-up to make to make sure the carb slide is seating all the way down once the throttle is backed off ?
And on that point, do you have a dual cable set-up ?
may not be it, but it's a quick easy one to check and eliminate.
Cheers
Rick
I have desmo disease, I just hope they never find a cure !!!
That is a perfect description of mine. On the road in town (30mph / 50kph) I cannot use anything less than 3rd gear and 2500rpm.abmartin wrote:If its any consolation, my SS has never had an even tickover either. I have to set the idle speed around 1800 revs on the tach otherwise it gradually slows and dies. Even at the best of times, the tickover (if you could call it that) is ragged and uneven. Once off idle, the bike pulls fine with excellent throttle response although it doesn't particularly like running below 3000 rpm.
I have a twin cable throttle (no longer the Darmah 1 into 2) and there is ample free play.
I have another issue which may be related. After a few months (after I took delivery in 1980), a small oil leak started somewhere in the rear cylinder head. I've never found the source - having checked all O rings, gaskets, dowels several times. But I never had a crack test done. Could it be that when hot, some oil is getting into the inlet tract and causing the slight black smoke? Both plugs always look the same and look to be running well.
I just live with the assumption that a good even low idle with 40s is difficult to achieve and that the motor shouldn't be run much below 3000 revs in any case. My SS is a first kick starter when cold and has no other carburation issues. I rarely ride in town so its not really an issue for me.AndyCapp wrote:That is a perfect description of mine. On the road in town (30mph / 50kph) I cannot use anything less than 3rd gear and 2500rpm.
Bruce
1979 900SS
Fredericton, NB
Canada
Fredericton, NB
Canada
Update
Long delay, but lots to report.
I have now measured the valves - they appear to be Mille (42mm & 38mm). Valve guides were original (bronze?) and a bit worn - also, oil appears to have been seeping past the guide/head and dripping into the cylinder where most of the oil was accumulating at the exhaust and seeping through the exhaust gasket. Although the guides had the notch for a rubber seal at the top, none were fitted. The new guides I have appear to be steel and I intend to loktite them in to stop the oil leaking past - unless somebody points out that is a bad idea. I have loktite 648 with a temperature range up to +175 C. That should be high enough ?
Rings and cylinders look and measure ok, so I can only assume the oil is getting in from above.
Fuel wise. One idle jet was clogged and I fit two new ones (65). With mixture screws out 2 and turns, I now have an idle. Not great, but at least I can pull up at junction without the engine dying.
I have now measured the valves - they appear to be Mille (42mm & 38mm). Valve guides were original (bronze?) and a bit worn - also, oil appears to have been seeping past the guide/head and dripping into the cylinder where most of the oil was accumulating at the exhaust and seeping through the exhaust gasket. Although the guides had the notch for a rubber seal at the top, none were fitted. The new guides I have appear to be steel and I intend to loktite them in to stop the oil leaking past - unless somebody points out that is a bad idea. I have loktite 648 with a temperature range up to +175 C. That should be high enough ?
Rings and cylinders look and measure ok, so I can only assume the oil is getting in from above.
Fuel wise. One idle jet was clogged and I fit two new ones (65). With mixture screws out 2 and turns, I now have an idle. Not great, but at least I can pull up at junction without the engine dying.