sorry i dont know much on the carbs as mine is an injection AL is font of knowledge on carbys you can check HT leads with a test meter they should read less than 5 ohms plugs are a bone of contention with me had loads of issues over the years with ngk R plugs (R being resistor) had new ones with a lovely spark but would not run right changed to bosch and never had a problem thats just my opinion anyway fault finding is tedious and annoying but worth it in the end read my thread on my 750 ss ie if you can find it lol could be something daft like float level slightly off have you tried a bit of choke to see if it makes any difference when revving if it does it narrows it to carbs maybe
Good suggestions. Will check plugs and leads and try the choke idea. I still wonder if the slides aren't lifting as everything seems ok up to 4k and then it goes to hell. When do slides start to lift?
Iridium plugs need a bigger gap, 1mm from memory, and I recall reading somewhere that they do not work so well on Ducatis of this vintage, something about a mismatch to the coils/igniter boxes. Changing the plugs would be my first port of call.
The pilot circuit controls the low revs running up to when the slides should be lifting so the main jets take over between 2500rpm and 3000rpm.....they may even start slightly as low as 2000rpm......it all depends on how well other settings are and the condition of components. NB. The float measurement has a considerable effect on the pilot circuit.....it seems that 14mm may be alright for the 900s but I found my 750 ran better at 12.5mm.
So my pilot circuit seems just fine. Starts, ticks over and runs fine up to 4K. Its a very definite change over point. Also, the motor bogs if I open the throttle rapidly. I'll do all the simple plug and lead checks but something tells me its the diaphragm slide actuation.
I feel your pain. I have just been thru these again checking, double checking and checking again every needle, jet snd setting setting. I’ve replaced the rings and found that thanks to a pic earlier in your thread that they still had Dynojet springs in which is no good to me as the bikes gone back to full stock. Will refit tomorrow and check the float levels with a tube. When they were last checked the levels were spot on by that method. Hopefully everything will work, but no road test for me yet as we have got snow everywhere. Nowadays I hate carbs. Good luck with sorting yours out.
Do both slides lift evenly from about 2500rpm up to the 4000rpm? If they don't it may be because they are out of balance or there is a hole / split in a diaphragm.....is the carbtop cover little O-ring in place? If they are lifting evenly, what do you see when you say the engine bogs down? Any black smoke from the exhaust? Any spitting back / backfiring? Is the main needle clip in the 4th slot from the top? Is the airbox cover fitted? With or without the trumpets? And a stupid question, possibly......are the throttle cables properly adjusted so the butterflies open fully? PS. I always use standard NGK plugs and caps, plus copper core HT leads.
Update! Got proper airbox lid and a new filter. But before I fitted them, I started bike up and got to nice tickover...even with no filter... so I could see slides. As you say, these started to lift evenly and equally from 2500 upwards. So I put airbox lid back on and decided to check plugs. Pulled horizontal cylinder cap off. Absolutely no effect on engine tickover at all! Pulled vertical cap off....engine stopped dead! Further checks. Spark plug OK Primary and secondary circuits OK Plug cap...no resistance! Duh!! its a plug cap. Probably been breaking down over some time which sent me on the wild goose chase of carburation. The original riding sensation felt like a fuel starvation but I bet it was that cap all along! Will let you know when I get a new cap!
The tickover wouldn't be a particularly good one if a spark plug cap was faulty......(well, actually it might be good....for a single cylinder engine). By having no resistance are you saying the plug cap had a multi-meter reading of 0...? if so, it should have been sparking better than with a good resistor cap. Depending on which HT leads (ie, copper core or carbon) a way to test for plug cap problems is take the plug caps off one at a time and connect the lead directly to the plug.....
OK, my noodle is now baked! Went back and took my time today (I was excited to have found the apparent cure to my problem yesterday!). Measured both cap resistances. Both about 5k Ohm Measured both primary circuit resistances. Both about 17k Ohm with caps, 12 without. Checked plugs for spark. Saw a spark for both cylinders. Swapped plugs top and bottom. Sparks seen. Buttoned it all back up again. Got it running and revved up to 8k but a bit rough. Let it warm up. Still a bit rough at top end but nothing like as bad. Pulled vertical cap off....engine ran the same. Put it back. Pulled horizontal cap off. Engine stopped. Rechecked vertical plug for spark...none. Checked resistance. Still 17!! So I seem to have an issue with vertical cylinder sparks. Guess I need to replace leads and plugs as well as caps. Fingers crossed Ive not got a coil breaking down.
Have you synchronised the carbs with vacuum gauges? You can do it visually looking at the butterflies and get it very close so both carbs are working in harmony...
swap plugs from one cylinder to the other se if fault follows is so its a duff plug if not can you swap the coils around and try the same cheaper than replacing to find out
I notice from your posts, one minute it is the vertical cylinder that stops firing and the next post it is the horizontal cylinder that does it (or vice versa)....so is that because you swapped the plug caps/plugs around? If it was my bike I would junk the HT leads, plugs and caps (particularly if they are OEM originals). Then buy 2.0m of decent copper core HT lead for a few quid......I had red ones. https://www.autoelectricsupplies.co.uk/product/100/category/22; Two new NGK spark plugs and two NGK plug caps with the rubber seals. Plugs can be bought for about £3.50 each. https://www.ngkntk.com/uk/products/ignition-leads-and-spark-plug-covers/ Also I would check the resistance of the coils... Coil HV circuit resistance: 3700 Ohm +/- 10% Coil LV circuit resistance: 0.34 Ohm +/- 10% Pickup resistance: 100 Ohm +/- 5% You may have to remove the alternator cover if all else fails and check the pickup air gaps.
Sorry Arquebus. Typing trouble. It is the vertical cylinder that is not sparking. Agree, I will be renewing all those. And doing more checks today.
OK. I have been out to the shed and done slow and methodical checks. Swapping over coils, pickups, leads plugs and caps and these all give a spark on horizontal cylinder and non on vertical. So then I tested to pickup resistance in the pin plug for each pick up. Horizontal cylinder 91 Ohm Vertical Cylinder no reading. So I assume I either have a broken wire or an issue with the pickups inside the alternator cover. It looks like the cover will have to come off?
might not have solved it yet but eliminated some parts as said before DUCATI MAKING MECHANICS OUT OF RIDERS lol
Indeed Johnboy. But I am enjoying the learning curve. I have invested in a shed, a proper lift and am building up[ the tools. Just need the knowledge to go with it! Thats why the forum is so valuable. Gotta say it though, its not my favourite machine to work on!
neither was mine at times nearly took a hammer to it on the odd occasion just walked away came back with a clear head however long it took but the ride the bike ........worth it because its not a honda lol
