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Discussion Starter #1
Hi guys, my bike runs like crap. A little background first. I did 200 miles a few weeks ago and started to notice very light surging at cruise, say 70ish on the road. It has become progressively worse in the midrange. Wide open it runs great and idle fine. The bike has NOS mikuni carbs from a 600 with all proper fuel internals for a ss. I pulled carbs yesterday and all passages are free from any debris, as it should, they’re new.
The ignition coils and wires were replaced yesterday also with zero improvement. The ignition has been replaced with Ignitech a few months ago. I still have stock pick-ups installed. Carbs get plenty of fuel. Any ideas?? I may try swapping ignition modules back to see if it make a difference.
 

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It could be an issue with the diaphragms which raise the carb slides as you turn the throttle. There is an air line going to the diaphragm chamber on each carb. Air must flow in and out through these hoses as the slides go up and down. If your hoses are original, they can kink or collapse. There is a small canister in each line. It’s a screen filter and it gets plugged. I replace those lines with double wall fuel line so it can’t gtcollapse or kink. I replace the little canisters with small fuel filters. Also, the diaphragms can leak and crack and won’t raise the slides evenly and smoothly. The diaphragm covers can leak, and the small o ring under the covers should be replaced. If the cover or o ring leaks the slides won’t raise smoothly. Remove your air filter and with the bike running slowly open the throttle. The slides should rise simultaneously and smoothly, or the above issues need to be addressed.
 
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Discussion Starter #3
Thanks, I’ll check the slides. The diaphragms are good and not leaking. Currently the carbs are off the bike, I plan to install them this evening and I’ll check them with the engine running. I checked the diaphragm vent hoses and installed new filters previously, they’re free from any restriction. I used Uni filters that push into the vent hoses.
 

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Don't forget to check your chain as well, if the chain has loose and tight spots you will feel a surge at cruise. Put it up on a rear stand and spin the wheel, if the chain run under the swingarm raises and falls then it might simply be time for a chain and sprockets or a bent output shaft. I just had a ST2 in the shop with the same symptoms that was a bent shaft.
 

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Discussion Starter #5 (Edited)
Thanks ducvet, I installed a new chain and sprockets within the year. The bike maybe has 350 miles or so since installation. The hesitation or surging is happening at steady cruise or after closing throttle then immediately open to1/4 to 1/2. Feels and acts like fuel because it won’t take the throttle transitioning from closed to 1/2. I guess a better description would be stuttering when transitioning.
 

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Sadly both lean and rich can cause surging so it can be tough to diagnose. Is your airbox lid open? try running with a open cover and then a closed cover just to feel which made it better, this may give you an idea of which way to go with jetting. Yes a dyno or gas tester would be better but save that for after you have tried all others.

So you used the 900 jets in the 600 carbs? were the pressed in air jets the same? Did you use new 900 needle jets (some times called emulsion tubes) of the right size?
What about the needles themselves are they stock or a jet kit? There are different base diameters on needles so that could be looked at.

Are the slides operating properly if you blow air in the vacuum hoses do the slides rise smoothly?
 

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Discussion Starter #7
The parts were switched from my previous carburetor. The bike ran great with no problems whatsoever when I installed the new carbs, in fact it it improved. The air box is stock with stock air filter. Like I said, the slides work properly when moving them manually, haven’t checked with the motor running yet as they are removed.


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If your needle jets from the 900 carbs were worn teardrop shape from the needles then you will have a rich surge, the amount of wear can be small to have the effect so look very carefully at the needle jet for any out of round. It helps to put the butt of a needle in the hole and then look to see if the gap around the needle is even all the way around, you are looking at the top part that is in the carb venturi.
 

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Discussion Starter #9
I’ve been that route as emulsion tubes were replaced when I made the swap along with needles. The carbs are stock jetting for a 900ss.

I disassembled them and used low pressure(45psi)compressed air to blow out the jet manifolds and double check the tiny jet passages with a jet file or cleaner. The carbs look new, because they are, inside. I’m definitely missing something as I’m at a loss at this point.


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If there's one thing I've learned from this forum, and especially @ducvet, it's that these bikes have a tendency to mask ignition issues as carb issues.

Look over your ignition again. Check your grounds from the block to the frame and frame to battery. Bench test your pick-ups, coils, plugs, wires and caps with a multi-meter. Maybe consider pulling the side cover off and checking your air-gap on the pick-ups.

Are you using the stock coils? I ask because more than one person, myself included, had a single coil in my pair from CA Cycle Works that wasn't functioning properly even though they were new and tested correctly. When this happened to me my issues were the near polar opposite of yours, which I find interesting: back/after fire at idle, felt like a rev-limiter kicked in at 7000 RPM, but mid-range was strong and healthy. I felt like I had waited too long to do anything about it and couldn't, in good conscience at least, try a warranty return on them, but others have had positive results by contacting Chris Kelly with their issue.
 
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Discussion Starter #11
Well, I have the carbs on and went for a test ride. I believe my issues were self inflicted. The pilot screw was 2 1/2 turns out, so I went to 3 1/2 and it seems to run much better. Surging is gone with only a very mild stumble from closed to 1/2 throttle. I’d say, it’s 95% out of 100. That said, I have #40 pilot jets installed instead of 37.5 for US SuperSports. I had #42 installed at one point, but imho it seemed too rich so back to 40’s.

On a side note, the CA Cycle works coils really put the “snap” back in the motor! Wish I would have done this earlier.

Thanks once again to everyone for your valuable input to my problem. I used most if not all the advice offered.
Sorry for driving y’all nuts!




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Glad you got it sorted. Thanks for letting us know what worked, many people don’t.
 
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