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Old Aug 8th, 2009, 8:29 pm   #1 (permalink)
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Advice sought, Keihin Carb Gurus

I am getting my sons 94 M944 monster (high comps, Keihin carbs, open airbox) running again. Sprage clutch went out. Have it all back together and bike turns over fine now. Problem is when its cold it turns over but wont start very easily. It also backfires as its turning over. I blew out all the passages I could find with compressed air in the carbs themselves. Didnt seem to change anything. I disassembled the timing plate getting to the sprage clutch. Upon assembly I noticed the light weight flywheel didnt have the timing marks like the stock flywheel. I kind of winged it when assembling the timing plate since I dont have the marks as a reference. I retarded the timing slightly, but was wondering if the timing was too much retarded if that would cause it to backfire when starting cold. When starting hot no problems at all. And the bike runs like a raped ape. I also backed out on the fuel mixture screws underneath the carbs about 1 turn from the recommended settings. When starting I twist the throttle 2 times then just crack the throttle to start. Any input would be appreciated as I am going to sell the bike and it needs to start easier.
99 ST4
05 Honda 450F
01 YZ 250
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Old Aug 9th, 2009, 6:53 pm   #2 (permalink)
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I'm not familiar with the spark and carbs of Ducatis, my general logic thinks too much advance will cause backfires, firing way before top dead center with maybe a valve open due to overlap, long duration, top end power, cam grind. Cam timing ok? Retarded will create excessive heat up in the rev range, & power flattens out, but easier to start. Over sized (low vacuum to pull fuel) or stock carbs? So many variables, enrichener (choke), slide cut (angle), needle shape (taper), jets (pilot and main) accelerator pump (if equipped), and idle air screw. I've used a sewing needle to clean out jets until I could see light thru them, body soaked in carb cleaner? How does it idle when warm? Spit on throttle application? How strong is your spark? High comp will affect the spark kernel & may tend to blow a weak spark out, strong coils, good wires, iridium plugs? Old gas? I have been surprised on how well the carb worked after I fixed a weak spark; TT 500, '65 XKE, fatboy, tiger cubs, & every used vehicle I bought needed some carb TLC

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Old Aug 9th, 2009, 9:06 pm   #3 (permalink)
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All Ducati's are cold blooded, I think. It could be the backfiring is due to unburnt gas in the exhaust. Perhaps too rich. But I think first thing is to stop winging it (sorry for the harshness) and get a regular flywheel to get timing dead on. Then fuel air mix can be adjusted. Cool that your doing this for your son; man that is great.

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Old Aug 9th, 2009, 9:32 pm   #4 (permalink)
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My 944 likes no throttle (while cranking) until started. Its a good idea to turn up the idle a bit until warm. They are not easy to start cold unless perfectly tuned. Every time you try to start it when cold it needs two or three shots of throttle. That's my story.
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Old Aug 10th, 2009, 5:42 am   #5 (permalink)
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thanks guys for the input. The Nichols lightened flywheel that is on the bike does not have all the timing marks on it like the stock flywheel. I may put the old one on which does have the marks to set the position of the sensors. The workshop manual says to put the horizontal cylinder at TDC and center of the pickups should line up with the marks on the fly wheel. In the past I have always retarded the timing 2 degrees as detonation has been an issue with the high comp pistons. I actually burned up a set of pistons and rings after about 20,000 miles.

The thing is right now after it is warm it starts very easily and idles very nice. The plugs look a good color and bike runs very smoothly. Only cold starting is an issue.

99 ST4
05 Honda 450F
01 YZ 250
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Old Aug 10th, 2009, 12:16 pm   #6 (permalink)
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turn the fuel screws out a 1\4 turn, turn the idle up slightly and blip the throttle 2-3 times before starting.
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Old Aug 11th, 2009, 3:53 pm   #7 (permalink)
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I put the flat sides on my 98 900 FE. Its cranky about starting cold , especially in the winter, backfires through the carb once in a while and the bike really doesn't run well until the oil temp gets to 100c. once its warm its great .
Kevin , Clearwater , Florida
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Old Aug 12th, 2009, 4:11 pm   #8 (permalink)
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My SV does exactly what you describe. I added a set of 41mm Keihin's to it and it runs great...but cold starting leaves something to be desired.

Have you had a chance to dyno the bike with the carbs? My tuner stated that I am more than likely running too rich off idle. Messing with the mixture screws can help, but it's tough to get right, as it starts fine when warm.

I need to get it on a dyno with a sniffer to set up just right.
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Old Aug 13th, 2009, 10:03 pm   #9 (permalink)
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If you have FCR (flatslide) Keihins, they are a racing carburettor, ie. they don't have a choke. The backfiring will be telling you the mixture is too lean when starting. With these you need to give the 2-3 "blips" of the throttle you mention to activate the accelerator pumps to give the richer mixture for starting. Too much can 'flood' the head and it won't start - essentially you need to experiment to find the right amount to start it, then warm it up on a steady throttle. Occasionally you may need to start it a second time when cold. Anyone buying the bike should have this explained to them on the basis that these are a vastly superior carb. once engine is warm. Stronger ignition coils can make starting less finicky, but basically you are dealing with the compromise of not having a choke. Pat
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