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Discussion Starter #1
900ss sp 1995
K&N filter
Unmodified air box w/o snorkels
FE exhaust
Pilot 40
Fuel screw 2.75 turns out
Factory pro needle, clip /2
127.5 main
Altitude 6000 feet, 80-90degrees, very low humidity

Bike starts easily enough. Good "blip." smooth acceleration, no flat spots. At wot it sounds a little flat but there's no lean surging. Some popping on deceleration. High speed roll ons from 70-90 are frighteningly fast.

Does that main sound right to you? I mean, factory is 140. It feels right, just sounds a little lean. I dunno. Should I try a bigger main, or just go ahead and put it on the dyno and see where I'm at?
 

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If you have Dyno access I'd go that route. If the operator knows his stuff, he'll be able to tell you if you need richer jets. If you let off the throttle very slightly from wfo, does the bike pick up or pull better ? If so go up one jet size. Really though, that main jet seems about right for your set up. Most people are rarely on the main jets anyway. Most of your riding is on the needles. What do your plugs look like ?
 

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Discussion Starter #3
If you have Dyno access I'd go that route. If the operator knows his stuff, he'll be able to tell you if you need richer jets. If you let off the throttle very slightly from wfo, does the bike pick up or pull better ? If so go up one jet size. Really though, that main jet seems about right for your set up. Most people are rarely on the main jets anyway. Most of your riding is on the needles. What do your plugs look like ?
Eh kinda tan. But I haven't done a full throttle chop. It doesn't surge on roll off.

If it sounds like I'm close I think I would rather invest the 60 bucks in a dyno run than dismantle the bike again to swap jets. Sheesh, you can swap jets on an el cheapo vm36 in about ten seconds.
 

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Discussion Starter #4
Dyno results

I'll keep this post updated since you don't see much data for high altitudes.

I think my exhaust has been cored; I look in there with a flashlight and I don't see any baffles, and it's loud as all ****. So here's the current setup:

K&N filter, stock air box w/o snorkels, cored exhaust

Pilot 40
Mix 2.75 turns out
Needle /2, no shim
Main 132.5

85 degrees, 6000ft, low humidity. (Density altitude 8671 feet!!!)

Bike feels fine, sounds a little flat

Dyno shows an air:fuel ratio of about 15 all the way across. Peak torque is 56 at 5100 and peak hp is 69 at 7500.

I kind of thought that setup sounded lean, although for the altitude the bike is making excellent power, maybe that's what Factory Pro is after but I think I'll richen it up for the sake of the engine. Gonna pull the needle up a notch (i.e., put the clip on /3) and try a 140 main if I can find one. Comments are welcome, I don't know what I'm doing.
 

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I'll be interested to know how this turns out. I've never dyno'd my bike but I would have thought the air:fuel ratio would dip down to the 12's under hard acceleration, Moving the needle up is what I'd try too
 

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As much as it is a PIA to change jetting on these bikes I would not recommend making two adjustments at once. Either pull the needle up a notch (or perhaps add a shim) or increase the jet size. Small increments usually have pronounced effects.
 

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Discussion Starter #7
Incidentally, parts are getting a little hard to come by for the mikuni cv carbs.
1. Soft parts (gaskets, o-rings) are available from Ducati.
2. Needles are discontinued. Based on my measurements with a vernier caliper, I don't see anything in the mikuni catalogue that will substitute. Only way to get needles is in a jet kit.
3. Needle jets are available from Ducati, and nice nickel plated ones from Factory Pro.
4. Mains are hard to find, many are on back order.

With regard to Dynojet vs Factory Pro, the former has a better selection of main jets and the latter has better emulsion tubes. If you have a stock setup I like Factory Pro, and if you're highly modified you should look at Dynojet.
 

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I would try sudco. I couldnt get some air jets from the dealer for some mikuni cv carbs on my GSXR. Discontinued. Went to sudco and they found me a suitable replacement. They were really helpful and have a ton of parts for all kinds of carbs.
 

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Harley used the same CV carbs in a few of their bikes, shouldn't be too tough to track those down at a harley shop.
 

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Discussion Starter #10
I would try sudco. I couldnt get some air jets from the dealer for some mikuni cv carbs on my GSXR. Discontinued. Went to sudco and they found me a suitable replacement. They were really helpful and have a ton of parts for all kinds of carbs.
Even Sudco is low, but they did have 140 and 150 which should get me close. Bike came with a 155.

When I first got the bike, it ran pretty well at wfo, but was rich as crap at 1/4 throttle, which I think was due to wear on the needle jet. So I think I'll put the 155 main back in, eliminate that variable, and work on the needle position for now.
 

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Your local Yamaha dealer shoulr be able to get you Mikuni parts too. Plenty of Yams use them.
 

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I think 92-93 yam TDM - 850, but not cheap and needle & seat available only as set for $ 60 ! Look up my old post for p / n .
 

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Jetting adjustments

Hey guys. Going along with the jetting questions...
I rebuilt my 97 carbs with the factory pro kit. My factory pro and Chris at ca-cycleworks recommended settings are stock main and pilot jets, the new needles at 2nd clip position (from the top), float heights at 14, air mix screw at 3 turns out. I installed new emulsion tubes as well. All resulting in a very, very lean condition. :mad:

I have cored Vance and Hines SS2r cans and K&N pod filters.

Whats the best next step as far as making bold adjustments to rectify the lean condition.

Thanks in advance.
 

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If the revs hang and come down slowly when you release throttle, screw your air screws out half turn at a time til revs drop quickly on release. Make sure the screwdriver fits in the hole. Or, replace pilot jet with larger size. Raise your needles one notch at a time unless plugs were bone white, then go two notches. This will profoundly effect fuel mileage, and will also richen the entire range. Better rich than too lean. I tuned mine at sea level. Gutted stock pipes, k&n with no lid. 40 low jet, 5 turns out on air screw, 140 main, Ti needles one notch from bottom, nickel emulsion tubes. I think this will get you close.Good Luck.
 

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If the revs hang and come down slowly when you release throttle, screw your air screws out half turn at a time til revs drop quickly on release. Make sure the screwdriver fits in the hole. Or, replace pilot jet with larger size. Raise your needles one notch at a time unless plugs were bone white, then go two notches. This will profoundly effect fuel mileage, and will also richen the entire range. Better rich than too lean. I tuned mine at sea level. Gutted stock pipes, k&n with no lid. 40 low jet, 5 turns out on air screw, 140 main, Ti needles one notch from bottom, nickel emulsion tubes. I think this will get you close.Good Luck.
made the adjustments. 40 pilot, 4.0 turns out, 144 main (my stock), 1 notch from bottem. I also sync'd the carbs this time. This really made a huge difference from before. However, low range is sluggish and blips, mid range is flat, top end sounds and feels okay. What can I do to regain that low end torque and eliminate the flat mid? Floats?
 

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Does it run better or worse in the low to mid range when warmed up ? If it runs worse, it's rich, if it runs better, it's lean.
 

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Does it run better or worse in the low to mid range when warmed up ? If it runs worse, it's rich, if it runs better, it's lean.
Took it out to full warm. Its bogging down through the whole power band. Idle is solid, but as I roll on power it takes a lot to get going. Im assuming im rich as I can also smell fuel and had a back fire at startup. Im still at 40, 144, 4 out, 1 from bottom.

Im thinking of dialing back the air screw first. Then go with the 132.5 Main that came with the kit. Then move up the clip. Thoughs?
 

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Discussion Starter #18
Hm. Messing with the needle, /2 is too lean and /3 is just a tad too rich. I put it back at /2 with a .02" shim and it feels pretty good.

Thanks to the good folks at Sudco I was able to locate a 140 main and that works like a dream.

I might re-dyno it but I figure I'm rich of peak any way you cut it.*

Observations:

When the main jet was rich, I noticed it transiently bogged down when I whacked the throttle open in pretty much any gear, most noticeable in low gear.

When the main jet was lean, the exhaust sounded kind of weird but it ran fine. Word to the wise.

When the needle/needle jet is too rich, it affects performance when you're cruising down the highway at 1/4 throttle. If you roll on the throttle, it's kind of sluggish; and more dramatically, when you let off the throttle suddenly it burbles and bogs down. With the degree of engine braking these bikes have, it really gets your attention. It feels like you're riding around in first gear; you have to whack the throttle to go, and when you ease off you almost go flying over the triples.

I wound up (surprise!) with exactly what Factory Pro recommended, which was unanticipated at this altitude, although I'll note that when I got the bike, it was set with the needle at /4 and a 155 main. Presumably for sea level.

Summary:
900ss (1995) Mikuni CV carbs
K&N filter, with lid, no snorkels
Cored cans
6000 ft, 85 degrees F, low humidity
Idle: 40, 2.75 turns out
Needle: /2 with .02 inch shim
New needle jet (emulsion tube) from dealer
Main 140



*I don't know about motorcycles, but aircraft reach peak exhaust gas temperatures and peak cylinder head temperatures at the theoretically ideal air fuel ratio of 14.8:1, which happens to be right where I was when I dyno'ed my bike. Running a few degrees "rich of peak" lets the engine run much cooler and dramatically reduces the likelihood of catastrophic engine failure. Plus, for reasons I can't comprehend, best power is usually a few degrees rich of peak. Since I richened both the needle and the main jet, I'm clearly rich of peak. I don't know what my margin is, however, and when it gets cooler this fall I might drop in a 145 or 150.
 

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Discussion Starter #19
Took it out to full warm. Its bogging down through the whole power band. Idle is solid, but as I roll on power it takes a lot to get going. Im assuming im rich as I can also smell fuel and had a back fire at startup. Im still at 40, 144, 4 out, 1 from bottom.

Im thinking of dialing back the air screw first. Then go with the 132.5 Main that came with the kit. Then move up the clip. Thoughs?
Some people go from top down, others from bottom up. I don't think taking things out of order is a very good idea. In your case, I would be leery about trying to jet your mains until you get the needle/needle jet thing tuned. You can easily tell if your idle mix is OK, but if you're whacking the throttle open from 1/4 to full, you are looking at a lot of variables there.

I didn't think these mains came in 144? 145 is probably OK but if you're going to do bottom up, you are actually better off excess capacity in your mains while you are working on idle and needle.

IM me your address and I'll send you my 155's if you would like to try them.
 

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"Plus, for reasons I can't comprehend, best power is usually a few degrees rich of peak"

Apparently this is because rich engines run cooler, lessening detonation and pre-ignition and also making sure all the oxygen gets burned.

I highly recommend the Haynes Fuel Systems Techbook, it as absolutely chock full of information about carbs and fuel injection from basic theory to the chemical makeup of the fuel itself.
 
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