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r/Fixxit•
achwas

Help with initial carb setup for Mikuni CVs on a 94 Ducati 900ss.

Solved

TLDR: Trying to adjust idle mixture and idle RPM. Can't get idle RPM high enough, without causing rev-hang when throttle is blipped. Leaning idle mixture via screws helps up to a point, but idle is still too low.

Hello carburetor experts, I'm just finishing a restauration of a 1994 900SS CR. The bike has the original Mikuni BDST 38mm CV carbs, they are in good condition, correctly assembled (no thanks to the previous owner...), and clean. In there is a "factory pro" jet kit that some previous owner installed, with a 130 main, 40 pilot, and 70 start jet. I think FP uses slightly different numbers than Mikuni, because I'm pretty sure the main jet is actually slightly bigger than the 140 Mikuni jet that they come with.

The needle jet is their nickeled tube, and the needle is a titanium replacement to fix the wear issue the original carbs often suffer from.

Bike has OEM exhaust and air filter box, with some unknown (but clean) oiled foam type filter in it. So no too drastic modifications to the intake or exhaust parts.

Anyway at this point I'm just trying to get the idle circuit to work right so I can take it on the road and do some proper plug reading. My problem is that the idle seems too low (indicated 1050 and "loping" a bit. Should be 1200), but if I turn the idle RPM up, it hangs at 3,000 RPM after I blip the throttle. I think this behavior can be caused by overly rich mixture at idle, so I adjusted the mixture screws in (lean) in a few step, from 3 turns open down to 1-3/4 turns open now. It tends to rev-hang less this way, but if I go under 1-1/2 turns the idle RPM drop further and I still can't turn them up without causing rev-hang.

What tests can I perform to find out what needs to change here? The plugs don't seem to tell me anything just yet, other than maybe that they could be a step too hot.

Posted Jan 21, 2018, 1:31 AM UTC

715
engineerrol12014 Ninja 1000::2004 ZX636::2006 Ninja 650::2005 CRF450X•

First off, this guide will get your carbs dialed in. Follow it and you'll be happy with the results.

Second, make absolutely certain you don't have any air leaks around the carburetor boots. Also ensure that you didn't pinch the vacuum diaphragms when you reassembled the carbs. Air leaks can cause a lean running condition that carburetor adjustment can not fix.

3
achwasOP•

Thanks. I know I'm going at this backwards by adjusting idle operation first, but i need to get this bike to work properly at idle and low speed before I can ride it somewhere to test the mains and needle jetting. The idle circuit shouldn't be affected much by the main or needle so I don't think it'll be a problem, also the main and needle should at least be close for the setup I have.

Interesting that the factory pro guide says RPM hang up is caused by too lean idle, whereas mine seemed to improve as I leaned it out more. I need to do another run with it tomorrow though and check it again properly.

The bike did have a diaphragm leak from a pinched diaphragm but I fixed that before this.

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Will_Piss_You_Off•

Perhaps it would be worth purchasing the stock jets, and starting with a blank slate?

I also found it interesting that there were no modifications to the OEM air system. Anecdotally, the jet kits I've installed usually required airbox modifications.

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achwasOP•

Well the airbox is oem-ish... The air filter is some other type, and the stock rubber funnels on the airbox are removed, which should mean the bike could theoretically use a little bit more jet than stock. But I put the stock pipes on because the open pipes it came with were China garbage and this thing is more than loud enough anyways. So who knows what that does to it...

You are right though, if I can't get this sorted I should probably source an oem air filter and at least the OEM pilot/start jets. The ones in there are close to the oem numbers, in fact the pilot is the same as the overseas model came with from the factory. But I don't know for sure if they are FP jets or Mikuni.

I'm about to do another test now. It's cold and I'm at sea level, so if it runs too rich here then something is really off.

1
furiousfart•

Bike hang when lean and bog when rich. Don't work Backwards, If everything else checks fine then work on just the idle. When you rebuilt did you clean them and check the idle jets weren't clogged?

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achwasOP•

Yes I cleaned out the carbs, and they had been cleaned before so they weren't gummed up at all.

I managed to get the idle sorted with the screws, allowing me to take it on the highway. I know it's backwards but I had to do that part first to be able to get out of my residential neighborhood...

It is a bit rich at partial and full throttle but not too bad, I might drop the needles one step, but probably leave the mains as they are.

Then a second problem showed up a few hundred km later, complete loss of idle when coming off the gas. I think I have a dead or dying coil or ignition module. Right now it's back on the stands and I have a few other things to sort out before further riding.

1
furiousfart•

Are you're floats adjusted/ not sticking? Also does this have the vacuum operated fuel pump? I'm sure you've checked but how are the vacuum lines?

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achwasOP•

Fuel is pumping and no vacuum valve on this. I checked the float levels as well via clear hose attached to drain ports, they are good.

The carbs had been cleaned and rebuilt with fresh seals by the previous owner. I took them apart anyways to check his work (and because the choke wasn't working). He had mixed up something leading to the starter jet being blocked, other than that everything was good.

I am now convinced that the carbs as such are close enough for the bike to run. The main jet may be too big, but it did run and respond well under load and partial throttle. I also did have it idling well on the second half of my test ride with the screws 2.5 turns out. The problems then came back later that day riding home in a rain storm, and the suspicion is that one of my ignition modules has packed it in. Evidence of this is that both plugs looked the same at my destination (sooty base ring but nothing too dramatic), while after the ride home where it was struggling to run again, the front plug was a lot browner and wet.

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yogi_boyy96' 944SS CR - SPLIT FCR 41s/44-38/FBF 11.1/ST2 CAMS•

The FP should not use different numbers than Mikuni the last I checked (October 2016). They should still be using Mikuni jets only. DynoJet uses their own sizing scale.

  • What stage is your FP kit?
  • What elevation are you?
  • Did you try removing the air-box lid? If so, what happened?
  • Did you test your compression? Carbs can never be properly dialed if compression is off. It's a quick and easy test. I spent 2 months fiddling with my Stage 2/FP Mikunis before I said, "Fuck it, I'll test the compression," even though I had no "logical" reason to do that. It was a measly 100; therein lied my problem.
  • Does the idle become better once the motor is hot, or become better when it is cold?
  • Did you sync the carbs?

When scouring the FP site, you will find the test results from the FP kit. IIRC, nobody used a 130. The main jet has an influence, although every minimal, upon the idle circuitry. I would switch to something between 120 and 125.

1-3/4 - I have never heard of our bikes needing such a lean idle. I think that's abnormally low. Mid 2s to high 3s is normal.

Call Salvisberg if you must; he's a stand-up guy willing to help you dial-in his kit. FP kits are amazing even when tuned to to average spec.

2
achwasOP•

Hello again!

The latest on this is that I probably have the carbs close enough, but I have an ignition issue.

First of all, I'm at sea level and it is cold, 40 Frankenstein or there about. I'm also running stock pipes, and stock airbox minus snorkels, but with an aftermarket foam filter in it (more on that below...) I don't know what FP kit this is (any way to check?) I just know the nickel emulsion tubes say FP, the needles are titanium I think, and the jets say 130, 40, and 70.

I dialed it in on a test ride at 2.5 turns out, with that it ran and idled well for the rest of the way. Plugs looked good at my destination, bit sooty but I expect that since this carb is probably jetted for open pipes and such.

On the trip home I had several die-at-idle events, then it started to die whenever the revs dropped below 3k... I limped home in a rain storm (or rather, ran home..) and found that...

A) my forward plug was now suddenly wet and brown, while they had been the same before the return trip

B) my air filter lid wasn't tight enough and let a bunch of water and sand in the airbox, thanks to the shit aftermarket air filter that is too thin. So my cylinders and pistons may now be pooched.

The wet plug leads me to believe that the forward plug simply wasn't firing all the time at lower revs. Since the problem got worse, and since one of the ignition boxes had already been swapped before, and since they respond to RPM, I think I have an ignition issue.

But the next step has to be a leakdown test, because if I really did wreck the cylinders on day one, then everything else is secondary..

1
xk1138•

I'd get your airbox and filter back to a stock setup, CV carbs behave erratically without the back pressure they're designed for. You should also go through the carbs and replace all of the orings, especially the ones around the air mixture screws. I'd also replace your choke plungers, they can mess with your idle as well.

2
achwasOP•

I wish I had followed that advice on Sunday, and not even because of the mixture... See my post above.

But yes I did go through the carbs with a fine comb before all this. All the seals are new and work, everything is factory clean, floats are set etc. And it turned out the stock setting of 2.5 turns on the idle mixture does work well, but I probably have a dead ignition module that is messing with my idle.

2
xk1138•

Aw man! That's not the update I was hoping for. You're probably right about the ignition since it's only happening after the bike is warmed up, coils can partially fail and still work well when cold. You can have them load tested or just replace them, and for a 94' 900ss just treat her with some new ones (that's my next bucket list bike btw, SP variant though.) If you think sand actually got in, I'd say it's time to inspect the cylinder and piston and start flushing the oil. Hopefully it's not bad enough a simple hone and some new rings won't fix, but I think you'll be better off in the long run approaching it as a worst case scenario. Pics of the bike would be great too!

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achwasOP•

Yeah I nearly had a heart attack when I saw the mess. I am waiting for a compression tester right now that will tell the truth. If it is ruined there are some options but they are all financially painful..

Here is the only complete photo I have, this is at work just before the disaster with the airbox happened.

https://s14.postimg.org/kkxwipyn5/PHOTO_20180121_132942.jpg

And this was under the air filter when I got home. The slides are wet, camera flash just makes it look like they are scratched.

https://s9.postimg.org/s76eauzr3/IMG_20180121_195817.jpg

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[deleted]•

[deleted]

1
achwasOP•

Update: I think I got it. Mixture was too lean at idle. I turned it up a bit which then made it run well, but once the engine got hot riding through some city traffic, it got too rich leading to more weird rev hang issues. Now I'm back to 2-1/2 turns open and it idles well when hot. It pulls good at any RPM without hesitation.

Occasionally it dies when going from highway speed to idle (clutch pulled), not sure what that is about. I'll synch the carbs next though.

1