[MotoRoboto]

Thanks for the previous help with the electrical gremlins.

Still doing shake down/fix/repeat runs. Currently fine tuning the carbs. It's a former parts bike '95 900SS/CR. Bike has stock carbs with a mystery jet kit, K&N filter with aluminum ring and Viper exhaust and stock headers. No other mechanical upgrades I'm aware of. It has 42.5 pilot jet, tapered kit needle with clip five from the top, main jet 142.5.

Issue #1 is when I'm at WOT it pulls really hard, like surprisingly so, but then hits a wall at 8k really abruptly and it's almost like engine braking. It will then start pulling again if I lower throttle slightly. Typically I would say that indicates it needs less main jet but I had 140 in it before and it didn't do this but it didn't pull anywhere near that hard either. Before I lower main jets I was wondering if it's indicative of a possible spark issue.

Issue #2 is when cruising at roughly 4k rpm and presumably on the needle I can't snap the throttle without it bogging. Temporarily adding fuel with the choke doesn't help but it also isn't popping out the exhaust. Is this typical of the 900SS stock carbs or does it seem like a carb tuning issue?

Familiar with these carbs but not familiar with them on this bike or the kit that's in it. Last Ducati was a 99 M900S that came with the 900SSie engine. I also had an FZR1000 that used these carbs.

Any advice or tips would be greatly appreciated.

Edited to add: I had a 145 in as well and Issue #1 was more pronounced and happened at 7500 instead of 8000. When I had 140 in it the bike ran poorly at WOT in general but responded almost perfectly with the choke fully applied at WOT. That was the reason I started going up on the main jets to begin with.

 

[OddBird]

All I know is what I have read. According to that info, most 900SS’s that are stock except for slip-ons and open airbox end up with a main jet below 140, like 127.5 or 132.5, etc… 140 was the stock main jet, and is considered to be too large for most, but not all situations. I am new to Mikunis and I’m getting ready to tune mine after installing a jet kit, so I have no empirical knowledge (yet) with these carbs. Although I don’t know much, I would say that you are running pretty rich at 8k at WOT. Additionally, I don’t know how much you should be expecting from these engines at 8k…would love to hear a more knowledgeable comment on this.

 

[MotoRoboto]

All I know is what I have read. According to that info, most 900SS’s that are stock except for slip-ons and open airbox end up with a main jet below 140, like 127.5 or 132.5, etc… 140 was the stock main jet, and is considered to be too large for most, but not all situations. I am new to Mikunis and I’m getting ready to tune mine after installing a jet kit, so I have no empirical knowledge (yet) with these carbs. Although I don’t know much, I would say that you are running pretty rich at 8k at WOT. Additionally, I don’t know how much you should be expecting from these engines at 8k…would love to hear a more knowledgeable comment on this.

The Factory Pro kit goes with a smaller main jet of 127.5 or 132.5 but it's due to the needle that they supply. You can't do that without their needle. The bike didn't come from the factory five main jets too rich. It wouldn't run if it did. The bike shouldn't feel like it's cutting off on the way to redline. My old '99 Monster 900 S had virtually the same engine with the same carbs and virtually the same setup and never did this. Will just keep looking into it. Thanks

 

[belter]

1/ if 140 to 142.5 made that much difference i'd think one of those jets isn't what it said it is. i dynod an ss years ago with open lid, jet kit and 140 and 145 and the only difference in the power curve was the fall away past the power peak. didn't feel any different on the road. 8k is done on these anyway. use it as a reminder to change gear.

2/ i'd lower the needle. i had a 900ss recently with homologated (fixed baffle) termis and it ended up on #2 to stop exactly that.

 

[MotoRoboto]

1/ if 140 to 142.5 made that much difference i'd think one of those jets isn't what it said it is. i dynod an ss years ago with open lid, jet kit and 140 and 145 and the only difference in the power curve was the fall away past the power peak. didn't feel any different on the road. 8k is done on these anyway. use it as a reminder to change gear.

2/ i'd lower the needle. i had a 900ss recently with homologated (fixed baffle) termis and it ended up on #2 to stop exactly that.

Interesting. I'll give that a try on the needle. My suspicion is that it's the Dynojet needle since it runs well enough in the stock 140 main jet range vs the Factory Pro which requires much smaller main jets. I'll also pop the 140 back in and see what happens since at the very least it didn't have the dead spot. The performance improvement could also be related to the fixes/improvements happening to the overall bike as I've been going thru it. The bike was in extremely rough shape when I got it.

 

[belter]

i'd always run a 140 with an open lid regardless of jet kit brand. once the slide's up it makes no difference what needle's in it.

 

[MotoRoboto]

i'd always run a 140 with an open lid regardless of jet kit brand. once the slide's up it makes no difference what needle's in it.

Yeah that's true. I'll def toss it back in there. Will make sure it's the genuine Mikuni ones as well. Came with a bag of three carb sets and random internal parts and jets from different suppliers apparently.

 

[duc96cr]

Are the vacuum slides working properly ? Rising together and smoothly without fluttering as you slowly open the throttle ? Have you checked float levels ? Are you sure the restrictor in the return line is installed right .?

 

[OddBird]

I need to make sure my restrictor is installed- where is it typically located, and what does it look like? Is it just a small orifice pushed into a hose end?

 

[MotoRoboto]

Are the vacuum slides working properly ? Rising together and smoothly without fluttering as you slowly open the throttle ? Have you checked float levels ? Are you sure the restrictor in the return line is installed right .?

I will double check the slides. The last time I had them out to re-clip the needles I did make sure there was no built up gunk etc to affect their function. Yes the float levels were set to 14mm using Factory Pro's trick for these carbs. I was not aware of a restrictor's existence until now. Will have to look into that. The two fuel lines could be switched for all I know.

 

[OddBird]

Pic of the restrictor:

 

[Serious Black]

On other bikes I've owned when running into a rev wall and the main jet stop responding the fuel tap has been limiting flow.

 

[MotoRoboto]

On other bikes I've owned when running into a rev wall and the main jet stop responding the fuel tap has been limiting flow.

That makes sense. The fuel valve is an odd one for sure compared to more conventional ones. Will look into that.

 

[Serious Black]

I must admit it never occurred to me due to the 900SS having a fuel pump. I always assumed that there would be enough fuel supplied. Might be worth experimenting with a float needle seat with a larger hole. I'm sure Belter has probably been there.

 

[MotoRoboto]

I must admit it never occurred to me due to the 900SS having a fuel pump. I always assumed that there would be enough fuel supplied. Might be worth experimenting with a float needle seat with a larger hole. I'm sure Belter has probably been there.

Fuel supply was one of the first vague thoughts that popped into my head when it happened on the bike. Like Obi-Wan telling Luke to use the Force but I brushed it off. lol. The increase in performance when rolling off throttle slightly would also apply to that as well. I've experienced fuel supply issues with hopped up cars before. Kinda similar effect. Surprising how thirsty a car gets with a supercharger.

Editing to add that I haven't been thru the internal tank plumbing yet since everything seemed fine. Will take a look and refresh all that soon regardless since likely original.

 

[spacey]

May not be related to your issue, but be mindful of the ring holding down the filter with open airbox. I had that setup and couldn't get the bike to run consistently or jet properly. Turns out the ring was not holding down the filter firmly enough, it would lift in certain situations and create a temporary lean condition. Went back to airbox lid with the snorkels removed.

 

[MotoRoboto]

May not be related to your issue, but be mindful of the ring holding down the filter with open airbox. I had that setup and couldn't get the bike to run consistently or jet properly. Turns out the ring was not holding down the filter firmly enough, it would lift in certain situations and create a temporary lean condition. Went back to airbox lid with the snorkels removed.

Noted. Will look into that as well. Neither of these bikes had any type of airbox lid when I got them. Had to source a used lid from Japan to get one in decent condition. Ran it without the snorkels and honestly the intake sounded like a small dirtbike. Prefer the sound of an open airbox and got a ring so as not to destroy a nice stock lid. As an experiment I'll run it with the stock lid and snorkels to rule out either issue being a lean jetting condition just for shits and giggles.

 

[ducvet]

Just curious what you are expecting past 8000 rpm on a carby 900?

Have you ever had it on the dyno?
many 900 caby motors peak close to 7500 rpm and start to fall off afterwards unless you have porting, cams etc.

 

[MotoRoboto]

Just curious what you are expecting past 8000 rpm on a carby 900?

Have you ever had it on the dyno?
many 900 caby motors peak close to 7500 rpm and start to fall off afterwards unless you have porting, cams etc.

View attachment 1062066

I'm expecting it to pull smoothly to the redline of 9k not completely drop off. Peak power does not mean the engine will seem to stall one RPM past that. The engine basically stops at 8k and throws me forward towards the tank. I can't imagine that is normal. I mentioned above I had a '99 M900S with virtually the same set up. Open box, jet kit, aftermarket exhaust. It ran perfectly fine all the way to redline. So well in fact you had to be careful not to hit the rev limiter since it would spin up to 9k so quick. The only difference between the two was the M900S came with the 900SSie motor so had the better cams. If that's the difference than I'm definitely picking up a set of 900SSie cams. Don't know that's why I'm asking if this is normal for the carb 900SS.

 

[ducvet]

A injected motor has short intake maniifolds vs the long manifolds of a carby. The long manifolds pretty much give you a drop in power where power falls off a cliff. Also fyi a carby has no rev limiter the red line on the tachometer is a suggestion that you went too far.

If you monster was a carby then it should be similar though ie cams were not stock if it was a carby. The ie cams + short manifolds were where the higher rpm power came from.

If I take a 900ss ie and put long manifolds on it, it too will drop power early. No stock carby 900 ever made power that high, I am not sure I have seen them make power to 8000 let alone 9000.

Are the ignition boxes oem kokosan?
spark plugs resistor type?
plug wires?
coils?
plug caps?
There are some poor boxes that do have a limiter but it often is closer to 6000, oem is best.
If you are jetting your main at wide open throttle you likely have had if open long enough it is not the jet size otherwise you would not get that far. If it is wide open from say 6000 and pulls clean to 8000 then it is not likely jetting unless you are draining the fuel bowls.