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At the rpms and throttle pulling away from a stop you are unlikely to be on the needle unless you are drag racing. I have not had to change but very few mikuni diaphragms because they start out more robust than say a keihin that are very soft and prone to failure. Do verify the o-rings are still in the diaphragm area and under the plastic slide cage.
 

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For those of you who have rebuilt these carbs, do you spray silicone on the slide or plunger diaphragms prior to assembly?
 

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Discussion Starter · #43 ·
For those of you who have rebuilt these carbs, do you spray silicone on the slide or plunger diaphragms prior to assembly?
I don’t. I don’t think hats necessary. Am I wrong about that?
 

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I'm using silicone spray on the o-rings. I don't know about using it on the rest of the seals, like the diaphragms.
 

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I have factory pro kit in the carbs right now. No, I haven't done the diaphragms yet, I haven't found the replacement (I saw one on eBay, but that's about it so far, also should I replace the slide as well? The diaphragm feels fine, by the way, very supple, but you are right, it is 25 years old after all). I don't think I have any issues with vacuum operated carbs, other than that I don't know enough about it.
You don’t need slides unless yours are damaged in some way. Just shine them up so they slide smoothly In the grooves.
 

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When the slide starts to rise and pull the needle up with it , fuel starts to be siphoned through the
orifice the needle is being pulled out of. Since engine vacuum is present , or the slide wouldn’t rise, it starts to pull fuel through the gap between the needle and jet.
 

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Discussion Starter · #47 ·
I can’t make heads or tails of it. I put the carbs back to “baseline” with 37.5 pilot, floats at bang on 14mm, everything cleaned again. Still don’t have new diaphragms yet, but still. It starts right up, idles just fine, it pops less from the carbs, a slight - not much, but definite - improvement as pulling away, but then now it pops from the exhaust as I pull away, which it never did before. The indicates that it’s rich. It makes no sense to me! Im beginning to wonder if it’s an ignition issue, particularly 3 degrees retardation at the pick up that I did for 944 kit.
 

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Are you sure the popping isn’t ignition related ? Also, fresh fuel with no additives ?
 

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Fuel screws ?
sync?
exhaust leaks?
Its not the timing, you are still in a part where it should not be popping if warm.
Verify the enrichener plungers are BOTH fully closed
 

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The stock spark is on the weak side. I’ve upgraded coils , wires, caps, and plugs. Fuel is actually a bigger factor than you might think.
 

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Discussion Starter · #51 ·
Thanks for advice, and keeping me company during my extremely frustrating process! If I were Elvis, I’d have shot it tonight. Well, I’m no Elvis, thank god.

I played with fuel screws, it seems to be happiest around 3.5 turns out. The enriched hasn’t been touched, and it’s closing as well as it before. I did my best looking into exhaust leak and found none, the fuel is new, coils and all that are renewed last year with ca cycle works exactfit unit. You know how it sounds when rich, and it does sound rich, and again, with the airbox lid off, I watched it carefully, and just right off idle, with throttle just barely cracked open, the slide starts to move, like 1400 rpm or thereabouts, that’s where it hesitate, bogs a bit (less so than with bigger pilots) but this time slightly above that is accompanied by”bop” from the exhaust. Just to test, I leaned out the needle, which made no difference. I’m absolutely stumped. And it still runs rather very well once it passed that. I’m giving up for a night, am gonna make a martini…
 

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"coils and all that are renewed last year with ca cycle works exactfit"

I may have this wrong, but wasn't there a bad batch of CA cycle works coils not too long ago? Some intermittent ground issue or something?
 
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Discussion Starter · #53 ·
"coils and all that are renewed last year with ca cycle works exactfit"

I may have this wrong, but wasn't there a bad batch of CA cycle works coils not too long ago? Some intermittent ground issue or something?
Hmmm…I do have good spark. How can I check the intermittent ground issue? It’s a part that was working fine prior to rebuild, so it’s low on the list of things to investigate, but it doesn’t hurt to check.

I must be missing something, I can’t think that it’s a component that worked before, it must be something I’ve done. It’s not the rebuilt itself, everything checks out fine on that front. I’ll go through the carbs again(!), I’m going to get the new diaphragms (they are wicked expensive!), double check the static timing, too (while at it, I may might as well do what belter suggests and modify the flywheel hump). It’s really been frustrating. I’m missing the riding weather.
 

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What (whos ) needle jet is in the bike iirc it should be Y-2 or Y-4 but should be verified. Is it worn, any wear will cause rich.
What is the needle?
Not just position but the butt of the needle that is metering fuel when mostly closed. aftermarket needles are usually thinner here than oem and there are 3 different diameters so you can get different results. If you have stock needles you might try them just to see if the off idle is better.
 

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Discussion Starter · #55 ·
What (whos ) needle jet is in the bike iirc it should be Y-2 or Y-4 but should be verified. Is it worn, any wear will cause rich.
What is the needle?
Not just position but the butt of the needle that is metering fuel when mostly closed. aftermarket needles are usually thinner here than oem and there are 3 different diameters so you can get different results. If you have stock needles you might try them just to see if the off idle is better.
When I went through the carbs this time, I put in the Factory Pro needle and emulsion tube, along with the rest of the kit. I just happened to have the unused kit at hand. Previously, I had Dynojet kit that came with the bike. So the Factory Pro kit wasn't tested before the engine rebuild.
 

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I feel like the Factory Pro is the best TUNING kit, the needle and emulsion tube , but I think it was the DynoJet kit that had the lighter slide springs , which really help throttle response. I think the needle and seat came from a Keyster kit, maybe the diaphragms too, can’t recall.
 

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So what size is the factory needle jet?
If you measure the factory needle at the butt and compare it to oem or the old dynojet needle in the same area what size is it?
There are plenty having great luck with factory jet kits and needle jets but imho when you start making changes without knowing what the change is you can end up lost. Oem needle jets are measured and marked different for 600,750,900 etc so what is the needle jet now marked for size? did you get the correct size?
 

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I don't know how much this might affect things but since you are not making any headway it might be something to check.

On my E900 I documented the stock setup when I put in the Factory Pro kit. The kit was not a good move for me as it was just an SS900 kit with a different sticker on the box. But the E900 had 750 heads (and other differences) so it was way rich.

There is an offset between the pilot screws and I assumed that was due to their wanting the rear cylinder to run slightly richer to keep the heat down and possibly to compensate for the difference in the angles of the intake manifolds.

In any case, the chart below shows the stock offset and the number of turns to keep the same offset. The guy inside me who splits hairs says that the percentage of offset would probably need change to maintain the actual A/F offset but this at least would give you something to try.

My thinking is that in addition to any lag in the diaphragms lifting if the offset is for the manifolds issue, not having the offset might also be impacting things.

There are also the specs for the E900 carbs which my understanding was that they were pretty much the same as the SS900 carbs except for the needle and jets. In the USA, the OEM E900 jetting was very lean and there was a pronounced stumble at 3800. The Euro jetting of the 42.5 pilot jets and the 5C19 needle eliminated that 100%.

As for the intermittent ground issue, if all of the connections to the coils and ignition boxes are tight, then the ground is not going to come and go and if it did it would be across the full RPM range, not just at a point off idle.

HTH.

Thanks,

M./

mikuni.pilot.screws.png e900.mikuni.specs.png
 
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Worn stock emulsion tubes at this point in a carbines life are inevitable. For some reason, the stock needles and emulsion tube would be the first thing I would replace, especially since it’s a a change in design of the emulsion tube and the taper of the needle, along with adjustability of needle. The emulsion tube is nickel plated and the needle is titanium, both to reduce wear.
 

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Of interest, My mods didn’t seem to affect fuel mileage much, if you care. Some of the DynoJet kits I’ve used in other makes resulted in really bad mileage.
 
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