Dropping Cylinder At Idle/low Load - Not The Common Culprits

That's good, worth keeping an eye on it.

Admire your perseverance BTW! Good luck with sorting it.

 

Running too rich on the idle or slow circuit will be due to blocked/dirty air filters and/or jet sizes that are too big.

Running rich on the fast circuit ie wide open throttle will be the same reasons.

Running rich on the interim circuit will be blocked/dirty air filters and/or a worn needle or worn main jet in which it moves up and down.

You mentioned a PO had installed an aftermarket exhaust, maybe they upjetted but went too far.

I would verify the sizes of the jets you have, it will be stamped on the side of them, compare that to what they should be as it left the factory, get some jet kits (2 sets) and go back to factory standard and work from there.

As for runnning well when you forget to turn the fuel on; you’re starving the carb bowls of fuel so the level drops and less fuel can get sucked up through the jets. Which, on the face of it, confirms your ‘it’s running too rich theory’.

The float needs to be a set height so there’s never too much or too little fuel in the bowl. Too little it will run lean or not at all, too much it will run rich or not at all and possibly overflow into the inlet manifold or piss out of the carb over the engine cases.

 

Good points. Do you actually have the original exhaust? If so, might be worth re-fitting it then stating with a standard set of needles/jets, as WCP suggests.

 

Assuming they’re Mikuni you can get jet kits that contain a range of idle and main jet sizes so you can play around with different sizes. They’re readily available on eBay and Amazon, genuine Mikuni will be a lot more expensive than some of the Chinese copies which most offered for sale are.

I am informed the machining tolerances of the genuine ones are a lot better than most (but not all) the Chinese ones, you pay your money and take your choice. That said, I’ve bought the Chinese ones for Keihin carbs, fitted them and they’ve been fine. Mind you that’s been on some 30 year old IL4s and other old stuff where the client balked at €20/jet.

 

For anyone reading this thread who wants to understand the inner workings of a carburettor I found this manual useful.

Hopefully it’s not too big to share here.

 

I 've also been watching this thread and I'm impressed with your persistence Cammy. I owned a '95 900SS for about 5 years a while back. The carbs are the same apart from marginal changes in the jet sizes. My bike had Bos open cans and I ran it with a Factory Pro Stage 2 jet kit. All that really did was change the needles to a different taper and the springs above the slide were less strong, presumably so that the slides reacted quicker to a change in throttle opening. Factory Pro jets, unlike Dynajet, use the same sizing numbers as Mikuni. The bike ran fine with the standard 140 size main jets.
However at around 18,000 miles it started to run poorly at smallish throttle openings, with a flat spot when opening up from a closed throttle. I found I could tune this out by leaning the pilot mixture but then the idle was crap.
The cause was worn emulsion tubes (needle jets) which is a very common problem with Ducatis running Mikuni carbs.
I fitted new emulsion tubes and all was fine again.
I ran this bike a 14mm float height (some folks prefer a different level) and with the pilot screws 3.5 turns out and it was one of the sweetest running carburated bikes I've ever owned.
The carb specs for your bike are:-

Type :BDST 38 B 261A
Main jet : 132.5 (or may be 140)
Starting jet : 70
Emulsion tube :Y6
Needle :5CJ01-4
Pilot Jet : 42.5​

I've attached a couple of files which, although not specific to your bike, you should find useful for getting the cabs sorted out.

 

Wasn't it always that way ?
...... and not just with Ducatis from that era .

I just took it to be all part of the "fun" .....

 

From the Factory Pro website

 

This is the text from below the picture that Derek posted - I copied it to make it easier for people following the thread to find:

It seems that you shouldn’t just replace the jets but also the plastic slide guide to prolong the life of the new jets.

“Top Left side - This is a picture of the needle jet area of an emulsion tube from a stock 38mm CV Mikuni carb as installed on Ducati, Yamaha TDM/TRX 850, YZF750, FZR1000, etc..
It has approximately ~5000 miles or 8000k usage on a Ducati or TDM 850 - WITH A STOCK Aluminum NEEDLE.
Notice that the center hole is noticeably worn at the "V", the 12 o'clock position, which would be towards the engine, as installed in the carburetor.
This causes 2 conditions. First, it allows more fuel to enter the carburetor, as in using a needle with a small base diameter AND the fuel that enters the carb - it enters in larger droplets, resulting in poor combustion.
We carefully check needle jet condition on the vehicles when we create and install carb recalibration kits.
If you install a kit and it is much too rich at low rpm, and lowering the fuel level 1mm doesn't fix it, check the needle jets for any wear - it's not uncommon.

Additional: Eventually, in the 36mm horizontal mount and 38mm downdraft Mik's, the carb body plastic insert, where it holds and guides the slide from rattling back and forth, will wear out - allowing the slide to rattle and bang the needle fore and aft in the needle jet orifice - causing accelerated NJ wear. This wear is biased towards the lowered slide position - as in idle and low rpm areas - as in "in-town", "stop and go" driving.
The gist of that is that you may only get 5,000 miles out of your needle jets in your VL / VS bike that you use for cruising 4th Street and hanging out at McDonalds - but your long distance , highway cruising buddy might get 25,000 miles before mileage deteriorates to 25mpg or worse.
I haven't heard of anybody who has got the "60,000 miles" that some Tech Bulletin claims is normal with a stock needle...... I WILL change this if I ever find that person who made it to 60,000 miles on his/her VS / VL and has the original needle jets/emulsion tubes. In fact - I'll GIVE any carb needle jet rebuild to any VL 1500 owner who makes it to 60,000 miles on the original needle jets! Marc

Slide Guide Wear:
Example: The original needle jet lasted 5,000 miles in your usage. After replacing the worn emulsion tube / needle jet, they only lasted 3,000 miles or less. That's because the slide guide slot in the carb body is worn - not because of the emulsion tube or needle is bad.
Even Suzuki knows that - even though their highly optimistic estimate of 60,000 mile life with a stock needle is based on perhaps a VL1500 or VS1400 run at high speed on a highway.

A "step" inside of the needle seat in the carb slide?
It's there in virtually every Mikuni CV carb. It's supposed to be there.
It's to try to keep the needle bearing onto only one side of the needle jet - otherwise, the needle bangs back and forth onto both sides of the needle jet at idle and low rpm, instead on only one side.
Of course, if one "notches" the plastic needle shim, they are lowering the needle about 2 clip positions and that will lean out the mixture, but that doesn't really permanently fix anything.”

 

Thanks for posting that. I tried to post a link to it but the Web page is made up of frames that I couldn't link to individually.

 

That website is like a visit to the late 90’s internet!

 

Carb rebuild kit part number is mk-bdst-38-b67 - as you need 2 of them it’s not cheap.

Allens Performance Ltd should be able to supply everything you need.

Carb assembly video from Brad the Bike Boy - has some good info as he talks through the process.