750ss Carb Model Not Starting

Mine had Bos race cans, a part open airbox and a Factory Pro jet kit. The fuelling was spot on with 14mm float heights until the emulsion tubes wore oval at around 20,000 miles, when it got at flat spot when coming off a closed throttle. Replacement ones from Factory Pro restored it to its former sweet running.

 

My 750 had airbox trumpets removed; Stage II Dynojet kit; straight through Laser cans (from a 750IE); possibly it had the OEM air filter but it was fairly new.

I replaced float assemblies; emulsion tubes; gaskets and O-rings; diaphragms.

Compression test on both cylinders were 160. I fitted NGK spark plugs and new wire core leads.

When I first stripped the carbs, I found the floats were set at 12mm and 9.5mm.

When I reassembled the carbs I had set the floats at the usual 14mm and it would not run right, even with the pilot screws out as far as possible (without them falling out) it still wouldn't run well.....weak as hell.

A bit of research on the Factory Pro website (that's where I think it was) indicated that they set the floats at 14mm on the square floats; but then I found another paragraph where they showed they were using the diamond float at 14mm.

This made no sense to me, so having owned a ZZR1100 (one of the first) which I de-restricted; I read my notes which I always make when making adjustments, just in case I ever have to go back to stock settings.

I worked out the difference between float height and fuel height....float height measurement is with the carbs upside down.

I also looked at the other method of checking the fuel height; by means of the lines and marks on the float bowls and the tube connected to the drain stub method.....IIRC I did that on the ZZR1100.

I knew that the pilot screw setting was dependent upon the fuel height; so I used the tube method on the 750 and found that 14mm was quite a way off the lines on the float bowls, making the fuel level too low giving a weak mixture.

That's why the pilot screws couldn't get a suitable setting and why the bike was mis-firing like mad.

I then set the float heights at 12.5mm and set the pilot screws at 4 turns out (regardless of the number of turns; don't expect both carbs to be the same); and thus the bike was running pretty much how it should.

In the end they were set at 4.25 turns out on the vert cylinder and just under 4 on the horz cylinder......

.....which is why I say the float heights would have been better at 12mm and the pilot screws nearer to 3.5 turns out (which is the Dynojet setting for Stage II..........the 900 Stage II has them at 4 turns out).

I don't have the answer as to why mine was wrong at 14mm........I can only assume it may have been something to do with the stance of the bike, which in my case was a nose down set-up; throwing the level in the float bowls out.

 

Forgot to mention; also a K&N air filter and it was 900SS.
Pilot screws at 3.5 turns out.

 

Forgot to mention I had a stage II Dynojet kit, a CCW coil kit and Magnecor leads.

I didn't get mis-fires but it did 'hang' quite badly if you revved it in neutral

 

Exploded diagram here, if it helps:
https://www.ducatiforum.co.uk/threads/900-mikuni-carb-rebiuld-kit.11940/

 

It's 20yrs ago but I clearly remember that on my mid-90s 900SS I had Bos cans, DynojetII & 'colander' airbox top. Never had a carburation glitch or issue after setting up both idle mixtures. Bike was less than 5yrs old at the time, so not surprising that it was all ok really - and I always have had the fortunate carb-tuning knack since my teens

 

Under the cross bar......

 

The biggest problem with carbs these days is the shitty fuel we have to use. I only use Shell in my bikes and never use supermarket fuels.

It seems that it goes off so damn quickly that I have resorted to using fuel stabilsers which I never used to.

It certainly isn't good after 6 months standing without an additive.....as my mowers were witness too.....the gunge that was left inside was terrible and a helluva job to get out.

My mate has also got a few classic cars and he has been dropping SU, Stromberg and Weber carbs round for me to deal with.
Even though he used some of them early to mid last year, some plastic components appear to have been attacked by ethanol.

 

Do not ever take your bike back to the arse at that bike shop.The state those carbs were in should of been seen very early on.

 

You have been unlucky, out of the frying pan into the fire. On that note I can't stress more strongly that the best and simplest way to check float levels is prior to stripping as per Brad Black. Often people get in there with their slide rules and just make things worse.

http://bradthebikeboy.blogspot.com/2016/03/setting-float-level-on-mikuni-bdst38.html

 

I only came across that fairly recently. I wish I'd known about it years ago.

 

Agree with Chris, I forgot to do the pipe check first time, really annoying!, carbs on the bench then thought shit if I’d just done the pipe check I might not have had the PIA job of taking them off!, if in future you do it make sure that when you’ve got the pipe attached you open the drain screw very slowly otherwise you’ll get air bubbles that throw the reading off, also wiggle the pipe around a bit til the level settles, I’d do this check when you’ve got the carbs back on anyway to confirm the setting you’ve done has worked, here’s a photo of mine, when the level lines up with the middle of the circular feature you know it’s correct.
(The feature to the right , circular boss with a pin in the middle)

 

Just caught up with this thread. Very informative and lots of good advice. Just to add my two penneth. I winterise my 900SS and have found that if I don't drain the float chambers the floats get stuck and fuel just pours through the carbs and out of the air intakes.
Also I don't think anyone mentioned the cock in the feed line at the tank. WhenI first started reading the threadI was thinking "the fuel Cock is closed" althoughas the story developed it became obvious that wasn't the problem.
Good luck Sideshow.

 

As I learnt to my time cost on my lc project..........