old problem ove rlast 20 years , having another go ??? 750ss carbs

[jerryz]

i have owned my 750ss  along time , but the carbs have always been a little of beat and rear cylinder is hesitant at idle , My other ducatis dont do that ..


The  Mikuni BDST 38s run great above 2k rpm all the way to over 125mph++ but are lumpy a at tickover and rear cylinder is hesitant at tickover until 1500 rpm then its fine all the way to 8k rpm . and always have been for last 20 years but its annoying me , so i have the carbs apart again , they were ultrasonic cleaned twice last year and i rebuilt them with new parts , gaskets ,orings , new emulsion tubes ,new diaphragms , new bypass rubbers ,, etc etc but it made no difference ,, only parts i did not change 5C19 was needles,,, and slow running jets , float height 14mm ,, this is bugging me for many years LOL
 
140 main jets,
70 pilot jet 
1.4 slow jet   
5C19 needles new emulsion tubes
13mm Float height
New airscrews  and Orings etc etc

NO AIR leaks , Engine Valves are perfect etc etc
electrics are also all new plugs , Ht leads, coils pickups etc etc ,,

bike runs lovely but once off idle ,,, I want it perfect like my Monster 750 only difference is M750 has stage 2 Dyno jet ....

[jerryz]

Ok so carbs are in A1 condition ,, all double checked again ,,   Going to check the rear cylinder valves again over easter ,,,  other than that im stumped LOL .. but its been an minor irritation for 20 years  so no real bother ...

Also got to put new tyres , chain and sprockets on the M750 . But i have the kawasaki W800 to ride   so not going without therapy 

[koko64]

That 70 jet is the starter/choke/enrichening circuit jet. The pilot jet is in the recess up into the jet block. It's a skinny little thing. They can easily get blocked as do the passageways.
They will probably be 42's. If you get plug fouling you can try 40's.
I've used as low as 37.5's in some 750/900's but only on occasion.
Use 14mm float height.


As a left field suggestion, don't discount the rear coil or too much resistance over the coil, plug and plug cap. In theory that would be 15k Ohms, each being 5k Ohm rated. However, if each item is closer to 6-7, then you could be closer to 19-20. This means using non resistor plug caps or non resistor plugs, but not both. So in summary, if each item is on the high acceptable range of resistance then the combined amount is getting too much under certain conditions. I go for a non resistor cap if needed.

If the plug fires at idle much better when you pull it off and have a 5-10mm gap to the top of the plug you'll know it's ignition and not fuel. Wear some thick gloves so you aren't Earth.

You're basically creating a capacitive discharge system test.
Howie will explain it better.