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.

[Howie]

Check the clearance and  resistance on the vertical pick up .6-.8mm, 95-105ohms.  Coils too, primary 3.8-5.2 ohms, secondary15.6 -23.4 ohms

[jerryz]

Check the clearance and  resistance on the vertical pick up .6-.8mm, 95-105ohms.  Coils too, primary 3.8-5.2 ohms, secondary15.6 -23.4 ohms

i have done that  , i also took the coils and pickups off the Monster 750  and tried them and it made no difference to the 750ss ,, when i put the     750ss coils and pickups and HT leads etc on the M750 it runs absolutely sweetly at tickover , even and crisp... 

Thats why i am sure its  not electrical  ..mind you when i swop the carbs to the M750 they also seem to run  better at tickover   Over last 5 years I have looked at this Anomaly several times ,,  I have even changed  the long intakes  from M750 to 750ss and all the rubber carb mounts and seal,s


even the cold compression is OK on both cylinders at 146psi front and 145 psi  rear  .

it make just be because the 750ss is a very early 1991 bike number 693   and the M750 is a year 2000???

im going to check the valve  clearances  on rear cylinder again, before i put the carbs back on  but I did them only 18 months ago and the bike has only done 700 miles in that time , as i have so many other rides ...

[jerryz]

KOKO the pilot jet is .40 ,, same in my M750 which is perfect   the manuals usually say 37.5  for 750ss

Also usually the Horizontal cylinder needs 3.5 turns on Airscrews like my M750  BUT the Vert cylinder has always needed  4- 4.25 turns   which is again an anomaly   ,   

But as i have said once off tickover the 750ss  pulls evenly and smooth like a rocket  up to over 125mph

[koko64]

I reckon I'll re-read the whole thread again to get my head around it. It certainly seems like you have gone through a long diagnostic list better than most.

A thought, tick over quality can sometimes be related to closing shim clearances or valve/valve seat condition.

If a bike has been rescued from a salvage yard and been sitting for years, a valve seat/valve face may be pitted due to the fall of the cams exposing it when the bike was turned off.

Seen and heard of rear exhaust valve seats (iirc) being weathered from bikes that sat. A couple of guys alerted me to that years ago when I bought a salvage bike from Japan.

[koko64]

Another thing. Carb synch is sometimes more of a compromise on some bikes than others. Some bikes, once carb synch is set, have an even synch all the way from idle to higher revs, while others require you to decide where the less than ideal synch should be. It may just be a 500 rpm zone, but If that zone can't be easily tuned out with jetting, cam or ignition changes, then you have to decide where to put it.

[koko64]

Check the clearance and  resistance on the vertical pick up .6-.8mm, 95-105ohms.  Coils too, primary 3.8-5.2 ohms, secondary15.6 -23.4 ohms

Maybe rear pick up gap is a bees dick wider. It has been suggested that inductive pick ups get a weaker signal at low rpm.

Fastbikesgear in NZ recommend a closer than oem clearance for the pick up coils to improve idle and low rpm running. They suggest 0.4 - 0.5mm due to the weaker signal at low rpm, or as close as possible plus a bees dick for thermal expansion.

[ducpainter]

I'm thinking back to the mid 80's when cars sucked. They had relatively primitive electronic ignitions, carburetors, and were basically pieces of shit.

The biggest problem Ford had during that period was valve guides, and when they were bad...every single one...they idled like shit.

Just sayin'