carb set up

Hi again got the ss running ok few weeks ago hi comp pistons twin plug heads etc had a slight stumble about 5-5500 rpm this has gradualy got worse wont often pull through this but when it does it hesitates almost like it is waiting for the slides to lift then another delay before fuel comes through other times it will just sit there just sort of spluttering/missing have got new plgs new fuel filter timing checked float hieght ok all jets clear dont know what to try next Steve

STEVE being slowly driven mad by my duc but loving it ------mostly

Fuel lines inside the tank all in good condition?
Did you tighten all the clips up when you fitted the new filter?

Wondering if the fuel feed is losing a bit of pressure just when you need it most.

Just a guess


It sounds like a simple (like they ever are!) jetting issue, especially if the bike then pulls through to 8k+. What carb set up are you running? Having done all that work, I’m assuming you have ‘de-restricted’ the exhaust in some way as well? If it’s all still completely standard, then the work you’ve done will only have exaggerated the stumble in the power curve. That 5k stumble (but not as severe as yours sounds) is exactly what disappears from a standard bike after fitting free flowing cans and stage 2 Dynojet kit

I’m trying to rememeber what individual mod cured the prob on my bike, but it’s now running Dynocoils (so 1.25mm plug gaps!), Stge 2 Dynojet, and ‘clean’ exhausts, and it pulls from tickover clean thru’ to the 8k ‘wall’ where the power disappears into the Matmos :frowning: Hence my request for a set of heads to help move to the next level :smiling_imp:

Hi wilbert have hi level silmotor pipes open air box dynojet st2 mix 5 turns out 14mm float hieght 144 mains just went out to give new mix settings ago batt flat and clutch not lifting went to bleed as lever came half way back before doing anything and ******** bleed nipple sheared arggggggggggg steve

You cannot believe how much empathy I have right now over that clutch :cry: Mine was ok until it got disturbed taking it off to get the crankcase off to get at the starter motor bolts (what a ridiculous place to put them!)… I cleaned the slave ylinder and put on a new seal - still leaked; cleaned it again and got a 4 lip seal - still leaked… I just come in from the garage after turning a gnats midge off the cylinder walls and polishing inside with flour paper before refitting the quad lip seal/piston … and now the forecast is crap so I wont know if it’s been a success until next week :frowning: I’ve got another (the better ones off a later model) in the post, which if my bodge works, will do as a spare…

On the subject, rather than spend £100 on what is basically a cylinder and a rubber sealed piston (so if it was off a Ford and called a ‘brake cylinder’ would be about £8 form any motor factors) has anyone on here ever refurbed the old ones by machining the cylinder out and turning a new piston? If so, did they go out to 27/28mm to get a lighter clutch? and if so, where did they get the right size seal from? There seems to be plenty of meat on the cylinder wall… I spy a business opportunity :wink:

I’m going to try something slightly different to get to 944 involving standard 1000 DS pistons and shaved barrels - but need some heads to mock it up first (!!!) I’ll keep you informed of how it goes…

As an aside Steve - your mention of Tim Radley in that email prompted me to check on something I knew he was doing… namely his ‘big bang’ Suzuki… what a lovely noise - just like a Duc :smiling_imp: Whilst at Oulton for a BSB round last year, one of the Rizla Suzukis sounded just like that, but only during warm-up… wonder what they were trying out !!! Why don’t they just admit it - Ducati have got the sound and power delivery right all along :laughing:

Hi Wil sorted clutch wemoto next day £4 instead of 12 locally carbs still not right have been playing with mixture screws yesterday 5 turns out ok till 5000rpm then stops revving 6 turns out flat low down but gets to 6000rpm before stopping also of idle throttle response is better no popping but low down is woffley Iam thinking bigger main jets may be in order here have 150 to try then i may be able to wind the mixture screws back in steve

Hi wil i think 1000ds barrells wont fit as ds is just air cooled no oil st2 barrels look the same on the outside

Does sound like you need more juice from mid to full throttle, and unscrewing the idle is slightly compensating, but useless at top end…

I’m not looking to use 1000DS barrels, just the pistons! They are 94mm with a raised crown, and a hell of a lot cheaper than proper high comp 900 ones because they’re plentiful (I paid £45 for a pair from an engine that wasn’t even run in - owner went for a 1078 kit from new). Unfortunately they are 2mm shorter from pin to crown - so, If I fit them staight into a standard barrel bored to 94mm, I get a comp ratio of about 8.6 - useless (unless I can find a turbo :smiling_imp: ) - but, and this is where I’ll need to do some head scratching… if I can lose that 2mm out of the standard barrel length by machining base/top/combination, I get 10.5:1 compression for £45 plus some machining, instead of £225 - and I’ll probably do some machining anyway to set the squish as tight as practicable (around 40 thou) … I’m after a low revving torqey set up …

Don’t get me wrong here - the cost of proper hi comp pistons is a steal for the 10% power/torque increase it delivers - I’m just one of lifes born tinkerer’s… hence the determination to do my own port work/twin spark as well…

with you now my heads have had work around the valves in the combstion camber to reduce shrouding anthe ports have been smoothed out most work around vale guide boss bump taken out soorry did not take any photsos will look for some pakaging for the heads over the weekend
have just had my carbs off and one of the slides is sticky if i push it up it wont come down is this my problem Steve

Slides should move as smooth as then can, both up and back - check if needles are catching on worn jets - more common than you might think. and wrecks any fuelling… the sticking would explain a stumble which clears at higher revs; and if it’s only on one carb, it’ll throw out the balance between the cylinders and the motor wont rev out! Sound familiar?

Have you seen what arc fabrication (USA) do to their race heads? They basically give up trying to correct the poor standard ports, and bore a 2" hole at 45 degrees through to the back of the valve throat :astonished: Very much one for the trackday bikes…

We got it sorted runs loads better now the air way from top of diaphram was blocked its a bit of a rocket now will have a bit of a play with the mixture to sort out the bottom end tommorrow thanks for all your help your project sounds really interesting Steve

Hi it works properly now blockage in air way from top of diaphram chamber will have to tweek mixture at bottom end your project sound interesting thanks for all your help Steve
Ive just realised i posted twice Icouldnt find it at firs didnt realise it was on page two sorry steve

Had a chance to take a better look at the heads you sent - I hope they were never on the same engine (read below) :astonished:

After saying what a mess the inlet port looked (I’d only looked at the vertical head), I put both heads on a very basic flow rig (vacuum motor and a tube leading to a set of carb balancing gauges) and the ‘front’ head flowed about 15-20% more than the vertical one with the cam at full lift… After getting over the shock, and more than double checking the gauges, I took the heads off the rig and had a good look inside both heads - and it looks like the inlet port work (guide boss pared back, guide 'bullet-nosed, valve stem ‘waisted’) has already been done on the front head :open_mouth: … and with quite good results… big bonus…

I haven’t set up a decent metered flow rig yet, and don’t know what velocities are like inside the ports, but 20% volume increase will do for starters :smiley: I’ll sort out the other head when the missus runs out of jobs to do around the house, then it’s rebuild time… :smiling_imp:

Hi Pete yes the head were both on the same motor do you realy think someone has had ago at just one of the heads Ive got quite a lot of history that came with the bike and no mention of this also when I did the top end job it looked like it was the first time the heads/barrels had been off curious eh Steve

One of the heads has a ‘90’ date stamp, the other ‘95’ so it looks likely that at some stage one of the heads on the original has been swapped for a spare, which has definitely been worked on… I’m guessing it was a '95 bike ? No matter to me - by time I’ve butchered the ports they’ll both be as bad as each other :laughing:

Well I never whoda thought it no onder carbs were out of balance with new heads on Steve