Dellorto ssi29d on my 250

[Bevel bob]

Now that our thoughts are focused on carbs, a puzzle. my 250 starts well and will idle a bit , the mixture shows on a colourtune plug as being bunsen blue(perfect) ,when the throttle is lifted a little it goes white(weak) and stays white. It rides fair but with a tendency to gasp and hesitate at small throttle openings so one would assume a smaller cutaway is needed,(a new snug fit 60 is fitted). After a ride the plug always looks a bit too sooty(dry black) round the mouth and i have thought to drop the needle a notch The pipe is blued.Most other 250 owners seem to favour bigger cutaways. Main jet is 108.Silencer is less restrictive than silentium.New needle and jet is fitted.Its all a bit contradictory??.Does anyone know about alterations to the emulsion tube air holes?And how big (in mm) is a 260 needle jet --2.6 mm?? Its possible a pattern part could be badly made so I'll check the size.I'm also thinking of filing down the slide to a smaller cutaway which would richen the just off pilot mixture and weaken the needle position , What would you try??.

[DewCatTea-Bob]

" It rides fair but with a tendency to gasp and hesitate at small throttle openings so one would assume a smaller cutaway is needed,(a new snug fit 60 is fitted). "

____ I never did get a good handle on any of the SSI-model carbs, and always wanted to lay blame for that on it's peculiar choke set-up !
__ The DUKE-models which came with SSIs that had throttle-slides with slash-cut sizes under #80 seem to run well enough when they were new but, when those models would come back into the shop with over 2 or 3 k-miles put on them, they had by then developed the common stammering/inconsistent 2stroker-like running, whenever the throttle was in a position that's in transition off of the idle-circuit and ridden at speeds at which the resulting air-resistance doesn't require more than half-throttle.
__ Solving THAT particular problem could be done by either swapping the #60-slide with either a #80 or #100 slide, or grinding-down the stock-slide to the equivalent of a #70 or #90 slide.
____ Since your Colourtune seems to indicate that you need to go richer rather than leaner, then I can't be sure of exactly what's causing your particular dissatisfaction with your SSI's carburetion. - (Perhaps your choke-lever is not in the right position.)


" After a ride the plug always looks a bit too sooty(dry black) round the mouth and i have thought to drop the needle a notch "

____ If those rides of yours are mostly at speeds which require less than half-throttle, then your sooty/black plug is consistent with a throttle-slide that's too rich !


" I'm also thinking of filing down the slide to a smaller cutaway which would richen the just off pilot mixture and weaken the needle position "

____ I would not suggest trying that to any slide that's in good-condition !
I've never done that to any slide but, it seems like it may be a worth-while experiment, since those stock-slides sometimes seem, (depending on hand-throttle & cable set-up), to be too tall to get their bottom/front-edge completely up & out of the airflow-port during full-throttle position,, and the idle setting-screw should still allow the slide to go low enough so as to not prevent proper setting of the idle-speed.


" What would you try?? "

____ Certainly before trying that idea of shortening-up the bottom-edge of your NEW-slide's skirt, (so as to effectively REDUCE it's cut-away), you should first try INCREASING the slant of your OLD #60-slide's cut-away, so as to make it effectively more like a #70 or #80 slide.
And then see if THAT modified slide doesn't reduce or even eliminate your particular issue. - (Which I think may just be the very-same common-issue which most SSI's with the #60-slides seem to develop.)


Hopeful-Cheers,
-Bob

[Bevel bob]

No choke on my carb, its been removed and a brass plug fitted at the top, I'm sure it was once a racer.The bike came with an 80 fitted,it didn't work well.I usually belt along on at least half throttle. Going for a bigger cutaway seems competely at odds to the indication I'm getting from the colourtune and blue pipe, you can see why I'm puzzled!!My next step will be to solder or glue a baffle inside the slide to reduce the cutaway just to test the theory, if it doesent work the slide wont be a loss.

[DewCatTea-Bob]

" No choke on my carb, its been removed and a brass plug fitted at the top, "

____ The stock slug-plug in the tube (on the side of the throttle-body), probably ought to still be kept inside, even though it's position never seemed to make much difference (at acting as a choke).


" The bike came with an 80 fitted,it didn't work well. "

____ I suggest that you try grinding that #80-slide to make it more like a #90 or #100, and then give it a try to see how it affects your running-issue.


" I usually belt along on at least half throttle. "

____ That's really not too easy to do, unless you're near roads that keep you above 50-MPH, away & back.
(Depending on the cut-away size, it has very little or no effect above half-throttle.)


" Going for a bigger cutaway seems competely at odds to the indication I'm getting from the colourtune and blue pipe, you can see why I'm puzzled!! "

____ Right... But I'd (instead) try a richer needle-setting, (having already made sure the needle-jet has no plugged holes), (to help make the Colourtune happy), in combo with a #100-slide. - (Providing that you do have that 'common-issue' that I'm aware of.)


" My next step will be to solder or glue a baffle inside the slide to reduce the cutaway just to test the theory "

____ If you ride far at under half-throttle, then I predict that your theory (of trying an even smaller cut-away) will result with an even blacker spark-plug !
You really should try my suggestion with your old-slide, before-hand.


Hopeful-Cheers,
DCT-Bob

[Bevel bob]

After a lot of thought I realised that at the position just off tickover the needle taper is nowhere near doing anything, and coming off pilot the extra air from the lifted slide was just not picking up fuel. The fuel level appears correct but in the interest of experiment i swivelled the float away from the frame spine and twisted the carb on the manifold clip to raise the float as far as it would go, carb top now hitting tank. Expected carb to drip fuel but no!, a little tickle and she fired up first kick and now ticks over better than ever before.I'll let you know how she rides.Maybe now i can drop the needlle a notch to cure mid throttle richness. SSi problems?--just tweek the float!!

[DewCatTea-Bob]

" The fuel level appears correct but in the interest of experiment i swivelled the float away from the frame spine and twisted the carb on the manifold clip to raise the float as far as it would go, "

____ I had expected that someone such as yourself would've already had the float-bowl all straightened-out, (before asking for any further advice).
__ The carb's main-body ought to be adjusted on it's manifold so as to hold the float-bowl vertically, and the bowl should not be either fully forward nor fully rearward (within it's range of swivel-placement onto the main-body).
It seems you've left your bowl at an extreme position, so as to raise the fuel-level to help compensate for another short-coming? _ If so, I don't think that DelLorto intended for that.
But if your Colourtune also likes it that way, then I guess it's okay.


" Maybe now i can drop the needlle a notch to cure mid throttle richness. "

____ Your needle should have 5 clip-positions, the top two would be considered 'Green thinking' settings, but with your less restrictive muffler, the center or the 4th from top -(2nd-richest) ought to be closest to best mixture.
The needle's exact position shouldn't matter much under 5/8ths-throttle and so speeds under 60-MPH, (unless you're a large-shouldered bloke), should not be using fuel that's only metered by just the needle-setting range of carburetion. _ In fact, such lower speeds (with less wind-resistance) should have the engine running on a fuel-mix that's been fairly-well affected by the throttle-slide's cut-away size !
So I think that what's considered to be "mid throttle" richness, is more the fault of the rich slide-number, rather than the needle's exact position-setting.
Keep in mind that the smaller the cut-away is on a slide, the wider range -(of throttle-travel) the slide's effect (on the mixture) is. _ (In other words, while both a #100 & a #60 will have there expected effect on the mixture down to idle,, the #60's (relatively small) cut-away will still be able to influence the mixture at throttle-points where the #100's has been lifted-up out of the picture.)


Dukaddy-DUKES,
DCT-Bob

[Bevel bob]

The carb was previously set up in the correct position and fuel level checked with tube alongside carb to the position shown in the Dellorto diagram, so i have always thought that was it. It seems to me that perhaps there is insuficient venturi effect (too big a cutaway?) or perhaps this carb is especially poor at coping with the strange stuff that they are calling petrol these days. I know it does not vaporise anything like as well as it used to,Spilled fuel takes ages to evaporate . Been out for a run today, starting and idle better,runs ok up to 9000 and flew up to 90 on the (rubbish!) clock.Pickup seemed a little slower but i also backed the timing off a bit so can't be sure,motor runs cooler so the raised level can stay. I have thought the carb would drip if the level too high.I need to do some plug chops to learn more.