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Was asked about this SSD carb problem some time ago by a fellow user here and was told the problem remains, even after trying numerous tuning (jetting) suggestions by myself and others in the group. This problem appears to be somewhat unique to SSD installations, so I've given it some more thought and also discussed it with some racers and Harley owners I know. Here's their consensus:
If you're sure that your idle (transition) jets are correctly sized, the only cure remaining is to enlarge the transition holes (not including the lowest idle hole) at the throttle plates. Both racers and Harley owners told me, and it makes perfect sense, that these holes (fixed orifices) may be too small for the transition mixture VOLUME requirement (i.e. correct mixture, but too little volume) of multi-cylinder-per-throat applications like our 4-cylinder SSD engines.
A lot of Harley owners use DHLA40s on hopped-up bikes and have had to enlarge their transition ports to get rid of off-idle stumble. If you think about it, each barrel has to feed ~700cc or more on a Harley and even more on our SSD multi-cylinder applications. ALL the other jets, etc. are variable, but the transition fuel/air mixture VOLUME is controlled by the size of fixed orifices (holes) and can only be changed by enlarging them.
Remember, these carbs were specifically designed for one-throat-per-cylinder, high RPM, limited displacement engines. Ideally, carb bore size is typically determined by and matched to an engine's single cylinder displacement flow requirement and are offered in a wide range of sizes for that reason. Though ideal and easily tuneable for one-throat-per-cylinder applications, they were never really designed for multi-cylinder-per-throat applications like ours.
Within their design specifications, they work beautifully even in their transition phase. In a multi-cylinder-per-throat, high displacement application, this design becomes somewhat compromised. In effect, each throat must now pass twice the mixture volume for each engine cycle (two revolutions). All the areas of the carb can be adjusted to tune for this except one, the transition phase, where maximum mixture VOLUME is controlled by fixed orifices (holes).
So, if you have an off-idle transition stumble in a SSD setup, you may want to think about slightly increasing the size of the transition holes . . . making sure that both the size changes and the order of them are identical in both barrels, of course.
In case you're curious about my 2.15 SSD carb, I bought my DHLA48 from a racer who had already done this. He basically just sold me his "spare" (only 48 I could find at the time) with my specified jetting in it, but without the shorty, 15mm trumpets (air horns) I'm using inside my fabricated cold air box.
If you're sure that your idle (transition) jets are correctly sized, the only cure remaining is to enlarge the transition holes (not including the lowest idle hole) at the throttle plates. Both racers and Harley owners told me, and it makes perfect sense, that these holes (fixed orifices) may be too small for the transition mixture VOLUME requirement (i.e. correct mixture, but too little volume) of multi-cylinder-per-throat applications like our 4-cylinder SSD engines.
A lot of Harley owners use DHLA40s on hopped-up bikes and have had to enlarge their transition ports to get rid of off-idle stumble. If you think about it, each barrel has to feed ~700cc or more on a Harley and even more on our SSD multi-cylinder applications. ALL the other jets, etc. are variable, but the transition fuel/air mixture VOLUME is controlled by the size of fixed orifices (holes) and can only be changed by enlarging them.
Remember, these carbs were specifically designed for one-throat-per-cylinder, high RPM, limited displacement engines. Ideally, carb bore size is typically determined by and matched to an engine's single cylinder displacement flow requirement and are offered in a wide range of sizes for that reason. Though ideal and easily tuneable for one-throat-per-cylinder applications, they were never really designed for multi-cylinder-per-throat applications like ours.
Within their design specifications, they work beautifully even in their transition phase. In a multi-cylinder-per-throat, high displacement application, this design becomes somewhat compromised. In effect, each throat must now pass twice the mixture volume for each engine cycle (two revolutions). All the areas of the carb can be adjusted to tune for this except one, the transition phase, where maximum mixture VOLUME is controlled by fixed orifices (holes).
So, if you have an off-idle transition stumble in a SSD setup, you may want to think about slightly increasing the size of the transition holes . . . making sure that both the size changes and the order of them are identical in both barrels, of course.
In case you're curious about my 2.15 SSD carb, I bought my DHLA48 from a racer who had already done this. He basically just sold me his "spare" (only 48 I could find at the time) with my specified jetting in it, but without the shorty, 15mm trumpets (air horns) I'm using inside my fabricated cold air box.
