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I've been playing with carb settup and have been unable to completely eliminate the occasional spit back on small throttle openings, I dont think its carb settings more a result of large carb (29D) and Mach1 cam timing which give poor air speed thro the carb and inefective atomisation. I note that there are no air bleed drillings in the needle jet which are a feature on other carbs. I'm getting some spare parts to experiment with.Anyone have any thoughts on the subject?.

Hi Bob

I don't have one of these and have not had one apart, but the drawings make it seem pretty much like others.

this link has lots of SSI info especially recommendations on set up for various displacements and conditions.

regard,
Mike

http://www.thisoldtractor.com/gtbender/dellorto_tuning_instructions.htm

" I've been playing with carb settup and have been unable to completely eliminate the occasional spit back on small throttle openings,
Anyone have any thoughts on the subject?. "

____ If "the subject" is actually the inconsistent back-spitting snapping & pop-firing out-through the intake-tract, then I have my own theory about that...
__ I think that that disturbing effect occurs when a couple of conditions come to meet each other... First, the previous charge (of perhaps slightly rich) air/fuel-mix has not finished burning when the intake-valve opens and exposes the next batch of relatively stagnant incoming air/fuel-mix, and second, that next batch of air/fuel-mix just happens to already be near perfectly atomized & mixed, so that it is very easily touched-off & explosively-burnt (within the nearly closed-off port), thus causing the undesired spit-back popping-effect, back-out through the intake-tract.
__ This situation is no-doubt increased due to a particular muffler -(of some combination of length & back-pressure), in combination with a cam which opens the intake-valve somewhat early.
If such is indeed the case, then when the condition seems to be fairly consistent, then just remove your muffler and see if your regular pop-firing quits or at least gets notably reduced.
____ I'm wondering exactly what muffler you're running which may be assisting the cause of the issue,
or if you've tried more than one muffler, without any notable difference (on the out-come) !??


Hopeful-Cheers,
-Bob

Before you do anything, spitting back typically is an indication of a couple of problems:

- Valves are too tight or sealing poorly. Double check this FIRST.

- Part throttle mixture is too lean. Try going up a couple of sizes in the low speed/idle circuit.

I am vaguely familiar with this as I just converted a four stroke (bleed type needle jet) UB carb to two stroke (solid type needle jet) spec.

If you have a close look you'll see that the needle jet has an air space around it where it enters the venturi. This is where the air "mixes" with the fuel. It's primitive and it doesn't work that well. Rather than trying to make a silk purse out of a sow's ear, why not fit a modern Dellorto? They have much, much more sophisticated metering circuits. SS1 carbs are gorgeous to look at but not the best metering choice out there. You can get polished intake horns for the modern Dells as well.

If you do decide to mess about drilling bleed holes in your SPARE needle jet, you will also have to alter the size of the air supply passage. If I remember correctly, it will need to be restricted. Mikuni air bleed jets are readily available so threading to suit them might be the best way to go.

I tried different slides, 60,80,100, and varied float levels ,richening and weakening pilot settings, valve seats are perfect seal but tight stems is a possibility,I also remember having a new Mach 1 with the same issues, I've tried 3 types of silencer and have found that the best by far is a cigar pattern Norton Dommy that has a bigger outlet and sounds loud!.I'm sure that a modern carb or a smaller SSI say 27mm would do the trick but don't like to give up till I've tried everything.If i was setting the bike up for the Giro or daily use I would fit a 24mm touring carb with a tickover and filter, but thats no challenge at all.I notice fuel staining on the tool box so maybee a bit of resonance tuning is needed.The mixture is indeed a bit rich at 1/4 throttle but going weaker has messed up the pick up off a closed throttle.

" I also remember having a new Mach 1 with the same issues, "

____ Right, the spit-firing is a common-problem with the SSI-carbs matched with wild cam-timing (which opens the intake-valve earlier).
____ Did you ever get to checking your actual valve-timing?
Perhaps retarding it the minimum-step would work better for you.


" I've tried 3 types of silencer and have found that the best by far is a cigar pattern Norton Dommy that has a bigger outlet and sounds loud!."

____ As ya no-doubt expected, those wilder engines really weren't expected to be run with restrictive mufflers.


" I'm sure that a modern carb or a smaller SSI say 27mm would do the trick "

____ I don't think so. _ (But the 24mm-carb & a smaller intake-port may help-out.)


Hopeful-Cheers,
-Bob

Just the nature of the beast (SS1) I'd say.

I run a V2 race desmo cam with a 33 mm carb and have a weakish, wandering 2400 rpm idle but no spit back. I have quite a bit more lift and overlap than yours and my carb is even bigger.