twin TB thumper

[BenDiesel]

so im working on converting my old 86 xr600 to efi with a microsquirt. the main reason im doing this is because there is no aftermarket cabs available for me and stock replacement parts for the carbs are next to impossible to find. the reason its not supported by the aftermarket is they are a twin carb bike. and only did it for 3 years. after that they changed the frame a bit to accommodate a single carb. i think twin 28mm is better then single 39 imo, but that's just me.

i been working with captain midnight (who efi a single carb xr a while back). he made the wheel for me and did some machine work on a pair of gsxr600 tb to get them to fit a spare intake boot i had.



you can see space is a little tight between the tb. any idea on how to sync them together? the carbs had the advantage of the throttle link edge being above the center line of the intake port.


the other advantage they were set up in a progressive link edge. kinda like a primary and secondary on a 4bbl carb. one carb would be used up to 1/3 throttle and then the other would open and both would max at wot. this was to help keep the front wheel on the ground should you snap the throttle open to fast and to keep air speed up though the carb for atomization.


first idea was to trim the throttle plate shaft, weld the sync lever to the shaft(for max clearance) bend and try to get to work. problem is the tb are at approximately 30 deg angel to each other. so the path each takes would be a bit weird.

the next idea is to turn both on side so injectors point out and build linkedge to tie them together. that would make a fuel rail interesting. was going to have to build one any way and thinking of using a fuel pump controller (http://www.madhu.com/content/Main/FuelPumpController) and wanted to mount the sensor to the fuel rail any way............

or am i just going crazy?

thanks in advance for your input.

[24c]

Re linkage, you could use a spherical joint & spigot detail, but why not pull cables connected to a bespoke pivoting assembly. This could have a cam action, one cable rotates a spindle & cam, which opens carb 1, but after the cam contacts the second cable it opens carb 2.

You can also get splitters that convert one single cable into two, so by varying the length or make two sliders in a bespoke case with an adjuster, you could achieve delayed opening action for left and right carbs (you might want to alter the leverage at the carb end to achieve full throttle on carb 2).

Now you're probably well confused but what you are trying to do was seen on older cars (when they had carbs).

PS I know you converted the Suzuki TBs now, but FYI an old XT of mine had twin carbs, one CV the other slide, to achieve a similar result re throttle response.If your TBs were CV types, they'd be probably better on part throttle, but anyway, a twin carb set up should work better than a single carb set up, they did on a Husaberg project years ago (640cc with 2x40mm Mikuni).

[BenDiesel]

Thanks that gave me some food for though.

Next question is if I do stage the throttle body's like the stock carbs, can I program ms to shut the right injector off till 33% throttle? I know if have to use both injector channels but that's OK seems how I only have 2 injectors.

[dontz125]

The base code doesn't support staging, but Matt pointed out in a recent thread that you could have each injector on its own channel with its own map.

[R100RT]

If you have a single throttle cable coming from your twist grip, you might use the parts found on aircooled BMW motorbikes. They run the initial cable into a little "splitter body" that allows twin cables to exit, one for each side (originally to each Bing carb on each inlet of the boxer twin). Lots of those parts around, probably used too. The sync adjustment is done down @ the throttle body in that case.
Lorne.

[slingy88]

As has been said, probably the cable splitter would be the easiest way. All multicylinder 2 strokes use them as they use slide carbs as well, so pretty easy to source parts and cables to suit.
I dont really see the need to stage the opening off the TB,s as on the carbs, this was mainly done to keep good airspeeds through the venturi at slow runnng to ensure good fuel atomising. This obviously is not a concern with efi. Even if you do, i take it you'll be using alpha-n to control fueling, so throttle possition will govern airflow into the engine, so just let both injectors share the fueling. Shouldnt make any difference if one butterfly is at idle and one is 1/3 throttle, all that matters is the right amount of fuel is injected by both injectors for the accumalative amount of air drawn in.
Keep it simple and it will be easier to tune and live with. You can always play later on if your not happy with the results, but dont see why it shouldnt work fine.

Cheers Slingy