Topic: tuning: the throttle max stop screw vs. the main jets

[grumpy]

when setting up 750K3 carbs at WOT, how does one know whether to tweak the max limit screw on the carb rack or to mess with the main jet size?

Example: if it's lean at WOT, how do you tell if the jets are too small of if the set screw is backed out too far?
One could open up the screw all the way & get some huge jets but then that screw exists for some reason, so.... ?

[bjbuchanan]

The big screw controls the idle. It moves the slides up all 4 at a time. Ride the bike until it is nice and warm and set the big adjuster knob to idle around 1200. Now leave it totally alone, even on a cold bike. None of that twisting it so you can gear up while it gets warm stuff

For me the main jet is an afterthought in my opinion. Stock engine, put some 110's in there and pay more attention to the other ranges. Pilot and needle position are more important, you actually use them. How often do you honestly go WOT? Or half throttle even?

[Jayelwin]

None of that twisting it so you can gear up while it gets warm stuff

I do this. Why not?

[grumpy]

The big screw controls the idle.

How often do you honestly go WOT? Or half throttle even?

nope, not that screw. there's another one that sets the max. I'll see if I can find it in the fiche.

and - as often as I can !! almost daily!
 

[LesterPiglet]

None of that twisting it so you can gear up while it gets warm stuff

I do this. Why not?

Why trouble yourself to find the correct idle speed every time you start your bike?
Fiddling with the idle screw is totally unnecessary as it takes the bike 30 seconds to ride from cold.

[grumpy]

#22 is the max throttle stop screw

(#40 is the idle screw)

[Don R]

What?

[andrewk]

You just need to make sure the max screw is tuned to open the slides just to the top of the carb bore.

I see where you're going with your thoughts there, but I would bet that the screw is fine and the jets need tweaking. There's a theory out there that these bikes are over-carbed in the upper ranges.

If the bike in question is the profile pic (4-4 stock box), set those carbs up stock, maybe go to 108's on the main. (assuming stock bore)  Everyone seems to want to put 120's in these things but they don't need it, especially with a full baffled exhaust and the stock air box. Giant jets don't make you go faster, or give you more power.  Just makes the thing lazy and fouls plugs.

[grumpy]

Giant jets don't make you go faster, or give you more power.  Just makes the thing lazy and fouls plugs.

yeah - I came to that realization a few years ago after weeks spent messing with jets (anybody wanna buy a pile of very lightly used mains?). I settled on 110s. It's bored to .5 over; second step.

my carbs were completely disassembled for cleaning so I don't know how the screw was/is supposed to be set. I think it's out too far.
so you're saying to set it so the slides stop at the point where they first clear the bore? IE when they're fully up & out of the air flow? that makes sense.

[Drummer]

There's a setting for that. I've seen it in a manual.

[andrewk]

Giant jets don't make you go faster, or give you more power.  Just makes the thing lazy and fouls plugs.

yeah - I came to that realization a few years ago after weeks spent messing with jets (anybody wanna buy a pile of very lightly used mains?). I settled on 110s. It's bored to .5 over; second step.

my carbs were completely disassembled for cleaning so I don't know how the screw was/is supposed to be set. I think it's out too far.
so you're saying to set it so the slides stop at the point where they first clear the bore? IE when they're fully up & out of the air flow? that makes sense.

I might be interested in your pile-o-mains, I tune quite a few of these every year.   PM me with sizes?

Page 196 of the FSM (K1-K4 supplement) states to set the throttle stop so that there is 1.28-1.29 inches (32.5-33mm) between the carb top and the bottom of the plastic bushing that the bellcrank forks fit into.  In my experience, that has been where the slide just clears the cylinder head side of the carb bore.

[Dream750]

Page 196 of the FSM (K1-K4 supplement) states to set the throttle stop so that there is 1.28-1.29 inches (32.5-33mm) between the carb top and the bottom of the plastic bushing that the bellcrank forks fit into.  In my experience, that has been where the slide just clears the cylinder head side of the carb bore.

I use the 750 service bulletin throttle stop adjustment setting. Either way, both methods will get the job done.

[grumpy]

Excellent!
Thanks!!

andrew - I'll see what I've got.