Andy VH said...
The procedure is to run the bike at a throttle position that gives the fuel system a chance to stabilize. That means at least a mile of running at full throttle. Pick a location where you can do that, in a gear that allows you to stay within the speed limit. For a bike like your Bandit, that may mean 2nd or 1st gear. Ride it at wide open throttle for at least a mile, then while at that speed and engine rpm, at the same time, pull in the clutch/close the throttle/and hit the engine kill switch. Let the bike coast down to a stop, and then remove the plugs to read the color. I'd do that first with the current main jets to see if the color shows it to be a bit rich.

Assuming that three of the the new main jet sizes are all larger than the stock main jets, I'd first opt for the smallest of the three jets to see how the bike responds. If you get the power back and the plugs are a light tan color I'd say its good. If you just put in the largest of the three main jet sizes, and it does not produce more power or response, then it needs to be jetted down to the smallest of the main jets size.

I once test rode a BMW K1200GT hard in 1st gear to clear it out after too much idling and putting around the shop. That bike would easily do 70mph in 1st gear before it would limit out against the rev limiter.

Andy said...
But you won't be able to ride around at 3,000 in any gear and be anywhere near wide open throttle.

skyhawk04kilo said...
Okay I'm a little confused as to how carbs work, ...

 

So the website is saying that when I open the throttle grip all the way, the engine is running on the main jets. But what you're saying is that even if the throttle grip is opened all the way, I can't be running on the main jets at low RPM. If that's the case, then I have a totally different problem, because my bike starts to hesitate when the throttle grip is opened all the way, and it doesn't necessarily stumble at the top of the RPM range.

Richard47 said...
There is not a lot of difference in the air flow through the carb when the butterfly is nearly fully open and fully open. But in your case it must be making some difference to the vacuum or the bike would run just the same. Was the bike running OK before you fitted the rebuild kit? Do you know the size of the main jets you took out? Is the needle exactly the same and did you fit the clip in the same position as before (assuming it has more than one groove)?

Andy VH said...
Skyhawk04kilo, your comment, "If I ride around at 3000rpm with the throttle all the way open, are the carbs drawing fuel from the main jets or one of the other jets?" is what I responded to. You can't ride at 3,000 rpm engine, in any gear, at wide open throttle. At most, maybe fifth or sixth gear at 3,000 rpm may be 1/3 throttle at most, until you reach a hill. I'm referring to the grip, which on your bike with CV carbs controls the butterfly, through which the vacuum signal determines how much the slide moves up.

As far as carb jetting, each circuit blends into the next. Idle (pilot) circuit blends in the midrange needle/jet at around 2,000 rpm. The needle/jet meters the fuel into the airstream through most of the mid-range engine rpm, from 2,000 up to around 5,000 rpm. The needle/jet blends into the main jet at about 6,000 rpm up to about 1,000 rpm of redline, at which point the fuel is metered entirely by the main jet. Most carbs will draw some fuel through the idle jet throughout the rpm range, but its affect is minimal much above idle.

Richard47 said...
You are most certainly not using full throttle in first gear at a constant 10K. Your Bandit will only need maybe 10-15bhp to maintain 10,000rpm in first and the amount of gas consumed to give that speed is not enough to bring the main jets into play. A plug chop needs to be conducted with the motor working hard so that the main jets are delivering fuel. Your plug chop in first seems to indicate that the bike is running a little lean with the slide/needle only a little off the stop, which is where it would have been in those circumstances.

Remember that with a CV carb you are only moving the butterfly with the twist grip, not the slide. The position of the slide is what counts and this is governed by the amount of vacuum created by the airflow in the venturi which is also the amount of vacuum at the top of the diaphragm. The difference between this vacuum and atmospheric pressure is what causes the slide to rise.

CaddmannQ said...
It's not just bike/engine speed or throttle position that determines jet requirements.

It's the load on the engine. More load requires more fuel.

To test this increase load without increasing test speed range.

Does the bike perform better or worse when going up a steep hill than on the flat?
Try it and see.

IMO, you should consider what Andy said about leanness.

You're not running right between 3/4 and WOT.

When running hard at 1/2 throttle, you're even a bit lean.

Now you want to correct this by leaning the main jet?

I think the needle isn't richening the upper mid-range quickly enough.

It's not coming out of the jet soon enough to supply fuel when you're not quite fully ready for the main jet.

Several things can make this happen faster:

Shorter needle

Faster taper on the needle

Raise the needle

Lighten the slides or reduce the slide spring rate

I don't think you have the skills to mod the slides or springs

I suggest tweaking the needles before condemning the main jet.

Andy VH said...
This proves the logic of only using a jet kit designed for the bike (as Skyhawk has done here) and for the way a bike is modified intake and exhaust wise. Anyone who claims he can simply jet up a bike without a planned kit is just shooting in the dark. And more than likely the bike will be running much worse than stock.

CaddmannQ said...
Adding shims may not be the answer, because the rate of taper on the needle is probably wrong.

Eventually, adding shims causes the needle not to close enough & then you have to back off the pilot/idle circuits to compensate, while it still doesn't open up fast enough in the midrange.

Also, adding shims adds weight to the slide in a situation where the slide really wants to be lighter.

Cutting springs changes the dynamic response of the slide: cutting a little is OK, but cutting more may make the slides start to bounce, because you've already added weight but have not added damping to the system.

I think the solution will be found in removing all the shims and putting in shorter, blunter (and therefore lighter) needles.

This will make the fuel come on earlier, stronger, and more suddenly.

Perhaps the selection is much better today than back in the old days (before I wised up and went to fuel injection), but I was never able to buy the exact needles I wanted so I always ended up just making them from small nails. This requires much patience to develop the correct shape and length and to get them both the same. All I had to work with was a small drill motor with a modified chuck, some sand paper and files, a caliper, and a micrometer. It was basically done by hand.

I was typically just tuning singles & twins. I never had to make 4 needles exactly the same (!) but certainly other people have done it.

Hopefully nowadays the selection is better & you can buy just what you need.