Hesitation: But ONLY at WOT Gear Shift

I have a 1979 750 twin (B4) just rebuilt it about 5000 miles back. It has the MAC headers, stock carbs and jets, and it runs perfect EXCEPT for one thing:

If I crank it up hard in a low gear with WOT, then let off the throttle to shift gears at maybe 5000 - 6000RPM, then when I crank the throttle back wide open there is a definite hesitation and then the power hits on hard.

The fuel levels are right, there is never any hesitation when I hammer the throttle under any other condition, it's just when I am winding it up, let off fully to shift, and snap the throttle back open.

Otherwise it pulls very strong at just about any condition where I snap the htrottle open.

The only thing not stock is I have put in different carb vacuum pistons (slides) because the rubbers on the originals were shot. The new ones are off a newer year and slightly different than stock, but I am using the stock needles (which are adjustable) set to the same height below the flat surface of the piston.

Checked ignition coil voltages? Running points? if so the capacitorsmay be failing. If electronic ignition, check the electrical connections for corrosion/loose wires.

Sparkplugs and wires good?

I just put on a new coil and the CDI ignition is sparking very well. This definitely has a carburetor feel to it.

I've seen that happen on more than one bike.

In your case, it might be something with the slides, like the cutaway, or sticking or something...

My advise is don't close the throttle.

You only need to blip the throttle back some to shift up without touching the clutch.
You certainly don't have to close the throttle, then slam it back open.

Add some fuel to your pilot circuit via the mixture screws, and see if that helps.

Bountyhunter, just a shot in the dark, But, c/v carb pistons have a vacumn control orifice located in the vicinity of needle jet hole. It's diameter controls slide rate & speed. (that's why some jet kits use different springs, & require drilling/restricting .) In some cases, diaphrams can be swapped. & may be an option. luck G

Raise your needles.

TeK9iNe wrote:

I've seen that happen on more than one bike.

In your case, it might be something with the slides, like the cutaway, or sticking or something...

My advise is don't close the throttle.

You only need to blip the throttle back some to shift up without touching the clutch.
You certainly don't have to close the throttle, then slam it back open.

Add some fuel to your pilot circuit via the mixture screws, and see if that helps.

Hmm, good ideas. The pilots are pretty much dead on, I actually have tried different jets and settings. I am using the stock 45's right now, 47.5 ran a shade rich.

gane wrote:

Bountyhunter, just a shot in the dark, But, c/v carb pistons have a vacumn control orifice located in the vicinity of needle jet hole. It's diameter controls slide rate & speed. (that's why some jet kits use different springs, & require drilling/restricting .) In some cases, diaphrams can be swapped. & may be an option. luck G

I was suspecting there might be a subtle difference between the slides. I don't want to try swapping the diaphragms because I am sure I would tear them. Screwing with 30 year old rubber is not for me.....

guitargeek wrote:

Raise your needles.

I have actually played around with needle height, my needles have five slots and I think I am on the one that runs best.

I've actually got a touch of the same condition with my bike cutting out a little after high rev shifts. I picked my racer/wrench friend's brain and got some good ideas. After describing the condition to her, she first suggested shimming the needles, then riding it to see how it had changed, then putting in some stronger slide return springs (though I don't know if you have CV or mechanical carbs on that twin).

It has the stock BS-38 Mikuni carbs.

bountyhunter, I have this old Honda cb350 twin I've been tinkering with, and it also has CV type carbs.
I hate them. They are THE most persnickity carbs I've ever messed with. Mostly because of the bizzare single throttle cable that splits into two, one for each carb. To synch them, your supposed to slowly turn the throttle, watching for each slide to start moving at the same time, then adjust the cables until they do start opening at the same exact time.
I've done that, but I still have similar symptoms as you do. And I think the reason for that is the diaphram springs might be slightly different from each other, so they're probably not really synched at all.
Do you have any kind of vacuum ports you can hook some guages up to them to see what's really happening?