How to 1978 CB750 slide needle shims

[Lucky 1]

IF you would like to shim your non adjustable needles on your 1978 CB750 here is a how to.

You will need calipers that measure in thousandths.
You will need needle nose pliers small, 8mm combo wrench, #2 phillips screw driver,
and a small flat blade electronics screw driver to remove a circlip.
Some music and a beverage.

Remove the carbs.
Place a clean towel on your work area to avoid losing small parts.
Drain all of the float bowls if the carbs had gas in them.

Do one carb at a time.
Now remove the top covers on all of the carbs.
Measure from the top edge of the #2 slide to the top edge of the carb body. No gasket.
Slide all the way down and idle screw backed off all the way.
WRITE DOWN this measurement. Should be about 1.205 thousandths.


Then remove this screw from the shaft completely.

It will allow you to lift the slide further.
Do not loosen all of the throttle shaft screws at one time.
Loosen the adjuster nut and use needle nose to remove the spring.




Make sure to measure the shims and remove the burr from the back side of the shim/washer.
When you put the needle hold down device inside the slide make sure that the spring is centered
on top of the needle.
Install the two screws and make sure the cutaway is facing the right way. (to the rear of the motorcycle)
Now that it is all back together screw the adjuster down to the approximate position and then measure from the top of the slide to the top of the carb body and make it match the measurement you WROTE down  and make it the same as the #2 carb.

Repeat on all of the carbs.
Don't forget you still have to take #2 apart and add the shim but it will not require adjustment,
(no adjuster).

Adding 2 shims .022 or so will raise the needle about = to one notch on a adjustable style (normal) needle with clips set up.

This raising of the needle will help richen the midrange on this non adjustable style slide set up
or if you live in a very cold area. If you have pods or velocity stacks you might want to get this part of the carb function to work better.

[Re-run]

Only thing I can add is I have been able to do this without touching the adjustment nut. Prevents me from having to hook up vacuum gauges.

Moving to Write ups column.

[Lucky 1]

Thanks for moving my post Re-Run.
If you measure these slide positions you won't need to synch the carbs.
The slides  will be within .001 thousandths of each other.

BTW....
IF you shimmed your needles can you tell us what intake devices, and exhaust you were using, and
what the thicknesses of the shims were and the results that you got. Thanks.

[Re-run]

Well I currently run 4-4 but not the original mufflers. I had to get new slip-ons and They seem more restrictive and always ran a bit rich. Then a couple years ago I dropped in a K&N into the box and that brought it back from being rich.

I have pulled my slides though when checking things out. I do remove the bolt of course and the spring but never had to loosen the adjustment nut. Measuring isn't a bad idea at all. My only recommendation would be a digital caliper for greater accuracy. I am betting though that loosening the nut makes it a bit easier to get the slides out. I will have to try it. I DO have a slight stumble if I roll on a bit too fast.

[Lucky 1]

The digital caliper is NOT more accurate.
It may lead you to believe it is more accurate because it is electronic
but the mechanics inside is the same.

If you grow up with push button electronic devices you will usually believe in them.

Any analog instrument can be read between graduations as a comparator, intrinsically impossible with digitals which have a fundamental digitisation error and can never be better than ± the least significant digit. Not forgetting the hysterisis built in to the change-over point to ensure that you get a stable reading.

Depends on the quality too.

Buy a Mitutoyo 6 inch caliper with 0-.100 in one revolution.
A good vernier caliper costs almost $100.

ANYWAY .001 thousandths is good enough.

[Re-run]

The greater accuracy comes from a person being able to read it easier. AND it displays the EXACT measurement in numbers. You do not have to worry about the dial sitting between lines. If you have a decent caliper, it will display to thousandths with no problem.

So bottom line, if your caliper or any other measuring device is accurate, the number it displays is the exact accurate number.

Oh, and FYI, I did not grow up with push button devices. i grew up on a farm learning on tools older than you most likely. I was out of high school before Windows 95 came out.

I now buy quality digital gauges are they are every bit as good and are easier to read. I would also bet that when most shops have to buy new tools, yup you guessed it, digital. Pretty sure NASA uses digital.
If it is good enough for them, it is good enough for me and you.

Hell, a simple $20 caliper from Lowes is accurate to +-.001 and that is plenty accurate for anything on these old bikes. Anything more than that requires a micrometer.

[shinyribs]

   Thanks for the info on the needles.That may come in handy one day

[mikefootusa]

Sounds like we got a Metrologist in our midst.  +1

[Lucky 1]

I agree basically.

"Oh, and FYI, I did not grow up with push button devices. i grew up on a farm learning on tools older than you most likely. I was out of high school before Windows 95 came out. "

I was using a slide rule to compute power on jet engines in 1969.
Nine years after computers got to the USA from England. LOL

[Re-run]

I was actually replaying to Lucky. See now it is confusing, we have Lucky AND Lucky1!

[Piute]

donald

[Lucky 1]

It is Lucky 1.

I had a reason to make the change sorry.

[shinyribs]

Lucky 1 wrote

It is Lucky 1.

I had a reason to make the change sorry.

On the lam again?

[pschilling]

Hey buddy, what was the ID and OD for the shims or washers you used to adjust the needles?

[shinyribs]

Lucky hasn't been around in a while, but maybe he'll see this and pipe up.

But any decent sized hardware store should have a good assortment of machine washers you can test fit. If you want really thin stuff, look in to shim stock. Though you'll have to drill and cut your own washers out. Shop for machine washers or machine shims. That should get you started. At least, that's what they're called at the place near me.

[pschilling]

Thanks boss, I am also a toolmaker/Injection mould maker, just don't have the shim punch kit and shims at home…was just trying to prepare various stock prior to a weekend of testing…
My velocity stacks will be finished tomorrow, so gather I will tear the carbs apart again to gain a strong midrange…

[shinyribs]

Ah, gotcha. Good luck and happy testing.