Author Topic: Shimming the jet needle PD42? (Read 1279 times)

RandyBMC · « **on:** April 26, 2016, 07:52:10 AM »

I'm asking for help as an ignoramus.

I've got a '78 CB750K which has the PD42 carbs ("keyhole" tops). I understand that the '77 has adjustable clips for the jet needle, but the '78 needs to be shimmed since it has no clip grooves. Since I have pods, a 4-1 without a baffle and an 836 kit, I've swapped out the main and slow jets to 135 and 42 respectively. From what I've read, it also requires a shim on the jet needle.

I cleaned the carbs with compressed air and carb cleaner and rebuilt the accelerator pump when I swapped jets. All of that was done without separating the carbs from each other. I would like to avoid pulling them apart from the carbholder if I can.

So the question, and I've used the search function for hours (again I'm an ignoramus), is how do I access the jet needle (I am assuming through the keyhole top) and can I do that without dissassembly? Any photos or videos would be helpful, but a good explanation written out would be really helpful too. I scoured all of my factory manuals, and they all show full dissassembly.

Thanks in advance for helping out!

flybox1 · « **Reply #1 on:** April 26, 2016, 08:04:31 AM »

your slides come out the top cap.
nothing to mess up or break here if you have a gentle hand, and its pretty straightforward
you will have to remove a screw to get the slide arm loose from the throttle linkage, gently remove the spring with hook-ends, and shimmy the slide linkage off the arm. the slide comes right out.
two screws inside the slide are removed to access the needle.
these two screws are often stripped unless you use the proper JIS bits.
everything goes back on in reverse order once the M2.5 shims go on the needle.

135's are pretty rich, even for pods and an open exhaust. How does your WOT plug chop look?
What test did you to do determine that you needed needle shims to accompany your existing jetting?

flybox1 · « **Reply #2 on:** April 26, 2016, 08:17:33 AM »

there is a detailed photo thread of the removal process. i'll try to find it.

here...http://forums.sohc4.net/index.php/topic,71245.0.html

RandyBMC · « **Reply #3 on:** April 26, 2016, 08:31:51 AM »

Thanks for the info.

The bike isn't running yet. I based it on what is working for a couple guys that have similar set ups on this forum. I figured I would start rich and tune from there. My experience with cars has always been to go that way instead of the other - safer. I know I may end up with 130 or 125 mains. There are two bikes that supposedly run great with the 135, so we'll see I guess.

I will open up the tops and check it out. I have JIS bits, and I will be careful (sounds like an opportunity for some swearing).

If anyone has photos, I would still appreciate that.

This is what I have: