Topic: Chasing the PD42b Carburetor Gremlins or Backfires Through the Intakes

[Crack On]

Ok moved from what Did you do to your bike today...

Installed a Dynatek DS1-2 but the backfiring through the carbs didn't go away... It does run noticeably better aside form that and since I needed new points it was the way to go....BTW got the best deal online through sohc/4 store (with amazon prime) so thanks to those who work behind the scenes to keep those of us who dig these crazy bikes man...Groovy baby YEAH!

With timing and advance correct, you'll need to dig into proper A/F mixture, starting with your idle circuit.

Put the pipe down, crack 
Drilling jets will never be accurate and even with PODS, you'll rarely need more than 42's, which equates to .42mm.
Anything other than OEM keihins will cause all sorts of tuning issues.  stick with Keihins.

Jetsrus.com has all these keihins jets, including the press-in pilot jets you'll need.

Also....be sure your fast idle cam and accel pump are in working order. 
They are there to assist with cold starts, among other things.  They work well if they are kept up.

Unless you have a frame kit, the engine needs to be pulled to re-torque.

So to continue...
 I have a 1978 CB750K (totally stock when I started with it.)

Before anything else I'm not perfect and I'm not saying anyone else is wrong, this is just what I have found so far...

 I have searched the net for a chart showing the SLOW jet hole sizes in decimal/inches and found nothing, fast jet up the wazoo so, to the Math...

I measured the jets' holes in these carbs before I drilled them and they were approx .043" using gauge pins,(Aircraft machinist for 35 years so I like to work in inches but no problem to convert) that's approx 1.0922 mm not .40mm as I've read here, so I'm already at .057" or 1.4478mm after drilling ,so the math I've found sofar that relates Keihin numbers to mm, and what I started with (A totally stock 78 cb750 with 21700 miles on it) doesn't match with what I had to begin with.

 If we go the other way, I started with something in the neighborhood of a # 110 SLOW jet, I don't think that's right (and yes I do know the difference between the slow and fast jets( the stock FAST jet was #110 now  #125 ) and now after drilling I have a #145 SLOW jet?... Still not buying it...
Something is amiss with the Keihin number equating to the hole diameter in tenths of a mm... It's possible however unlikely that the SLOW jets were drilled before I got the bike but I don't think it would run if that were the case and it ran and drove fine once I cleaned the carbs and put in fresh gas and a Battery and aired up the tyres, so not very probable...

"Jetsrus.com has all these keihins jets, including the press-in pilot jets you'll need."

True, they list the right ones however, at $10.99 each I'd really like to get this right size the first time, $43.96 (plus tax and shipping?) isn't something  want to throw away at this point, I'm in over $5k at this point and I think I may have to spen more but thta's another saga...

"Also....be sure your fast idle cam and accel pump are in working order. 
They are there to assist with cold starts, among other things.  They work well if they are kept up."

Both are working well, fast idle needs to be put back up a bit but since I live in SoCal, cold here is 60def F and a choke is rarely needed in the depth of our version of winter...I Replaced and adjusted the Accelerator pump when I rebuilt the carbs (sourced Honda kits and parts), verified all 4 nozzles are clean and squirt and adjusted the tab to factory specs for engagement timing... off idle the bike runs and pulls sweet! At full throttle it almost feels like my old Chevelle Super Sport with a Holley 750 double pumper and mechanical secondaries, you can feel the difference in your keester. when it hits, it's nice!

No frame kit so it suck to be me atm...

[flybox1]

you have the main jet and pilot jet confused.
Main jet is a screw in, into the brass emulsion tube.  typically 100, 105, 110 etc
pilot jet is a press in, typically in the range of 38, 40, 42
All of these jet numbers relate to 100ths of a millimeter.  Japanese use metric 
If your jets are not stamped with the number along with a Keihin K AND the size in 100ths of a mm, they are aftermarket garbage.

L to R
Main jet, (Main jet in emulsion tube) and Pilot jet.
Notice the Keihin K logo on the main in the emulsion tube.

[Crack On]

Negative, I'm not confused about which is which "lemme esplain"

I physically put a .043" gauge pin into the slow jet and it touched the shoulder where the larger diameter(at the bottom of the jet) and the jets' smallest diameter meet, the  with a little wiggle it went farther until it stopped and I could not move it, approx. 22.5mm deep, BEFORE I changed anything from what was there originally. Hopefully the drawing will upload with this post as an attachment...As per the drawing, there are only 2 diameters in the jet so unless the PD jets are different than the drawing I'm correct and I have drilled them through to.057" dia...Like I said the FAST jet is a 125 and I bought a kit with 15 different sizes I didn't drill the fast jets...

[Crack On]

oh  I missed part of your reply...yes I know the identification symbol and both of the original jets have the boxy K symbol stamped on them next to their  size...

[flybox1]

Negative, I'm not confused about which is which "lemme esplain"

I physically put a .043" gauge pin into the slow jet and it touched the shoulder where the larger diameter(at the bottom of the jet) and the jets' smallest diameter meet, the  with a little wiggle it went farther until it stopped and I could not move it, approx. 22.5mm deep, BEFORE I changed anything from what was there originally. Hopefully the drawing will upload with this post as an attachment...As per the drawing, there are only 2 diameters in the jet so unless the PD jets are different than the drawing I'm correct and I have drilled them through to.057" dia...Like I said the FAST jet is a 125 and I bought a kit with 15 different sizes I didn't drill the fast jets...

 
If you are using gauge pins to measure ID, they must go all the way thru...not stop at 22.5mm depth/shoulder, to get the actual throughput ID.  If you have drilled through the pilot jet you pictured above to .057" (1.44 mm, or equivalent to a #145)  you aint going nowhere fast.  Thats 3x + the needed fuel supply.
At most you will need a .42mm pilot jet...even with pods/stacks.

[Crack On]

Well I guess I'm going to have to remove them and see what exactly I do have and then change them...

Anyway if you wouldn't mind, you said #42 slow jet would be about it, have you run pods with a MAC 4 into 1 and a Kerker with a quiet cone?
And how did you get the jets out, I tried pliers but I didn't want to ruin them so I didn't get to energetic with them...

[flybox1]

Pilot jets will pull/twist straight out.  As long as you dont crush them, your bike wont care if the outside gets a little marred.
My jetting is in my signature below...

[Crack On]

Ok thanks, working on 2.5 hours sleep in the last 48 so not reading that well apparently..
I'll try some 42's and see how it goes, thanks again ttyl...

[Crack On]

Ok, disregard my previously sleep deprived posts...

    After 7 hrs sleep and the mandatory coffee, I removed the slow jets and could see what I was working with, it all became clear... I had #35 slow jets and as you said, holes still .35mm (not .40mm, that was in a 200x 3 wheeler i was doing over the weekend...yikes !!!) I drilled up to .40mm, a nice improvement in idle and 1/4 throttle, but still some(noticeably less) backfiring,...

I re-set all valves to .002/.003" while I was there and once I had it running again I checked for air leaks in the manifolds with carb cleaner, none found...going to try adjusting the idle jets and re-syncing...

Thanks again for you help

[flybox1]

Cool.  Now use your idle mixture screws to further tune the idle mixture.
Combustion deposits on clean white plug porcelain (after a 4-5 minute idle(no throttle)) will tell you which way to tune.
If your plugs dont show enough color after 3 turns out of the IMS, you'll have to go to .42mm

[Crack On]

Great Thanks..

Since this is my first set of PD 42b's, once i get the idle sorted, what about the fast jet... I see you're using a 110...I have 125's in there now, my question is:

When you go full throttle, is there a lag? Unless I'm up at 6,000 rpm when I open it up, I get the feeling I'm getting too much fuel... Feels like it bogs down... Over 6k and it's good so is that typical or I still need to adjust the fast jet too?