Author Topic: 77-78 CB750 PD42A Carb Rebuild Info (Read 71034 times)

brewsky · « **Reply #25 on:** November 10, 2013, 01:40:33 AM »

Quote from: eurban on March 28, 2009, 07:38:09 PM

Nice pics and write up. I do however think that you are making a mistake with the 42 pilots. Jumping up to 42s is a huge step from a #35 and I don't think you are really correct with your assessment that these carbs are lean from the the factory. I would venture to say that most of these 30 year old machines will have impairment to their idle circuits and will require a very thorough cleaning, perhaps contributing a cold blooded perception. Without some coaxing, few will perform the required step of pulling and cleaning the pressed in idle jets along with the passages behind them. . . Have you modified your bike? Typically the decision to go to larger pilot is based on actual performance observations. If when turning out (counterclockwise) the idle mixture screw from the factory baseline starting point (between 1.5 and 1.75 turns or so) idle and off idle performance continues to improve as you approach about 4 turns out (the end of the screws effective adjustment range) then the next step larger size pilot jet would be in order. In my personal experience with the PD carbs both in stock applications and in performance ones, the stock 35 pilots are well up to the task with the sweet spot on the idle mix screw ranging between 1 and 3 turns out depending on the MODs (my 836 kitted bike actually likes a setting of about 1 turn out) You have choosen a pilot that is essentially 3 steps larger. I would expect that your results will be poor. BTW, where did you find a #42 pressed in pilot??

Be aware that the press in pilots from sci may be way bigger than the size indicates.
See this thread...
http://forums.sohc4.net/index.php?topic=112370.0;nowap

flybox1 · « **Reply #26 on:** November 10, 2013, 01:34:29 PM »

Pulling your choke cable actuated your fast idle cam.
Do a search of the forum for it. There are many detailed posts on it's function and tuning it for proper operation.

Sdsbassist · « **Reply #27 on:** April 08, 2014, 06:20:19 PM »

In the picture, item #1, do you clean it from inside the carb bowl? Mine are just two solid posts from the outside....

flybox1 · « **Reply #28 on:** April 08, 2014, 06:49:31 PM »

Quote from: Sdsbassist on April 08, 2014, 06:20:19 PM

In the picture, item #1, do you clean it from inside the carb bowl? Mine are just two solid posts from the outside....

Which photo are you referring to?

Sdsbassist · « **Reply #29 on:** April 08, 2014, 07:10:25 PM »

Photo 1, #1

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Sdsbassist · « **Reply #30 on:** April 08, 2014, 08:28:46 PM »

Isnt that just a metal tube solid?

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dlhoulton · « **Reply #31 on:** April 09, 2014, 04:01:13 AM »

Sdsbassist, I have to admit my stupidity on this one. I too thought it was a "solid" tube. Then I kept reading to use a "high E guitar string" to clean it. I couldn't find a hole!!!! Then I had the carbs fully broken down and from the BACK side of the picture shown (Photo 1, tube #1) I could see a hole. Picture a light bulb coming on over my head!!!! Cleaned the hole using said metal wire string and sure enough next time I squirted carb cleaner through it shot/squirted fuel out!!!!! I thought it was supposed to squirt fuel straight up, but what it does is squirt fuel back (into throat of carbs). I felt so stupid, but hey, lesson learned.

Sdsbassist · « **Reply #32 on:** April 09, 2014, 06:37:48 AM »

So you have to pull the slide up to get at it?! Weird

would this affect starting? still can't get mine to go.

dlhoulton · « **Reply #33 on:** April 09, 2014, 08:26:17 AM »

You might get one of those "dental mirrors" and then put a bend in the wire to get at it. Buy the way when I went to my local music store looking for the "High E" guitar string the guy said what size. There are at least 10 sizes of "High E". What you need is a #10 which is 0.010 in thickness.
Otherwise I'm thinking you are going to have to at least remove or raise your slides.

Sdsbassist · « **Reply #34 on:** April 09, 2014, 08:38:03 AM »

If those spray gas directly into the engine when you twist the throttle, that would absolutely affect starting wouldnt it?

DG

flybox1 · « **Reply #35 on:** April 09, 2014, 09:03:15 AM »

Quote from: Sdsbassist on April 09, 2014, 08:38:03 AM

If those spray gas directly into the engine when you twist the throttle, that would absolutely affect starting wouldnt it?

DG

DING DING DING DING!!
we have a winner

those are the nozzles for the accel pump.
In a functioning accel pump system, when you actuate the throttle, the pump squirts fuel out these nozzles into the carb throats. it should be a forceful squirt.
helps with better accel from a stop, and also primes the carbs for cold starts.

dig into the accel pump system.
start with carb bowl #2. remove the smaller bowl on its lower edge. note the path of fuel into and out of this small bowl, and how it gets to these brass nozzles.
note the check valves and which way fuel does and doesnt flow, and the path

dlhoulton · « **Reply #36 on:** April 09, 2014, 09:06:32 AM »

During normal startup it's not used. Only time it is "spraying gas" is during "QUICK" throttle use. On carb #2 there is a small "pump" that is part of the fuel bowl. It has an arm that runs up through the outside of carb #2 and is only activated during "QUICK" throttle twisting/usage. Hopefully others with way more knowledge will add to this.
Thanks flybox1 for posting. You answered before I was through with my post/answer. Much better explanation.

flybox1 · « **Reply #37 on:** April 09, 2014, 09:08:25 AM »

Quote from: Sdsbassist on April 09, 2014, 06:37:48 AM

So you have to pull the slide up to get at it?! Weird

would this affect starting? still can't get mine to go.

no, you dont need to pull the slide to get at it.
you can either bend a hook into a piece of wire to poke around in its hole, OR, as a last resort, pull them out. they are press in.

BUT!!!...if you are working on clearing a clog, you need to look at the check valves first. clogs typically happen here. there is one in the bottom of the accel pump bowl under the diaphragm, and there is another at the top rim of carb bowl #2(note the small oring required around this check valve. dont lose it. replace it if its dry and flat)
the accel pump diaphragm has two mickey mouse ears. both of these need to be perfectly clean, AND the diaphragm cannot be cracked or torn for it to work properly. a new one with fresh rubber is advised. carb cleaner eats diaphragms, so be warned!

flybox1 · « **Reply #38 on:** April 09, 2014, 09:10:34 AM »

Quote from: dlhoulton on April 09, 2014, 09:06:32 AM

During normal startup it's not used. Only time it is "spraying gas" is during "QUICK" throttle use. On carb #2 there is a small "pump" that is part of the fuel bowl. It has an arm that runs up through the outside of carb #2 and is only activated during "QUICK" throttle twisting/usage. Hopefully others with way more knowledge will add to this.

during startup is SHOULD be used

so you're not relying on carb vaccum alone to start the bike.
one of the nice things about PD carbs

choke knob pulled.
1-2 twists of the throttle to prime the carbs
kick it or hit the starter.
off and running

Sdsbassist · « **Reply #39 on:** April 09, 2014, 02:28:30 PM »

So I am feeling like im missing the oring and the diaphram isnt too healthy either.

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flybox1 · « **Reply #40 on:** April 09, 2014, 02:42:18 PM »

um..yeah..thats foooked! replace immediately
continue this over here... http://forums.sohc4.net/index.php?topic=136153.0

Sdsbassist · « **Reply #41 on:** April 09, 2014, 02:53:33 PM »

... and the spring goes on top of the diaphragm during reinstall right?

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flybox1 · « **Reply #42 on:** April 09, 2014, 03:06:04 PM »

http://forums.sohc4.net/index.php?topic=136153.0

TommyLud · « **Reply #43 on:** September 01, 2016, 08:16:22 AM »

I'm so happy I found this forum! The info, links and pics are great! I just bought a 1978 CB750F from a guys shed. Turns over, so I"m happy with the project. I"m rebuilding the carbs. Anything I look up is telling me the float height is 26mm. However they all seem to naturally sit at 15mm without me adjusting them. Maybe the previous owner already made adjustments. That's fine but I can't find out if this is the correct height. Any input?

flybox1 · « **Reply #44 on:** September 01, 2016, 08:28:00 AM »

Which model carbs do you have?

harisuluv · « **Reply #45 on:** September 01, 2016, 08:28:39 AM »

The CB750 float height of 26mm is correct, however not for your year.

The carbs for the year 76 and before are not the same as for the years 77-78. The float height for your carbs is 14.5mm. Some are listed as 12.5mm for specific castings/models.

TommyLud · « **Reply #46 on:** September 01, 2016, 08:30:58 AM »

KeiHin PD 42APG. I guess it's the PD series. I've been looking up specs and shop manuals. I've been feeling like I have UNIQUE carbs until I saw this thread. haha

TommyLud · « **Reply #47 on:** September 01, 2016, 08:33:23 AM »

Wow! Did not know that. I wonder how these carbs ended up on this bike. Maybe he didn't know the year of the bike. Maybe I brain farted and didn't check the year myself.

Time to do my dummy checks..

Thank you very much. 14.5mm makes much more sense!

flybox1 · « **Reply #48 on:** September 01, 2016, 08:49:01 AM »

Quote from: TommyLud on September 01, 2016, 08:33:23 AM

Wow! Did not know that. I wonder how these carbs ended up on this bike. Maybe he didn't know the year of the bike. Maybe I brain farted and didn't check the year myself.

Time to do my dummy checks..

Thank you very much. 14.5mm makes much more sense!

Yeah...this is why carb model is more important than the bike year.
If you have 'K' stamped on your jets, they are OEM Keihins...and your pilot jet body is also 14.5mm tall.
Makes verifying their height an easy task

Mathewslg · « **Reply #49 on:** March 27, 2018, 05:28:35 PM »

Question about your setup Lucky, what exhaust are you using and do you have any filtration on your Velocity stacks?

I am also doing a Velocity stack setup using the Lossa Engineering quiet core exhaust. I used your carb settings, but am still running incredibly lean. I’m gonna try and jump up to #45 / #130 with the single shim.

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Author Topic: 77-78 CB750 PD42A Carb Rebuild Info (Read 71034 times)