Author Topic: 77-78 CB750 PD42A Carb Rebuild Info  (Read 24918 times)

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Offline madmtnmotors

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Re: 77-78 CB750 PD42A Carb Rebuild Info
« Reply #125 on: October 12, 2018, 08:13:54 pm »
How do I get those scratches off that wheel???

That's a tough one. I think the Comstars are anodized. Any sanding, steel wool, or anything else will likely cause more scratches or cut clear through the anodizing creating a noticeable difference in the finish. I made the mistake of trying to "clean one up" and ended up having to wet sand and polish the entire rim to get a uniform finish.

Our discussion about tube type or tubeless does not matter now, I ain't mountin' any more tires but thanks for the help.

LOL! I struggled with tire changes when I was younger. Now I know tire lube (Ru-Glyde) and rim savers can make the job much easier. Powder inside the tire, inflating and then deflating the tube prior to attempting to lever over the second bead makes the process much less dramatic. I pinched my share of tubes in my early years...
TAMTF...


Wilbur



My most recent project "Evolution": http://forums.sohc4.net/index.php?topic=100352.0
F2/F3 O-rings: http://forums.sohc4.net/index.php?topic=113672.msg1300721#msg1300721
Cam Tower Studs: https://www.mcmaster.com/#93210a017/=t19sgp
Clean up that nasty harness: http://forums.sohc4.net/index.php?topic=137351.msg1549191#msg1549191
                                           http://forums.sohc4.net/index.php/topic,148188.msg1688494.html#msg1688494
                                           http://forums.sohc4.net/index.php/topic,139544.msg1579364.html#msg1579364
                                           http://oldmanhonda.com/MC/wiring750K1.html
Charging system diagnosis: http://forums.sohc4.net/index.php?topic=1012.msg8345#msg8345
Get the manuals: http://manuals.sohc4.net/cb750k/
The Dragon: http://forums.sohc4.net/index.php?topic=45183.msg1571675#msg1571675
Headlight Switch: http://forums.sohc4.net/index.php?topic=113986.msg1283236#msg1283236
Branden's leak free top end thread: http://forums.sohc4.net/index.php?topic=107040.0
Engine Lifting Made Easy: http://forums.sohc4.net/index.php/topic,58210.msg1684742.html#msg1684742
                                      http://forums.sohc4.net/index.php/topic,100352.msg1675840.html#msg1675840
Static and Dynamic Timing: http://www.hondachopper.com/garage/carb_info/timing/timing1.html
Airbox Gasket Replacement: http://forums.sohc4.net/index.php/topic,114485.msg1290000.html#msg1290000
"Café" : http://forums.sohc4.net/index.php/topic,84697.msg953814.html#msg953814
PD Carb Choke Linkage: http://forums.sohc4.net/index.php/topic,100352.msg1669248.html#msg1669248
                                    http://forums.sohc4.net/index.php/topic,110931.msg1248354.html#msg1248354
Follow up on your damn posts: http://forums.sohc4.net/index.php/topic,144305.msg1791605.html#msg1791605
Taiwanese Cam Chain Tensioners:  http://forums.sohc4.net/index.php/topic,155043.msg1774841.html#msg1774841
Gumtwo Seat Cover: http://forums.sohc4.net/index.php/topic,164440.msg1897366.html#msg1897366
Primary Drive: http://forums.sohc4.net/index.php/topic,166063.msg1919278.html#msg1919278
Tank Latch: http://forums.sohc4.net/index.php/topic,165975.msg1919495.html#msg1919495
Shorten your forks: http://vintage-and-classic-honda-s.456789.n3.nabble.com/How-to-shorten-forks-td4042465.html DO NOT CUT THE SPRINGS!
Clutch How To: http://vintage-and-classic-honda-s.456789.n3.nabble.com/How-to-change-and-adjust-a-clutch-SOHC-td4040391.html
Sent from my Tandy TRS-80!

Offline Don R

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Re: 77-78 CB750 PD42A Carb Rebuild Info
« Reply #126 on: October 13, 2018, 08:00:34 pm »
 I found a set of PD42a's on the ground at a swap meet for $20. They look really clean, I haven't opened them yet. I've cleaned a set in the past, my brother had to pull a brass fitting out to open an accelerator circuit even after several trips through the ultrasonic cleaner, that's all I remember. Anyway those carbs have sat for 4 years probably with gas in them. Hopefully he at least put some stabil in it.

 Some of this should be sticky'd somewhere if it isn't already. FAQ's or tips and tricks.
« Last Edit: October 13, 2018, 08:02:16 pm by Don R »
If the things you own end up owning you, it's better to be owned by some cool things.

Offline hondacr

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Re: 77-78 CB750 PD42A Carb Rebuild Info
« Reply #127 on: November 02, 2018, 03:30:24 am »
Hey everybody, just wanted to say that I've been lurking for a couple months now and figured I should probably CONTRIBUTE for once... Well here's a quick intro: Bought this bike a few years back, a 1978 cb750F, and it never really ran very well. I always knew it needed carb work, but I was hesitant to take the jump and really get in there and open them up.  But because of everyone on this site, I got the guts to tear them down last October and I'm am really glad I did.  You wouldn't believe some of the stuff I found going on in there.. but that's another story!  So, I've had the carbs off all winter, and they've been updated with new gaskets and o-rings, #125 mains, and #42 slows.  Cleaning them was a PITA but the stuff I've learned in the process because of everyone's info has made it really worth it.  So, before I put the carbs back on the bike, I figured I'd take a few pics and label them with the main parts, and a brief description of what you should see when cleaning them, just to provide a really basic starting point to convince others to get in there!!

First, the intake side of the #2 carb:



1 --  Accelerator Pump Nozzle -- You should see fuel squirting out of this litle brass post into the carb under quick throttle application.  This was probably the HARDEST part to clean.  I used a high E string from an old guitar and that worked pretty well.
2 --  Slow circuit -- Spray carb cleaner into this hole and watch the slow jet located in the underside of the carb body.  You should see fluid squirting through the brass slow jet.
3 --  Main Circuit -- Spray cleaner through this hole, and make sure you see fluid exit from the main jet located in the underside of the carb body. 

Underside of the #1 Carb:



1 -- Float pin --  Works with the float to maintain a constant level of fuel in the float bowl.
2 -- Main Jet -- Screws into an emulsifier tube, which screws into the carb body.  When I opened up this carb for the first time.. this WAS LAYING IN THE BOTTOM OF THE BOWL. Wow. Unscrew and make sure it's clean.
3 -- Slow Jet -- Pressed into the carb body and comes out with a gentle tug of some pliers. These are also very hard to clean because of the small passageway. Make sure they are PRISTINE.
4 -- Pilot Jet/Needle -- A pin, spring, washer, and O-ring that is basically a fine tuning air/fuel mixture screw. Make sure the taper on the needle is clean.

Well, thanks again to everyone, and if anyone has ANY requests for pics in the next few weeks before they go back on the bike, please don't hesitate to ask!

What about the #2 carb accelerator pump check balls????

Has anyone successfully cleaned the jets with a spray cleaner Without removing them #CB 750 Keihin PD Carbs K7 ?
Also has anyone used / tried "Forte" fuel line cleaner ?
https://www.ebay.co.uk/itm/FORTE-ADVANCED-FORMULA-GAS-PETROL-TREATMENT-FUEL-CLEANER-400ML-BOTTLE/123057179680?hash=item1ca6c78c20:g:6ygAAOSwk4taw7a6:rk:4:pf:1

Any tips welcome / advice
« Last Edit: November 02, 2018, 03:32:08 am by hondacr »

Offline flybox1

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Re: 77-78 CB750 PD42A Carb Rebuild Info
« Reply #128 on: November 02, 2018, 06:48:20 am »
No bottle will clean accel pump system (check valves and jets) debris or clogs....period.
No bottle will scrub or poke emulsion tube and pilot jet holes clean...period.

Get in there and do it right.
'78 750K (F3 engine) PD42b's, Modified airbox w/K&N  filter, 40/110 jets, 1 needle shim, IMS@ 1 turn out. Kerker + Cone 18" QuietCore

Past Bikes
1974 550K0 (stock), 1973 CB350F (stock), 1983 Yamaha XS400K (POS)
77/78 cool 2 member #3
"Knowledge without mileage equals bullsh!t" - Henry Rollins

"This is my CB. There are many like it, but this one is mine…"

Offline hondacr

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Re: 77-78 CB750 PD42A Carb Rebuild Info
« Reply #129 on: November 03, 2018, 02:55:48 am »
No bottle will clean accel pump system (check valves and jets) debris or clogs....period.
No bottle will scrub or poke emulsion tube and pilot jet holes clean...period.

Get in there and do it right.

If "getting in there" means removing the carbs then it will have to go to someone who has the experience & patience to do it, thats not me.. :-\
Cylinder 4 is the only one that keeps on fowling up the plug hence number 4 gives up after 40 mile-ish ... All new genuine Honda coils fitted, plug leads - suppressor caps & new plugs fitted [tried different plugs] at the moment are D8EA, air/fuel screws all brand new set at 1/2 turn out, new accelerator pump, 135 main jets- velocity stacks + Aluminium Fuel Tank.
I use  Esso super unleaded petrol (Synergy Supreme+ Unleaded 97) is ethanol free..

I only had this problem with pot 4 for a few weeks   
« Last Edit: November 03, 2018, 03:03:56 am by hondacr »

Offline flybox1

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Re: 77-78 CB750 PD42A Carb Rebuild Info
« Reply #130 on: November 05, 2018, 11:07:07 am »
If "getting in there" means removing the carbs then it will have to go to someone who has the experience & patience to do it, thats not me.. :-\

forreals?  :o  ok, then
...how about clear tube test bowl #4.  show us what the fuel level is.
'78 750K (F3 engine) PD42b's, Modified airbox w/K&N  filter, 40/110 jets, 1 needle shim, IMS@ 1 turn out. Kerker + Cone 18" QuietCore

Past Bikes
1974 550K0 (stock), 1973 CB350F (stock), 1983 Yamaha XS400K (POS)
77/78 cool 2 member #3
"Knowledge without mileage equals bullsh!t" - Henry Rollins

"This is my CB. There are many like it, but this one is mine…"

Offline hondacr

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Re: 77-78 CB750 PD42A Carb Rebuild Info
« Reply #131 on: November 05, 2018, 01:29:12 pm »
If "getting in there" means removing the carbs then it will have to go to someone who has the experience & patience to do it, thats not me.. :-\

forreals?  :o  ok, then
...how about clear tube test bowl #4.  show us what the fuel level is.

how about clear tube test bowl #4.  ::)  show us what the fuel level is:-[
wheres is that ? dont you mean float bowl ? if so how do you test the level ?
sorry if If i'm not up your standard of being a selfy mechanic my freind

Offline flybox1

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Re: 77-78 CB750 PD42A Carb Rebuild Info
« Reply #132 on: November 05, 2018, 01:52:02 pm »
Not trying to be condescending, but cleaning carbs is basic bike maintenance.
Youve fitted coils and an electronic ignition and done timing......and you wont touch carbs?!?!?  c'mon! ;D

the clear tube test is used to determine where the fuel level is in the bowl.
fuel level too high can contribute to a rich running cylinder.

If you complete this simple test, and see the fuel level is too high, a float height adjustment may be all you need to correct your issue.
your clear tube will connect to the overflow pipe out of the bottom of the bowl...then, crack open the drain screw...see where the fuel level stops compared to the top of the carb bowl. easy peasy. 
If its closer to the top than 3mm....you'll need to make a float adjustment to correct it.  see below.


'78 750K (F3 engine) PD42b's, Modified airbox w/K&N  filter, 40/110 jets, 1 needle shim, IMS@ 1 turn out. Kerker + Cone 18" QuietCore

Past Bikes
1974 550K0 (stock), 1973 CB350F (stock), 1983 Yamaha XS400K (POS)
77/78 cool 2 member #3
"Knowledge without mileage equals bullsh!t" - Henry Rollins

"This is my CB. There are many like it, but this one is mine…"

Offline hondacr

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Re: 77-78 CB750 PD42A Carb Rebuild Info
« Reply #133 on: November 06, 2018, 04:13:55 am »
Hi Flybox1

Thanks for the info very helpful...I have actually played around with the carbs but not the float level, all new fuel / air mixture screws. main jets & acc' pump fitted by a mechanic

I can manage that simple test you mention.
I am just recovering from lower back spinal surgery so at the moment i'm very soar so its not possible right now, but as soon as I am fit enough I will give it a go...thanks again for your patience & the info + diagram.   

I will let you know the result eventually
« Last Edit: November 06, 2018, 04:15:56 am by hondacr »

Offline madmtnmotors

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Re: 77-78 CB750 PD42A Carb Rebuild Info
« Reply #134 on: November 06, 2018, 06:27:57 am »
The only difference between the diagram of the early model carbs that was posted and the later PD carbs that you have is that the drain and the overflow are the same. This means you will connect the clear tube directly to the overflow/drain tube on the bottom of the PD carb and then open the drain screw. This makes the clear tube test much easier to perform on the later PD carbs IMHO.
TAMTF...


Wilbur



My most recent project "Evolution": http://forums.sohc4.net/index.php?topic=100352.0
F2/F3 O-rings: http://forums.sohc4.net/index.php?topic=113672.msg1300721#msg1300721
Cam Tower Studs: https://www.mcmaster.com/#93210a017/=t19sgp
Clean up that nasty harness: http://forums.sohc4.net/index.php?topic=137351.msg1549191#msg1549191
                                           http://forums.sohc4.net/index.php/topic,148188.msg1688494.html#msg1688494
                                           http://forums.sohc4.net/index.php/topic,139544.msg1579364.html#msg1579364
                                           http://oldmanhonda.com/MC/wiring750K1.html
Charging system diagnosis: http://forums.sohc4.net/index.php?topic=1012.msg8345#msg8345
Get the manuals: http://manuals.sohc4.net/cb750k/
The Dragon: http://forums.sohc4.net/index.php?topic=45183.msg1571675#msg1571675
Headlight Switch: http://forums.sohc4.net/index.php?topic=113986.msg1283236#msg1283236
Branden's leak free top end thread: http://forums.sohc4.net/index.php?topic=107040.0
Engine Lifting Made Easy: http://forums.sohc4.net/index.php/topic,58210.msg1684742.html#msg1684742
                                      http://forums.sohc4.net/index.php/topic,100352.msg1675840.html#msg1675840
Static and Dynamic Timing: http://www.hondachopper.com/garage/carb_info/timing/timing1.html
Airbox Gasket Replacement: http://forums.sohc4.net/index.php/topic,114485.msg1290000.html#msg1290000
"Café" : http://forums.sohc4.net/index.php/topic,84697.msg953814.html#msg953814
PD Carb Choke Linkage: http://forums.sohc4.net/index.php/topic,100352.msg1669248.html#msg1669248
                                    http://forums.sohc4.net/index.php/topic,110931.msg1248354.html#msg1248354
Follow up on your damn posts: http://forums.sohc4.net/index.php/topic,144305.msg1791605.html#msg1791605
Taiwanese Cam Chain Tensioners:  http://forums.sohc4.net/index.php/topic,155043.msg1774841.html#msg1774841
Gumtwo Seat Cover: http://forums.sohc4.net/index.php/topic,164440.msg1897366.html#msg1897366
Primary Drive: http://forums.sohc4.net/index.php/topic,166063.msg1919278.html#msg1919278
Tank Latch: http://forums.sohc4.net/index.php/topic,165975.msg1919495.html#msg1919495
Shorten your forks: http://vintage-and-classic-honda-s.456789.n3.nabble.com/How-to-shorten-forks-td4042465.html DO NOT CUT THE SPRINGS!
Clutch How To: http://vintage-and-classic-honda-s.456789.n3.nabble.com/How-to-change-and-adjust-a-clutch-SOHC-td4040391.html
Sent from my Tandy TRS-80!

Offline hondacr

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Re: 77-78 CB750 PD42A Carb Rebuild Info
« Reply #135 on: November 08, 2018, 04:16:42 am »
The only difference between the diagram of the early model carbs that was posted and the later PD carbs that you have is that the drain and the overflow are the same. This means you will connect the clear tube directly to the overflow/drain tube on the bottom of the PD carb and then open the drain screw. This makes the clear tube test much easier to perform on the later PD carbs IMHO.

when connecting the clear tube to the drain do I leave the fuel cock off or on ?

Offline flybox1

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Re: 77-78 CB750 PD42A Carb Rebuild Info
« Reply #136 on: November 08, 2018, 06:41:57 am »
Fuel has to flow into bowl, and, into the tube, to get an accurate reading. So leave the petcock on.
Otherwise, you're just draining the bowl into the clear tube, and you'll always get a low reading.

« Last Edit: November 08, 2018, 06:44:08 am by flybox1 »
'78 750K (F3 engine) PD42b's, Modified airbox w/K&N  filter, 40/110 jets, 1 needle shim, IMS@ 1 turn out. Kerker + Cone 18" QuietCore

Past Bikes
1974 550K0 (stock), 1973 CB350F (stock), 1983 Yamaha XS400K (POS)
77/78 cool 2 member #3
"Knowledge without mileage equals bullsh!t" - Henry Rollins

"This is my CB. There are many like it, but this one is mine…"

Offline hondacr

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Re: 77-78 CB750 PD42A Carb Rebuild Info
« Reply #137 on: November 08, 2018, 08:31:54 am »
Fuel has to flow into bowl, and, into the tube, to get an accurate reading. So leave the petcock on.
Otherwise, you're just draining the bowl into the clear tube, and you'll always get a low reading.

Thanks very much to all of you for all your help its much appreciated ! I cant wait until I can get my hands dirty again...
still suffering pain just now from post Op' spine surgery... I wished I had looked after my lower back much more when I was younger...take care of yours Guys !

 

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