Topic: 77-78 CB750 PD42A Carb Rebuild Info

[madmtnmotors]

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...

[Don R]

 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.

[hondacr]

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

[flybox1]

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.

[hondacr]

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   

[flybox1]

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

[hondacr]

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?    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

[flybox1]

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!

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.

[hondacr]

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

[madmtnmotors]

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.

[hondacr]

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 ?

[flybox1]

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.

[hondacr]

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 !

[hondacr]

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.

I have just done this check & its all good....Thanks for the info'

now i'm trying screwing the fuel/air mix screw right in [but not to the very top], also fitting D7EA plugs .

[flybox1]

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.

I have just done this check & its all good....Thanks for the info'

now i'm trying screwing the fuel/air mix screw right in [but not to the very top], also fitting D7EA plugs .

The mixture screws should be screwed all the way in, and then backed out the desired turns.
D8EA, DR8ES-L or Denso x24es-u are the proper plugs for your CB

[hondacr]

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.

I have just done this check & its all good....Thanks for the info'

now i'm trying screwing the fuel/air mix screw right in [but not to the very top], also fitting D7EA plugs .

The mixture screws should be screwed all the way in, and then backed out the desired turns.
D8EA, DR8ES-L or Denso x24es-u are the proper plugs for your CB

The DR8ES-L  are resister plugs [don't use them with the resistor caps] ive tried several sets of the above including the D8EA but no good,
I am using the D7EA which are a hotter plug...up to now all Good...

[flybox1]

With good coils, plug wires and the proper caps, the plugs I listed provide the proper spark.
Anything else is a bandaid for another issue.

[hondacr]

I have recently fitted brand new OEM Honda coils [Dave Silver] with wires & plug caps, but still had a problem, tried all the plugs you mentioned + iridium, adjusted the carbs as suggested by others still & had the problem .. now got the NGK D7EA & these seem to have sorted it, now starts 1st push of the button, idles very smooth at 1100rpm, but not had a real good run on it yet so only time will tell.

PS.. its well known that there are a lot of snide NGK plugs on the market

[flybox1]

"The SOHC4 bikes all came with "resistor caps" for the sparkplug connections.
On the bikes prior to 1976, the ohms value of the resistor inside was 7500 (7.5k) ohms from the factory. Over time, they slowly burn out and increase in value: at 9500 (9.5k) ohms, they are considered to be "burned out" and can cause several problems, including hard starting and fouled sparkplugs. This can also make the spark jump to the engine or frame (or your leg, in the rain), making for some pretty night effects. 

From 1976 until the end of the SOHC4 series, the ohms value was 10,000 (10k) from the factory, and the coils came with thicker spark wires because of this higher resistance (to prevent pretty night effects!). This later design was intended to reduce AM radio interference for cars next to your bike, and for riders who were sporting fairings with built-in radios for touring. It also made the plug cap life shorter, as the "burnout limit" was (is) only 11,000 (11k) ohms. These coils can be retrofitted to the earlier plug cap resistances with improved spark being the result, as the internal parts of the coils did not change.

Today, we can get plug caps from NGK in 5000 (5k) ohm values, which will slowly burn out over time to the 9500 (9.5k) limit over a much longer period. The newer plastic and rubber boots are superior quality: a bargain for their $5 price tag. They come in the straight, 90 degree, and 135 degree angled shapes for all of the SOHC4 bikes. Use these as replacements for superior performance on your bike.

NOTE: NGK makes caps in 0 ohms, 5000 (5k) ohms, and 10,000 (10k) ohms types. Pick the right one!

An alternate approach for the CB750, CB350F/400F and the CB650 bikes, and touring with the CB500/550, is to use the 0 ohm caps from NGK and the resistor sparkplug DR8ES-L from NGK. This heatrange is ideal for today's gasolines that are laced with ethanol: it is halfway between the colder D8EA that fouls easily and the D7EA that fits the CB500/550 engines. Don't use both resistor caps and this resistor-type sparkplug, though, or you will have too much resistance in the spark circuit. There must be at least 5000 ohms in this circuit, whether you use Honda or Dyna or Accel coils, to make things work properly."

[TwoTired]

The CB750 will happily use D7 heat range plugs, especially in cold weather.  I wouldn't use them in summer heat, though.  Prefer D8 when the motor is hot and making power.

You can use resistor plugs and resistor caps, so long as the 5K ohm resistor plugs are matched to 5K plug caps or 0 ohm).  Don't really want more than 10K ohm added per spark lead. 
I've seen plugs spark even with completely open plug caps.  Not ideal, but it still works...for a while.  An open plug cap adds another spark gap to the circuit.  The internal arching gradually increases/erodes the gap further and further, until the total number of gaps are to large to spark reliably with available voltage.  Gaps not at the spark tips are bad.

Cheers,

[hondacr]

I have a question about the NGK plug supressor caps, is the rubber on the plug-end of the cap important ? 
The reason I ask is because of the problem I have been having recently with Cylinder 4 plug fowling up after a decent run....
I decide to try NGK 7's instead of 8's & also fitted a new NGK plug Cap, when removing the old Cap[but not that old] I noticed that the old Cap for cylinder 4 did not have the rubber sleeve at the plug end so I wondered then could that have caused an inferior spark on this plug , the other 3 plug caps for cyliners 1.2.3. were in place ?
obviously when fitting the new plug cap to cylinder 4 I made sure the rubber was in place.. ive not had chance to try out yet ..

Prior to all this I set the air/fuel mixture to be as lean as possible, I checked the float bowls for over fuelling & all was good. 

[sixthwisconsin]

Prior to all this I set the air/fuel mixture to be as lean as possible, I checked the float bowls for over fuelling & all was good.

Did the mechanic that installed your jets and accelerator pump thoroughly clean the carbs and insure all passages were open? These PD carbs have some very tiny passages that can clog easily. I just finished a set for a buddy but even after sonic cleaning and assuring liquid passed through all circuits, after assembly #1 accelerator pump circuit was clogged. It took several shots of carb cleaner and compressed air to get it to reopen.

My point is, I think you are chasing other perceived issues with the spark plugs before assuring the carbs are clean and set up properly. Unless you are running pods and/or open exhaust, put the carbs back to factory settings and go from there. Set the air screws at 1.5 turns out, bench sync and set float height with clear tube method.

Now you will have a starting point for tuning.

[hondacr]

On the Keihin pd Carbs the air / fuel mixture screw shown below has a spring, washer &  O-ring... in what order does the washer & the o-ring go on the screw & spring ?

 sixthwisconsin thanks for your input, in answer to your question yes the carbs were cleaned new accelerator pump .etc. he also fitted all 4 new air/fuel mixture screws & all was good initially, its only recently that Cylinder 4 plug has started fowling up, I am thinking of removing the air fuel screw & taking a look, I have to admit that I don't like messing around with fiddly things like this has my patience is not good just like my lower back  , but would like to check this screw on carb'4 & make sure the washers are in situ ...I remember when the mechanic removed the old screw to fit the new ones sometime ago that the rubber o-ring on carb'4 had to flushed out but only part of it came out & no matter what he did he could not get any more debris out despite many flushes however the bike did run OK after that.
also after the total rebuild the bike had K&N's fitted & larger main jets 135's & 4 into 2 open pipes from CycleX USA, but the K&N's proved to cause a problem with air flow & the bike would start baulking at around 50/ 60 mph, so I tried fitting open velocity stacks with gauze filters & they transformed the bike.. its just pot 4 that's recently thrown up this issue.

[bryanj]

Spring, washer, O ring

[sixthwisconsin]

Spring, washer, O ring

+1
I'm also guessing you were over fueling with the 135 jets even with the free flowing K&N pods.