Topic: 77-78 CB750 PD42A Carb Rebuild Info

[madmtnmotors]

Be careful though....the connector tubes each contain two brass restrictor tubes inside.  these are necessary to the system as they take up volume. Be sure to clean them and put them back in the new tubes.

+1

I'm pretty sure there is just one brass restrictor in each rubber connecting tube, but be sure to remove them from the old tubes and put them back in the new rubber connecting tubes. 

[flybox1]

Two in each. 6 total.  Got these little buggers in my hand 
The fuel tube needs to bend slightly.. otherwise you cant maneuver them into place over the barbs
2mmID x 4mmOD x12mm

Just be sure they go back on the carbs 

I also took a sewing needle and put a 180deg bend in the end of it....so i could get to the front of the accel pump squirter and dislodge stuff while i sprayed into the fuel barb.  Just put a finger over the opposite side barb when going into 2 and 3...lol

[Old Okie]

Frustration is setting in, she is still not running. So far I have checked the compression, 120 psi, low but should run. New spark plugs. checked valve clearance. removed carbs from the engine(left the rubber on the engine) and cleaned every thing I could clean. Slow jets were plugged, was able to push Marvel Mystery Oil through the accel ports, MMO because my eyesight is very poor and I can see the red oil, new accel pump kit, set floar heights, did not need adjusting, started her uo, same story, starts OK on choke, runs on 1/2 half choke dies on no choke. No fuel through the accel ports.
Throttle cables hang up with wheel turned right, OK other wise, I can fix that. I can see that these carbs have been worked on, probably several times. I am beginning to think some of the problem is in the carb linkage, the cam not pushing the accel pump, slides seem to work OK but the running on choke has me stumped. Also the float valv on #4 leaks a full stream, has been leakie before. So here I am looking for advice and assistance.   may have mentioned it before, I thought the starter was noisy. The problem is that tin can starter cover is bolted to the starter and makes a good resonator. Without the cover the starter is quiet.  Any how, that's my story, got any suggestions.??

[Remcod]

Running on choke means lean mixture. Either float height incorrect or slow jet plugged. Did you pull the slow jet and run a wire true it? You can check float hight with transparent tube also. Still original jets?

Have had the same for 2 reasons:
1 clogged slow jets
2 replacement slow jet from keyster was shorter than original thus not getting fuel

Pull the slow jet with pliers, just a little tug will do

[Old Okie]

I did pull the slow jets and clean them, original jets.  Float heights measured OK but I will check with the clear tubing. Carb cleaner blows both ways through the slow jets. The only thing I could get through the slow jet was a high "E" guitar string, .010 ", is that right, I can ee light through them. The accel pump is still a puzzle. I may be learning why the bike was setting for a while.
The slow jet is a #35, what size wire is that??

[Remcod]

That all sound ok to me. I assume you also cleaned the tiny holes at the side etc

The float height must be 12.5 mm or 0.492 in

Check indeed with the clear tube to rule out fuel starvation; inline fuel filter perhaps? Or the filter in the tank clogged? Stuck float valve?

I don’t think that the accelerator pump has anything to do with it. This one only squirts in extra fuel when you pull the gas quickly ie when passing a car or so, but does not control the normal acceleration. This to prevent the choking of the engine at fast acceleration that you get with round top carbs

[flybox1]

Float height is 14.5mm.  Same height as the top of the pilot jet.
Clear tube test will confirm this...

[Old Okie]

I am seeing 12.5 mm and 14.5 mm for float heights, so which is it???
I am at 14.5 mm now and not running.

[madmtnmotors]

14.5mm per the manual.

[TwoTired]

I am seeing 12.5 mm and 14.5 mm for float heights, so which is it???
I am at 14.5 mm now and not running.

Neither mechanical measurement will prevent the bike from running.  But, unless you are using original Honda parts, the mechanical measurement may not result in actual proper fuel level height in the float bowls.  That is why the clear tube, sight gauge measurement is superior, in order to know what fuel level is actually in the carb bowls.

Knowing the fuel level is a stepping stone of knowledge gathering to help determine why you motor won't run.

Do you know fuel is collecting in the bowls?  A sight gauge confirms that.

Cheers,

[Old Okie]

I will be doing the clear tube float level test today.
Not sure my slow jets are clear, I can see light through them but just barely.
what is the actual; size of the opening in that jet, is there a restriction inside
 the tube or is it just a tube??? 

[flybox1]

I will be doing the clear tube float level test today.
Not sure my slow jets are clear, I can see light through them but just barely.
what is the actual; size of the opening in that jet, is there a restriction inside
 the tube or is it just a tube???

The jet size is just that...in millimeters.
if you have a #40 pilot jet...the ID is .40mm

110 main jet = 1.10mm ID

[Remcod]

12.5 mm as per my Honda shop manual

Page 257, carburetor setting table, supplement to k7

Can’t post a picture because I refuse to login  with google/Facebook in Tapatalk

[Remcod]

Ok to be more precise:
K7 supplement p.257
Main jet 115
Slow jet 35
Float height 12.5

F2 no carb supplement thus I assume the same

F3 supplement p.282
Main jet 105
Slow 35
Float 14.5

Hope this helps

[flybox1]

OP has a '78.

14.5mm


Post pictures using the icons above the text field.

[Old Okie]

My pics are poor so I did not post them. Some progress, all accel jets work.
By the clear tube method #1 fuel leves is nearly 1/4 inch below the top of the bowl, #4 is slightly higher??  I will het some better tubing and try the fuel level test again.

[flybox1]

for the sake of ease in adjustments, i hope you are doing this off the bike with an aux fuel source.

Continue to adjust the floats till all fuel levels are 3-4mm under the top edge of the bowl

[Old Okie]

The carbs are on the bike, accel system working but still won't idle.
I checked carb #1 and #4 fuel level with the clear tube. #1 is about 1/4 inch
below the top of the bowl, #4 is a little higher. Slow jets not getting fuel???
What now??

[Old Okie]

I will do the floats tomorrow. Got to get new tubing, mine was not fuel
resistant and the gasoline ate it up.  The upper throttle cable binds up
with wheel to the right, can I route the cable over the coil to gain a little length
or get a longer cable??

[flybox1]

If your cable rides smooth when disconnected from the carbs and hanging towards the floor, its in good shape so just play with different routing. 
Over the coils worked for me..

[Old Okie]

The cable is OK just too short. I think a new route will fix it, I may have a longer cable.

[Old Okie]

Maybe the only thing I have not figured out is the tiny holes in front of the
carb slides. Some one asked if I cleaned them and I asked where they went so i
could clean them. My eyesight is really poor and I can not see those "tiny holes"
without a bright light and a mirror. So where do those tiny holes go so that
I can clean them with carb spray???

[Old Okie]

I'm back. I have cleaned everything I can and tested the fuel level with the clear plastic tube method. All bowls check withing the 2mm to 4mm below the top of the
float bowl, so that is good. I can see no reason that this bike will not idle. I did see one bit of info that may help me understand this problem. It seems these bikes are cold natures because of running so lean. I may be expecting the bike to idle before it is properly warmed up. I will put the carbs back on later today and see how it goes.

[flybox1]

Yes, these bikes can be somewhat cold blooded, but that is negated when your carbs are clean, your fast idle cam is functioning, your jetting is correct for your intake and exhaust, your IMS are set correctly, and your idle set screw holds your +/-1200rpm idle.

Youve only got the carbs clean...so on to the rest 
Set your IMS to 1 turn out from seated.

Install the carbs, throttle and choke cables, airbox....pull the choke and start it up.
do not mess with the idle set screw near carb #4....yet. 
If it starts, leave the choke pulled until the rpms start to race..start pushing it in to keep the rpms near 2000.
If you get it depressed all the way, and the rpms are still high, now start turning out your idle set screw out to keep you 1200rpm or so.
At this point, there really is no reason to further adjust your idle set screw until the bike is fully warmed up.
After this last adjustment...it is left alone, and your throttle/choke is used to keep the rpms up while the bike is warming up.

[Old Okie]

What am I doing wrong??? I can not get the air box on.
It does not go far enough forward to meed the mounting bolts??
Is there a trick?? Will I ever get this thing to run??