Topic: 1977 CB 750 accelerator pump question, again.

[jcmatin]

This is my first post on the forum, so I hope I make it a good one.  I bought a 77 CB 750 K that I hope to make a good runner.  When I bought it, it ran poorly and only at full choke.  After a lot of research I figured out that it is the accelerator pump on the carbs.  I did everything possible that can be done to it, and now it is correct, well almost correct.  The problem that I am having is that the rod does not touch the throttle cam.  It is as though the rod is too short or the part of the cam that touches it needs adjusted.  I see a nut down there that can be loosened, hopefully allowing the piece that actuates the pump rod to move.  The issue is, I cannot get to it without taking the entire rack apart, which I do not want to do.  Please help!  I have had the carbs on and off the bike 5 times in the last 24 hours, each time thinking I have got it, only to be let down.  Then I realized that the pump is good, but it is just not being activated.  If I have to take the rack apart, I will, but I prefer an easier fix if possible.  I replaced the diaphragm with a new one, and it is the same length as the old one.  Maybe they are both too short?  Thanks for the help.  I look forward to getting it running right.
                                                               Joe

[flybox1]

Yes, there are diaphragm pumps with shorter stems.  I dont recall if it was a bad batch, or poor QC from the aftermarket mfrs, as these are used in 76-78 F/K and A's.  IIWM, stick with HONDA brand.

How much space do you have between the tip of the rod, and the tang?
Does the tang actually move full stroke with throttle twist?
If yes, does it touch the rot tip at all?

A picture would be nice.
-fly

[jcmatin]

There is a significant distance between the rod and where it should touch, about 1/4 inch.  As the throttle rotates, it gets closer to the rod, but it should be touching at all times.  I believe the rod is too short.  If anyone has a measurement for what it should be, that would be appreciated.  I will try to send a picture later.  Very frustrating.  Thank you for the reply.
                                                                   

[flybox1]

With that much distance it sounds like you got a short rod.
There should be some space (<1mm) between the tang and rod tip.
I have one at home to measure, but it wont be till the end of the day...sorry.

[jcmatin]

Short rod eh?  So I've been told.  On a serious note, it would be great to get that measurement.  I am in no hurry as I am at work all day.  Thanks again for the time.
                                                                                    Joe

[TwoTired]

Honda CB750 shop manual, chapter 24 supplement, pg 258  states the rod to pump arm clearance is 0- .2mm (0 to 0.008 in) clearance.  Adjust by bending pump arm tang.
It also states the arm to carburetor stay with throttle closed clearance as 9.5 to 10.5mm (0.374 to 0.413 in.) Adjust by bending the pump arm.

If you have a short rod pump, fix that rather than try and bend the arms.

Cheers,

[jcmatin]

I must have the wrong rod length.  As the carb rack sits on the bench, there is about 5/16 in from the rod end to the tab.  I have to really open the throttle quite far to even get it close.  I am assuming that this rod should be activated throughout the throttle range?  The bike did seem to smooth out the higher the revs were, maybe where the rod was actually being activated?   Very annoying, as I am sure I cannot return the part now.  I am growing tired of the aftermarket parts game.  Does anybody have a link to where I can get the correct part?  Also, any pictures of others bike would be helpful, to be sure I am understanding it correctly.  I apologize for the lengthy and annoying line of questions, but I do not want to be defeated by this thing.  Thank you all for the replies so far and for any new ones in the future.
                                          Joe

[TwoTired]

Have you thought about extending the rod on the pump you have now?  Silver solder a rod extension and perhaps a sleeve coupling for the extension?

Or...
16021-393-004
https://www.amazon.com/Honda-CB750-Diaphragm-16021-393-004-16021-393-014/dp/B00UNMXP8E

Cheers,

[jcmatin]

Two tired,
   That is the part I purchased from amazon.  I am wondering if something has loosened and moved on the carb, maybe the tab where the rod is activated.  There seems to be a nut down in there that can be adjusted, but I cannot get to it as the rack sits, may have to take it apart, gross.  Silver solder is a last resort.  I have some at home, but it is a pig to get the temp right for the melt.  Thanks, as always.
                                                                                      Joe

[flybox1]

JC....didn't get home till 10pm last night.  Sorry 
I have a reminder in my calendar for when i'm home at 5:30 today.  I'll try again.

[jcmatin]

No worries flybox, I'm good.  I'm gonna get this thing figured out if it kills me.  First japanese bike, and I am trying to not get frustrated with it.  I know that when it gets right, it will be amazing.  I have old brit iron now, and they are just a bit less complicated.

[TwoTired]

Two tired,
   That is the part I purchased from amazon.

That's worrisome.

I thought there was something odd about it.  The Honda Parts fiche shows that number including the bottom pump casting (as an assembly replacement), which the Amazon one did not include.

The assembly versions are available upon search.  But the cheapest I found was $80.   Less attractive than $18.

I wonder if a complaint to Amazon is in order?

Odd, though.  I did purchase a replacement diaphragm (only) from Honda, way back.  It's still packaged and waiting in my parts cache.  ...700 miles away at the moment.  I wonder what the part number is on that?

Cheers,

[flybox1]

Ive never seen a 750s PD carbs with a removable diaphragm stem
2 OEM and 2 replacement diaphragms in my hands all had fixed stems 

[jcmatin]

Well, I am an idiot.  The part is correct, the mechanic is faulty.  I had it together correctly, no, the spring was in the wrong spot.  What a dope.  But, does it run now?  No.  I believe the accelerator pump jets are blocked, as I cannot get any fuel through them.  I have read about the pump method with the syringe, I may try that.  I have guitar strings to poke through the holes, but I cannot reach them, as they are facing the inside of the carb.  Can they be removed?  I really feel silly for the mistake, but as they say, live and learn.  As a side note, if anyone wants to have a contest to see how fast one can remove the fuel tank and carb bank, I'm your huckleberry.
                                           Dopey Joe

[madmtnmotors]

the spring was in the wrong spot.

I was going to ask since that was about all I could figure would leave a 1/4" gap. So from the carb body, the little rubber boot goes in, then the stem with the diaphragm, then the spring, then the little "mini bowl" so the spring is between the diaphragm and the little bowl. So you had the spring between the carb body and the diaphragm? We've all made at least one mistake, there's even a thread dedicated to just that somewhere here on the forum (for those that are willing to admit!). 

[jcmatin]

You guessed it.  Now, I have to hope that the jets are clogged.  If not, I am beaten.  Well, I take that back.  I will not be beaten by this lump.  I know that when it runs right, it will be a blast.  I am balancing time between it and rebuilding a triumph 500 motor.  I think I will focus on the triumph tomorrow, then hit the Honda back up on Thursday.  I feel that I am so close.  I really want to take the entire carb bank apart and clean everything, then re assemble.  What I am afraid of is all of the linkage, springs, etc that make it go.  I don't want to get rim rocked and have to farm it out.  The bike was cheap, $600 with a title and it runs, sort of.  I want to keep the cost down, and money spent on carbs could be pricey.  As a little back story, my friend was going to buy the bike to build a brat style bike, but his wifey said no, so, I picked it up.  I may keep with the brat theme, but with modest changes, no missing oil tank and such.  I just need to have it running well before I go tearing it down.  I just need to feel that 750 pull the way it is supposed to.  Great motor when it is right, but mine is far from right.  A final, more interesting side note: my uncle has a 69 sand cast that he may GIVE ME.  That's correct, give to me.  That, I would keep original.  Fingers crossed on that one and the 77.  Thanks again for all of the help and support.
                                         Joe

[flybox1]

OK 
Nice to know a brother didnt get the short shaft   

Cleaning the accel pump circuit.  Follow the steps in order. 
If any of these steps does not work, repeat the previous step.  Wear eye protection .   
Can of carb cleaner w red straw, sewing needle/pin are needed.
We are assuming your carbs have been thoroughly cleaned at this point.

Accel Pump Diaphragm Bowl: spray carb cleaner through the tiny side hole to ensure the check valve in the center of the bowl is clear.
#2 Carb Bowl: On the underside of the carb bowl, where the accel pump bowl attaches, spray carb cleaner into the small hole to verify the check valve on the TOP rim of the carb is clear.

Remove the three black accel pump fuel circuit tubes from their barbs.  Be careful as each of these contain two brass restrictor tubes.   Replace the fuel hose as needed, but be sure to install these restrictor tubes in the new fuel line.

Cleaning the accel pump jets: While these fuel tubes are off, you will need to spray carb cleaner into each one to verify a stream comes out of the accel pump jet in the carb throat.  Carbs #2 and 3 you will need to finger-close the opposing side.
If no fuel squirts out of an accel pump jet, you will need to fashion a hook out of a sewing needle , or get a long shank'd fish hook, to carefully poke/clear the small hole of the accel pump jet whilst shooting carb cleaner into the fuel line barb.  Repeat as necessary as these NEED to be clean.
#2 Carb bowl to carb body check valve interface.  This is a tough one, as its a TINY hole.  Find the brass plug on the carb body which mates to the check valve at the top of carb bowl #2.  Please wear eye prot for this one.
You will need to spray air, or carb cleaner into this tiny hole, to make sure it fuel can pass thru.  Fuel will come out one of the fuel line barbs of carb body #2
Once done, reinstall the three accel pump fuel connector tubes containing the brass restrictors.

Bench test.  do these in order pls.
Install all carb fuel bowls EXCEPT for #2
Fit the diaphragm and spring into the accel pump bowl, and affix it to carb bowl #2.
Put a small amount of CLEAN fuel into the bottom of #2 carb bowl, and depress the accel pump rod until fuel squirts out of the TOP edge of carb bowl #2
Install the small o-ring on the top edge of carb bowl #2, and install the bowl onto the carbs.
Connect carbs to an aux fuel source, and let the bowls fill.( A  PERFECT time to clear tube test them  )
Use a std screwdriver (or actuate the throttle bell) on top of the accel pump rod to depress it until fuel squirts out ALL of the accel pump jets in the carb throats.
This might take 10-20 depressions of the diaphragm rod.
If NO fuel travels, you have a blockage somewhere.
First...check the o-ring at the top of #2 bowl. 
If this o-ring is not OEM Honda, ive found that the off-size, compressed, can prevent any fuel from passing thru the bowl rim check valve.
To see if this is the case, remove ONE of the fuel lines between 1&2, or 2&3, and then depress the pump rod.  If no fuel exits out the fuel barb, revisit the o-ring/check valve/brass interface insert here.

Dont put the carbs back on the bike until you get all these to squirt.
PM me if you have any issues working thru this...
Rob

[jcmatin]

Well, the saga continues.  I did all that Rob suggested, and I know that the accelerator pump is working.  But, I am only getting a drizzle of fuel from the accelerator jet nozel on carb 1.  All 4 jets are clear, as I can spray carb cleaner through them very easily.  All 3 hoses that connect the float bowls are clear.  The mains are clear.  The jet on 1 does not spray, but leaks fuel out of it.  When I spray the carb cleaner through all of the jets, it rockets through.  With the hoses and float bowls on, nothing.  Clearly there is an issue of fuel getting to where it needs to go.  Why would accelerator jet 1 let fuel through, but the other 3 will not? Are the carbs being starved of fuel somewhere along the way?  I checked the petcock, and it flows pretty freely.  I did replace the inline fuel filter with an autozone version.  Could that be starving the entire system?  Where else could I be losing fuel, or fuel pressure?  I feel that I am so close right now.  Thank you for all of the help.  Rob, great post.
                                                                 Joe

[jcmatin]

I just thought of one more thing.  When I got the bike, the air screws were at least 3 turns out.  I screwed them in and backed them all out 1.5 turn, and the bike would not start.  As I backed them out more, a 1/2 turn each, it would get better and at least start, but idle very high and only at full choke.  I hope this helps the diagnosis.  The owner mentioned that he put bigger jets in it due to the baffle being removed from the 4-1 exhaust.  Thanks.
                                                                             Joe

[jcmatin]

The main jets are 125.  I checked the float needle orifices, and they are clean.  When I sprayed carb cleaner through each one, it exited through the fuel line/filter.  The floats may be a little off, need to check again how to measure that. The entire lower end of the carb seems clean and functional.  Yet, only a drizzle through the far left carb accelerator jet.  I'm dying here man!
                                               Joe

[flybox1]

Do all the connector tubes have the brass restrictors installed?
What happens when you continually depress and release the diaphragm rod....does it eventually squirt out the other carbs?

I purposefully switched to clear tubing on my accel pump fuel tubes so i could see them prime with fuel

[madmtnmotors]

125 main jets are awfully large. My K8 has been bored to 836cc using a MAC 4 into 1 with a straight through baffle and I had to drop back to 110 mains. These don't have much effect at idle though.

Neither does the accelerator pump

The sole purpose of the accelerator pump is to help overcome the slight lean condition when the throttle is applied. At idle, or even at a steady cruising speed, the accelerator pump is dormant. The accelerator pump circuit only pumps fuel under acceleration and has no bearing on the carburetor performance at idle nor at steady rpm in any range.

If the bike won't idle you need to be looking at several things:
>Compression
>Timing. Have you checked static timing?
>Fuel flow. Measure the flow rate from the fuel line into a graduated container. How much fuel flows after 1 minute? After 3 minutes?
>Valve lash
>Carb idle circuits
>Proper choke operation. Look into the carbs and verify that when the choke cable is pulled that all the choke plates close smoothly and completely.
>Check carb sync once you can get it to idle well enough to do so.

If none of these checks reveal the problem then the only thing left is spark. What else have you done to this bike? Did you replace the spark plugs?

[jcmatin]

Thanks boys.  I am going to go back through it all again tonight.  I am going to check the fuel petcock and the venting of the tank to see if the problem starts at the top.  I am 99% sure that all of the passages are free, as I have cleaned them all individually.  After that, I will start with the spark, then go through the rest of the progression.  Fingers crossed. 
                                                                                           Joe

[jcmatin]

Update.  I am getting somewhere.  I have discovered that the gas cap is not venting properly, so there is a vapor lock.  I took the screen out of the tank, it was clean.  I checked the petcock, flow is good.  When I removed the fuel cap, gas started flowing in to the fuel filter quickly, as it had not done before.  So, I started the bike and it ran a little better.  The next issue is that gas was pouring out of the hoses that come off the bottom of the float bowl.  I made sure that all of the screws are screwed in tight, but no luck.  It was not coming from all of the hoses, just 2, I think.  There is also some gas leaking from the float bowl gasket on the carb with the accelerator pump.  I am guessing that the float needles are not working properly, or the screws on the float bowls are not sealing, or both.  Either way, time for a rebuild kit for all 4.  I also checked the spark, as a matter of course, and found that the far left cylinder had none.  Got a new plug and now it fires.  I am still not sure if the accelerator pump is working properly, but at this point, I think I have too many other problems to worry about it.  I have to get the fuel going everywhere issue figured out.  I come home from work, change clothes, mess with the bike, and then smell like gas all night.  My wife really loves it.  Please advise on the fuel issue, as I feel as though I am nearing a conclusion.  As a side note, I also re set the air screws to 1.5 turns, as they were about 4-5.  Thanks.
                                                   Joe

[flybox1]

When you had the carbs off, did you check/set the float heights?
did you install new viton-tipped float valves?
If the bowls leak, yes, new bowl gaskets are needed.  Get the shaped viton ones.
if the overflfow tubes leak, the float valves are not doing their job...either because the height is wrong, the fooat is caught ot stuck, the valve tip is worn, or, there is debris in the gas boocking the valve.
Once you know the float valves close, you will need to verify the fuel level using the clear tube test.

All of this can be checked and verified with the carbs off the bike...including the accel pump.