Topic: Ready for a fire sale

[vistirus]

O.K. - '78 cb750k- new points, plugs, condensor. Fuel tank has been cleaned out - twice. Carbs taken off, completely dismantled and cleaned (removed all the jets and cleaned with wire and liquid. Stuff squrts out everyplace it is supposed to). New accelerator pump. Bench sync'd the carbs, set the float levels, pilot screw back 1.5 turns. Stock air box (cleaned the filter), factory exhaust (4 in to 4).

Pull the choke out, turn the key, hit the button and it fires right up. Immediately goes to 4,000 rpm and stays there. Idle thumb screw back out till it doesn't touch.  I can use the choke control like a throttle.  Push it in and it revs go down, but if you try to turn the throttle, it instantly dies. I guess I could use the choke cable for a throttle, or I could leave it at 4,000 rpm and pop the clutch.

Or I could dump the 5 gallons of gas in the tank all over the top of the thing and light a match.

BTW,  the previous owner couldn't have done anything to the bike. I bought it new in 1978.

Any suggestions? Any estimates - will I have to pull these carbs 12 more times or do you think I can get by with 10?

Everyone on this forum has been a great help, but I guess I just don't get it.

[cmonSTART]

Intake air leak is the first thing that comes to mind.  Are your clamps good and tight?  Boots in good condition?

[mycb750k6]

Make sure you take a video of the fire    I feel like doing that from time to time.

[cmonSTART]

Make sure you take a video of the fire    I feel like doing that from time to time.

Don't we all!

[flybox1]

the way you wrote your procedure, if true, is confused.
try this...

turn in idle screw
pull out choke
start button
let warm
PUSH IN CHOKE
.... then turn down idle set screw to a good idle.

dont turn the idle screw all the way out, it will die. 
it drops your slides and cuts off vacuum...

but if not, i got dibs on your exhaust before you light the match 

[cmonSTART]

Reading what flybox1 said, is the bike at 4000RPM with the choke on or off?

[crazypj]

Just because carb cleaner comes out 'everywhere it should' is no indication carbs are 'clean'
Emulsion tubes and jets are brass. you get oxidation in all the holes which restricts the flow and makes bike run lean.
Some of the holes are only about 0.015" and biggest is only about 0.046" (most are around 0.025")
Check throttle cable set up and routing, could be kinked or too tight?

[flybox1]

EDIT: crazypj is right, but I can only assume they are as clean and right as he says they are, and its something else.

i read it as choke pulled ON, and idle screw all the way OUT.
should be the other way around once warmed up. 
choke off, and idle screw holding idle(turned in)

[rangerboy2788]

I take it as he is using the choke to rich-en the fuel/ air mixture and its bringing the revs back down? this in fact would indicate an extremely lean mixture.

[phil71]

yeah man, those carbs are still dirty. Lots of people forget to pull the idle jets out.. because they aren't as obvious as other pilot jets.. and it sounds like yours are fkd. Those PDs are a bear to clean well.

[Really?]

Before you decide to burn it, sell it to me!

[TwoTired]

Pull the choke out, turn the key, hit the button and it fires right up. Immediately goes to 4,000 rpm and stays there. Idle thumb screw back out till it doesn't touch.
 I can use the choke control like a throttle.  Push it in and it revs go down, but if you try to turn the throttle, it instantly dies. I guess I could use the choke cable for a throttle, or I could leave it at 4,000 rpm and pop the clutch.

That's the way it is supposed to work.  It's not going to idle at 1000 RPM when cold.  The choke is coupled to its own fast idle cam that move in unison with the choke.  The big idle knob only sets the idle with choke disengaged after a thorough warm up (choke completely turned off).  The fast idle cam has it's own speed adjustment screw and it sound like someone tweaked it to add more revs, or didn't put the choke cable on to attain full travel operation to allow for fast idle cam disengagement.

Or I could dump the 5 gallons of gas in the tank all over the top of the thing and light a match.

...Or, you could RTFM to learn how it should work and adjust it correctly.  But, it your toy to abuse as you see fit.

but I guess I just don't get it.

I suspect if you read Chapter 24 of the CB750 Honda Shop manual, you could "get it".  In there, it says that the fast idle speed can be set "3000~4000 rpm"

This assumes the pressed in pilot jets are clean and clear.  The shop manual does NOT tell you to remove those, but if they are plugged, you won't get an idle after warm up and the choke pushed in.

The carb pulling cycle will continue until the job is done right.  It can be done in one cycle, though.  But, a stern stare and splashing of cleaner doesn't guarantee it.

Good luck!

[vistirus]

I did pull and clean all the jets - including (if it is the one that is pressed in) the idle jet. To clarify, the choke is pulled all the way out, at start, and, originally the idle thumb screw was turned in (maybe 5 turns). I turned the idle screw all the way out after it started and I thought it was goint to blow up.  Pushing the choke in, and leaning the mixture, brings the idle down.  Pull the choke back out and the rpm goes back up to 4,000.  Push it in and it goes back down. But you still
can't  touch the throttle. If you do, it  stalls and shuts off.  Should I let it run  at 4,000 until it warms up and then try to push the choke back in?  I am using the original carb boots. I believe they are clamped tight, but they are rock hard (used some heat to remove/install carbs).  I read another posting from a member with a '78 cb550k who had a lot of idle problems(sounded like mine)  and fixed them by installing a new o-ring (his description) that goes between the intake manifold  (or head) and I assume where the carb boot is clamped. I couldn't find this part on any exploded views in the service manual (the Honda manual posted on this site). Is there such a thing on a 750?

When I was young (45 years ago) I campaigned a drag car up and down the east coast. Did pretty well.  Never had trouble like this. Any one know of an adaptor to mount a Holley 4 barrel?

Seriously, I know this is a rant, but I have spent a long, frustrating day pulling out what hair I have left.
I appreciate any advise I can get.

[dave500]

with your bench sync did you make sure all the slides can shut fully?you might have them all adjusted too high off the idle stop,and theyll never drop down to it,a four barrel is easier to set up than multiple carbs,i prefer the rochester qjet.

[TwoTired]

I turned the idle screw all the way out after it started and I thought it was goint to blow up. 

With the choke pulled out, the big idle adjust screw does nothing.  There is a fast idle cam and adjust screw that takes over slide position until the choke is pushed off.

Pushing the choke in, and leaning the mixture, brings the idle down.   

Leaning the mixture?  What screw(s) are you turning?

Pull the choke back out and the rpm goes back up to 4,000.  Push it in and it goes back down.

Yup, that's exactly what a fast idle cam is supposed to do. 

But you still  can't  touch the throttle. If you do, it  stalls and shuts off. 

Sounds like the accelerator pump isn't working, or you haven't let the engine get to operating temp.

Should I let it run  at 4,000 until it warms up and then try to push the choke back in?

At partial choke, the rpm should be less and still run. 2500 is fine.  After warm up the single big idle adjust knob is set for 1000-1100 rpm with the choke off.

[flybox1]

Start it. Leave the idle screw turned in.
As it warms up, push the choke knob in.
As it warms up further, turn down the idle screw to bring the rpms down.
Turn the throttle slowly.
If the throttle still kills it, dig into your carbs and accel pump again.

[cmonSTART]

Ya, so far it sounds like fairly normal operation - nothing to get too excited over yet.  Run the bike with partial choke as said above until it warms up more.  It's fairly normal to run a bike with partial choke for the first couple miles before it warms up. 

[grcamna2]

It probably wouldn't hurt to soften up the intake rubbers by immersing them in a special solution as documented here on the forum..Wintergreen Oil + another liquid to be sure they're making a pliable tight grip seal on the intake manifolds.
It would sure make taking the carbs on and off 110% easier  .

[kpier883]

I am pretty sure the op understands normal startup operations as his first post stated that he bought the bike new in 1978.  Of course I am making an assumption that it was actually ridden for some years prior to getting gummed up carbs. 

The bench synch advice and the pilot jets and associated passages check sound promising.  Also, if you fill the bowl of the carb where the accelerator pump is located then operate the throttle, do the squirters in each carb actually spray fuel?  I am not acquainted with the accelerator pump equipped cb carbs but my CBX is equipped with an accelerator pump.  It has a bit of adjustability for the point at which accelerator  pump action begins.  Maybe others could comment whether there is a way to adjust this?

[crazypj]

One thing that has been happening with accelerator pump diaphragms, since the introduction of E10 fuel, it breaks down the diaphragm pretty quick (around 4~6 months daily use)
The bits get into passages they shouldn't get into 

[grcamna2]

I am pretty sure the op understands normal startup operations as his first post stated that he bought the bike new in 1978.  Of course I am making an assumption that it was actually ridden for some years prior to getting gummed up carbs. 

The bench synch advice and the pilot jets and associated passages check sound promising.  Also, if you fill the bowl of the carb where the accelerator pump is located then operate the throttle, do the squirters in each carb actually spray fuel?  I am not acquainted with the accelerator pump equipped cb carbs but my CBX is equipped with an accelerator pump.  It has a bit of adjustability for the point at which accelerator  pump action begins.  Maybe others could comment whether there is a way to adjust this?

AS far as the adjustment for the accel.pump there is a metal engagement tang that's part of the throttle shaft/spring cable reel,etc. that you bend to a specified gap to engage the accel.pump shaft...I've always bent the tang to where it just touches the accel.pump "peg" which cut's down on that lag of acceleration just off an idle.

[flybox1]

AS far as the adjustment for the accel.pump there is a metal engagement tang that's part of the throttle shaft/spring cable reel,etc. that you bend to a specified gap to engage the accel.pump shaft...I've always bent the tang to where it just touches the accel.pump "peg" which cut's down on that lag of acceleration just off an idle.

nice tidbit!   
My carbs are coming off soon for new airbox boots and.  I will check this. 
Thanks

[lucky]

I am sorry that you have not gotten the help that you need,

So many ideas thrown at you you must be exhausted. No wonder you want to burn the bike!

Lets start from the beginning.
There is no O ring on the intake.
You have cleaned everything but
size the idle jet is?
size main jet you have?
Needle?
On the 1978 there is no adjustment.
On the 1977 it is an adjustable needle.
You have a stock air box .
You have 4 into 4 exhausts. Baffles?
Mixture screws 1.5 turns out.

Still three unknown things, for the mechanic.


First check.
With the engine off, Air filters off, Float bowls full, look into the intakes of the carbs with a flashlight and turn the throttle a couple times to see if fuel is squirting from the brass nozzles. Make sure ALL 4 of them work.

With this exhaust and stock air filter and stock jets this bike would run very lean at idle and the idle would come down very slow and be over revving. Too lean.
That is why I would like to know what size the idle jet is.

So please report back and let us know about the functioning of the accelerator jets and if you can get the other info it will help establish all the facts.
I know this will take some work. But it has to get done.

[lucky]

I take it as he is using the choke to rich-en the fuel/ air mixture and its bringing the revs back down? this in fact would indicate an extremely lean mixture.

Yes...exactly,, now we just need to know about the size of the idle jets and their state of cleanliness.

[lucky]

AS far as the adjustment for the accel.pump there is a metal engagement tang that's part of the throttle shaft/spring cable reel,etc. that you bend to a specified gap to engage the accel.pump shaft...I've always bent the tang to where it just touches the accel.pump "peg" which cut's down on that lag of acceleration just off an idle.

nice tidbit!   
My carbs are coming off soon for new airbox boots and.  I will check this. 
Thanks

Yes take a look at that then look at the photo in the Clymer's manual. It even has an specification for adjustment. But here is the reality.

That tang is so strong and heavy duty you are not going to bend it with pliers thats for sure! I doubt it would ever get out of adjustment because of how it is made.

If you even tried to bend it you would probably damage part of the carbs.
I would leave that alone until you have actually checked the function of the nozzles and other parts. Go take a careful look. See what i mean.

Also the fast idle cam is very hard to get to and most of all of the ones I have seen have never been touche since new. I would leave it alone. Maybe if you lived and rode in the snow all the time it might be worth messing with.

But this is all getting too exotic!!! LOl..lol

[lucky]

I would like to add that going back over this story several times this hint is very powerful.

QUOTE from OP:
"but if you try to turn the throttle, it instantly dies."

That is almost always too lean problem if fully warmed up.
This configuration is going to take a #42 or #40 idle jet.

The stock #35 or even #38 will not work, with this configuration, even if the mixture screw is turned out.
It has been tried by at least two other forum members and would not work on these 1978 carbs. -LUCKY

[TwoTired]

I would like to add that going back over this story several times this hint is very powerful.

QUOTE from OP:
"but if you try to turn the throttle, it instantly dies."

That is almost always too lean problem if fully warmed up.
This configuration is going to take a #42 or #40 idle jet.

The stock #35 or even #38 will not work, with this configuration, even if the mixture screw is turned out.
It has been tried by at least two other forum members and would not work on these 1978 carbs. -LUCKY

You seem to be arguing that the original bike would not run well as driven off the showroom floor.  I'm sorry, but I cannot believe that.
The original jets and adjustments were just fine when new.  If that same configuration today is not working, then something is broken, or not meeting "as new" spec.

The Shop manual for the PD carbs, describes tuning each IMS for maximum rpm at idle speed.  This corresponds to the leanest setting at idle.  The accelerator pump was provided to compensate for this lean mixture, as when the throttle is twisted, it causes the metering jets to flow less, while more air is available to flow through the carbs.

The accelerator pump shooting raw fuel into the inlets should pretty much compensate for any lean mixture the pilot jets and IMS at set for, including if the pilot jets are plugged solid. (No, it won't hold idle with the pilots plugged.  But, it will dang sure accelerate with that strong shot of gas entering the carb throats and the slides open.)

For the accel pump test, it is not enough to simply observe fuel coming from the  squirters, it matters a great deal HOW MUCH is coming from the squirters.
I had them "working" on my 77 CB750F.  And fuel was getting pumped.  However, I had a check valve that was leaking which reduces pressure, and the diaphragm hole in the Mickey mouse ears wear swelled/smashed nearly shut.  Fixing the valve and passage holes, changed (increased) the volume they shot into the carb throats.  Those squirter posts shot a steady stream at least six inches after proper repair, and totally cured throttle twist acceleration.  Even later when the pilot jets plugged again and it would not idle, if you could catch it before the RPM dropped below 800, a throttle twist would still rev the engine.

QUOTE from OP:
"but if you try to turn the throttle, it instantly dies."

That is almost always too lean problem if fully warmed up.
This configuration is going to take a #42 or #40 idle jet.

While true that twisting the throttle causes a lean A/F mixture, that is exactly what the accelerator pumps are supposed to cure on the PD style carbs.

Ideally, you would want idle speed to be >AT< stoichiometric.  But then, any slide opening would go over lean and produce very weak to no power.  The accel pump overcomes this if operating correctly.

As to using a larger pilot jet, this actually IS true to obtain idle pickup with non-accelerator pump carbs.  Mechanical slide carbs must be tuned over rich at idle, so that when the slides are opened, the mixture remains in the combustion range while more air and less vacuum are present in the carb throats.

There are other things to consider within the carb design.  But, the details must be framed about the specific carbs in question and not applied to all carbs in general.
Factors to consider:
How far away is the fuel from the carb throat fuel entry point?  (Actual float bowl fuel level?.  This effects how easy it is to get fuel into the carb throats.)
What is the fuel volume/weight of the emulsion tube chamber contents?  (I don't expect many readers will understand the significance of this.  But, a carb designer must, and a carb alterer, should.)

Cheers,
 

[Duke McDukiedook]

I vote for another carb cleaning with compressed air in all orifices.
That and run the spray test at the carb boots.

[lucky]

HOLD ON...... I think I see the confusion here.

He said he had 4 into 4 exhaust system.
There are aftermarket 4 into 4 exhaust systems right?

He did not say it is the stock exhaust system.

Is it the stock exhaust system???

[lucky]

I vote for another carb cleaning with compressed air in all orifices.
That and run the spray test at the carb boots.

Yes....agreed - pump action and nozzles need to have a visual function test.

[Plummer]

Start it , spray carb cleaner around the boots ,engine side ! If it effects it , you have a leak ! start looking for new ones ! scratch that , I looked for 3 weeks "78" ! I found some used wintergreened ones ! Also new clamps !

[vistirus]

Thanks to everyone for the great response to the questions about my starting problems.  I haven't had much time to work on the bike. I did take a shot and start the bike, with the choke pulled out all the way and the idle thumbscrew turned in 3-4 turns after touching the arm it goes against. Like before, it started at 4,000 rpm (I just knew the piston rods were going to explode through the case) and I was determined to let it warm up. Wound up nursing the choke slowly in until, after 10 minutes, I was able to take the thumbscrew and adjust the idle, with the choke now fully in, to about 1100-1200 rpm. It sat there and idled and I could advance the throttle, in short blips or up to high rpm's, with no problems. I thought, what the heck, so I knocked it off the center stand, dropped  it into first and rode around the  neighborhood.  First time on the road in 25 years and I remembered how much fun it was. Pulled it back in the garage and it still idled OK.  After shutdown, though, I smelled fuel. Pulled the tank and saw gas leaking around the three rubber tubes that connect the carbs and I think this is where fuel would be pushed by the accelerator pump.( BTW, new pump installed when I had the carbs apart. And the arm was adjusted so it just kissed the pump rod and fuel squirted out in all four carbs.) These tubes that were leaking are new, but I think an incorrect size. A little bigger then factory (I saved those) and wouldn't clamp tight. But I still thought it was close enough that it wouldn't leak. Guess I was wrong. I put the factory tubes back on, but haven't started it yet. Anyway, could these tubes leaking have affected the starting in any way? I would like to put new tubing on the bike, but this was the closest in size I could find. The Honda dealer is quite a ways from me.  And the questions about jet size - all I can say is they are the jets that were in there when it rolled out of the dealership in 1978. Everything on the bike (except for the handlebars - had to go a little higher, I was bent over too far)is factory stock. What ever baffles, jets, springs, etc. the guys in Japan put in the bike back then are what is in it now.
Another quick question, I have been very lucky with electrics on this bike. Everything has worked (hi/lo beam, all the lights, turn signals still go beep-beep-beep, front and rear brake light switches) except for the horn. Can I test this by running wires from the horn terminals to the battery? Would give me a hint about either the horn or the switch being bad. I am REALLY lost when it comes to electrics.
Again, thanks to every one for all the advice. Don't know if I would have attempted this if I hadn't have found this site.

[grcamna2]

vistirus,
     I had a set of 3 OEM accel.pump tubes that I just sent to a member in England a few months ago.I know they're NLA through Honda,but you'll find the correct size(or undersize) that'll fit your application.The sohc4 members here will help ! 

[TwoTired]

Vistirus,
Do yourself a favor and remove the fuses, clean and polish where they contact the fuse ends.  Polish the fuses, too.  This will save you from melting fuses and possibly stranding you out on the highway when the fuse overheats and parts (= dead bike).

The interlink accelerator pump tubes should be 3.5mm I.D.

Are you using ethanol blended fuel?

With an 8500 red line, why would you worry about 4000 RPM?  Regardless, if you really find that objectionable, then find the fast idle speed adjust screw, and back that off a bit to reduce fast idle speed.  I did.

The horn button contact itself just might be oxidized and repeated pressing might wear through the oxidation.
Do you have dual horns as on the F model?
Anyway, after exercising the horn button a few cycles. Tap on the horn itself while depressing the horn button. It may come back to life and there is another contact inside the horn that can oxidize.  Tapping may break through the surface film.  If no joy, other avenues can be investigated.

Cheers,

[Duke McDukiedook]

I vote for another carb cleaning with compressed air in all orifices.
That and run the spray test at the carb boots.

Yes....agreed - pump action and nozzles need to have a visual function test.

You misunderstood me, read Plummer's comment. That is what I meant.

[grcamna2]

Are you still going to have the "fire sale" ? w/ those accel. tubes leaking it could indeed get fired up in the wrong way.