Topic: 1978 CB750k is driving me CRAZY!!! UPDATED

[tinyrobot]

Okay,  as stated in the title the bike is a CB750K from 78 with the only mod is Honda 2-4 exhaust.  I purchased from PO who said he was riding all last year until winter season and it was running a bit "weird" at the end.  When I purchased it he had the carbs and tank stating he was going to clean them over the winter, but decided to sell of some of his projects.

Presently:  I rebuilt the carbs to factory spec using the 77-78 Honda factory manual and did all the proper "Pre ride inspections and tune-ups".  New plugs went in.  I was having trouble starting the bike, as in an hour to get started and really only turned over with the kick start.  Once it started it was very hard to keep running and would not idle.  Pipes 1 and 4 were cold.  Since then I have installed a Dyna S ignition and replaced the plugs yet again.  Readjusted the valves and cleaned/synced the carbs.  I was able to get it to idle (although rough) and even ride it up and down the alley a few times.  1 is cold now and 4 is hot but not blazing like 2 and 3.  I pulled the plugs to find they are all whiteish and bike is running lean.  I get to work on this maybe twice a week and each time I try to start it it still takes 1/2 hour or so, but once it does run it will start up no problem with electric start.  Carbs are back off and getting a cleaning although they look brand new at this point.

I have been reading and searching since purchase back in Feb but now I'm ready for the blatant "Help Me" post.  This is my first CB750 and road bike so bare with me on this.  I just want to ride 


As of 5/11
Yesterday I pulled the slow jets and cleaned them out, rebuilt carbs, check accel pump again, put the carbs back on.  Then I checked the spark plugs and cleaned them, checked wires, checked timing, adjusted the valves, and got fresh gas in the tank.

Hooray!!!! It lives, I almost got it started and then realized the choke was down.  Pull it, and BAM instant start and idle.  Adjusted the idle screw to 1100 rpms.  Seemed to have some trouble below that.  Took it for a quick spin up and down the alley and then pulled the plugs (not bad).  I had my friend who has a license take it for a spin to the gas station to fill the tires (I will get mine end of may after attending a MSF course here).  When he got back I pulled the plugs again and they a bit whiteish again.  Next change I get I will adjust the mix screws another 1/4 turn out.  Until then thanks again everyone!!

[hcritz]

One and Four are fired by the same coil...could be problems there or with the plug wires.
Check for a good connection at the end cap on the wires...they just thread on...should have a clean tight connection...no green corrosion on the wire.
Did you remove the low speed jets and clean them??? Seems they are the culprit when these bikes are hard to start and won't Idle.
Also they need to be synced to get a decent smooth idle.
I've got a 77...it has the same carbs as yours and it has some idiosyncrasies starting when cold...I just had to learn the propper procedure.

[my78k]

First off never put 1978 CB750K and Help in the same sentence when posting as manay many other others in here are under the false belief that their years of bikes are far superior to what was really the onnacle of the CB line of bikes...that being ofcourse the 78k! No doubt you will get replies pointing out that if you had any other year you would not have any problems at all!  

Having said that, obviously the relationship of the 2 offending cylinders leads me to think that it could be electrical. Do you have good spark at both those plugs? Having said that with these bikes you can never rule out carbs...does it run any better on choke? Have the carbs been re-sync'ed after the rebuild? Were new idle jets put in? Were the mains returned to stock? IF so that could affect it since the aftermarket exhaust though I wouldn't suspect it causing that drastic of a problem.

Dennis

[jdigga]

It's weird that your #1 pipe is cold and #4 is only warm but the plugs are white.  If those cylinders aren't firing, shouldn't they be fouled?

You didn't say explicitly, but can we assume your timing is correct?

[tinyrobot]

Check for a good connection at the end cap on the wires...they just thread on...should have a clean tight connection...no green corrosion on the wire.
Did you remove the low speed jets and clean them??? Seems they are the culprit when these bikes are hard to start and won't Idle.
Also they need to be synced to get a decent smooth idle.

Wire connections are good, no green, nice white to blue spark on all 4.  I did not remove them  (fear of pulling them out and damaging them) but I did spray them with carb clean and blow them out with the air compressor 3 times.  I have also bench synced and synced with gauges while at rough idle.

Do you have good spark at both those plugs? Having said that with these bikes you can never rule out carbs...does it run any better on choke? Have the carbs been re-sync'ed after the rebuild? Were new idle jets put in? Were the mains returned to stock? IF so that could affect it since the aftermarket exhaust though I wouldn't suspect it causing that drastic of a problem.

Dennis

Runs only on full choke until really warmed up (it will die almost instantly if I try and rush the choke), then starts to raise rpms to which I back off the choke until it idled choke full off.  Carbs have been re synced.  Old idle jets, new stock Main jets.

It's weird that your #1 pipe is cold and #4 is only warm but the plugs are white.  If those cylinders aren't firing, shouldn't they be fouled?

You didn't say explicitly, but can we assume your timing is correct?

Thats what I assumed, but pipes state otherwise.

[tinyrobot]

And I do love the pre 77 flame war that goes on.     I bought this with out any expectations and really just wanted a hobby for the summer.

[hcritz]

Hey Tiny....
If you got good spark at the plugs...it's more than likely the low speed jets...
Lots of post here on how to remove and clean them...
I would guess at this point you have rechecked the valve clearance etc.

[tinyrobot]

yep, adjusted the valves after I did the timing with the Dyna S.  I will pull the idle jets out this time and clean them out.  I was just afriad of causing harm since it was a bit unclear in many threads.

[hcritz]

They are pressed in, but can be removed fairly easily...Read through some of the post.
Some people us a small E guitar string to clean them,Some use a single strand of copper wire...the holes are very small...just be careful not to enlarge the holes when cleaning them!!! The copper is probably the safest since it's softer than the jet material.

[BobbyR]

I too have a 78K and that is when Honda corrected all their prior mistakes. Plug end caps can be a booger if they are really old. I put some new NGKs on there and the bike idled better. I also cut a couple of mm off the end of the wire so the screw would have cleaner copper to bite into. The slow jets plug of easily I have worked on 3 sets of these carbs and a whitish deposit was present on all of them. Hondaman claims it was the MBE they had in the gas. You can pull them easily with a good plier using a twisting action. Be sure to clean the circuit they feed when you take them out. A bit of wire and some spray carb cleaner works pretty good, if you have some good compressed air that will help also. We have accel pumps. I found that they also plug up and the gas stored there goes bad quickly. This of course will not help you idle, but will help you when you are running and want to dodge a Soccer Mom in an SUV. While you have the carbs of you may want to check that out. Of course check for spark and enjoy the  SOHC that was what the what it was meant to be. Oh yeah our idle screws actually adjust the mixture, so play with them a bit. Let us know how you are doing.

[eldar]

If you just got the bike, then you have made a good start. You will have to check your carbs completely.

Remove them and then dis-assemble them from each other or at least pull of 1 and 4 and all the tubes.

Clean everything, everything, make sure to pull the accel pump on #2 off and clean it and check for tears in the diaphragm.

Clean out all passagesways with some soft wire and carb spray, dont worry about compressed air right now. Pull the jets, all of them, even the mains. Clean all areas inside the jet housings. Pull of the tops and remove the slides. clean those and the bores.

Then pull the fuel screws and clean those. Watch out for the screw, spring, and washer and oring in there.

Pull the floats and valves. Clean the pivots and valves and the valve seats. Put them back on and measure at the middle of the float from the side, 14-15MM.  When doing this, observe the valves, there are little pins there, these must NOT be pushed in, so you hold the carbs at an angle just so the valves close but not push in the pins.

Put it all back together.  make your bench sync so all 4 slides are even. Set you fuel screws to 1.5 turns out. Check your boots for cuts or splits. Remount carbs and check for any gaps in the boots. Put on the upper part of the airbox and then the filter and bottom. If you use an uni or K&N filter, turn the fuel screws to 1.75 turns out. If pods, start at 1.75 but be prepared to turn out more.

OUT is RICHER and IN is LEANER.

Make sure timing is on, should be pretty easy with a dyna. make sure battery is fully charged and have at er. You should start pretty easy with choke and be able to drop it after a minute of 2 if the air is warm.

[Quail "Owner of the comfortable k8"]

Damn Eldar you said that with such authority I almost walked out to the garage and ripped the carbs off just to give them a good spring cleaning!  I found myself  standing there in my driveway in my skivies with a can of Seafoam spray.  Then I remembered "Hey I have a K8 and I fixed the carbs YEARS ago", I went back into the house.

Tinyrobot you bought a k8, it will run and run well.  some pre k7 motorhead just messed with perfection and you just have to find where.  (It is the carbs 9 of of 10 times) 

[tinyrobot]

Thanks everyone for the info.  Eldar - I took the carbs all the way down so every lit piece was off cataloged repaired/replaced and then put back together.  The only part I did not do was pull the slow jets in fear of damaging them.  Last night I did it and - YES 1 and 4 were clogged.    I lose.

Hopefully after work today will be the turning point in the project and I can move on to replacing bearings and bushings and following Hondaman's tips to improve the bike heavy feel.

[eldar]

So you checked and gapped the accel pump? Very important as that provides fuel when opening the throttle at idle. With the enigne off but the fuel on, hit the throttle and you should see a small jet of fuel from EACH opening on the carbs. This jet comes from the little brass tips that stick up in the opening of the carbs and shoots into the throat.

[tinyrobot]

yup, checked, cleaned and gapped the accel pump

[jesusno2]

Iis the accel pump working is the next question??? i check them buy filling the bowl up with carb cleaner and manually push on the lever it should shoot a fuel stream about 3 feet in the air if ya push hard enough watch your eyes LOL. I actually love these carbs compared to the older ones but i have spent many many hours with jet drill's, shims and different springs and such, to make them "almost" like a 29mm smoothbore mikuni, on various different model bikes which none of them were stock. the factory leaned the hell outta these bikes back in the day to pass emmison's i live at sea level, and a completeley stock bike runs so lean here your lucky #1 if you can get it to start and #2, i have yet to have one that i can ever take the choke off completley unless its a 80 degree day or so. but i have spent alot of tme trial and error with these things to make them work in stock form they pretty much suck.

[eldar]

I would refrain from pushing that hard. Old diaphragms dont like that much and accel pump kits can cost quite a bit at some places. Being as how it also shoots directly into the carbs, unless your slides are missing, it wont shoot 3 feet but you might get dispersal spray from the gas when it hits the slide.

[hcritz]

Hey Tiny...
Glad you found the problem...Bet it runs better now.
My 77 starts are runs very nicely, but when very cold out (below 40)  it won't start unless you use the double secrete combination...then it starts instantly...I think every bike has it's own Double Secret Combination...so you may have to learn what your bike likes.
Normally, if it's not cold out,  I can give the throttle a couple of twist to shoot some fuel in from the accelerator. pump and I don't have to use the choke at all. Below the magic temp...I have to use full choke and Full throttle...fires instantly that way...Just using the choke with it's fast idle just won't get it to start.

I'm testing on of Honda Man's electronic ignitions...and that definitely improves the idle and low speed.
I keep the idle set at 1000-1100 to get a nice smooth quiet Idle. Even with perfectly balanced carbs, if I go lower it get a bit lump and noisy. I would think that with the Dyna you will have the same thing.
Make sure your advance mechanism is free and lubricated...bet you've already done that<G>

[eldar]

Yeah you just have to find the sweet spot. When cold, I yank the choke all the way and hit it and it fires. If I hit the throttle at all, it floods some but will start and cough a bit.

[tinyrobot]

It lives!!!!!  All 4 pipes are hot and running 

[dusterdude]

K8K8K8K8K8K8,yikes,if i had any hair i`d pull it out.

[eldar]

After riding a harley and older K bikes, I understand why you have no hair!

Now you just have to hook up a tach and fine tune it and you will be golden. Maybe run some seafoam thru it just to finish cleaning things up.

[tinyrobot]

It would be nice another cb750 to compare it to sound wise.  The engine kinds sounds tinny and loose or something?

Do you just pour the seafoam into the tank?

[eldar]

Yup just dump in about a 1/3 to 1/2 at most and fill er up.  Well remember, these engines are air cooled and so make more noise than a water cooled engine.

Do you get more noise sitting in neutral with the clutch out? That can cause a little extra noise if the carbs are not fully synced. Which probably happened with the carb work. Make sure the cam chain is tensioned properly. You might have to take the adjustment housing of and clean it, every common really. Oil gets in there and goops things up and the rod and spring do not move right and so keep the tension from setting properly.

[mick750F]

the factory leaned the hell outta these bikes back in the day to pass emmison's i live at sea level, and a completeley stock bike runs so lean here your lucky #1 if you can get it to start and #2, i have yet to have one that i can ever take the choke off completley unless its a 80 degree day or so. but i have spent alot of tme trial and error with these things to make them work in stock form they pretty much suck.

   If you are having these problems then something isn't right. My completely stock carbs have no problem starting up here at sea level and the choke is only needed until it's warmed up. These carbs get a bad rap from some people but they work perfectly fine when properly cleaned and adjusted.

Mike