Topic: 78' CB750 acceleration issues

[754]

If you are not postive that the low speed circuit is clean, then you do not understand carbs.

It should not need the choke after warmup, if it does, it shows that it is not getting fuel when it should be...

[zeus87]

got ya, EVERYTHING is clean...like i said, i just wanted clarification as to what is included in the slow circuit.....not that i didnt clean the barbs spottless, just wanted to know what was included.  Im not trying to be a dick, so dont think im arguing with you, i appreciate every bit pf information you have to offer.  I am just curious as to what else it could be besides carbs...i cleaned those things spottless including the slow jets, i pulled them and they were in good shape to begin with).  thanks again.

[754]

Nobody is saying you were a dick..

 some of those parts have to come out and get cleaned, if they are  clean, it should run..

You already PROVED it is not ignition by the fact that the bike does run with choke on, meaning it is a fuel problem..

[bender01]

What about needle position? Ive been moving mine on a 350f. Does the 78 750 have needles? I dont have a choke prob but have been mangling an old exhaust and now have a new mac. I was suprised the difference one notch up and down from middle made on my plugs and throttle response ,cold starting... Do you have a baffle in your pipe? Just a thought...

[zeus87]

alright, got ya...
Now i have a factory in tank filter, and i added an inline filter after treating the tank with phosphorus acid.  Do you think this could be an issue?  it obviously isnt an air screw adjustment...it was running lean, now its looking real good (plug reading).  Like i said, the floats looked good after checking height with aquarium hose, but now i am leaking from carb #1 overflow.  Floats responsible?  Thanks!

I am running stock 4-4 exchaust, and no there is no needle adjustment.  Thanks

[bender01]

Do you have enough gas in the tank? are you on reserve? Ive had just a little gas in the past not help tuning.

[zeus87]

yeah, i have about 2 gallons in it...running it with petcock ON.  thanks

[eurban]

A few thoughts here . . .New plugs in a bike driven around the block a few times should in no way resemble the pictures you may have seen in the spark plug guides like those in the Clymer / Haynes manual or online etc. It will take many miles of road driving before your plugs will develop a look that you can compare to those guides.  Basically, using the plugs appearance after a short toot around the block as a guide for adjusting your IMS screws a waste of time.  Once you have things running better, either adjust all the screws evenly in small increments and test ride for seat of the pants performance,  or adjust individually for highest idle speed as outlined in the manual.  Given that your bike is currently (basically) stock then it should run well with the carbs set to stock specs.  1.5 turns out on the IMS screws should have you pretty close.  If your stock bike requires close to 3 (3 turns out is just about the end of the working range of these screws and there is no reason for a properly functioning stock bike to require a setting this far out!) turns out on these screws to run well (which it actually still doesn't) then you should be still looking for problems that need to be fixed before you can properly set your mix screws.  You might for instance have an air leak, an impaired slow circuit or insufficient fuel supply.  Did you pay much attention to how you routed your fuel line with its inline filter. A short constantly descending run to the carb is what you want.  The 78's gas tank is pretty large and I wouldn't be surprised if you had issues with fuel delivery with only 2 or so gallons in the tank and a poorly routed fuel line. Set your petcock to reserve and see if that helps . . . Certainly if you have an overflowing carb then that should be fixed but it shouldn't result in a lean condition requiring choke to run OK below 4000rpms. . . .Oh yeah, are your slides installed backwards?  The smaller gap between the slide and the bore should be on the engine side . . .

[zeus87]

i will check, you have given me some things to look at.  Thanks

[zeus87]

Alright, i pulled the carbs...RUST everywhere, pulled everything apart and did a thorough cleaning (nothing was really plugged, including the pressed slow jets).  Found the floats were off terribly, fixed them to factory measurement.  In the process i treated the tank with evaporust and sealed it with por15 and then put a paper inline filter on.  Anyway i put it all back together (air screws set at 2 turns out) and im still having the same issue...runs awesome (idles great a 900) but sputters and lacks any throttle response below 2000 rpms, if i pull the choke it responds awesome.  GRRR well i rode it around for about 10 miles, stalled a few times with the low end flat spot.  Got it home and pulled the plugs....  What is going on?  im running 91 octane and dp8ea plugs.  Here is a picture of the plugs.. THANKS guys  (again bone stock except k&n factory style filter) (valves,timing,chain,points, were all adjusted)

[TwoTired]

Is your accelerator pump still working correctly?

[jjwaller]

alright, got ya...
Now i have a factory in tank filter, and i added an inline filter after treating the tank with phosphorus acid.  Do you think this could be an issue?  it obviously isnt an air screw adjustment...it was running lean, now its looking real good (plug reading).  Like i said, the floats looked good after checking height with aquarium hose, but now i am leaking from carb #1 overflow.  Floats responsible?  Thanks!

I am running stock 4-4 exchaust, and no there is no needle adjustment.  Thanks

First, double check that there is not an air bubble in the fuel filter.  Can cut off fuel to carbs.
Needs to be routed so still direct path to fuel inlet on carbs. 
Second,  are you running at least a 115 main jet?  You probably need a 120.
K+N's will move more air so need more fuel.  Sorry, forgot what bike you have, but the later
years were ridiculously lean from the factory for EPA reasons.
Third, Check that Accel pump again.  Sounds like that might be part of the problem.
Remember, it is spring actuated and the spring might be stretched out so it is not actuated at
reasonable throttle twists.

[zeus87]

routing looks good, Accelerator is working great (that was the first problem, the little o-ring was bad, but now its spraying like a fire hydrant).  I have the factory main, i believe it is a 115, someone can probably veryify this (1978 cb750k). Thanks, should i try going 2.5 out on the air screws?

[BobbyR]

I too have a 78K and I have 120 mains and I do not soot up the plugs. The PO must have put them in. 900 is a low RPM for these bikes to idle at if yours runs that low smoothly your idle circuit is working above average. Most people report 1200 to be norm. 
Is your timing advance working smoothly? How fast are you cracking the throttle when you get the hesitation? Even with accel pumps the engine vacumn has to draw the mixture into the cylinders so there is some response time.

[scondon]

   Sorry if any of this has been covered before, I did skim through the posts.

   Before tuning carbs:

   Warm bike:  Adjust cam chain tensioner

   Cold bike:  Adjust intake/exhaust valve gaps

   running bike: adjust ignition timing so that timing mark hits "F" at 1000rpm and between 1[]4 and 2[]3 at 2500rpm+

     Check for carb boot leaks by spraying carb cleaner on them and see if engine speed changes.  No leaks?  Synch carbs. Confirm fuel levels in bowls with clear tube attached to bottom overflow nipple, should be at or just below bowl gasket and even across all four carbs.

     When you cleaned the pilot jets you also removed the "air" screws(we will now call them fuel screws cause that's what they are on the 77-78) and spring and o-ring and washer and gave a good blast of cleaner through them and the air jets pressed into the carb intake throats,yes?

  1 1/2 turns out on the fuel screws should be enough if all systems are go, as eurban points out. If all systems are not go then you can try and compensate by adjusting each fuel screw per the manual and don't be surprised if they end up at varied turns out.

[TwoTired]

OK, let's review:

« Reply #1 on: April 06, 2008, 12:06:20 am »
Sounds like the accelerator pump is not squirting..

« Reply #13 on: April 06, 2008, 09:38:50 pm »
got it!! the o ring inside the pump cover was shot, i saw bubbles coming from the outside.  replaced it and it runs GREAT.  thanks all!

« Reply #26 on: April 13, 2008, 11:07:06 pm »
got ya, EVERYTHING is clean...like i said

« Reply #34 on: Today at 01:31:16 am »
Alright, i pulled the carbs...RUST everywhere,

« Reply #35 on: Today at 03:19:41 am »
Is your accelerator pump STILL working correctly?

Reply #37

routing looks good, Accelerator is working great (that was the first problem, the little o-ring was bad, but now its spraying like a fire hydrant).  I have the factory main, i believe it is a 115, someone can probably veryify this (1978 cb750k). Thanks, should i try going 2.5 out on the air screws?

In reply #26 you said "everything was clean".  Reply #34 there was "rust everywhere".

When was the last time you checked the accelerator pump output?  Before or after the latest cleaning?

[chknitout]

Hey, check to see if your carbs are sagging down in the rear,( the air box boots won't line up correctly) the carbs must be leaning tward the front of the motor a couple of degrees. they must NOT be level with the ground as seen from the side, but pitched forward a little. see if this is happening to your bike ( it does to most) lift the rear of the carbs up,I did it by running a hose clamp from the frame to the rear most bracket on the carbs. you'll be surprized at the differance it makes. hope this helps.

the chan man

[zeus87]

I checked timing, but not advanced.
Float level was checked and adjusted
Cam chain adjusted
Tappets Adjusted
Carbs are not sagging(bolts pull the airbox upwards)
Points adjusted

(i dont know for sure about the mains, this bike is pretty much untouched as far as mechanically, so im sure they are stock)

Its not so much just throttle response (, when i take off from a stop it chugs and sputters untill i get to about 1800 rpm.  It really hesitates when it is cold, but still rough when it is warmed up.  I thought i could deal with it, but when i stalled when the light went green i knew it was time to really figure it out.  If i hold the throttle back just a hair, it will sputter lightly and slowly increase rpms, but if i give it choke while it is sputtering and slowly rising in rpm, it will clean up and rev perfectly smooth.  Same thing with accelerating from a stop, pull the choke real quick and it has amazingly smooth and fast response.

Thanks everyone, i cant wait to figure this out

[BobbyR]

This may sound dumb, but try raising the idle to 1200 and see if the rideability improves. It woiuld appear you are having a problem when you are moving from the idle circuit to the mains.

[scondon]

 The needles don't have clip positions but you can raise them by shimming them with small washers. If you're using pod filters you will definitely want to do this and it can help the transition from idle to main. Can be done with carbs on the bike, check synch afterwards.

[zeus87]

The idle helps, but not greatly.  I am not using pods, but still might be interested in this.  There is no way new needles and old seats could cause this problem correct? 

[jjwaller]

New needles and old seats should, in an ideal world, make it slightly richer.
(Worn out Seats -bigger holes)
Still sounds like you have clogged low speed circuits.  Think it's time for
one more cleaning.  Especially if you found rust in the carbs.  OR, check actual
level of fuel in bowls.  Hook clear tube to bowl drain ports and open drains.
Run short lengths of clear tubes above carbs so they will act as sight-glasses.
Best way to verify float level settings.  If fuel is too low in bowls will result
in extremely lean response from carbs.  Something else to check while on bench.

[eurban]

Still sounds like you have clogged low speed circuits.  Think it's time for
one more cleaning.  Especially if you found rust in the carbs. 

Right!  Zeus, you seem to be all over the place with this problem; I would guess it is time to really go focus on your idle circuit.  Either you have damaged or incorrectly installed parts or there are still cleaning issues with the pilot jets and or the associated passages.   Did you check to see if you had installed your slides correctly?  Reread the advice already given on this subject and give it a try again.  As I tried to point out before, you essentially have a stock setup and the cure to your problems should not require much deviating from the stock carb specs.  Things like swapping to different size main jets, shimming main jet needles, or taking your idle mix screws out close to three turns should not be necessary.  It really seems that there is still something functionally wrong with your carbs.  Fix the sickness, rather than trying to put a band aid on the symptom!

[upperlake04]

   From my limited experience and from what I've read here, it's not uncommon to have to clean the carbs perfectly at least three times before they're perfectly clean.

[zeus87]

hahaa, oh brother....