Topic: 78' CB750K Poor low rpm running (me again)

[zeus87]

RECAP:  1978 cb750k with 35,000 miles, purchased it from the original owner.  new clutch, carb rebuild kits (completly cleaned, pulled all jets, measured floats etc), valve adjustment and timing is done, running stock airbox with k&n factory style filter.  Bike fires up instantly and will idle great at 1200rpms when warm, when cold it runs rough and real loud and clunky).  For the first 5 minutes, the bike is pretty much non rideable, it will die when trying to accelerate from a stop.  Throttle responds very little untill rpms are above 2500, but when warm it runs WAY better but still hesitates in the low rpm ranges (it will clear right up if the choke is pulled while riding, but then it lacks at higher rpms).  The carb angles are fine etc...any ideas here? im stuck!!  Is there a possibility that the valves are just that warn out?  that is needs to really heat up before it runs good (not only does it run better, but alot of the odd noises go away also).  Thanks

Also, the stock exaust is in rough shape internally, some have baffles, others dont.  Is there anything i should do about this? cut the cones off and pull the guts maybe? 

[TwoTired]

Sounds like the accelerator pump on the carbs isn't working.

The pump is on #2 carb.  Each carb has a squirter post near the choke butterfly.  Twist the throttle and raw gas gets squirted into the carb throat.

The diaphragm can be bad. Either check valves for the pump can be stuck, or the squirters can be clogged.  I know you wanted to tear into the carbs again.   

Just remember, "A job worth doing is worth doing twice!"

Cheers,

[zeus87]

works great, strong stream to each carb.  Thanks though!

[TwoTired]

Still think it is a carburetion issue.  Even clean carbs can get dirty.
Are they vacuum synched?

If you suspect valves, do a compression check. Wet and dry when cold, then later when warm.  If compression checks out, it ain't the valves.

Cheers,

[zeus87]

yes they are vaccum synched.  I just dont understand how it could be carbs.. They were cleaned 3 times by myself, then after this problem continued to occur i dropped the bike off to a mechanic friend of mine to go through the carbs...Still same symptoms.  How can it be carbs if this issue is dependent on motor temp?  Runs like #$%* cold, runs decent warm.  HELP!! im sick of stalling from red lights haha

[zeus87]

Someone must have expirience with this....Possibly ignition system? what would faulty points/condensor do?

[jjwaller]

Are you still running stock jets?
Might try going up 1 size on pilots and up to 115's or 120's on mains.
Where are your idle mixture screws at?  Try 2 to 2 1/2 turns for an
experiment.   BUT,  78's are very lean from the factory and extremely
cold-blooded.  My first 78F (new in 78)  and my latest 78F were and are still
cold-blooded, and need choke for at least 5 minutes before riding, and then
I usually leave choke on partially for the first few minutes of a ride.
Your deteriorated pipes could have an effect on breathing and need a little
more gas.
If it runs fine after warm-up though, sounds like it's fine.
HTH
John

[zeus87]

hmm sounds alright...yeah, when its warm it runs pretty good.  I am going to make a new post with my plug findings....ooooof

[TwoTired]

I know the 78 cb550 is very sensitive to back pressure changes. With a loss in back pressure, cylinder scavenging is more complete making more oxygen available.  This will result in a lean mixture, and have the most impact during warm up.  If you want to drive immediately after cold start up, you will have to do so with choke on.

There should be some compensation by turning the Idle mixture screws out.  The adjustment range is limited, though.  If you require more compensation than it can deliver, then you would have to increase the slow jet orifice size.

The K&N filter is probably exacerbating the problem, as it has less restriction than stock, making the carb throat vacuum less apt to draw fuel from the jets.  Possibly, the stock paper filter would improve your cold operation, as would restoring the stock exhaust restriction.

D7EA plugs could also help your cold operation.  And, I would be comfortable using those unless you are running the bike hard in hot weather.

Vacuum leaks should be checked at the intake couplers.

Vacuum synch.  I  would also do a compression check, to see if you have even compression across the bank.  If not, that would explain the uneven idle.

Cheers,

[754]

Are you sure #1 does not have less compression?

Your plug pic points strongly to something in the 1:4 side of the ignition.

Could you baffle be so much more rotted on #1 that your settings will be off on that carb?

 I would go thought the 1:4 parts of the ignition thoroughly...

[zeus87]

I checked timing again, it is spot on.  Im not sure what it could be, im very new with this machine.  I know that the air screw was overtightened in carb #1 (the black plug in the picture) and broke off inside...that might be the cause of soot.  Im going to try d9ea plugs for the hell of it.  Let me know what you think i should do. THanks

[jjwaller]

I checked timing again, it is spot on.  Im not sure what it could be, im very new with this machine.  I know that the air screw was overtightened in carb #1 (the black plug in the picture) and broke off inside...that might be the cause of soot.  Im going to try d9ea plugs for the hell of it.  Let me know what you think i should do. THanks

Now this might be a significant part of any problem.

Those are idle mixture screws.  i.e. Regulate low speed Fuel, not air.
If one of your needles is still plugging the orifice, then you effectively don't
have a low speed circuit on that carb. 

[TwoTired]

I checked timing again, it is spot on.  Im not sure what it could be, im very new with this machine.  I know that the air screw was overtightened in carb #1 (the black plug in the picture) and broke off inside...that might be the cause of soot.  Im going to try d9ea plugs for the hell of it.  Let me know what you think i should do. THanks

Now this might be a significant part of any problem.

Those are idle mixture screws.  i.e. Regulate low speed Fuel, not air.
If one of your needles is still plugging the orifice, then you effectively don't
have a low speed circuit on that carb. 

BINGO!!!

You have a low speed circuit that is not working at all, being plugged by the broken needle tip.

[754]

So you tell us you cleaned the carbs 3 times..

AND ONLY NOW mention that???

[zeus87]

The broken air screw was replaced....the orface is not plugged.

[TwoTired]

The broken air screw was replaced....the orface is not plugged.

Replacing the idle screw is not enough.  The tip has a very shallow taper and wedges into the carb body.  When the  over-tightened screw is removed, the tip breaks off, leaving the slow circuit feed hole plugged where it should deliver into the carb throat.  Replacing the adjuster screw won't fix that.  The debris has to be cleared from the carb body BEFORE the new adjuster screw is inserted.  I would expect that, if not done in that order, the new adjuster screw is now also damaged and in need of replacement.  The jet orifice is in a different part of the slow circuit.  It won't matter one whit if that is clear, if the exit delivery hole to the cab throat is plugged by the old broken tip of the adjuster screw.

When you cleaned the carbs, did you verify each slow circuit in the carb body for clear passage from/to each of the four holes into that passageway? 

Cheers,

[zeus87]

The hole was cleared obviously, and yes the 4 passage holes flow freely.  Do you think i should replace the factory mains, with the aftermarket jets that came with the rebuild?  That is the only part i didnt replace when rebuilding them.

[TwoTired]

With the damaged exit hole, have you set all the Idle mixture screws to the same number of turns?  Or, have you set them for peak RPM at idle?
If the former, that could explain your rough idle.

What are your compression check numbers?

How confident are you that all your exhaust pipes present the same back pressure?

[zeus87]

rpm check, deffenitly not the same back pressure.  Some contain no baffles others are cleared.  Didnt do a compression check, looking to find the proper adapter.  Thanks