Topic: CB750K4 Carburation Problems - poor idle/throttle response when hot

[gp_st3]

After many hours of reading and a couple of weeks working on my 750K4 I’ve concluded that I need some help please.

First up, should say that I’ve done all the forum FAQ’s, searched and read everything I could find on carburation and fuelling (have the ‘My CB750 book’ as well) but not found the answer to my problem.

So to the details - have stripped and thoroughly cleaned the carbs (twice), set the float height (first to 26mm, then 27mm, finally 28mm), balanced using vacuum gauges, checked and confirmed points gap and timing are spot on, made sure no rubber boot leaks, checked coils and plug caps resistance, fitted new spark plugs and air filter. 

The symptoms - starts easily first/second go on the button but only with no choke at all; runs smoothly and quietly as warms up settles to a steady tick-over, ready for setting off.

Go for a ride, initially runs perfectly with good acceleration off a closed throttle and ticks-over nicely when stopped.  As engine gets hotter, a ‘hesitation’ develops when accelerating from closed throttle (less noticeable if revs are kept above 3000rpm) and there is a kind of ‘fluffing/stumbling’ when riding along at say 30mph with throttle at very small opening.

When hot and stationary, engine will not idle without blipping the throttle - otherwise revs gradually drop until it stalls. Restarts without any problem, but still refuses to idle.

Other points of note - turning the air mixture screws, whether in or out makes no apparent difference to either the idle speed or inability to maintain the idle when hot (screwing in the main throttle stop screw to least 1500rpm will force it to idle but unevenly).

Checking the plugs shows cylinders 1/4 very sooty all over and 2/3 sooty around the rim but light brown electrode ceramic.

As mentioned at the top, on the assumption that slow running was/is significantly over-rich, I’ve tried progressively raising the float height to try to lean out the mixture at idle but it’s clearly not worked and I’m not aware of any other way to change the fuel mixture, given that the air screws are ineffective.

Apologies for a long first post but feel like I’ve reached a dead-end so any thoughts or suggestions would be most welcome.

Thanks.

[ekpent]

What size jets, main and slow, do you have in there now and are they genuine Kehin brass or aftermarket ?  What style of pipe and air filter ? On my 750's I need the choke unless already warmed up. You do know that handle up is choke on and handle down is off.
PS- Welcome to the madness, eerr I mean forum !!

[Davez134]

I had the exact same symptoms while trying to fine tune the carbs on my 72. Response was good intil warmed up, then fell off. It was a overly rich idle circuit. I was convinced that the air screws were doing nothing, as a chance in idle never happened by turning them, as suggested in some manuals.

The bike should not easily start cold with NO choke. That alone tells me idle circuit is rich. What are your screws set at? Are they genuine, factory brass? Or aftermarket? also as asked previously, tell us the rest of your setup. Try giving them a big chance, like half turn out and see if any change at all. May not be ideal, but it will show you if it changes.

Mine were set at about 7/8 turns when I was having your same issues. I now have them about 1 3/8, so about half turn from where they were, and it is perfection. My setup is stock airbox with k&n filter,  4-4 no number exhaust, ported head. 40 idle jets, 122 main, clip in 4th position.

Only other thing of concern is difference in plug color. Could be due to airflow (if you have pod filters) or ignition issues, like advancer shaft being bent. ( this one I know from personal experience)

[gp_st3]

Hi - thanks both for responding so quickly and the forum welcome!

Some background - going by the frame and engine numbers, the bike is a K4 engine in a K2 frame!  Anyway, I bought it about 3 years ago from a reputable classic bike dealer in the UK; it’s all standard 4 pipes/airbox and was serviced by them at the time. For various reasons, after doing maybe 1000 miles on it in the first year (when it also exhibited the throttle hesitation but idled reasonably although plugs were sooty and needed regular cleaning), it sat unridden for the next two and I only recently did the clean up and got it back on the road.
Feeling a little foolish for not checking the slow jets but I remember looking at the mains when I first got the bike and they were 110.  I will strip the carbs again tomorrow afternoon (getting good at it with so much practice) and confirm make and size of all jets.
Choke (down) definitely not need for starting, in fact it won’t start at all with it up.

Have tried air mixture screws everywhere from 1/4 turn to 2 turns out from closed but doesn't seem to make any difference (hard to believe I know) - they look genuine (have the small 'k' on them)
I do have a timing strobe light but haven't yet checked the advance is correct; the points themselves and back plate look brand new but don't know about the advance mechanism.

[harisuluv]

Pictures.  Pictures.  Pictures.

[gp_st3]

Update -
Carbs are 657A;
Main jets 110;
Pilot jets 40;

Having said previously that 2 & 3 plugs were less sooty than 1 & 4 as you will see from picture, the difference is marginal and all are bad!

Have taken pics as requested but not sure that it will help much in the diagnosis.

If the jets are all original and the correct ones for the carbs, it’s hard to see where to go next - should I be changing the pilots for a smaller size (no idea if one is even available)?

I could run with no airbox I suppose, to see if that makes any difference but seems unlikely to have a major effect.

Any sage advice from those on the forum who have ’seen it all before’?

Thanks again.

[Davez134]

Hmmm, looks like all the right brass parts. Have you messed with the clips on jet needles?

[gp_st3]

No, as far as I know, the needles are where they should be (in the middle) - reason I say that is one of the slides has a rounded off needle holder retaining screw so I cannot see the clip position and couldn't change it either...  thing is, the bike accelerates strongly and all is good at speed, the problem certainly seems to be with the idle mixture.  Since the majority of my recent riding has been in town, the plugs get sooty really fast. I could clean them up every day!

[harisuluv]

Kudos for the GREAT pictures.  You will benefit far more than without them.

For example:  The nipples between 2&3 are to remain OPEN.  Meaning two lines about 8 inches long not connected at all.  The way it is right now:  Imagine turning a container of fluid upside down:  glop glop glop.  Now punch a hole in the other end.  Comes out smooth and fast now.  Now punch another hole right next to it and attach them with a hose. 

Plugs look fouled across the board, just some slightly less than others.

Looks like your choke is not synchronized, some of the flaps would slightly cover at WOT range--you can adjust this easy.

Are you running a throttle return spring or no? - Maybe your poor idle is partly because it's not fully ever really returning to idle position.

[harisuluv]

No, as far as I know, the needles are where they should be (in the middle) - reason I say that is one of the slides has a rounded off needle holder retaining screw so I cannot see the clip position and couldn't change it either...  thing is, the bike accelerates strongly and all is good at speed, the problem certainly seems to be with the idle mixture.  Since the majority of my recent riding has been in town, the plugs get sooty really fast. I could clean them up every day!

Well, sometimes these idle get "cleaned" improperly or even drilled out.  You can troubleshoot by buying a new set of OEM idle jets and then you will know if it is the jet.

Before you do that you might want to do some rudimentary voltage checks/3000 mi tuneup list.  Check your plug cap resistance/coils/etc.

[ekpent]

Good eye and yes get rid of the loop where the vent tubes are supposed to go and source up a spring before you kill yourself with a stuck open throttle.As mentioned look for the jets being drilled bigger and try some new plugs instead of cleaning and fussing with the old if they look fouled. If you removed the slides again make sure the rod isn't flopping around on its mount. That mount on the slide top can get bent and tweaked if anybody in the past forced open the slides if they were varnished or stuck. Set the idle when the bike is warm with the big idle adjuster knob if your having problems with it. Check for air leaks around the manifolds while running.  If the carbs , petcock and fuel vent are good to go maybe look at points and condensers if you still have them.  How many miles are on the bike ?  Hope you find an easy fix.

[flybox1]

I quickly read thru the posts, and didnt see anyone mention how HUGE your pilot jet holes are 
I only have the 35/40/42's form my PD carbs to reference. 
these look like they might not mix gas well enough for a complete burn....

[harisuluv]

I quickly read thru the posts, and didnt see anyone mention how HUGE your pilot jet holes are 
I only have the 35/40/42's form my PD carbs to reference. 
these look like they might not mix gas well enough for a complete burn....

PD press in jets have a hole at the tip very comparable to the very small hole which actually corresponds to the jet size.  With this type, the hole at the end is totally different from the internal hole which is the real jet size.

[ekpent]

Cutaway of pre 1977 jets for round top carbs

[gp_st3]

The nipples between 2&3 are to remain OPEN.  Meaning two lines about 8 inches long not connected at all.  The way it is right now:  Imagine turning a container of fluid upside down:  glop glop glop.  Now punch a hole in the other end.  Comes out smooth and fast now.  Now punch another hole right next to it and attach them with a hose. 

Well spotted - no idea why a previous owner would have connected the vents and as you say, there is currently no path to atmosphere for the bowls since are they are all connected together!  As it happens, I think you've fixed problem that I hadn't even mentioned - namely that I need to switch on the fuel tap a good 5 minutes before starting if the bike has been left longer than a couple of days since riding otherwise she takes several seconds on the button before firing up (float bowls filling slowly due to no vent).

So, will correct that and while logically it would seem that a restricted flow into the bowls would imply a lean mixture you never know!!

Well, sometimes these idle get "cleaned" improperly or even drilled out.  You can troubleshoot by buying a new set of OEM idle jets and then you will know if it is the jet.

Before you do that you might want to do some rudimentary voltage checks/3000 mi tuneup list.  Check your plug cap resistance/coils/etc.

Cutaway of pre 1977 jets for round top carbs

Had another look at the idle jets and the cutaway picture is interesting however I could not get even a very fine wire all the way through so while it's possible that they have been drilled out it seems unlikely so will save changing them for new until all other avenues are exhausted.

source up a spring before you kill yourself with a stuck open throttle.As mentioned look for the jets being drilled bigger and try some new plugs instead of cleaning and fussing with the old if they look fouled.If you removed the slides again make sure the rod isn't flopping around on its mount. That mount on the slide top can get bent and tweaked if anybody in the past forced open the slides if they were varnished or stuck. Set the idle when the bike is warm with the big idle adjuster knob if your having problems with it. Check for air leaks around the manifolds while running.  If the carbs , petcock and fuel vent are good to go maybe look at points and condensers if you still have them.  How many miles are on the bike ?  Hope you find an easy fix.

Don't be concerned, I remove the throttle return spring for convenience during inspection/cleaning as get tired of holding slides open, agree it would be dangerous to ride without.  For the plugs, I alternate two sets that are relatively new (< 300 miles) and a quick clean with a dry tooth brush removes all the sootiness - wouldn't fitting a brand new set before fixing the richness be a waste?
Coils and plug cap resistance already checked and no air leaks that I can find.
Miles on the odometer says 56k miles but probably not reliable, as by engine/frame number it would appear to be K4 engine in a K2 frame.

Will report back on how things are after vents pipes are corrected.

[ekpent]

You can see light through the slow jets though I hope ? When I deal with a really bad set of varnished or clogged slow jets after soaking in real carb cleaner/dunk I find, though not necessarily recommended, that a stripped bread wire wrapper wire can be worked through and the end the plastic I did not strip off is good to hold on to.

[gp_st3]

Well with some luck due to this topic from 2012 -

http://forums.sohc4.net/index.php?topic=108701.0

being refreshed on Monday and catching my attention, the rich idle (and running) issue seems to be sorted.

It’s clear that I was fixated on the idle mixture being the issue, when the culprit was worn needle jets all along!

As it happens, I was fortunate to already have a set of 4 replacement needles + jets from SMEBike (smebike.com) bought when I acquired the bike in 2012.  As mention earlier, one of the slide needle holders has a rounded screw head and (at the time) because I couldn’t change them all I decided to put the the new set away for a later date.

Of course if I had simply replaced the jets then, I could have saved myself a lot of effort and not troubled this forum!

Anyway, here are a couple of pictures that speak for themselves…

The old jets came out easily using the wooden dowel method and the new ones just drop in, to be held in place by the main jet. I reset the float height back to 26mm and installed the two bowl vents (thanks to harisuluv for that one).

First good sign was that I now need the choke to start from cold and with the air mix screws set to one turn out for starters, the tick over is stable when warmed up.  Went for a ride and suffice to say that I now know what a cb750K should be like to ride - not enjoyed myself so much in quite some time! Pulled one of the plugs when got home (#1 which had been the sootiest) and it has a nice tan colour for a change.
Will vacuum balance the carbs again and then tweak the air screws but getting some more miles in first is the priority…

So thanks to jimbojangles for the 2012 topic and also to those who took an interest in my problem to suggest possible solutions.