Topic: Cb750 lean cylinder help.

[Davez134]

So I've searched and searched, everything that I've found it seems like I tried already.

The bike has been running great. No hesitating, smooth idle, starts easy with choke, no problems noticed. (Except occasionally when I give it full throttle from an easy cruise I hear a split second of rattle not always though) I pulled the plugs just to look the other day after 20 miles on the freeway and saw this:  from the left, 4-3-2-1

Lighting is weird, but in person they all look nice and tan except for 3. It's white and has white on strap. Didn't look like this last oil change. Didn't change anything.


I pulled after idling (while double checking timing) and it was brown again, but after another longer ride it shows the same lean condition.

1972CB750
836cc (wiseco)
Webcam41, just double checked valve lash
Airbox with k&n filter, no number 4-4exhaust
Floats set to 26mm, just rechecked (levels all look the same using clear tube after setting with tool)
All genuine brass parts with 122main, 40idle, clips in 4th position, air screws at 1 1/4 turn out.
Pamco ignition just set timing with light, spot on at idle, full advance it's exactly on the left mark of the 2.

Seems weird to me that it's just that cylinder and I don't know what else to try. I even pulled the exhaust to check for leaks and replace gasket, and I saw the valve stem looking through exhaust port had white deposit on it. Don't really want to ride until I get this figured out, could cause damage i think. Any ideas? I'm all ears!



Sent from my iPhone using Tapatalk

[Jerry Rxman Griffin aka MuthaF'er]

Intake manifold leak?! Good fresh set of manifolds and clamps? Start there maybe? What rpm range did you do the plug chop?

Oh, how did your carb sync go?

[Davez134]

Intake manifold leak?! Good fresh set of manifolds and clamps? Start there maybe? What rpm range did you do the plug chop?

Oh, how did your carb sync go?

The manifolds are OEM honda, soft when I opened the bags just over a year ago. I haven't re-synched the carbs since 1000 miles ago, but all were equal on my gauges then. Good idea to try next thanks!

[ekpent]

Wonder if that 'rattle' is detonation or pinging.

[PeWe]

Try 130 main jets.
I had my CB750K6-76 OEM carbs very rich jetted but still worked very fine. 145mains, 40 pilots, needles 3:rd notch from top (1 richer than std) 836, ported head (34mm in valves) moderate cam that was written to fit stock engine, KN pod filters. Perfect long touring machine on Autobahn, 2 persons and plenty of luggage. See photo from 1985 below.
145 mains or even 147.5... 

I have noticed with my latest carbs that engine was prone to ping when twisting the throttle just a little, retarded ignition and lost the power. Increased pilots and mains, power back or even better and no pinging. I had earlier jetted the carbs to fit full throttle on dyno. Different when riding in city or nice roads not giving full throttle.  I think that the acc pump made it to work well when WOT, too lean when doing smoother accelerations and low throttle lift in general. Acc pump stroke now limited.

[Dino]

Pull the float bowl on #3 and take out the main jet. Check for any restriction in the passage up to the barrel. Also a good idea to check the needle height in relation to the other cylinders. Finally check the idle jet and passages to make sure there's no restrictions. It's for sure lean and that's what's cause the pinging/detonation. A lean cylinder will also run hotter than the rest.

[Davez134]

Pull the float bowl on #3 and take out the main jet. Check for any restriction in the passage up to the barrel. Also a good idea to check the needle height in relation to the other cylinders. Finally check the idle jet and passages to make sure there's no restrictions. It's for sure lean and that's what's cause the pinging/detonation. A lean cylinder will also run hotter than the rest.

I did that yesterday, pulled the idle jet, main jet and emulsifier tube. cleaned out all passages with copper wire just to be sure. I know the needle height is the same on all 4, as I rebuilt them myself but I will check 1 more time before I synch again.

Wonder if that 'rattle' is detonation or pinging.

Yes, thats why I'm worried, I know that sound. I'm sure it is.

Also regarding intake manifold leak, when I had one on my last bike, the cylinder with the leak ran rich. Isn't this usually the case with these carbs/motors?

Have a list of a few things to try. At work until tomorrow morning, so will post any updates after I re-synch, check clip position and re-check manifolds for leaks.

[Davez134]

One more question, while I'm trying to wrap my head around this while I'm at work. If it was an ignition/timing issue, wouldn't I most likely see the lean symptoms on 2 as well as 3? I'm thinking it is carb related since it is just isolated to number 3, but trying to rule other things out too. Looking at the pictures again, I really think I am beyond just lean and on the verge of causing damage.

[ekpent]

Without looking again I thought 3 of your plugs had some fairly good color and the one did look lean. It may be an anomaly in that one carb. Pinging though is a warning if you are hearing or feeling that. One thing I will say is I have not heard of a streetbike engine dying for being too rich and some good black smoke when you nail it  on a hot rod is not a bad thing, smelled it a few times.
   Listen to others not me and let your plugs and other factors be your guide 

[Davez134]

I totally agree that being a little on the rich side is "safer" for the engine. I am very happy with the condition of all the other plugs with the exception to number 3. That along with the death rattle (even though short lasting) is my concern. Instead of just throwing bigger jets or moving the clip on all carbs, I'd like to figure out the underlying issue. Wii start with a vacuum synch tomorrow, and if all even, I will pull the rack, go through it all again and inspect boots.

Air filter was cleaned, but is there any issues I should check with the airbox itself? Boots are new on there too, as well as gasket between the halves, but that's all I know to look at.

[scottly]

You've localized the issue to the #3 carb. Stay focused..

[edwardmorris]

Any change at all if you turn the air screw in for that carb? I've always wondered if the air screw setting has to be the same on all four carbs or can they be individually set? On my K2, it just so happened that they're all at 7/8 turns out (Chicago area) when they finally started behaving well, eventhough they were giving me uneven readings before the carb sync at 1 turn out on all four.

[harisuluv]

Any change at all if you turn the air screw in for that carb? I've always wondered if the air screw setting has to be the same on all four carbs or can they be individually set? On my K2, it just so happened that they're all at 7/8 turns out (Chicago area) when they finally started behaving well, eventhough they were giving me uneven readings before the carb sync at 1 turn out on all four.

They don't have to be set exactly the same across the board.  Just like when you vacuum sync and due to different compression or air flow whatever, you have all the slides at different heights (possibly).  The objective is to get the same mixture in the cylinder whatever it takes to get there isn't always exactly the same.

[Davez134]

Any change at all if you turn the air screw in for that carb? I've always wondered if the air screw setting has to be the same on all four carbs or can they be individually set? On my K2, it just so happened that they're all at 7/8 turns out (Chicago area) when they finally started behaving well, eventhough they were giving me uneven readings before the carb sync at 1 turn out on all four.

I will check that tomorrow as well. Although I don't believe the problem is with the idle circuit. (Plugs were pretty even colored darker brown when pulled after idling while setting timing.

[Davez134]

This morning I rechecked float levels, needle position,  let the bike warm up, double checked timing, turn in the air screw on number 3 carb. It's at approx 7/8 turn, rest are just a hair over 1turn out. The I did a synch. 1,2,4 are at just under 8 inches vacuum, number 3 is at 6 inches vacuum. All done at 1100 rpm idle. So I found a difference there. I'm going to catch some sleep since I was up all night at work I'll get back to it later and pull the plugs. Since all this was done at idle, at least I can see that circuit. Is that enough of a vacuum difference at idle to cause that cylinder to be so lean??

[flybox1]

Is that enough of a vacuum difference at idle to cause that cylinder to be so lean??

Definitely.

[Davez134]

Well, I synched the carbs tonight. Adjusting number 3, all are right at 8 1/3 inches vacuum. found a nice long road and rode with constant throttle position, mimicking my usual cruising position/speed, then cut it off. Number 3 is still leaner than the rest but now more grey than white. Made some progress but still a bit off on that cylinder.


Sent from my iPhone using Tapatalk

[edwardmorris]

I'd say try the air mix screw on just that carb some more, now that they're all sync'd up...

[Davez134]

Will mess with at screw some more today, but would that effect my mixture at cruising where I'm at 1/3+ throttle position? I may have to pull and inspect the needle.

[flybox1]

MUCH better plug color, but yes, still lean.

Yes, a small mixture change at idle( air screw) WILL make the rest of the range change as well.
What is in our carbs that shuts off the idle or pilot circuit from supplying fuel at WOT? 
Yes, the main jet supplies the majority of fuel at WOT, but vacuum pulls fuel and air from all sources at WOT.
Richen that carb by 1/3 turn on the air screw and retest.
If your results are good, you'll need to do an idle plug chop to see that idle is not too rich.  if plugs come out too rich, then your main, or needle circuit will need to be adjusted.
Are you sure you dont have a tear in that carb boot?

[Davez134]

Cool thanks. I will mess with the idle circuit then next. I am sure I do not have a tear in that boot. They are 1 year old, soft, and I checked them all very closely. I will get some ride time in tomorrow. Been busy hanging cabinets all day! (I'd much rather be in the garage!)

[Davez134]

I turned the air screw in some more on number 3 carb, went on a ride, no changes from the last post. After letting it idle for a bit then pulling the plugs, it was darker and sooty. Again, all the other plugs look perfect but 3 is still way lean after a ride. I'm thinking all that's left to look at is the needle/seat.

[harisuluv]

So the problem is that #3 was leaner than all the others.  You adjusted the screw and now it's too rich? 

You adjusted the idle screw and pretty clearly went in the absolute right direction.  Why are you now dismissing adjusting it back a bit?  Why would you say ""i'm thinking all that's left to look at is the needle/seat?"

How much did you adjust the air screw?

[Davez134]

So the problem is that #3 was leaner than all the others.  You adjusted the screw and now it's too rich? 

You adjusted the idle screw and pretty clearly went in the absolute right direction.  Why are you now dismissing adjusting it back a bit?  Why would you say ""i'm thinking all that's left to look at is the needle/seat?"

How much did you adjust the air screw?

It did go in the right direction when I checked the plugs after idling for a bit (I had it at about 7/8, and went a quarter turn in more at the recommendation of flybox1) I'm not dismissing that. What I was trying to say is that my problem of being lean under cruising conditions, 1/3 throttle+ is still there. After cruising for 5 miles at that position, plug is still lean. So turning screw in did change the color at idle, but did nothing for where I use the bike most. Still way too lean. That's what leads me to think I need to check needle/seat. Never really had a problem with idle circuit. Does that make sense? Not sure if last post was clear...
Edit: just read my last post, and yeah it was misleading. Sounded like I was concerned about just idle circuit. Changed it

[flybox1]

it certainly looks richer to me 
take a look at the top picture, and the outside ring of the threads....looks white and ashen, while the new plug looks sooty.
I think you're getting closer.
But more important...how did the 'seat of your pants dyno' come out?  perform any better?