Topic: Cb750 lean cylinder help.

[Davez134]

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?

I didn't feel much difference in accereration, the bike felt strong before and it still does. I did not notice/hear any detonation though this time. Idle circuit is pretty rich on that cylinder, and I'm a little less than 3/4 turn out. Still ashen on ground strap but plug does look much better than before, I agree

[Davez134]

See if this changes anything. It's the only thing left I can think of.


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[Davez134]

So changing out needle/seat made no difference, air screw made no difference (except at idle plug chop)
Float levels triple checked, all equal. All passages cleaned, even replaced rubber boot with a new one I had. Number 3 is still lean after a ride! I don't know what else to do!
Should I get pictures of float levels and post? I have no other guesses, I feel like I could be missing something simple and easily overlooked?


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[Dino]

Have you removed the idle jet to see if it's totally clean of debris? Also a good idea to remove it and the air screw and blast some carb cleaner through the passages in both directions. Put the air screw back in and spray some carb cleaner up through the hole where the idle jet goes. Look in the barrel and make sure a good stream of cleaner comes through. This is the only other thing I can think of to check.

Another thought that you may have already covered; how even is the compression across the cylinders?

[Davez134]

Have you removed the idle jet to see if it's totally clean of debris? Also a good idea to remove it and the air screw and blast some carb cleaner through the passages in both directions. Put the air screw back in and spray some carb cleaner up through the hole where the idle jet goes. Look in the barrel and make sure a good stream of cleaner comes through. This is the only other thing I can think of to check.

Another thought that you may have already covered; how even is the compression across the cylinders?

All valid points. Yeah, I removed the idle jet, main jet, emulsifier tube. Cleaned them all with a strand of copper wire. When I had the carb apart for replacing needle jet and seat, I sprayed carb cleaner through every passage, including up through where idle jet screws in, then back again against the normal flow just in case.

I too thought about compression, I checked that last week when I had all the plugs out. I can't remember the exact number, I believe it was around 130 cold on all 4 cylinders.

I checked valve lash again just to make sure. I'm going to do another clear tube test of floats after I get home tomorrow and post up pictures of the levels. I set them mechanically again though.

[DH]

Where on the carb body are you measuring your float level? From the actual bowl gasket surface, or the "notch"in the wall
that runs along the side the bowl gasket surface? I'll get flamed for this, but a Honda tech from back in the day told me to always measure from the gasket surface. I measured level using this method on my k5 with good results, and followed up with a
good sync. Make sure that All carbs are reacting at the same rate, none getting lifted early or later than the others, with vac readings as close to each other as possible at all throttle positions.  Make sure there's no slop in any of the throttle rods due to a bent needle plate, #3 especially. If the plate is bent, the needle height can change constantly. Just a guess,  these things can affect mixture, not sure if it's the cure.

[Davez134]

Yeah I am measuring from gasket surface. And again, all others are great. What part are you talking about when you mentioned bent needle plate? I haven't considered anything being bent, but I'm not sure what you're referring to. Thanks.

[DH]

Yeah I am measuring from gasket surface. And again, all others are great. What part are you talking about when you mentioned bent needle plate? I haven't considered anything being bent, but I'm not sure what you're referring to. Thanks.

After review, Plate probly not an issue, and #3 would most likely
have been hard to sync with the rest of the carbs,  with bad drivability as a result if plate was bent.. The plate is connected to the throttle valve (by those 2 little screws you had to remove to change the needle)on its bottom end, and links to the throttle lifter rod on its top end. The throttle rod and throttle valve must move at the same time. if the plate is bent, both the rod and valve can move independently of one another.
Again, tho, the engine wouldn't run good at all like that, which doesn't seem to be the case.
Are the mixture screws in all 4 carbs the same type/taper? Is #3 different compared to the others?

[ekpent]

If you have a bent plate the needle will have a sloppy fit compared to the rest,it will be loose and want to tip etc.when still connected to the slide. Have had to fix a couple,usually happens when people force carbs that were seized with varnish.

[Davez134]

Yeah I am measuring from gasket surface. And again, all others are great. What part are you talking about when you mentioned bent needle plate? I haven't considered anything being bent, but I'm not sure what you're referring to. Thanks.

After review, Plate probly not an issue, and #3 would most likely
have been hard to sync with the rest of the carbs,  with bad drivability as a result if plate was bent.. The plate is connected to the throttle valve (by those 2 little screws you had to remove to change the needle)on its bottom end, and links to the throttle lifter rod on its top end. The throttle rod and throttle valve must move at the same time. if the plate is bent, both the rod and valve can move independently of one another.
Again, tho, the engine wouldn't run good at all like that, which doesn't seem to be the case.
Are the mixture screws in all 4 carbs the same type/taper? Is #3 different compared to the others?

Oh, ok. Yeah, that is definitely not the issue then. It is straight like the rest of the carbs, I know because I just removed it to replace needle. As far as the mixture screws, yeah, they are all the same, I switched out between 3/4 with no changes. Im home from work, after a quick nap, Ill get back at it!

[Davez134]

So starting with the float levels...here is the level of number 4, plug looks perfect through all ranges

Here is number 3, the problem child
looks a bit higher if anything.
I was asked about compression tween cylinders, I thought it was 130 ish but just checked them all again. Here is number 3, they are all the same.



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[Jerry Rxman Griffin aka MuthaF'er]

You're running out of options. For #$%*s and grins try swapping carbs or parts/jets between that carb and another. Air screws. Go big jet first. That's the easiest. Get a reading. Slow jet is easy too but don't do 2 swaps at the same time. Next you gotta start removing carbs. PITA! Try needle swaps. Then try needle jets. The needle jets are not THAT hard to remove. A wooden dowel sized to fit in the emulsion tube and small hammer and you can tap them out from above then reinstall the same way from below.

[Davez134]

You're running out of options. For #$%*s and grins try swapping carbs or parts/jets between that carb and another. Air screws. Go big jet first. That's the easiest. Get a reading. Slow jet is easy too but don't do 2 swaps at the same time. Next you gotta start removing carbs. PITA! Try needle swaps. Then try needle jets. The needle jets are not THAT hard to remove. A wooden dowel sized to fit in the emulsion tube and small hammer and you can tap them out from above then reinstall the same way from below.

I replaced the needle and jet in that carb a few days ago. I used a pencil inserted from top and gave it a few taps. Came right out. So yeah, I'm at my wits end. I guess jet swaps are next

[Davez134]

Ok I'm done. Posted in WTB for #3 carb
Also the one part I haven't even considered changing out is the throttle/slide. I never thought to check the surface closely for damage (even though nothing obvious stood out)

[Davez134]

I'm pretty sure I have this fixed, but I want to get a few more rides/plug chops in to be sure. I bought a NOS throttle slide on ebay, still in original packaging, for a stupid low price. Compared to the new one, the one I had in there showed a bit of wear on the cutaway area, a few small nicks too. Nothing major though. I had to re synch again too after installing. After today's ride, Ive got a nice light tan color (pics tomorrow) and I noticed the engine has almost no vibration at all through the whole rev range!

[Davez134]

Picture of slide cutaway

Plugs 3(problem child) and 4 now. Lighting makes it hard to see but 3 is very close to the rest now

With flash makes it look lean, but here it is anyway.



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[Jerry Rxman Griffin aka MuthaF'er]

Looks like congrats are in order. Was the throttle sticking? Loose where the rod attaches above the carb? 

[Davez134]

It wasn't sticking, throttle rod felt solid. Honestly, I just replaced slide with nos one because it was all that was left to do and I got it so cheap. I only noticed damage because new one looked so good.

[Dino]

A fine example of how persistence and determination alone are omnipotent! Well done!

[ekpent]

Looks like somebody in the past was prying on the old one with a screwdriver. People need to use heat and chemicals not force.

[Davez134]

Cylinder 3 is still leaner than the others on the bike, not dangerously lean anymore as shown in the above posts, but still quite a bit leaner. Everything is equal as far as jetting, float level goes. The idle mixture screws are set at nearly the extremes, 1 1/8 turn out on 1,2,4 and 1/2 turn on 3. As you can see by everything in this post I have gone through everything and today I thought of something else. My head has been ported. Maybe differences in the porting and airflow could cause the symptoms. Possible? Plausible? If this is the case, the only way I can think to even it out is to change the jetting or needle clip position on #3 only. Has anybody done this? Lots of questions, I know.

[harisuluv]

Looks like congrats are in order. Was the throttle sticking? Loose where the rod attaches above the carb? 

I know it seems very odd, but all the cb750 slides have a little nick in them in the same place.  Now in that picture there is more than one, but if the profile isn't really affected, it's probably not this.

Source:  I noticed this phenomenon too, and inspected multiple sets across different years and they all have said nick in the same spot. 

[Davez134]

Looks like congrats are in order. Was the throttle sticking? Loose where the rod attaches above the carb? 

I know it seems very odd, but all the cb750 slides have a little nick in them in the same place.  Now in that picture there is more than one, but if the profile isn't really affected, it's probably not this.

Source:  I noticed this phenomenon too, and inspected multiple sets across different years and they all have said nick in the same spot.

I did notice a nick in all the slides when I took them apart last. I honestly replaced the slide because it was the only thing I hadn't replaced and I got it NOS for $10. It did make a difference in my 1/4 to 1/2 throttle plug chops, but I agree now that this is not the root of the problem. That is why I started thinking about the ports in my head may differ. If it could be in the porting, I was thinking of changing the idle jet in only that carb to a 42 or moving the clip down 1 more (since that's the throttle position/positions I'm noticing the difference) But I'd like to get some input from some experts here. I know I'm being picky, the bike runs great, I just believe it should be even across all cylinders.

Edit: I will post another pic in a bit of the difference in color again. (it is running richer than before across all cylinders because the last pics were taken when it was about 65-70 degrees, and it has been 110 all week!)

Ok, numbers 4 and 3 respectively:

[harisuluv]

I was looking back on your thread.  I feel like you overlooked a lot of simple stuff.  Most of the stuff could be eliminated with any pictures of carbs.

Often times I can identify problems with just clear, well framed, focused, lighted pictures of the carb assembly.  However, for whatever reason, that is very hard to get out of people.

Have you tried swapping the idle screws to eliminate that?  Have you checked all the emulsion tubes to see that the holes are the same?  The emulsion tubes are NOT all the same.  The number of holes and WHERE the holes are dictate the throttle range the mixture is most enriched.  Basically, every casting of carb has a different pattern of emulsion tubes.

Air screws, while they may look the same, often times have different part numbers and the profile of the taper.

If you can post pictures I would happily look at them for you, surprised you didn't post any TBH, considering it seems like you've had the carbs apart and off and on so much.

[Davez134]

It may seem like I overlooked a lot of simple things, but I really don't think I did. I did swap the idle screws, even though they looked the same visually, I understand they could have a different taper. I unscrewed and compared the emulsion tubes, where the holes were making sure all were clear, not bent from over tightening. Same with 40 idle jets. I will gladly take some good pictures of everything you mentioned and post them. I research a lot before asking questions, but sometimes I admit I only know enough to get myself in trouble. Thanks for the reply, and I'll get some pictures up in the next day or so!