Topic: CB750A Mixture screws all the way and still too rich!

[Ved]

OK, to make a long story short... 1977 CB750A. Straight pipes, stock airbox, carbs are good to go (jets are clear, float levels correct/no sticking)

I figured she would be running lean with the straight pipes, and I would either have to baffle em or re-jet... but still rich as hell.

With mixture screws screwed all the way in, she is still running rich. I mean really rich! Plugs are black, she smokes really bad when letting off the throttle at highway speed. Drinks gas like crazy. Tried to colortune and found that carbs 1 and 3 need to be all the way in just to achieve blue, carb 2 is 3/4 of a turn out, and carb 4 is 1&1/4 out. It's a mess, and driving me crazy. Why with straight pipes/stock airbox/stock air filter/straight pipes be making her run so rich?

I even made baffles using the washer technique, and it did quiet her down a bit, but still too rich.

Any thoughts would be much appreciated.

[co425]

Is your choke off?

Your mixture screws are only for idle as well. 1 1/4 to 1 1/2 turns out from full seat. To set slowly turn out screw until you idle tops out. Then turn in 1/8 turn.

Your problem sounds like wrong jets. Or needle clip set wayyyyy to low.

[Ved]

Jets haven't been changed. Shop manual says 1 turn out for this particular model. If a needle clip was set too low then why am I getting black smoke out of both pipes when letting off the throttle when cruising?

*edit* and yes.. choke is off 

[co425]

did you have these carbs apart??

I dont know if the A has the ability to have the slides in backwards like the older cb's. If it can that would probably be your issue.

[Ved]

This what ya talking about?

[co425]

Exactly if you have the slide( throttle valve) in backwards it will be choked all the time running rich as hell.

[Inigo Montoya]

Most likely your idle circuit is plugged up. Best thing to do is pull the carbs off and clean them. Pull out the jets, mains and idle. Remove all the tubes and passages for fuel. Pull the slides out. Clean everything. Spray liberally with carb cleaner. Run copper wire through the passages on the jets. In order to pull the idle jets, wrap with cloth, grab with pliers and twist while pulling. Do not wiggle back and forth as that could crack the jet housing. Pull out the idle mix screws, keep track of the screws and washers and springs and o-rings. Clean all fuel passages. Remove the accel pump and check for cracks and clean all the gunk out of there.

Now, do it a second time. Trust me, many think they get everything clean but end up doing it a second time. Save your self some time and do it right away.

Reassemble and adjust the gap between the pump rod and actuator. Should be .008 gap.
Turn out the idle mix screws to factory. Set the clip to middle position. Use a smooth rod such as the back end of a drill bit to set the slide height uniformly.
Get the carbs on tight to prevent air leaks.

See how it goes now.
I have to correct a previous poster. the idle mix affects a wide range of rpm. From idle to upper mid range it plays a big part. This idle influence being plugged is most likely why you are running rich.

[Ved]

Most likely your idle circuit is plugged up. Best thing to do is pull the carbs off and clean them. Pull out the jets, mains and idle. Remove all the tubes and passages for fuel. Pull the slides out. Clean everything. Spray liberally with carb cleaner. Run copper wire through the passages on the jets. In order to pull the idle jets, wrap with cloth, grab with pliers and twist while pulling. Do not wiggle back and forth as that could crack the jet housing. Pull out the idle mix screws, keep track of the screws and washers and springs and o-rings. Clean all fuel passages. Remove the accel pump and check for cracks and clean all the gunk out of there.

Now, do it a second time. Trust me, many think they get everything clean but end up doing it a second time. Save your self some time and do it right away.

Reassemble and adjust the gap between the pump rod and actuator. Should be .008 gap.
Turn out the idle mix screws to factory. Set the clip to middle position. Use a smooth rod such as the back end of a drill bit to set the slide height uniformly.
Get the carbs on tight to prevent air leaks.

That was the first thing I did when I got the bike. Had to use the high E string off my guitar to get through the idle jets. Cleaned the carbs 3 times before I realized three out of the 4 idle jets were clogged. Found an air leak using a propane torch. Hard boots were the problem. Soaked boots in Wintergreen oil and xylene and she is air tight now. No air leaks. I still have to check if the PO put the throttle valves in backwards, and make sure the needles are set to the middle position yet. Will do that tomorrow.


Today I checked timing with a test light, she was a tiny bit advanced, so I corrected it, but of course didn't solve the rich problem. On a side note, just for sh*ts and giggles I set all 4 mixture screws to 1 turn out, and took her for a 15 minute ride. Got back and pulled each plug. Plugs 1 and 3 were black, 2 and 4 were perfect caramel brown.    Cleaned plugs up, turned #1 and #3 mixture screws all the way in and took her for another ride. Got back and pulled the plugs again and now all 4 are nice and brown. My buddy who rode behind me said she still smokes like a diesel truck when I let of the throttle coming to a stop.  

I have to correct a previous poster. the idle mix affects a wide range of rpm. From idle to upper mid range it plays a big part. This idle influence being plugged is most likely why you are running rich.

 

[Don R]

My po cleaned jets with a drill. Moral of the story, don't trust the numbers.

[Inigo Montoya]

In a 77, the plugs should not be a caramel brown, that is too rich for these carbs. What brand of air filter do you have?

[tlbranth]

Try running with the air cleaner out.

[raymond10078]

On my 78 CB750A, it was not possible to put the carb slides in backwards.  It'll only go in one way.  FYI.

I'd still check to see that the chokes blades are fully open (visually).  When I put mine back together, I noted that they can be put together wrong, and could (and did) bind a little (until all of the alignment issues were resolved).  The blades would move, but not fully . . . .

[Inigo Montoya]

I had the same issue on the flaps. There are 2 separate rods that have 2 flaps each. If the alignment is off, it binds and I think the right side (#3 &4) stick.

[Ved]

OK, after 7 hours on it today... Checked slides, they were fine and in fact cannot be put in backwards (as kandrtech stated). Re-checked all jest both main and idle, all fine. They also do not appear to be drilled out. Slides passed the clunk test. Needles are set to middle notch. Seems as though no one has been into the top of these carbs ever. Gaskets were original, and are now new. Rechecked float heights and did a bench test with bowls off to assure they are working properly. Checked choke then did clear tube test with bowls on to check fuel height in the bowls. All checked out good. Accel pump gap checked and adjusted. Also took accel pump apart and made sure the diaphragm was in good shape. Chokes blades working perfectly, no binding.

Once the carbs were deemed "good to go" put em back on the bike, witch was cake compared to fighting the air box in, but with the aid of a ratchet strap I got her good and tight. Set all 4 mixture screws to 1 turn out via the shop manual. She fired right up half choke. I let it warm up for a minute then put choke in and gave her some throttle. Let off and what do ya know... smoke...both pipes. Checked for air leaks with my trusty propane torch. None found. Took her for a run and she ran tight. Goes from 50-90 like a bat out of hell, but nothing more. Full throttle and she wont go over 90.(I know I know, just checking the red line). Got back to the shop, pulled the plugs. 1 & 3 were rich, 2 & 4 were good. I then proceeded to colortune. Started at plug one. Way too rich. Had to screw mixture all the way in just to achieve a slight blue flame. Carb 2 was fine at about 1 & 1/8th turn out. Carb 3, same as carb one, had to screw mixture screw all the way in. Carb 4 about a turn out.

I'm baffled...  

[Rio_CB750]

To ask the same question as Inigo Montoya, what air filter do you have in?  I know from experience that the early K's like I have need to have OEM or K/N, the cheap aftermarket don't allow enough air = rich.

[Ved]

well... It's some cheapo I paid 12 bux from here> https://www.partsnmore.com/cat_index.php?model=cb750_sohc&category=carb

High Quality Stock Style Replacement Filter.

 

Gonna check the points again tomorrow, and do a compression test. This bike is driving me nuts, and is beginning to be more headache than it's worth.

My new cologne is 93 octane...

[Rio_CB750]

The filter is junk, take it out and set it on fire, my bike ran so much better after I got the right filter, it's blocking to much air, hence your very rich mixture, do yourself a favor and get an oem or k/n filter.

[tlbranth]

The filter is junk, take it out and set it on fire, my bike ran so much better after I got the right filter, it's blocking to much air, hence your very rich mixture, do yourself a favor and get an oem or k/n filter.

You know this for sure about the PartsNmore filter? Reason I ask is ....that's what I have and I'm running a bit rich.

[Rio_CB750]

I have no clue about the partsnmore filter, other than knowing that I was told, and it was spot on, that cheap filters are just that, cheap filters. They don't allow enough air in, go with oem or k/n if you have a few more bucks. I trusted my bike with a cheap once the first time.  The bike needs air, spark, compression and fuel to run, make sure that it gets the best of all of them, imho.

[Inigo Montoya]

Get rid of that filter! I had that same one! And it DID make things too rich! Your issue will likely clear up A LOT with a honda filter.
The filter is an emgo, it is well built, just too tight.

[tlbranth]

Finally put on a K&N filter. Runs leaner.

[jweeks]

How are the valve guides sealing? You may have more than one issue...

                                                   Jon Weeks

[tomsweb1]

VED, I had the EXACT same problem on my 750A. It was so bad that you smelled like gasoline when you got of the bike. Hold your hand a few inches away from the pipes and it it would turn black within seconds. And she ran like crap. After many many months of swearing and head scratching I found the problem. There are tiny little o-rings underneath the pilot screws. They were so corroded, for lack of a better word, that they were the same colour as the aluminium surrounding them. I didn't even know they were there. Discovered it looking at a schematic (during one of my swearing/head scratching sessions) and there they were. Replaced them, and she purrs like a kitten.

[Danno]

get a k&n filter for it and bump the slow jets down .02 and bump the main jets down 10 someone put a standard 750k carb set on your bike or they used 750k jets and the exhaust has little to do with it if your intake is restricted there are aftermarket suppliers that are selling replacement filters for these bikes that are too restrictive that wont help either but i am sure you got a rebuild kit for a 750 k not for a 750A