Topic: 71 cb750 running rich no matter what I do

[kangbang]

June 1st EDIT:

Things that I have done:
- Clean the carb using carb cleaner, compressed air, ultrasonic cleaner.
- Have added about 10oz of seafoam to a 3 gal of fuel.
- Bench synced
- Float level adjusted  to 26mm
- New carb boot/clamps.
- Used #40 keihin pilot jet from Harisuluv
- NEW #120 keihin main jet from Harisuluv
- needles are keihin and at stock setting (middle clip)
- NEW spark plug wires, 7mm copper core
- have measured resistance of the coils and spark plug caps (they seem to be correct)
- Adjusted the timing
- mixture screw is 2 turns out.
- UNI pod filters
- Mac 4-1 exhaust with modified internals to reduce back pressure.
- Did a valve adjustment
- Checked charging system


Things I have not that done (that I can think of):
- Have not synced the carbs using sync tool (it is bench synced though)
- Have not

Current Setup:
-Dyna Coil 5 ohms
-Dyna ignition
-NGK spark plug cap
-D8EA (i have tried denso prior with same result)
-7mm copper core wire
-#40 slow
-#120 Main
-Middle clip needle
-2 turn out on mixture screw
-7A carb

The bike is running a little better now with the MAC muffler baffles removed. However, spark plugs are still looking sooty.


If you have any recommendation, please let me know. Thank you

EDIT:additional info:

I think the idle circuit is running rich because, it smells rich at the exhaust (not very technical), it dies if stayed on idle too long (doesn’t necessary mean idle is rich), I have cleaned the spark plugs and reinserted and idle for a min or two to checked the spark plugs and found them sooty. The idle screw (not the air mixture screw but the one that you adjust to change the rpm of idle) is not turned up too much so it is likely activating main jet fuel circuit at idle. That is why I think the idle circuit is running rich. At Idle, the bike RPM is around 1000-1500. Any lower it seems to want to die.

EDIT pics

[iron_worker]

What are the symptoms or indications that your bike is running rich?

IW

[ekpent]

Are the jets known stock Kehin brand and marked with their logo or aftermarket. Any chance someone in the past may have drilled out the mains to a bigger size.

[kangbang]

What are the symptoms or indications that your bike is running rich?

IW

Smells rich, black sooty spark plugs, dies when I turn choke on, also fouls spark plugs pretty often. :/

[kangbang]

Are the jets known stock Kehin brand and marked with their logo or aftermarket. Any chance someone in the past may have drilled out the mains to a bigger size.

Unfortunately, they are aftermarket. When I bought the bike, it didn't have the keihin jets. I have purchased some not-so-good carb rebuilt kit off the internet, which had new jets. Didn't seem to fix the problem.

Should I try #38 pilot jet? I am just afraid the problem is else where, since I should be running leaner with this current setup with stock numbered jets. Also, what are some trust worthy place to purchase jets for these bikes?

[ekpent]

A good unmolested set of stock brass Kehin #40 slow and #120 mains would be a good base to start over. Some of that aftermarket stuff I have seen is pretty skeetchy ,I try to avoid it.  Have you confirmed with the serial numbers that your engine is a true 71 ?  Since later stock bikes had mains as small as #105 you can drop the 120 down if needed also.

[kangbang]

A good unmolested set of stock brass Kehin #40 slow and #120 mains would be a good base to start over. Some of that aftermarket stuff I have seen is pretty skeetchy ,I try to avoid it.  Have you confirmed with the serial numbers that your engine is a true 71 ?  Since later stock bikes had mains as small as #105 you can drop the 120 down if needed also.

CB750E-1101490 which I believe is the 71 model. So called Keihin pilot jets on ebays are replicas or maybe I just haven't found the right one yet.

[harisuluv]

what is the casting for your carbs?  you can do a search of where to find it.

did you replace the needle jets from the kit too?  Almost always better off with the original ones.

Also, "cleaned the carbs" means something different to everyone.  If you could go into more detail.

Did you oil your pods?

[ekpent]

A good unmolested set of stock brass Kehin #40 slow and #120 mains would be a good base to start over. Some of that aftermarket stuff I have seen is pretty skeetchy ,I try to avoid it.  Have you confirmed with the serial numbers that your engine is a true 71 ?  Since later stock bikes had mains as small as #105 you can drop the 120 down if needed also.

CB750E-1101490 which I believe is the 71 model. So called Keihin pilot jets on ebays are replicas or maybe I just haven't found the right one yet.

The carbs are made by Keihin and true stock brass parts are marked with a stylized "K" logo. Be sure to check the float levels. Clip position on the needle should be check also if there are problems. Member Harisuluv has joined the conversation now and he is  Guru. 

[kangbang]

what is the casting for your carbs?  you can do a search of where to find it.

did you replace the needle jets from the kit too?  Almost always better off with the original ones.

Also, "cleaned the carbs" means something different to everyone.  If you could go into more detail.

Did you oil your pods?

what is the casting for your carbs?  you can do a search of where to find it.

did you replace the needle jets from the kit too?  Almost always better off with the original ones.

Also, "cleaned the carbs" means something different to everyone.  If you could go into more detail.

Did you oil your pods?

the carb model is 7A JF. I will give the "original", pilot jet that came with the bike on purchase, another try.

I haven't oiled the pods myself, but I bought them recently and I think they come pre-oiled (atleast they are greasy to my touch). Should I oiled them again?

As for cleaning the carbs:
I disassembled all parts off of the carb and performed ultrasonic cleaning.
Shot carb/compressed air cleaner through fuel and air paths.
Replaced some orings
New fuel line and filters

And I did all that twice in the last 6 months

Recently ordered two new floats, should be here on monday. The carbs came with two brass and two foam floats. One of the brass was leaking, so I emptied it and sealed with a epoxy that is resistant to hydrocarbon for now.
I tried 26mm float level, I have also tried 27mm and 28mm. I thought maybe the increase the float level will lean out the overall mixture. Which it kinda did, but 28mm was too lean (making weird tick/pop noises). Currently at 27mm, at least it kinda stays running now. Funny thing, when I had it at 27 mm with 1 turn on idle air screw, it would idle low and when I blip the throttle, it would idle high for about 20-30 seconds. I thought maybe I have a leak somewhere but I have brand new carb boots and I re-clamped after it was warmed up. After I turned out the idle screw about 3 turns, problem went away. Doesn't seem normal to have the idle screw turned out 3 turns tho...

 

[ekpent]

The reason He asked about oiling the filters was because I think He is concerned you may have used too much. Those old leaky brass floats can be swapped out with the plastic. Everything up to snuff with the ignition-coils-plug caps etc. Out of curiosity what brand/number of plug are you running.

[kangbang]

A good unmolested set of stock brass Kehin #40 slow and #120 mains would be a good base to start over. Some of that aftermarket stuff I have seen is pretty skeetchy ,I try to avoid it.  Have you confirmed with the serial numbers that your engine is a true 71 ?  Since later stock bikes had mains as small as #105 you can drop the 120 down if needed also.

CB750E-1101490 which I believe is the 71 model. So called Keihin pilot jets on ebays are replicas or maybe I just haven't found the right one yet.

The carbs are made by Keihin and true stock brass parts are marked with a stylized "K" logo. Be sure to check the float levels. Clip position on the needle should be check also if there are problems. Member Harisuluv has joined the conversation now and he is  Guru. 

Yea I found a few stating Keihin pilot jets but when I look in the description, it says "These are aftermarket jets made in Taiwan, they are quality reproductions." I will keep searching. I called couple local motorcycle shops for them but no luck :/

Yea I messed with float level few times, take look at the reply to Harisuluv above. I left clip position in the middle for now, since it should't mess with the idle portion anyway. or... i think it shouldn't?

PLEASE SAVE ME Harisuluv!

[kangbang]

The reason He asked about oiling the filters was because I think He is concerned you may have used too much. Those old leaky brass floats can be swapped out with the plastic. Everything up to snuff with the ignition-coils-plug caps etc. Out of curiosity what brand/number of plug are you running.

I am using dyna s ignition (new), dyna coil (from previous owner) with NGK plugs, denso x24es-u spark plugs. Not sure what the wires are... I should prob get new wires at some point anyway. Do you know where I could get some good plug wires? or which kinda for that matter..

Chris-

[harisuluv]

Did you replace the needle jets?

Or have you verified they are kei hin needle jets.  This is a big deal!

I wish I woulda known you needed odds and ends.  I have everything, just let me know what you need. 

Pricing depends on how close to NOS you want to get. 

I would also make a reference to Hondaman's book where he basically says that with our slide type carbs (round top style) contrary to popular belief, things such as vacuum leak (or maybe more air via pods) can actually make things RICHER as opposed to leaner as you would think.

[kangbang]

Did you replace the needle jets?

Or have you verified they are kei hin needle jets.  This is a big deal!

I wish I woulda known you needed odds and ends.  I have everything, just let me know what you need. 

Pricing depends on how close to NOS you want to get. 

I would also make a reference to Hondaman's book where he basically says that with our slide type carbs (round top style) contrary to popular belief, things such as vacuum leak (or maybe more air via pods) can actually make things RICHER as opposed to leaner as you would think.

Needle jet meaning the needle? or the main jet? or main jet holder?. As far as I know the needle is the originial Keihin, I did used the one that came with the rebuilt kit but reverted back to the original needle. The main jet is not Keihin. Main jet holder looks like it from Keihin, quite old looking and one is in sorta corroded on the surface.

Wouldn't having "leak" on the engine side be different from "leak" on the intake filter side? Also isn't the choke doing exactly that? limiting flow at the filter side to increase fuel ratio. Unless the response to flow is not linear :/

[kangbang]

additional info:

I think the idle circuit is running rich because, it smells rich at the exhaust (not very technical), it dies if stayed on idle too long (doesn’t necessary mean idle is rich), I have cleaned the spark plugs and reinserted and idle for a min or two to checked the spark plugs and found them sooty. The idle screw (not the air mixture screw but the one that you adjust to change the rpm of idle) is not turned up too much so it is likely activating main jet fuel circuit at idle. That is why I think the idle circuit is running rich. At Idle, the bike RPM is around 1000-1500. Any lower it seems to want to die.

[chewbacca5000]

I had a similar situation with a set of earlier carbs that I vigorously ultrasonically cleaned.  Long story short one of the brass balls that seal the idle circuit came loose.  I fixed this with some clear gas resistant sealer very similar to red kote which is also mek based.

It worked like a charm.  After letting dry for a day or so the air screw worked as it should.  Maybe even better.

As for the pods you can test that with plug chops.  That will tell you if you got too much oil on them.

[kangbang]

I had a similar situation with a set of earlier carbs that I vigorously ultrasonically cleaned.  Long story short one of the brass balls that seal the idle circuit came loose.  I fixed this with some clear gas resistant sealer very similar to red kote which is also mek based.

It worked like a charm.  After letting dry for a day or so the air screw worked as it should.  Maybe even better.

As for the pods you can test that with plug chops.  That will tell you if you got too much oil on them.

I have attached a picture. Is that what you are referring to? Also, was it pretty obvious that it was lose by just touching it?

Chris-

[kangbang]

I just found something weird about my carbs. If you look at the picture I have attached. you can see that the carb 2 is almost a mirror version of the carb 3. However, the fuel circuit to the emulsion tube is thicker on the carb 3. I notice this because the float bowl was not seating very well on carb 3 and 4. both of the carb 3 and 4 have this "issue". The red arrow points at the fuel circuit and the yellow arrow points at the dent that over flow needle made on the carb body.... they both have 7A stamps. Does anyone else have this problem?

[ekpent]

An experience I had once with aftermarket jets was when I broke down a set of carbs after a new buy. When I went to clean and inspect the jets I noticed that the aftermarket slow jets were WAY bigger than the stock Keihin #40. My special little cleaning wire for plugged/gummed up ones just wallowed around in it big time and was visibly larger looking through it.. Wish you had a stocker on hand for comparison.

[kangbang]

An experience I had once with aftermarket jets was when I broke down a set of carbs after a new buy. When I went to clean and inspect the jets I noticed that the aftermarket slow jets were WAY bigger than the stock Keihin #40. My special little cleaning wire for plugged/gummed up ones just wallowed around in it big time and was visibly larger looking through it.. Wish you had a stocker on hand for comparison.

I order used stock jettings from harisuluv last night! hopefully they will get here by next week and can update you guys on the verdict.

[ekpent]

An experience I had once with aftermarket jets was when I broke down a set of carbs after a new buy. When I went to clean and inspect the jets I noticed that the aftermarket slow jets were WAY bigger than the stock Keihin #40. My special little cleaning wire for plugged/gummed up ones just wallowed around in it big time and was visibly larger looking through it.. Wish you had a stocker on hand for comparison.

I order used stock jettings from harisuluv last night! hopefully they will get here by next week and can update you guys on the verdict.

Sounds like a good plan that will hopefully put you on a good base setting with known good parts to continue your troubleshooting. Be looking forward to your report and comparisons.

[Ericcb750]

I recently had this exact same issue after re-building my carbs with Keyster kits. This is my first cb750 so I thought "these nice new jets (same size) will be better than the old ones". Boy was I wrong, I spent over a month of yanking carbs, cleaning, tightening boots, "tuning", pulling plugs, etc. It was f-ing ridiculous, I finally created a thread here and some of the experienced guys really helped me out. I ended up cleaning the Keihin Jets nicely and put them back in.....within a day or two I had them tuned up great and actually was able to ride the bike for the first time since I bought it. I'm no veteran with these but that fixed my problem and It sounds identical to yours. I hope that helps when you get the keihin jets. Good Luck .

[kangbang]

 Sounds like a good plan that will hopefully put you on a good base setting with known good parts to continue your troubleshooting. Be looking forward to your report and comparisons.
[/quote]

Got the new foam floats to replace my leaky brass ones and new/used main jet and pilot jet from Harisuluv! Thank you Harisuluv! I recommend anyone to get carb parts from him, very knowledgeable and stuff arrived here in 2-3 business day.

While I was waiting for parts to arrive, I rechecked my ignition system. My previous setup was Dyna S coil 5 Ohms, silicone plug wires, NGK plug caps, Denso spark plugs. Previous owner did not do a very good job terminating the boots on the plug wires so I decided to replace them with new copper core wires. From what I understand from reading sohc4, I should be using copper core wires anyway. (not sure if that is true with aftermarket coils though..)

I got the jets and the floats on the carbs and readjusted the floats to 26mm. Went for a short ride (only had time to ride around the block last night). From my vague diagnostic, its still running rich? I didn't get to bring my bike up to full temperature so not the most accurate assumption though. It definitely felt cleaner and snappier on mid-high ends. Idle was still little rough and whenever I turn on the choke, the  bike died immediately. If my bike was running lean from being cold or running just right, should my rpm go up when I slowly turn the choke on? Bike seems to have no response to choke until it is fully shut (and it just dies). I have checked the little flappy doors on the choke and they all are functional. I also checked the spark plug and they are still sooty. 

Reason why I think it is still running rich: sooty spark, dies when choke is on.

I am going to go on a longer ride today and borrow a air/fuel mixture monitoring tool from a friend to double check my assumptions.

[ekpent]

Don't use the choke as any kind of a 'guide' on the old round top carbs on the 750. Yes it will die when you close them.That would be normal. It is a very primitive affair with no fast idle cam built in etc like more modern chokes. I don't even leave them on when cold,just for initial start up and then feather them on and off a little as needed while I keep the rpm's up a little with the throttle till warmed up.