Topic: One cylinder rich = performance issue?

[jimbojangles]

Hey everyone, I need some carb experts.

I'm working on a 73 CB750 that sat in a warehouse for 20 years. I've got it running pretty well and can even commute on it a bit, but it's not PERFECT! It's driving me and in turn my wife crazy. She knows more about motorcycles than she ever wanted.

The problem is that Cylinder #4 is constantly rich. I can turn the air mixture screw out to about 2.5. to 3 turns and it will lean up a bit (spark plug not nearly as carbon fouled), but it will idle like crap. It idles happiest at about 1.5. to 2 turns out. I've taken the carb bank off 3 times now to clean jets, set float heights and try to clean out all of the little air passages. I'm confident that my floats are set right and my jets are clean. Not sure on the airways. I've also replaced most of the ignition system: plugs, wires, coils, points and condensers. Cylinders 1,2 & 3 run great. Plugs are always a nice light tan.

Is this rich mixture related to my throttle response when I barely touch the gas? For instance pulling out of my driveway or trying to maintain a constant speed of about 30 through our local speed trap. The bike burps, stutters and sounds rough. However a little more throttle and it takes off.

Any ideas would be greatly appreciated. I plan to dissect the carbs even further this weekend. Any recommendations on what to look for?

[wrenchmuch]

You might try using an aerosol carb cleaner to shoot though the air passages . Blocked or partially blocked air passages will cause a rich condition . Did you use any wire or drill bits to clean the jets ? This kind of cleaning method can enlarge the jets which will cause a rich condition . If you want to send something through the jets or air passages try some stranded copper wire . For the idle air circuit try passing the wire through in stages from the hole just outside of the choke plate to the idle air screw and then from the air screw to the idle fuel jet . Use carb cleaner in conjunction with the wire . Do you have brass floats ? They can leak over time causing a high bowl fuel level and rich condition . Even composite floats can loose buoyancy over time . The only way to check bowl levels for sure is to put a fitting where the bowl drain screw is and attach a piece of clear tubing to this . When held up next to the bowl with the fuel on it shows the level of fuel in the bowl . It might also be an ignition problem . Weak spark = incomplete burn and sooty plugs . Is the wire to plug boot connection
good ? Is the resister in the plug boot in speck (probably 5 k ohms ,check with a meter)?
Hope this helps .
Mike

[jimbojangles]

Thanks
I've blown carb cleaner through every hole I can find. I was pretty careful with the jets, but tried switching them to another carb to see if the problem moved with them. I will try finding an ultra thin wire to run through the air passages. Is one type of Aerosol carb cleaner any better than the rest?

Is it possible that this can be caused by the needle valve clip position? That's the only thing I can think of that I haven't checked. I thought that maybe it is running rich at part throttle and then causing me problems back at idle. After that, the only other thing I can think of is that maybe it has to do with my old rusted Mac 4-2 exhaust. I just don't know how.

[Spanner 1]

Did you operate the choke while the carbs were off?... When I got my K1 the choke , which has adjustable linkages between each carb, was all out of wack so that when choke was open ( off), the #1 carb was 'off', #2 was 'on ' 1/4th. #3 was 'on' 1/3 and #4 was 'on' 1/2  !!!  ... I'd be lookin' at dat.

[bjohnsonassoc]

Have you check compression, adjusted valves to spec and synchronized carbs. If you have done all of this and you are sure that all the passages are clear, make sure you have the correct main jets and they haven,t been drilled by a previous owner,that all of the clips are in the proper position on the main jet rods.  If you have done all of this and the floats are adjusted the proper height on both sides of the floats, make sure they are even side to side and not tweaked. then you can do a proper plug check with new plugs. I have found that the factory settings with stock air box and stock pipes will make your bike run smooth. Different air filters and pipe require some additional work, usually larger main jets and and moving the clips on the main needles down 1 click which causes the needle to let more fuel in quicker, works. The idle or low end jets and air screws set to factory will be extremely close even with aftermarket air filters and aftermarket pipe.
Assume nothing check everything and check it again.
I build my carbs on the bench with these guidlines and 95% of the time I only need to synchronize them and leave them alone, until the next tune up.

[bjohnsonassoc]

and yeah what he said should be checked also

[jimbojangles]

Well, I took the bike for about a 180 mile round trip drive to my dad's and back through the Ozark Mountains. Every time a strong gust of wind hit me the motor would cut out. After four stops on the side of the road without cell phone coverage I discovered I had a vacuum leak on Cylinder #4. After all this time of running rich, the thing was super lean. Unfortunately I didn't bring the tools to fix it and what should have been a 1.5 hour drive to my dad's turned into a 3 hour drive. When I finally arrived my dad was on the phone with highway patrol and my wife had been calling all the nearby hospitals. They all thought I had wrecked and died! I have some big apologizing to do.

It turns out that one of the carb boots was just a little loose and it only took 5 minutes to fix at my dads. I was never so happy to see Cylinder #4 running rich again.

So now I'm back to my original problem.

OK, I took the carbs apart AGAIN and cleaned all the airways as much as possible. This time I decided to switch all the internals from carb #4 (Rich cylinder) to carb #1. I don't have a way to check compression at this time. #4 is still rich. I'm thinking about upgrading to the Dyna ignition to get my timing just right and maybe get a bit stronger spark to see if things begin to clear up. Any other ideas?

[bjohnsonassoc]

I would think if it was an ignition problem it would show up on #2 since that is the same coil.Dont overthink it, just make sure everything is correct all of the settings, jets, screws, float height, and needle settings.
Check and make sure your chokes are all open the same too.

[nancy]

Hey Jimbojangles - snap! I have been wrestling one rich cylinder on my K2 750 for months now - number 3. I did all the prerequisite tuning stuff including several carb synchs - still no improvement. Turned my idle mix screws out another 1/2 turn on all pots - 1,2,4 are spot on and 3 is still sooty. So I pulled off the carb rack and removed all floats and jets and did a good clean with carb cleaner etc. Checked the float levels and they measured 26mm on all = correct, but when I picked up the rack - upside down, I noticed that number 3 float was 2-3mm higher than the others. Could not understand that - so re-measured the float setting = still 26mm. What the F? So I removed all the float valves and noticed that number 3 was odd - it was a newer part with rubber tip and the others were older brass tips. Swapped in an old but good brass tipped valve = eureka....now if I held the rack upside down, all the floats were level and all measured 26mm! Seems to me at a guess to be a diff in the spring pressure in the newer rubber tip part. Reassembled - and put on a new air filter while in there. Fixed. Grey/brown on number 3. Whhrroommm.
Good luck with yours..
Mark

[jimbojangles]

OK, so it's been a while since I posted, but I still haven't figured things out. However things have changed a bit. I had the bike running pretty well the other day so I pulled plugs just to check how things were really going. Now it appears cylinders 3 & 4 are running rich. How could that be?? I've also noticed that the problem seems to be worse when I have a full tank of gas. Maybe I'm imagining this, but it sure does stumble and pop a lot more on my way to work this morning after filling up. Cylinder #4 is still a the biggest offender.

So, do carbs 3 & 4 have anything in common other than the fuel line coming in?

Also, could the added pressure of a full tank of gas be overwhelming the float needles? I haven't seen anything come out the overflow tubes unless it sits overnight.

THANKS

[Spanner 1]

Still think it's your choke linkage ( like I suggested 6 weeks ago ).. haven't heard that you looked at it's operation... could easily be at least 1/2 way 'on', on #4 carb and now partially 'on' on #3 as well.........with the lever in the fully 'off' position.

[jimbojangles]

I did check this. sorry. All 4 slides open and close at the same time and to the same degree.

Since I began this post I did discover that the needles were two clips rich. That has helped tremendously when driving around at about 30 MPH. My only problem now is that it skips a beat every once in a while at idle and still fouls #4.

We were able to make it to a local bike rally and loved being one of the few people who weren't driving a Harley or a metric knock off. It's just these little things that are driving me crazy.

I plan to try to run some fishing line through the air passage ways this evening just to make sure they are open. Guitar string wouldn't make the bends.

[nancy]

Jimbo
I did some more work on my motor as I still was having richness problems and found number 3 float was out and needed redoing the float tang thing. This in spite of me doing the float level check TWICE before. What I did diif this time, is check the fuel level while carbs on the bike - didn't use a plastic gauge - I just emptied each bowl into a spirit nip glass and marked the measure for each carb. Found 1,2 & 4 to be very close but 3 was about 3mm lower. When I took the carbs apart I checked with a float measure - looked good. So I checked again with my glasses cleaned and leaned the rack (intact) so it rested on bits of timber so lean was just right to see the float valve tip just being kissed by the float tang. Sure enough found no 3 was off. reset it. Also found my number 4 throttle valve was scoured/scraped - swapped that out and I reset my needle positions to 2nd slot down - to match 120 mains I fitted. I now have 4 x grey plugs.
Mark

[jimbojangles]

OK, I think the bike just likes to screw with me. My ride home from work is mixed city and interstate for about 10 miles. The bike always does fine at higher speeds, the idle is just a little choppy.
Well I got stuck in traffic at the interstate exit and the bike spent quite some time idling. I actually turned it off a couple of times to keep it from getting too hot. When I finally made it home all of the plugs looked fine. #4 was only a little black around the edges. With as much time as I spent in traffic #4 should have been solid black with tons of carbon build up. So what gives?

I guess I'm not going to mess with success and leave everything alone for now. I just wish I know why it decided it was happy.

[tlbranth]

Pull the floats on the 'rich' cylinders and give them a shke. listen for gas inside. My K0 had 3 out of 4 leaking.

[jimbojangles]

I tried pouring out the carb bowls into a clear plastic cup and measuring and found that 4 was actually very low. I corrected this and my skipping a beat problem went away. I guess it wasn't getting enough gas. However the plug is fouling wore than ever. Carb #3 is also fouling pretty bad at idle. This is even with the idle screw 2.5 turns out. So I finally gave up and bought a can of carb dip on my way home and plan to completely tear the carbs down and rebuild them. No more messing around with spray can cleaner. When I took the carbs off last night after one last ditch effort adjustment I noticed something weird.

The carb vent hose between #3 & #4 was wet at the end with gasoline. What could cause this? and could this be what is causing me problems on #3 & #4?  Before anyone asks; I'm not talking about the overflow tubes. I'm taking about the tube the comes from between the carbs and runs up over the air box.

Either way I will still do the total rebuild, but maybe there's something I should look for?

[jimbojangles]

Oh, Tlbranth. My floats are the black foam? type. But thanks for the idea.

[HondaMan]

Set the air screws to 1 turn out, +/- 1/8 turn, no more, for starters.

Next, make dead sure the hose clamps at the head and the carbs are tight. On the troublesome one, consider changing them to wider clamps, as the hoses are old and hard. A small air leak on that cylinder's hose will cause dark spark plugs. I know that sounds counterintuitive, but it is true on these engines because of the type of carbs they have. 

[jimbojangles]

Will do. I didn't consider that because I thought a leak would only make it lean. That carbs are in about 100 pieces right now, but I will double check the boots when I reinstall. It may take a few days since I'm working 14 hours days for a while.

[Hondawggie]

OK, I took the carbs apart AGAIN and cleaned all the airways as much as possible. This time I decided to switch all the internals from carb #4 (Rich cylinder) to carb #1. I don't have a way to check compression at this time. #4 is still rich. I'm thinking about upgrading to the Dyna ignition to get my timing just right and maybe get a bit stronger spark to see if things begin to clear up. Any other ideas?

Did you try moving the float out of #4 and a known-good float from another carb into
the #4 carb?

Another thing you can try, temporarily:  switch #4 to a hotter plug and see if it lightens up.

I have another bike I'm working on that came with 107.5 main jets stock from the factory.
They had been drilled out -- to about 155.  Didn't find out until I went to the parts counter
to buy a new set of mains -- then discovered they'd been drilled.  The only way to tell was --
I compared the inner diameter of my "107.5" with 115s that I wanted to move up to -- and
found the 115s were SMALLER....then I checked 122.5...still smaller than my "107.5".....etc.
(The turkey failed to check the rubber intake boots, which were cracked and leaking, and
in trying to fix his lean problem he must have went to town on the stock 107.5's.)


The problem with drilled-out jets sitting in a mechanic's box of his collection of 'rejetting stuff'
is you can't tell from the label on the jet anymore.  One carb might get a drilled-out main jet
-- so maybe do a side-by-side comparison of the inner diameters of your #4 main and the other mains.  LONGSHOT?   Yep.  But a guy who drills jets to save money will also save everything like a packrat after finishing one project bike and possibly mix 'em up by accident on his next project.

[Duke McDukiedook]

Check static float height of gas with a clear tube hooked up to the drain screw to find out the true static gas level.