Topic: 78 CB750 Rich - I did a search already

[rgwebster3]

Okay guys...I need some help. I have been working on this for 2 months now trying to combat a super rich condition. Main jet and needle position seem to be okay. 112 main. Dyno acceleration tests show that full and 50% part throttle sitting at about 13.1 /1 A/F. Now, steady state cruise part throttle (70mph loaded test) & idle show super rich. Both read at about 10.0 /1 A/F. Running the stock 35 pilot and the mixture screw set at 1.5 turns out. Bike does have pod filters.

We have stripped the carbs and dipped them in Chem-Dip. Rebuilt and re-installed but still the rich condition. Bought a second set of carbs. Cleaned and setup the same thinking there was a problem with the 1st set. Installed and still the same problem with the second carb.

One thing I notice is that the idle set knob that raises the mechanical slides must be screwed in almost all the way for the bike to even idle. At this point any mixture screw setting has no effect when performing the idle mixture set as described in the manual. I can run them all the way in or go 5 turns out and nothing changes on the bike.

Question..I know that pod filters lean the mixture and provides very poor idle or off idle throttle response. What I think is happening is that because the bike has the pods and stock pilot jet...I am having to raise the slide height (idle set) just to get the bike to run. I think that by having to run the idle set screw almost all the way, thus raising the slide, thus raising the needle out of the needle jet, the bike is now pulling fuel from the jet needle in addition to the pilot circuit. If I back out the idle set knob, lowering the slide, the combination of the pilot circuit being to small (35) for the pod filters actually leans it out so bad that it will not run. What do you guys think?

To test my theory, tomorrow I am going to set the idle set screw about half of it's travel and then try to start the bike with the choke on. If it will run with the choke on but die at idle with it off...then my problem should be to small a pilot right?

For those of you running pod filters and have about the same setup can you please share you pilot jet size, main jet size, mixture screw turns, and idle set turn (from slides all the way down)

Thanks all for any help you can provide.

[cleveland]

I have a 77 CB750K that I switched over to pods and I just ordered jets for it.  The jets are 125's with 4 into 1 pipes and I am hoping this solves the same problem you are having.  The jets should be here this week, so I can let you know if that made the difference.  Are your pipes stock, or after-market?

cleveland

[eldar]

Pods should not require screwing in the idle knob likie that. It is something else causing the issue. Chemdip is worthless when it comes to the slow jets.  You NEED to pull these out and use a small soft wire such as brass, to clean them. There is a hole through them and a couple of holes on the sides. If these are not MANUALLY cleaned, they wont be clean.  yank the carbs, pull off the float bowls and floats. Take a piece of cloth and a pliers and grip the slow jet tightly and ROTATE and PULL. They should come out. If not, you can try rocking them back and forth BUT be VERY careful as you could warp the housing requiring adhesive to hold in the jet then. Once those are clean and you can see light through them. Make reassemble and make sure your accel pump is spraying fuel from the 4 brass posts at the front of the carbs, on the airbox side, into all 4 carbs.

[scondon]

Nice thing about having the pods is it's easy to check the slide height with the carbs on the bike

    I see your thinking on the idle screw causing the bike to run off the mains and therefor causing a rich condition but the slide goes up as well and should allow more air in to compensate. If you were to raise just the needles(by adding washers-'78 carbs, or changing the clip position-'77 carbs) then this theory would make sense.

     A couple things come to mind. One is that your choke arm may be hanging up on the pods causing the choke to be partially on at all times. Remove either #1 or #4 pod and check to see that the butterfly plates are perfectly horizontal and not tilted in any way.

     The other thing is the accelerator pump. If you take off a pod and twist the throttle you should see a geyser of fuel shoot out of the pipe in the back of the carb. This can certainly show up on your AFR reading as a rich condition, I got around 10/1 AFR for a bit as well but that was during a high speed, full rpm range dyno. This should have burned away in a few seconds during your steady state cruise test, but if your butterflies aren't fully open then I imagine it could take much longer.

      Your ignition timing and plug condition can certainly affect how well your bike burns fuel, not sure if unburnt fuel changes the AFR readings so just a thought.

     You should be running lean, lean, LEAN with those carbs and the fact that you've tried two different sets with the same result has me puzzled. Some more information on your bike would be helpful.

  '78 F or K?
  Stock engine or modded?
  Stock exhaust or aftermarket?
  What type of ignition/coils?
  etc.....

[fourplay]

I hear one needs to go up two jet sizes on carbs. I am about to put pods on my 77' also.

I have a Kerker four into one, so I imagine I will have to rejet.

Where can you get jets ? Is there an online place to get them ? All I seem to find is gaskets or packing kits.

I still need to rebuild my carbs. My half-assed attempt still left me with leaking or overflowing carbs.
I have a can of carb dip with the basket. Gonna do a complete rebuild after I get another accelerator pump.

Eldar, thanks for the low speed jet cleaning tip.

Vance

[eurban]

I'm with Eldar on this one.  In my experience the stock 35 idle jets on the 77/78 carbs are more than up to the task of providing fuel for a podded bike.  While you have your carbs apart to pull and clean the pressed in idle jets, bench synchronize them.  This means that at a given throttle setting all the slides are at the exact same height.  Adjust the idle speed knob so that a convenient object like a small drill bit will fit between the slides bottom and the bore of the carb on the #2 (non adjustable) carb.  Now make the others all the same.  Check that the "idle speed bump mechanism" that comes into effect when the choke is fully on is adjusted properly.  You may also want to use clear tubing on the overflow tubes to check the level of fuel in each of your carbs.  Simply connect the tubing to the brass nubs, hold the tubing up along the carb bodys and then open the drain screws a  bit  (this works on 77/78carbs only) The height of fuel in the tube will indicate the actual height of fuel in the bowl. They should be the same and just a bit below the bowl to carb seam.  Bottom line is if the carbs are set to stock specs / baselines and functioning properly that your bike should idle OK even with pods and that the idle mix screws should have an effect.  Something ain't right. . . . .Fourplay- don't worry about changing jet sizes until you do a whole assed job of carb cleaning and adjusting:)   Cycle recycle has Keihin jets I believe but do yourself a favor start with a functioning set of carbs before you fiddle with carb tuning . . . .

[rgwebster3]

I failed to mention that when we dipped the carbs...all the jets were removed including the pilots. We also sprayed the passages and jets with carb cleaner and shot compressed air through the passages. All are clean and clear including the 35 pilots that were removed for the dip. The choke butterflies have been checked and all are open completely. The accell pump works fine and in my opinion is not the problem. During idle or steady state cruise you can see that fuel is not coming out until the snap of the throttle. We have checked the fuel level in the float bowls using the tube and it is where you said it to be.

We did a complete top end rebuild on this bike and also installed new rings. Compression is excellent on the motor. To eliminate the ignition system...we replaced the existing Dyna electronic ignition and Dyno coils with NEW ones....set timing and still rich. No change. So ignition is not the problem.

What is meant by the  "idle speed bump mechanism". Are you referring to the accelerator pump?

As you can see we have pretty much exhausted every idea we have. I appreciate everyone's input thus far...but I still don't think we have nailed the problem. Please help.

How many turns in do you guys have your idle screw set?

[eldar]

Just because you  blew air through the jet does not mean it is clean. Many here have done that only to find they were still plugged. Also as eurban noted, the choke could be getting stuck and not opening all the way. As for the fuel screws, they start at 1.5 turns for the K and 1.75 for the F. Out makes the carbs RICHER while in makes them leaner. Your 112 mains are larger than stock also, which may contribute to this issue but not likely.


Just to add, I had this happen too.  I got a little set of jet cleaners from honda. Ran those wires through and set my accel pump and floats and I was golden. Dip and air just does not do it completely for the slows.

[Roach Carver]

i second eldar. Take the idle jets off and physically place a small wire through them. the orifice is tiny and I would bet money they are stopped up.

[rgwebster3]

What I meant was....we took the pilot jets out before the dip....they were removed when we sprayed them with carb cleaner during the re-assembly....I physically verified that the pilots were not plugged....the emulsion holes in the pilot were open...you could see daylight through the pilots.

Also, with the pilots removed...when I sprayed carb cleaner through the pilot seat, carb cleaner came out through the air jet in the front of the carb, through the pilot orifice in the throat of the carb behind the slide, and through the mixture needle seat....this is what I meant about the passages being cleaned.

What is meant by the "idle speed bump mechanism". Are you referring to the accelerator pump?

[eurban]

Not the accelerator pump.  When the choke is fully applied a cam bumps the idle speed up a tad to keep the engine from stalling on full choke.  If this is way out of wack it might be messing with your idle speed setting but not real likely.  Again your setup should work at least halfway decently with stock functioning carbs so you need to figure out what is not up to snuff.  Hows your ignition system?

[eldar]

Yeah there are only 2 things that could be affecting this.  Carbs not cleaned fully or assembled correctly, or your ignition is not functioning properly. Those are really the only 2 things that could be causing the issue. 9 times out of 10 it is the carbs not set up right or clean.

[Lumbee]

...just to throw my 2 cents in, my chop is a 78K, and I'm running pods.  I have 125 jets.  Never did a dyno, but the plugs are always golden brown.  I agree that running pods, and 110's, you should be wayyyy lean.  What do the plugs look like?  I'm not doubting the dyno numbers, but just for a sanity check, what do they look like.  Couple of other things.  You mentioned the slides being screwed all the way in, all the 77/78 carbs I've synced always ended up with the 1, 3-4 slides screwed way down, so that sounds about right.  I'm assuming u'r using a sync tool to do this, and not just "how it sounds".  If u'r not using a sync tool, get one.  Just another thing, check and be sure when u assebled the carbs, that you put the slides back in the correct way.  That is with the curved portion of the slide facing toward the back of the bike.  If you put them in backwards, that would be a problem...keep us updated...

[rgwebster3]

As I mentioned...the entire ignition system was replaced with new just to eliminate ignition from being the problem. It was still doing the same thing. What curved portion of the slide are you talking about? The slide is curved on both sides. Also, where is this cam that you guys are talking about?

Thanks again all.

By the way...the plugs are black and sooty....we are fouling them out so bad that we have replaced the sets five times.

[Lumbee]

The slide is curved on both sides

...not so!  The slides have a curve on the bottom of one side.  If you pull the carbs, and look in the front (facing the engine) the slide should be flat, if you look from the rear (intake from the air filter/pods) there should be a curve...

[scondon]

If the slides were turned 180 degrees it would certainly explain why the idle screw needs to be turned in so far to get the bike to run. There is a small hole in each slide, next to where the needle holder screws in. When the slides are in the correct position these holes are all on the same side, left I think.

    The "cam" is located just above the choke arm at the rear of the carbs. There is a small adjustment screw that should have a little yellow paint still on it. Lift the choke arm all the way while looking down at the top of the carbs and you can see the linkage move at the last bit of choke travel.

[eurban]

So are some/ all of your slides in backwards???

[Bob Wessner]

Are the carbs designed differently on the later bikes? I ask, because on my K0 I accidentally let one of the slides rotate 180 degrees while installing and the needle doesn't even line up. The one slide was essentially at full-throttle because it was resting on the needle tip. Very hard to tune that way, by the way.

[scondon]

Are the carbs designed differently on the later bikes? I ask, because on my K0 I accidentally let one of the slides rotate 180 degrees while installing and the needle doesn't even line up. The one slide was essentially at full-throttle because it was resting on the needle tip. Very hard to tune that way, by the way.

  Yes, the slides are different on the '78(maybe '77 too). There are no slots on the side to line up with the pin in the carb body like on your carbs. Very easy to install backwards if you're not paying attention.

[Bob Wessner]

Pretty clever of me to thwart Honda's nearly foolproof method of installing the slides correctly, eh?

[mrbreeze]

You know......this is a great example of why I like this forum. I'll bet Lumbee was right about the slides being backwards. Bob & I wouldn't have caught that because ours can only be installed one way. If you get enough heads together when these problems arise,it can usually be worked out. Especially when dealing with systems that changed through the years. When RG said there is a cut-out on both sides of the slide, I said to myself......SELF!!!!......WTF??. After Lumbee's post, I  thought,OH!!!!.....OK!!!

[medic09]

You know......this is a great example of why I like this forum. I'll bet Lumbee was right about the slides being backwards. Bob & I wouldn't have caught that because ours can only be installed one way. If you get enough heads together when these problems arise,it can usually be worked out. Especially when dealing with systems that changed through the years. When RG said there is a cut-out on both sides of the slide, I said to myself......SELF!!!!......WTF??. After Lumbee's post, I  thought,OH!!!!.....OK!!!

Sounds like you're off your meds.

Are you off your meds?

Has "SELF" been answering, or carrying on a different conversation?

 

[mrbreeze]