Topic: Continued CB750 K0 tuning experience

[zoo mob]

I've had my 750 K0 back for about a week now and though its been great being able to get it on the road a couple of times, it is not running close to properly. I've been doing some reading of the faq's etc. on tuning, but am still kind of struggling with figuring out my approach.

The bike will keep a decent idle, and runs on all 4, at 1/4 throttle or just a mild cruise, it runs horrible, constantly having "poof,poof,poof" type backfires that seem to be coming through the carbs (there is no apparent gas smell in the air filter however). At half throttle, it runs a bit better, but still kind of blatters and hesitates until I really open it up and it blasts past 5k rpms where it seems to actually come to life and run well.

Yesterday I did my first proper plug chop; I warmed the bike up, swapped out for some new D8EAs, rode it through the city for a few miles through the city to an open stretch of highway and proceeded to run it at 6k or so in 5th gear for a couple of miles before cutting the ignition and pulling the clutch. Upon inspecting the plugs, all four had a significant white residue on the tip.

I know that that means it is running lean, but how should I proceed? The bike is running a stock airbox with an Emgo filter which should be a bit more restrictive, unfortunately, the last time the carbs were put back together, it was by someone else, so I don't know what the needle settings are, or what jets its running, I'm pretty sure that they are stock though.

Should I pull the jets and swap in bigger ones right away? Go to a different plug? Thanks for your help, I finally feel like I'm getting somewhere with this thing.

[nippon]

stock jet size is #120. In which position did you hang the jet needles? If not already there, try the middle position with #120 jets.

nippon

[Johnie]

When you did the vac. sync did you do it at about 3,000 RPM. If you have the OEM KO carbs you have to sync the low idle with those big screws on the side of each carb and the high idle with the carb top cable screws. It is pretty easy to get the jets out to check the sizing with the bowls off. It is also very easy to check the needle position on the KO carbs. You can do it all without having to remove them from the bike. How do you feel about buying an OEM air filter there. It was jetted for the OEM. The EMGO could be causing some issues there.

[zoo mob]

I've got no problem going with an OEM filter, just had the Emgo lying around, but it was my understanding that they were more restrictive so I'm assuming that's not the problem. You may remember me talking about somebody else doing the sync and having the bike forever, so it looks like I'll do it over myself; not something I've done before, but I'm sure I can figure it out.

I'll have to check jet sizes and needle position when I get home. Wish me luck.

[Johnie]

Keep in mind the sync is the last thing you do...after the valves, points, condensor, timing. I take it you have a reliable sync gauge as you will need it. It took me a few times to sync my KO. Be sure to put a fan on the motor to cool it while doing the sync. I agree...if the previoius owner did it that means he did it...not necessarily did it right. I would not rush out to swap jets since the bike was made to run on supposedly what you have there...120 mains and 40 slow. Air screw 1 turn out from lightly seated. I would for sure check the needle position though. You just do not know what the PO did there. When you get these bikes it seems you have to go through it and fix everything someone else messed up. I bet once you get those sync gauges on you will find the high idle out of wack. Let us know how it goes.

[zoo mob]

Got the bowls off and the jets are indeed 120s. How can you check needle position w/o removing the carbs from the bike? Can you just eyeball it from below?

[Johnie]

Got the bowls off and the jets are indeed 120s. How can you check needle position w/o removing the carbs from the bike? Can you just eyeball it from below?

To check the needle position you have to unscrew the top of the carb. Just like unscrewing a peanut butter jar. When you do that you will pull out the slide and spring with the needle at the very bottom. Be sure to pay attention to which way the slide comes out as there is a front and back on them. Do them one at a time. Once the slide is out you will have to bunch up that spring so you can slide the cable off. Then there is a holding clip on the top of the slide you remove. Then you can see top of the needle and pull it up. Then take a look at the needle and be sure the clip on the top of the needle is in the middle position. There are 5 clip positions total. To be honest, if the first one you try is in the middle I would expect the others are too. I would be surprised if someone has those set in different spots. You can decide if you want to check them all. And just to be sure your KO carbs should look like the ones in the pics. Sometimes people put K1 carbs on the KO so just want to be sure we are talking the same carb set-up here.

[zoo mob]

I wish they were K1 carbs! Just pulled the plugs again to switch to B7EAs for a ride tonight, and 1,2, and 4 are lean, and 3 is rich (this is after a low-speed ride. Hmmmmm.......?

[zoo mob]

All right, took a week off from messing with the sled; after tonight, I feel like I have an idea as to where the rich condition is coming from.......how do the airbox side boots fit to the carb and to the airbox? Should the boots be loose to the box or tight? If they are supposed to be tight I'd considered using some RTV to make a seal as only one seems to snap into place. Pics below.

[Johnie]

The positioning is correct as they do snap into the holes, but you will notice there is a specific angle to each so they match up to the angle of the carbs. When I changed mine out I marked the top of each one so I knew how the new ones should seat. When you put them back on the carbs just try to match the angles by spinning them as needed. You will notice there is 2 seams to each boot. One of the seams goes up on each boot. You will have to figure out which one it is on each by the carb angle needed.

[Spanner 1]

With respect Zoo and posters, but air leaks at the air-box will not cause rich/lean condition.... the white deposits on the plugs you are seeing are rich deposits at lower throttle running finally burnt-off at higher speed/ throttle.... the prob. can be fixed by taking-off the carbs ( sorry ) and verifying the slow air-bleeds in each carb are clear... thats my fix.... anyone else ?

[Johnie]

"how do the airbox side boots fit to the carb and to the airbox? Should the boots be loose to the box or tight?"

The boots should be tight. My KO boots were old and very loose so I bought new ones from Honda and they have a very tight seal now.

[fishhead]

When you did the vac. sync did you do it at about 3,000 RPM. If you have the OEM KO carbs you have to sync the low idle with those big screws on the side of each carb and the high idle with the carb top cable screws.

 Just to elaborate on Johnie's point..

   The idle speed sync is done with the big screws on the sides of the carbs. The large screws on the sides of the carbs adjust the throttle/slides opening at IDLE ONLY(adjust these screws to get the same vacuum reading at idle only).

 After the idle sync is done, the individual throttle cables need to be adjusted to get the "anything other than idle throttle positions" synced . This is adjusted by the length of the cables going into the carbs (adjusted by the nuts at the tops of the carbs that the cable runs through). The cables need to be in sync in order for the 4 slides to open in unison. Having the cables at different lengths(slides opening at different times/heights) can cause the weird running that you are experiencing, especially at the lower throttle positions.

 After doing the Idle sync, keep your gauges hooked up to the bike. When you give it any/slight throttle, the gauges should rise in unison. If not, the cables will need to be adjusted to get the throttle(s) to open in unison. Generally, you want to adjust (synchronize) the throttle cables at 3,000 Rpm as this is a "typical-ish" cruising Rpm.

 You do want some slack in the cables at idle(1/8 inch or so- or see what the manual says) because if there is no slack in the cables, the slides may not return to the idle position, the (tight)cables could keep the slides from coming back to a rest on the large screws on the carbs (the idle sync screws) causing a high idle and inoperative idle(sync) screws.

[zoo mob]

With respect Zoo and posters, but air leaks at the air-box will not cause rich/lean condition.... the white deposits on the plugs you are seeing are rich deposits at lower throttle running finally burnt-off at higher speed/ throttle.... the prob. can be fixed by taking-off the carbs ( sorry ) and verifying the slow air-bleeds in each carb are clear... thats my fix.... anyone else ?

If the white deposits are in fact soot being burned off at higher speed, what do actual lean conditions look like?

[HondaMan]

If your inlet hoses have shrunk that much, I'd bet $5 that your carb-to-head hoses are hard and leaking. This will cause some cylinders to run rich while others run lean, and no amount of tuning will fix it. The big clue: the soft 'poof-poof-poof' you are hearing is a vacuum leak, causing the fuel mixture to alternate, in one or more cylinders, between being lean and then normal.

There is a simple, 100% solution (short of just buying new hoses on both ends of the carbs). Go to Amazon.com and buy 8 ounces of Wintergreen Oil, then go to Ace Hardward and buy a quart of Xylol. Remove all 8 rubber parts and soak then for 2 days each in the mixture of wintergreen and 1/2 of that quart of xylol (I use a 2-lb coffee can, fits 4 parts at once: cover the can). They come out bigger than stock at first, and very soft. Let them sit for 2 days and they shrink back to supple stock dimensions. I've now fixed 5 bikes this way in the last year.

New hoses are not all that bad: Z1 Enterprises sells them with less shipping than Honda. For about $150 you can get an entire set on both ends.

The clamps: chances are, yours are pretty stretched by now. Look closely at the area where the screw pulls the faces shut: they will be bowed inward. Flatten this out, then get some #4 washers and trim them oval to fit within the sides of the clamps, and install on the screws. This will give better closure and more life. (Or, just buy new clamps...). 

[zoo mob]

Thanks, I'm thinking that's the next thing to do. The carb to engine hoses seem ok, I tried spraying carb cleaner on them while it was idling and did not see any bubbles.

[zoo mob]

Another thing; just noticed that the #1 & #4 carb tops are switched, could this be part of my problem?

[HondaMan]

Thanks, I'm thinking that's the next thing to do. The carb to engine hoses seem ok, I tried spraying carb cleaner on them while it was idling and did not see any bubbles.

Ummm...you're not looking for bubbles when you do that, but for an engine speed change. If the carb cleaner fluid gets into the airstream, it makes the engine either stumble or speed up, depending on what kind of carb cleaner you are using.. It must be: engine held at steady speed (slow), then slowly by generously spray fluid around each joint of each connection on downstream hoses. Wait about 5-10 seconds for results. Spray also along bottom of hoses: they often split there with tiny through-the-rubber cracks.

Personally, I don't use this method: it makes a big mess of the engine below and doesn't seem to work reliably. I've done it before with "good" results, only to later disassemble the carbs for other reasons, then found obvious leaks as dust trails into the spigot side of the head. 

The method I most often use it to install wider hose clamps, crank them down, and see if they make a difference. If so, then I start changing hoses or soaking them.

[zoo mob]

I'll give that a whirl, the evergreen oil is on the way if needed.

[Don R]

WINTERgreen maybe wintogreen but not evergreen

[zoo mob]

Yeah, that's what I meant.