Topic: CB750K1 carb problem - SOLVED -

[Johnie]

Been working this problem for a year now off and on. A little history...last spring when I got this K1 I went over the entire bike with the usual points, condensors, plugs, caps, timing, valves, etc. Deracked the #7A carbs and soaked in Gunk carb cleaner, added new 120 jets and new 40 slows. Cleaned the needles and left in middle postion clip. 26mm floats and new float needles and seats. Air screw 1 turn out. Bench synced and then Morgan synced. Tank Red Coated and clean. Standard air box with OEM filter. So I did the stuff you would ask and here is the problem: the bike starts and idles perfect and runs like the wind at all speeds. But when I am at about 50MPH+ and try to hold that speed I can feel a loss of power. At the lower speeds no loss of power...only about 50 and holding is when I can feel a slight miss. If I goose it at 50 or 60 she takes off good. Only trying to hold and cruise a higher speed I notice the loss of power. Moving the choke around at that speed makes no difference with the problem. I have the carbs off again to try to find this problem. I did check the small holes opposite the air screw holes and they are clear when I blow through them with the straw from the carb cleaner can. A side note, when decelerating I do get some backfire and small puff backfire only on #1 exhaust when sitting at stop light. I do have another rack I may stick on, but would sure like some tips on what I may be missing here.

[Gordon]

My first suggestion would be to do a plug chop at the speed and throttle opening where the problem occurs to find out if you have a mixture problem, but you already have the carbs off again. 

[Magpie]

Johnie, this is a stretch but I had a problem with mine years ago that seems similar. I had routed the carb overflow hoses through the little bracket on the back of the motor and then between the swing arm and the motor. They seemed to get pinched there at times causing the bike to seem to starve for fuel. How have you routed the overflows? Maybe......
Cliff.

[Johnie]

Johnie, this is a stretch but I had a problem with mine years ago that seems similar. I had routed the carb overflow hoses through the little bracket on the back of the motor and then between the swing arm and the motor. They seemed to get pinched there at times causing the bike to seem to starve for fuel. How have you routed the overflows? Maybe......
Cliff.

Yes, I did have the overflows in the bracket on the back of the motor. I also should note that if I am going into a headwind it seems worse. My vent tubes are stock and over the air box. And when I did try to change the air screws from 1 turn out to more and less there was no difference.

[754]

 Are needles in stock position?
 Did you try puling aircleaner, when it was doing it?

[Spanner 1]

Hi Johnie, my K1 is set-up very similar to yours except for HM341 pipes with the last chamber drilled-out ( on Hondamans advise), same 7A carbs too. Occasionally when running steady as you describe I get a loss of power that feels like I'm running out of fuel, no misfire tho'. With a little increase in throttle it goes away... don't know if that's the same effect ?   Also, what grade of gas do you use?
Could be caused by a 'slow' gas cap vent ?

[Johnie]

Are needles in stock position?
 Did you try puling aircleaner, when it was doing it?

Yes, needles in stock middle position. I did try the needles in #4 position and it was a bit worse. Never did try it without the aircleaner. You mean take the box completely off or do you mean just take out the filter and try it.

[Johnie]

Johnie, this is a stretch but I had a problem with mine years ago that seems similar. I had routed the carb overflow hoses through the little bracket on the back of the motor and then between the swing arm and the motor. They seemed to get pinched there at times causing the bike to seem to starve for fuel. How have you routed the overflows? Maybe......
Cliff.

Cliff...I took a closer look and my K1 does not have the bracket for the overflow tubes. My tubes go in front of the swing arm and in front of the center stand post. They do not seem that tight but I will pay attention to it when I get them back on.

[Johnie]

Hi Johnie, my K1 is set-up very similar to yours except for HM341 pipes with the last chamber drilled-out ( on Hondamans advise), same 7A carbs too. Occasionally when running steady as you describe I get a loss of power that feels like I'm running out of fuel, no misfire tho'. With a little increase in throttle it goes away... don't know if that's the same effect ?   Also, what grade of gas do you use?
Could be caused by a 'slow' gas cap vent ?

Spanner...it does just as you describe. More of a loss of power rather than a miss. And it does feel like a fuel starvation issue in at least 1 carb. I use octane 87 Mobil. I thought I checked the gas vent, but will check again just to be sure. All great tips. I would really like to get this thing right.

[Johnie]

And like I mentioned, it seems worse if I am getting a strong head wind in my face. I thought it may have something to do with the vent tubes but they are stock tubes over the air box which was the OEM placement. They are clear too. I did see the PO had removed or lost the rubber grommet between the 2 halves of the air box. I wonder if he took it out for a reason? I put the new one back in there.

[754]

What I meant was drop the element. That should lean it slightly if problem get worse, shows which direction to go.
 Are the pilots bigger? Are you running HM300s with baffles out?

 Just thinking with no exhaust change, you may not have to change jetting..

 Does it change as you move your knees from against the tank to away rom the tank?

[Simpson]

About to suggest you move your needle position to 1 or 2 because it sounds like as the engine heats up the a/f ratio is too rich. After reading your comment about the head wind, then I would suspect a air box issue causing a lean condition.
I would experiment with both and see if you can change the engine performance.

[754]

 I find it a bit odd as they are usually well beheved with the box on. I did not mean leaving filter out(wouldnt bother me) rather induce a leaner condition mommentarily to see how it affects the glitch, to  help diagnose.
 I will say this, we have seemed to find in the past, that a very fresh engine may like certain jetting, but at a much higher mieage, may runned better leaned a bit.
 
 But I am not sure of the mileage on it or wether the baffles are pulled out.

[Johnie]

I find it a bit odd as they are usually well beheved with the box on. I did not mean leaving filter out(wouldnt bother me) rather induce a leaner condition mommentarily to see how it affects the glitch, to  help diagnose.
 I will say this, we have seemed to find in the past, that a very fresh engine may like certain jetting, but at a much higher mieage, may runned better leaned a bit.
 
 But I am not sure of the mileage on it or wether the baffles are pulled out.

Miles on the bike is 20,000. I am running the HM300's with OEM baffles in place. I have not tried to move my knees around to see if it changes. I have the 4 overflow tubes about 2 inches below the center stand post in fast flowing air. Could that be to low and maybe causing some problems in the carbs from the air flow at those higher speeds. Maybe a suction type effect on the bowls? Keeping in mind that city driving or under 40 or so it is fine. The carbs are sitting on my bench so I can digest your ideas and move from there. I will be checking float height again and maybe run a wire through some of the orfices to be sure all is clear. Once I get them back on I can try the bike with the air cleaner removed to see what response I get.

[sandcastcb750]

If your fuel mixture is bad, you might see a yellowing/blueing of the head pipes. Then again, you may have an ignition problem too. You could have a bad advance mechanism at the points/condenser area.

[Johnie]

About to suggest you move your needle position to 1 or 2 because it sounds like as the engine heats up the a/f ratio is too rich. After reading your comment about the head wind, then I would suspect a air box issue causing a lean condition.
I would experiment with both and see if you can change the engine performance.

If I have the positioning right as far as the numbers go, I did move the needle last summer from #3 to the next position to move the needles higher one notch thinking it needed more gas. Is this the 4th position. #1 being the clip slot on the top, right?

[hondaseven]

I had a similar issue with my k1, it was driving me insane,in desperation I swapped the carbys off my k4 and it ran perfect.I have felt the same surging as you are,and in my case it was the two halves of the airbox not coming together as they should letting too much air in at a steady throttle,try another set of carbys.

[Hinomaru]

Have you visually checked the fuel level in all four float bowls?

That is, removing the float bowls one by one and checking to see how much fuel is present. They should be close to being full and have equal amounts of fuel for smooth high speed operation. 

[Johnie]

Have you visually checked the fuel level in all four float bowls?

That is, removing the float bowls one by one and checking to see how much fuel is present. They should be close to being full and have equal amounts of fuel for smooth high speed operation. 

I have checked the bowls and all were full. I also checked float levels for a 3rd time just to be sure 26mm. Plugs are nice and light brown with no oil residue. Header pipes are chrome with no discoloration. Looks like a process of elimination here. I could also put on some extra carbs to see what they do. They are set up to OEM specs mentioned above...needle in middle position, air screws 1 turn out, mains are 120's running OEM HM300's with baffles and OEM air filter in place.

[Johnie]

OK, I think we are making some progress here. I was just blowing out the jets since I had the bowls off. Checking the brass float height with the gauge Myles (mycb750K6) made for me. Found a brass float with gas in it. Then another that either has a little gas in it or else it is solder in there. But one leaking for sure. Funny thing is the carb never spit out the overflow. I had this same problem on my KO and it spit on the overflow. So now to decide if I buy brass for $17 or plastic for $37 each. Or try some used plastic floats I have on a set of extra K4 carbs and get rid of the brass completely. But...could this cause the loss of power issue? With a sinking float it would put to much gas in the carb where it seems I was not getting enough?

[Spanner 1]

The minimal gravity feed of gas on our bikes is weird.... all kinds of strange effects can happen if one carb will allow a bigger/longer flow to it, as it would be in your case, Johnie. Even tho' there is gas in the float it will shut-off with a higher fuel level and a higher fuel flow to the motor on that cyl. causing that cyl. to be 'dominant' and causing low fuel flow to the other carbs....IMHO.
BTW... I have a mix of brass and plastic floats, works A.O.K.

[Johnie]

Thanks Spanner...I plan to get these carbs back on today and give it a test. I will report back...

[Old Scrambler]

Spanner is onto it with the fuel flow............What size inside diameter are your fuel lines?  Honda actually made the lines smaller to increase fuel flow at road speeds. Something to do with gravity and the flow-rate when under power. Static volume sitting in the lines takes more power to make it flow. The slightly smaller line size makes for a higher-speed straw and results in more fuel being delivered when its needed!  Another aspect of this problem is the amount of fuel in your tank.

[shorterdanny]

how spooky

this is almost exactly the same symptoms i have with my k7 750...

50-60mph get a apparent power loss, not feeling like a miss fire, then power through and clears up.
appears worse into wind..
idles nicely at about 1000rpm
slight backfire when low rpm/overrun

have stripped the carbs now have 125 mains and strangle enough float no 1 is on its way out as well...as well as CMS electronic ignition. pod air filters, and a 4 into 1 exhaust.

with me though this only happens 75% of the time..

tomorrow going to recheck the carb insulators, tappets, and plugs (inc plug chop)

i was thinking that due to the problem being intermittant that the problem might be electrical loose connection coil giving up, but as spanner says feelsmore like fuel starvation, will try running with the gas cap off tomorrow, also using 95 octane fuel. but i am still happy it is usable. loving the feel of an old bike rather than my modern crotch rocket..

[Johnie]

So I got the high speed hesitation fixed and you will not believe what it was. Here is the deal. I know for sure there was at least one brass float leaking. So I did replace all the brass with plastic. Put it all back together and took a test ride. Ran great until I hit the head wind again which was pretty bad today. Then had the same hesitation at 50+MPH. I removed the OEM air filter element as suggested and put the air box back together for another test. No difference. I know earlier in one of my posts I inquired if the overflow tubes hanging down below the center stand post in turbulant air could cause a vacuum effect on the carb bowls. So as you can see by the picture I tucked those 4 overflow hoses up over the top of the center stand post and gave it a test with the air filter element still out. No hesitation all the way to 85MPH into that head wind. I then replaced the air cleaner element and had no hesitation. So it was the positioning of the overflow tubes in that turbulant air causing either positive or negative air pressure in the bowl area.

Now I have to figure out just how to run those tubes as I am not keen on bending them over the center stand post. Maybe put them behind the lower case? It was a great learing thread here. Thanks for all your posts and help. Click the pic to see how I moved those tubes...