Topic: Carb tuning on a CB 750 K1

[Kevin D]

It runs nearly perfect when cleaning Sparkplugs but this is good for 20 to 30 kms

Cleaned plugs very often

Phillippe, that was my experience also when I had the 120's in. City driving would cause the plugs to foul and sputter on accelleration. A high speed blast on the expressway, say, 70mph or better for 15 minutes would clean the carbon out and restore smooth accelleration.
 It is some work to change the needle clips, so I haven't done it.

Thanks for the photos. My bucket list includes motorcycling on all of the best roads and I think maybe you have one there. But for the meantime I am content to look at photo of all the best roads. 

[Mushu]

I received Denso sparkplus but I will not mount them until I get 115 main jets.

I received a gauge to measure pistons compressions.

I did three tests :

• Engine Cold
• Engine hot
• Engine hot adding oil on the piston

------------------------------------

Piston 1

Piston 2

Piston 3

Piston 4

Engine Cold

136

85

100

140

Engine hot

125

110

120

120

Engine hot with oil

160

152

140

215

As the pressure is higher with oil, it should tell there is no valve leak even on the valve guides. Is this diagnostic correct?

Pressure values are to low. It should be between 145 and 175 PSI from Honda workshop manual.

I was wondering to see lower pressure on pistons 1 and 2 to explain my carbs 1 and 2 problems but it is not the case.

Anyway, I will have to rebore and change pistons/rings. Is my diagnostic correct?

From Hondaman book, I saw that tee providing fuel on carbs 1-2 and 3-4 may leak fuel or air giving problems at 3000 rpm.
I will check o rings which are on the tees when putting new 115 main jets.

[Henning]

Mushu, a couple of things:

Firstly, you have the air screws for cylinders one and two quite far in, and in is rich, as you well know. Also, you are fouling plugs on one and two. Putting these two facts together suggests to me you need to turn the air screw out to one turn, ignore the popping, take it out for a good  run and see if it clears up the plug fouling.

Secondly, carbs one and two have two things in common - the fuel line and that small vent thingy that goes up over the airbox. So maybe the fuel line is routed badly so you aren't getting enough fuel while adjusting the carbs (leading to popping), or else the vent is blocked. I think the vent is there to let air in and out as the fuel level rises and falls in the carbs, but am not sure what effect it will have if blocked.

Good luck, mate.

[Mushu]

Many thanks Henning,

I already looked at the air vent for each carb as recommended in Hondaman book but I did not look to tees and tubes outside of the carbs.

I will do it.

After cleaning plugs, I tried already to use one turn for air screw for carbs one and two but idle is crap and stay bad. I did not look at the plugs after doing this.

Philippe

[Mushu]

I have been cleaning again the air vent on carb 1 and 2. I have been disassembling them to remove air tube between them. I sprayed carb cleaner inside and it was OK.

I had a look to the fuel tee between carb 1 and 2.
I did not remembered that they had double O rings.
They are correct.
I put some grease on they to be sure not having air coming in.
I never seen any fuel leak on the tees.

I received 115 main jet in Honda bags and I installed them.

I changed sparkplugs following recommendations and used Denso X24E6-U.

I changed float balls setting coming back to 26 mm from your recommendation instead of 26.5/27

I modified setting of air screws for carb 1 and 2 to get one turn out from your recommendations.

I drove for 40 km in the valley. It's nearly freezing now.

Idle is not correct and did not stabilized after the trip meaning that I should be turning air screw 0.5 turn in to get correct idle.

You will find hereafter new plugs after the run.

I can see improvements when driving. It's smoother but I still perceive small hesitation (less than before) when accelerating from idle under load.

IMGP3347 by mushuchat, sur Flickr

Let me know what you think about the result

On the picture, you may not see that sparkplug 2 is oily around.

For one and 4 It's black but I am not sure if it is oil deposit.

For 3,let me know if it is too lean.

As we say in French (Le canard est toujours vivant). The duck is steel alive. Problem on carb 1 and 2 is still there (airscrew setting to 0.5 turn out to get correct idle and hesitation when accelerating between 1000 rpm and 2500 rpm).

For the next test, I will swap air screws from 1 to 4 and 2 to 3.
I also plan to swap rubbers connecting carbs to engine.

Your input will be appreciated.

From my previous post, I did compression test. Do you agree with my conclusions?

[Mushu]

As we say in French (Le canard est toujours vivant). The duck is steel alive. 
Is there somebody which could help me?

Philippe

[Henning]

Mushu, it's notoriously tricky to measure the compression correctly, especially on the inner cylinders. What you are looking for are similar numbers across all cylinders, and I would say you have that with the hot compression test. I don't think your problems are due to lack of compression, and if you do rebore, it's probably more because you want to rather than you need to.

For comparison, I rode a K1 from '78 to '82. When I parked it with 90 000 hard km's on the clock, I could easily kickstart it left foot flat on the ground (= really low compression), and that's when I weighed a scrawny 65 kg. Still, it started easily, idled beautifully and happily revved to the red line.

I'm kind of out of suggestions as to what you should try now. Maybe you should try to locate another set of carbs and see if that makes a difference. There was another member who did that to solve chronic rich running.

[vfourfreak]

I have been putting oil where needed on the cam and the small part which absorb oil (feutre). I don't know the name in English.

Feutre = felt

Kev

[Mushu]

Thanks Henning for your experience.

Thanks vfourfreak. On other word to remember.

Philippe

[guitarkev66]

Are the brass internal parts in the carburetors original or did they come from a rebuild kit?  Getting the bike to idle has a lot to do with more than the carburetors as you probably know but make sure your points are adjusted properly and your timing and dwell is correct.  Also check coil,wires and spark plug caps.  I always have used 40 pilot jets I've never varied.  Sorry if you already knew all of this info just trying to help

[Mushu]

Hi Guitarkev66,

Main jets, pilot jets, needle jet, jet needle are Honda original parts.

Ignition timing has been checked with stroboscopic lamp.

I use an electronic ignition from Hondaman. As far as I understand, dwell is generated by this electronic box.

Sync is perfect and checked with Carbtune.

Compression with hot engine seems low but pressures are nearly the same on the 4 cylinders.

Carb 3 and 4 are set to 1 turn out and idle is correct on 3 and 4.

Carb 1 and 2 are set to 0.5 to idle correctly and under load engine hesitate a short time to take rev in the range of 1100 to 3000 rpm.
All the recent changes gave me improvements to get less hesitation of the engine ( 115 main jets to lean the mixture and Denso Sparkplugs.
The duck is still alive.

My thinking is the following:

If I have to reduce air screw this is because:

1 fuel is not suck enough.
This may be clog hose between the top of the pilot jet and the small hole with inject mixture to the intake. (I have difficulty to clean these part with guitar strings but carb cleaner spray goes out)
Pilot jet may be clog. Inspection seems correct. To be sure I swap pilot jets from carbs 1-2 to 3-4 with no change.

2 Some air is coming from outside:
Rubbers  between carbs and engine intake have been changed 10 years ago and are still soft. Any way, I will swap 1 to 4 and 2 to 3.
Is there other possibilities?

3 Air screws could be worn:
I swap air screws from 1-2 to 3-4 with no change.

Concerning coils as 1-4 and 2-3 work together, I have difficulties to think that It can be a cause for air screw set up to 0.5 turn out on carb one and two.

For sure, I need some help.

Philippe

.

[Mushu]

I got for a short period of time an air compressor.

It was an opportunity to try to clean in a better way all small jet and specially hoses between pilot jet and intake.

Using carb cleaner spray and compressed air has been the way to solve the problem of carb 1 and 2.

I am now able to turn air screw at 0.75 turn out on carb one and two.

Idle is correct and there engine does not hesitate to restart from idle a under load.

I will be able to sleep on my two ears :-)

Many thanks for your help.

Philippe

[seanbarney41]

sounds like the duck is, at least, well on it's way to the oven