Topic: Carb tuning on a CB 750 K1

[Mushu]

I Try to tune my CB 750 K1 using the forum.

I made good progress but I now need some help.

My bike runs too rich and the engine hesitate a little bit when moving from idle to higher rpm under load.
Idle is correct at 1200 rpm after 5 mn and move to 1400 rpm after using it for more than a quarter of an hour.
It runs great from 3000 rpm to 8000 rpm

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

I don't know total mileage of the engine.
I did 30000 kms with it

Standard airbox and HM 341 are used

Pilot jet is 40 and main jet 120.

I verified:

• Tappet clearance
  ignition timing with stroboscopic lamp
  Cleaned plugs very often
  Install Hondaman Electronic Ignition (Good Improvement)
  Cleaned Carb using guitar strings. (I got a significant improvement doing this)
  Used carb cleaner spray to clean idle air jets and main air jet
  Adjust float balls to 26.5 mm trying to lean a little bit
  Sync carburators with Carbtune
  Adjust carb air screws to:
  

• 0.4 turn cylinder 1
  0.5 turn cylinder 2
  1 turn cylinder 3
  1.1 turn cylinder 4
  

Above these values exhaust starts popping on cylinder one and two.

You will find hereafter a picture of sparkplugs after my last trip in the mountains (220 kms)
Plugs are NGK D8EA
From the left to the right Cylinder 1 to Cylinder 4.

IMGP3164 by mushuchat, sur Flickr

I would be pleased to get your advise to improve the tuning.

I am not sure that I have been able to clean properly the horizontal tube or jet connect the top of pilot jet and small hole which spray mixture for idle  but I saw the carb cleaner spray going out (Excuse by bad English language).

Philippe






 

[edwardmorris]

........

Standard airbox and HM 341 are used

Pilot jet is 40 and main jet 120.
.............

For that set up, 120s are too big, try 110s or 105s for mains, then try 7/8-15/16 open on air screws.

[Mushu]

Thanks for your reply Edward.

I am not sure that a change of the main jet will solve hesitation of the engine to move from idle but it could be one point to look at later on.
I was also thinking later on to modify HM341 exhausts to gain power with 120 jets

As said, I cannot turn air screw more than 0.4 turn for cylinder one and 0.5 for cylinder 2 without getting unstable idle.

[Kevin D]

Mushu, I have denso plugs (x24es-u) on Hondaman recommendation, 115 main, and no more plug fouling. 1 turn, 38 slow jet, 341 pipes with end plug pulled, stock airbox with foam filter, 48MPG, most riding at 50-55mph (+/-).
Top gear roll on is good, but you cant crank it WFO lickety-split, easy roll on does it good.

http://forums.sohc4.net/index.php?topic=57005.0

[Mushu]

Hi Kevin,

Thanks for your reply.

Reading the thread on sparkplugs, I will move to Denso plugs X24es u. This will help.

I forgot to say that I use NGK plug caps with 5000 ohms resistor.

I would like to solve the idle air screw problem as I cannot get 1 turn on air screw for cylinder 1 and 2.

After it, I will concider changing idle air jet to 38 as well as main jet to 115 or 110.

[guitarkev66]

Mushu, I have denso plugs (x24es-u) on Hondaman recommendation, 115 main, and no more plug fouling. 1 turn, 38 slow jet, 341 pipes with end plug pulled, stock airbox with foam filter, 48MPG, most riding at 50-55mph (+/-).
Top gear roll on is good, but you cant crank it WFO lickety-split, easy roll on does it good.

http://forums.sohc4.net/index.php?topic=57005.0

+1

[Kevin D]

The 38 slo jet was changed a long time ago and didn't fix the plug fouling. The 115 mains were installed more recently and did the job along with the Densos. Good plug caps are also important too, I have failed a few of them over the years.
Sorry can't help you with the air screw problem, but I know that many here replace the carb rubbers due to leaks (I haven't). Also, I watched the boss mechanic tighten the carb tops all the time.
My idle isn't smooth as silk, but i'm more concerned how it goes down the road.

[Mushu]

Thanks Kevin,

I also forgot to mention that I changed rubbers 7 years ago.
Without this change, it was impossible to get carb synchronized.

On the road in the mountains, the problem is that engine hesitates to move from idle when going out of bends.
It's not very safe.

Philippe

[Kevin D]

Sorry again, I have nothing to offer regarding mountain carb tuning. What altitude causes the problem? What altitude range do you cover? I'm a flatlander, 800ft above sea level. There are others her who regularly ride CB750 at high altitudes. Many passes in Colorado are above 10000ft, and I think it is member Wobbly riding in the Alps. Where do you ride?

[Mushu]

Hi Kevin,

I am living near Grenoble. Altitude is 250 meters.

Around Grenoble, funny roads are in the mountains and altitude may be up to 2000 meters but in November it is safer to ride at 1000 meters.

The problem occurs also in the valley.

Philippe

[Henning]

Hi Mushu, I have a similar setup, K1 with the 341's and otherwise stock. I run 115 main jets, 40 idle jets, needle in the middle position and the idle air screws set to 1 1/2 turns out. The 120 main is definitely too big for your setup; 115 or 110 will be better. Definitely think you need to turn the idle air screws out a lot - that's what finally helped with the rich running on mine.

I take it you are using the stock Keihin jets and needles? People seem to have no end of trouble with the Keyster stuff.

[Mushu]

Hi Henning,

I suppose that idle pilot jet and main jet are stock.
I changed some years ago needles and needles jet with original Honda parts.

Needle is in the middle position.

Philippe

[Kevin D]

Philippe, I hope we can get this sorted out and then you might bring us some spectacular photos of your CB along the Route Napoleon N85 

When I was running the NGKs I put new plugs in twice? per year. The Densos have been in for several years, no problems.

[seanbarney41]

Mushu, are you familiar with the clear tube method of checking the float level?  That is what I would try next.

[Mushu]

Hi SeanBarney,

I tried to verify the float level but it was a little bit tricky as I don't have the proper screw to replace the purge screws.

I will try to find it.

Do you have any idea why I have to adjust idle air screw to 0.5 turn on cylinder 1 and 2?

For Kevin, I have been driving on part of Route Napoleon but not from La cote d'Azur.

Find hereafter, a picture taken from Semnoz mountain near Annecy on November 9 and one with the bike taken at Col du Rousset in Le Vercors mountain + the road to Le Rousset Pass (col) on November 2. Weather is very good. It's what you call Indian summer.

Sortie Moto Annecy by mushuchat, sur Flickr

Sortie Moto Die by mushuchat, sur Flickr

Sortie Moto Die by mushuchat, sur Flickr

[seanbarney41]

Many different devices can be devised to do the clear tube test...I have used a plastic vacuum nipple, force threaded into the screw hole.  The soft plastic will not damage the threads in the bowl.  Many others have heated the end of a clear tube and threaded it in while soft.  It does not take much to create a temporary seal here.
     As far as your mixture screws go, I am not sure.  I have had to use similar settings before, but only with aftermarket open filter element.  Getting your fuel levels straightened out may solve your running issues and change the air screw settings required.

[Mushu]

Hi SeanBarney,

Following your recommendations, I did the clear tube test.

Measurements are as following:

Carb 1: 6mm
Carb 2: 6mm
Carb 3: 5.5 mm
Carb 4: 5.5 mm

Values are greater than what we should normally expect (4mm).
It was intentional to try to lean the mixture.
Bike idle correctly even on the side stand.
I still have air screw at 0.4 turn on Carb 1, 0.5 turn on carb 2, 1 turn on carb 3 and 1.1 turn on carb 4.

I have doing a trip in the mountains yesterday for 180 km.
Preparing this trip, I cleaned again the plugs, putting a little bit more advance on ignition to burn a liitle bit more the mixture. I would say that it is one mm before the F mark.
I also used a carb cleaner mixed with the gasoline.
I feel that it improved engine response during the trip but  during last part of the trip, I had to go down from a pass during 10 km.
At the end, I was loosing cylinder 1 with plug full of gasoline.

Main problem would be to solve carb one and two with air screw below the 1 turn standard value.
I feel that it is the reason why engine hesitate to move from idle to upper rpm.
This is critical when accelerating at the end of the bend when climbing in the mountains

Do you have any suggestion?

Following this, I plan to reduce gasoline in the mixture:

Advises have been to reduce main jet to 115 and I found were to buy genuine parts.
Alternatively, I was thinking to modify needle clip position to 2 from the top instead of 3 to lean the mixture.

What 's your advise?

Advises have been also to reduce size of pilot jet.

I found 35 or 38 pilot jets (genuine).

What do you recommend?

I should receive Denso plugs during this week.

Philippe

[seanbarney41]

I would try what you have planned...other than that, I am stumped, your problems may be beyond my experience.  The only other thing I can suggest is to set your fuel levels 3-4mm from the gasket surface, turn your mixture screws to about 7/8 of a turn out and jet from there.  In my experience, these carbs just do not work right with any other fuel level, and lowering that level in an effort to lean things out is just masking a problem elsewhere in the fuel or ignition system.

[Mushu]

I will exchange pilot jet 1 with pilot jet 3 and pilot jet 3 with pilot jet 2.

After this, I will see if I can change air screw setting.

I will let you know the result.

30000 km ago when I got the bike, I opened the crankcase to inspect/measure all pieces.

Pistons are at origin diameter but I saw that cylinders where at the limit and  a bore to 0.25 would have been necessary.

Could a lower compression on cylinder one and two be the cause for setting air screw at 0.5 turn out even if I have been able to synchronize correctly the 4 carbs?

[seanbarney41]

Could a lower compression on cylinder one and two be the cause for setting air screw at 0.5 turn out even if I have been able to synchronize correctly the 4 carbs?

I suppose its possible, but I actually have had no experience with a cb750 that has significantly low compression.  What process are you using to set the air screws?

[Mushu]

To set air screws, I first used 1 turn out setting, then I have been trying to obtain the highest and stable idle rpm.
Exhausts 1 and two were popping which means too lean mixture.
I turned in air screw just at the limit between good idle and popping for carbs one and two (0.4 and 0.5 turn)

Philippe

[Henning]

Mushu, when working on the carbs you need to change one thing at a time and then test; I'm sure you know this already.

On my K1 I battled rich running for a long time. The plugs were black, but never bad enough to foul a plug. One of the things I tried was moving the needle clip from middle position (third from top) to second from top to lean it out. It ran a lot hotter, I got great mileage, but it didn't run quite right. I ended up moving the clip back to the middle position. What helped was turning the idle air screw out to now 1 1/2 turns.

Problem with the needles is that you need to remove the carbs to adjust them, so it's quite a big job, and be really careful when removing the small screws holding the needle in place. But then you can check you have the correct needles, should be K27201, and check for wear. I take it you have the 7A carbs?

Nobody seems to change the idle jets on the early bikes away from #40, but I suppose it's worth a try. At least you can swap them with the carb rack on.

Have you check the ignition advance mechanism recently, as in removed, inspected and oiled it? That sometimes causes problems and I'm thinking that may have to do with the bike falling flat off idle.

[Mushu]

Hi Henning,

Thanks for your help.
Ignition has been verified last week with stroboscopic lamp.  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.

Carbs are 7A with air screw without hole.

I understand that needle must stay in 3rd position.

I will order the 115 main jets after testing the swap of pilot jets.

Philippe

[Mushu]

I have been doing swap of pilot jets 1>4 4>1 2>3 3>2.

Before to do it, I verified that jet holes were all the same using a needle for sewing with a diameter a little greater than the jet hole.
When inserting the needle in the jet hole, it stops where the diameter was becoming to large.
Then I measures the length of the needle still out of the pilot jet.
They were all at the same measurement.

Doing a test on the road I got no change.
 At idle I have not been able to  do a one turn out for air screw on carb one and two.
Carb three and four still needed a one turn out.

If somebody has a brilliant idea to solve this problem, I will be more than happy.

To lean the mixture on all carbs, I will order 115 main jets right now.

Philippe

[edwardmorris]

I still think 115 with HM341s and stock air box will run rich. Order a set of 110 along with the 115s to have them ready just in case. I'm running 7As with needle in the middle position, 105 mains and air screws at 7/8 turns on all four. This is at the Chicago area elevation though, not sure what has to change for higher altitudes. Even 110s are slightly rich and require high RPMs to keep the plugs clean.