Topic: Help in syncing / adjusting carbs

[dave500]

yeah try and get use of one,itll be so much more a thorough job then!

[Kyytsis]

Ok,

had a moment to do some work on the bike again (wife went out with the kids).. First pulled the slow jets and cleaned them. #2 and #4 were good, but #1 + #3 were all clogged up. Didn't have compressed air now (I did get the point of using that though) but used a quantity of carb cleaner to flush the passages.

Next pulled the slide and needle from #1. The needle is set to the 1st notch - or last...


The bike "boomed" with large throttle openings. Could it be that the #125 main and this setting make it too rich? Also, the slow jet is #35 - would this call more fuel from the fuel screw?

Open to suggestions!

Thanks,
Ville

[ealanm]

I have a compressor so I've never tried this, but I wonder if it might work to use one of those cans of compressed gas that computer technicians use to blow clean electronics?

Also, make sure you give the jets a thorough cleaning.  When I got my CB750K2 engine the jets were clogged with rock-hard gunk.

[Kyytsis]

I decided to see how this works now. Changed the needle clip to middle position. Now I have room to play to both directions when I know how the setup works. This was very educational, now I have a feeling I understand how the carburetors work and what can be wrong with them. They are not that difficult or complex to work with, you just need some very basic tools and some time.

I'm hoping to have a chance to put them on the bike soon to see how I did... At least the few clogged jets I cleaned, the bench sync and change in needle position should contribute to some change. Hopefully an improvement.

Thanks for all the advice, the saga continues

Ville

[dave500]

well done,you of course know each time you adjust the needle clip the vacuum sync must be re set by nature of the action?

[Kyytsis]

Yep, full dynamic sync is on the list of to-do.

[dave500]

yeah cool,carbs are basicly a controled syphon system.have you got a vacuum guage set or a friend has maybe?

[Kyytsis]

Yep, I can borrow a carbtune and there is a manometer in the garage 

Here is a video of the first run: First run after carb clean

I still could not get the idle down, as the fuel screws are 2.5 turns out, would that make the mixture too rich calling for more air --> more rpm? Closing the screws 1 turn to see if that helps.

Cheers,
Ville

[lucky]

Oops..Got ahead of me. You cleaned those things fast!
Did you check to see that the accelerator pump and nozzles are working?

To do that you will need the pods removed and using a bright LED pocket flashlight look into the carb opening (Engine OFF) and turn the throttle a couple times.
You should see a squirt of gas coming from that brass nozzle sticking up in the throat of the carb and the squirt of gas should be going inside the engine.
Check all 4 of them.

Facts of this set up.:
CB750 K7.
Pods.
4 into 1 exhaust pipe.
Main jet?
Idle jet?
Needle now set on the middle position.

[lucky]

Oops..Got ahead of me. You cleaned those things fast!
Did you check to see that the accelerator pump and nozzles are working?

To do that you will need the pods removed and using a bright LED pocket flashlight look into the carb opening (Engine OFF) and turn the throttle a couple times.
You should see a squirt of gas coming from that brass nozzle sticking up in the throat of the carb and the squirt of gas should be going inside the engine.
Check all 4 of them.

Facts of this set up.:
CB750 K7.
Pods.
4 into 1 exhaust pipe.
Main jet? Stock was#115 mm.
Idle jet?  ////stock was #35 mm,
Needle now set on the middle position. (Stock was 1st groove down from the top)
Mixture screw turns open, (Stock 1.5 turns)


You will not want to hear this but it will be way too rich with the needle on the middle position or 3rd grove down from the top. (Always count from the top.

You will need to get #42 idle jets for this Pod and 4 into 1 set up.
Mixture screws about 7/8 turn open.
I would change the main jets to #120's. But hold up on that for now. Stay with the #115's .
#115's may work since you have that adjustable slide needle on this 1977.

[lucky]

Ok,

had a moment to do some work on the bike again (wife went out with the kids).. First pulled the slow jets and cleaned them. #2 and #4 were good, but #1 + #3 were all clogged up. Didn't have compressed air now (I did get the point of using that though) but used a quantity of carb cleaner to flush the passages.

Next pulled the slide and needle from #1. The needle is set to the 1st notch - or last...


The bike "boomed" with large throttle openings. Could it be that the #125 main and this setting make it too rich? Also, the slow jet is #35 - would this call more fuel from the fuel screw?

Open to suggestions!

Thanks,
Ville

Those "booms" back fires are because the clip on the needle is wrong. Very lean.
The stock position is 1 groove down from the top, not from the bottom.
The previous owner made a mistake. Must have been running very poorly if at all.

[lucky]

Yep, I can borrow a carbtune and there is a manometer in the garage 

Here is a video of the first run: First run after carb clean

I still could not get the idle down, as the fuel screws are 2.5 turns out, would that make the mixture too rich calling for more air --> more rpm? Closing the screws 1 turn to see if that helps.

Cheers,
Ville

Sounds terrible. Shut it off.
At least it starts and runs.

Hear that sound in the video?
That is called 4 stroking.
With #125 mains and the needle clip raising the needle 2 positions it
is way too rich. Always count from the top of the needle.

What needs to happen now is to take off the carbs  and make sure the accelerator pump system and nozzles work. Then ( and I know you won't like this), you need to take out all of the needles and move the clips to the 2nd groove down from the top.

One step richer than stock.

Then I would put that all back together while you
 wait to get some larger idle jets.

[lucky]

UPDATE:


Facts of this set up now:
CB750 K7.
Pods.
4 into 1 exhaust pipe.
Main jet? #125 main jet
Idle jet?  ////stock was #35 mm,
Needle now set on the middle position. (Stock was 1st groove down from the top)
Mixture screw turns open, 2.5 open

[bollingball]

I wonder if you turn one of those cans of air upside down if that would flush it out better than right side up? For Gods sake sync those carbs before you do any thing else. After you get the needles where you want them. When I use my air comp. I put a football needle on the gun with the end clipped off so the air comes out the end instead of the side of the needle. It fits up to those little holes real nice.

Ken

[Kyytsis]

Knowledge = pain 

The accelerator pump is working and there is a squirt of gas when rolling on the throttle on all four carbs.

Facts of this set up now:
CB750 K7.
Pods.
4 into 1 exhaust pipe.
Main jet? #125 main jet
Idle jet?  stock #35 mm,
Needle now set on the middle position. (Stock was 1st groove down from the top; used to be last notch)
Mixture screw turns open, 2.5 open


Need to source the larger slow jets or could I just drill the original #35's to fit?

Have to say that it is not that terrible sounding in nature, the HTC I used to record this has probably the poorest microphone I've seen on a camera, phone, whateveritis.

[Kyytsis]

Size 45 seems to be available, 42 not... Well, of course for K0-K6 there is, but for K7/8 or F2...not.

How much can I play with the fuel screw? Is that just for balancing the carbs, a bit like syncing the slides?

[lucky]

I wonder if you turn one of those cans of air upside down if that would flush it out better than right side up? For Gods sake sync those carbs before you do any thing else. After you get the needles where you want them. When I use my air comp. I put a football needle on the gun with the end clipped off so the air comes out the end instead of the side of the needle. It fits up to those little holes real nice.

Ken

Great idea Ken! I have one of those too.
I should get it out and get it ready for use.

[lucky]

Size 45 seems to be available, 42 not... Well, of course for K0-K6 there is, but for K7/8 or F2...not.

How much can I play with the fuel screw? Is that just for balancing the carbs, a bit like syncing the slides?

Good, the accelerator pump and nozzles are working!

The #45 idle jet is way too large.

That needle clip needs to b on the 2nd notch down, NOT the third or middle notch.

[lucky]

Some progress last night...

I first removed the tops and did a bench sync with a drill bit. I did this by first putting the bit between the #2 slide and carb housing and then adjusted other slides to match that opening. They were clearly off, now they are aligned.



You need to lower the #2 slide down till it hits bottom and there will still be a opening for the air.

Next I removed the float bowls and was surprised how clean everything was.



I removed the floats, checked the float valves, and removed the jet and cleaned all the holes. The one in the center says 125 - is that the main jet? The top one says 20, that sounds really small if that is the slow jet, I thought it should be like #40?


I opened and cleaned the fuel screws and then set them to 2.5 turns from fully closed.

The next thing I'm guessing I should do is to remove the slides and the needle through the top of the carb? That means un-hooking them from the adjustment system...so I get to do the sync again. I looks quite simle, removing a spring and then pushing the connecting "rod" to the side and then lifting the slide needle assembly?

To which groove should I put the needle?

Thanks!
Ville

[lucky]

Now I am wondering if you checked you float heights.
Click on the photo once for a larger image.

BTW that mixture screw. Take each one out and make sure the rubber o ring is not damaged or squished. Make sure it has the little aluminum washer that sits on top of the O ring.

Look at this photo of the float.
That straight edge across the idle jet is where the top of the float should be without depressing the float needle spring at all. It just so happens that that straight edge across that idle jet is the correct measurement. Just a coincidence.
Also notice in the bottom photo the number stamped on the idle jet.?
Thats why I know it will not work. I tried it.

[lucky]

Smear just a little bit of Vasaline on the float bowl O ring before installing the float bowl .It helps to initially seal it, and keeps the rubber from sticking to the aluminum after it gets hot.

[Kyytsis]

Now I am wondering if you checked you float heights.

I checked that they were all the same, but didn't take accurate measurement of what they were.

BTW that mixture screw. Take each one out and make sure the rubber o ring is not damaged or squished. Make sure it has the little aluminum washer that sits on top of the O ring.

Did this when cleaning the carbs, pulled the spring out as well and they all looked good and had the washers on them. Dropped one and spent some time finding it...

Look at this photo of the float.
That straight edge across the idle jet is where the top of the float should be without depressing the float needle spring at all. It just so happens that that straight edge across that idle jet is the correct measurement. Just a coincidence.

Ok, thanks. Will check when I pull the carbs for the idle jet change + change of the clip position.

Drove the bike to work today, worked a lot better than before but it still booms with large throttle openings. So lean it is.

[Kyytsis]

Looks like I need to order them from Germany. For anyone looking for this, try this in Google: KHS-49A042

[MRieck]

If you are near Turku I can recommend someone.

[lucky]

Now I am wondering if you checked you float heights.

I checked that they were all the same, but didn't take accurate measurement of what they were.

BTW that mixture screw. Take each one out and make sure the rubber o ring is not damaged or squished. Make sure it has the little aluminum washer that sits on top of the O ring.

Did this when cleaning the carbs, pulled the spring out as well and they all looked good and had the washers on them. Dropped one and spent some time finding it...

Look at this photo of the float.
That straight edge across the idle jet is where the top of the float should be without depressing the float needle spring at all. It just so happens that that straight edge across that idle jet is the correct measurement. Just a coincidence.

Ok, thanks. Will check when I pull the carbs for the idle jet change + change of the clip position.

Drove the bike to work today, worked a lot better than before but it still booms with large throttle openings. So lean it is.

It would not be lean if the clip is on the third groove down.
 It would be extra rich (1/4-WOT), but that #35 idle jet would never work at idle . It is just too small.

You just need to get that needle clip in the right position and the idle jet right and it will come together.
You are getting to know the ins and outs of those carbs.


So now you will have checked your float heights.
Checked your mixture screws for crushed O rings and missing parts.
And the needle will have the clip one step richer instead of two steps richer.
The accelerator pump and nozzle system is clean and working.

You are getting very close now.  Excellent.

When you get the #42 idle jets open the mixture screws 3/4 turn, and if you hear any backfiring on decleration ( while parked) open then to 7/8 of a turn each.


Remember get it to run good in the shop first then take it out on the road.

Generally if it runs poorly in the shop, it will not be any better out on the road.