Float Bowl Adjustment

[Myumi-2]

I'm progressively working through the issues with my 1972/74 CB750 and each solution unearths another problem on the path to ideal performance.  

The most recent issue is when operating at between 3000 and 4000 revs the performance is unsteady.  The bike feels jittery as if one cylinder is only sparking occasionally.  i.e. when maintaining a constant speed of 60kms/hr it will gradually slow down, then when I increase the throttle slightly, it will come back to life with gusto and rapidly accelerate to 70km.  If I change down a gear and maintain a constant speed of 60km/h with 6000 revs there are no performance issues.  

Also when acclerating slowly from a standing start there is a pause, right at the start of the acceleration, before the engine kicks in with full power.  Whilst under hard acceleration is problem is not apparent.  

Whilst the immediate solution might appear to be that I just need to ride this bike hard and keep the revs up, I'd like to resolve this little problem.

I suspect that the issue is Carb related.  Ive had a series of events with my number 3 carb.  First, it was overflowing (I adjusted the float), then it seemed to develop a bloakage and no fuel was making it to the bowl (I was able to fix this on the side of the road by removing the fuel line from the tank and blowing through the line). So tonight I'm going to pull the carbs out again and clean them, re-sync and see what the outcome is.

Ive cleaned the carbs before, I've not re-jetted as they appear fine, and synced the carbs with noticable improvement.  

I have a suspicion that I may have over compensated for the overflow problem and adjusted the float bowl to far causing the one jet to be starved of fuel at speeds.

My question is what is the correct adjustment/height for the float?

Cheers
Tim

[brcree]

 Oye....this may help. I have a 76 cb750 k6 and had problems with leaking as it sat and ideled. To adj carbs, remove, remove bowels, make a guage out of a small slip of wood or plastic and measure out a 26 mm line from the end. With carbs in hand tilt them so the float just rests on the needle valve. Take measure stick and place it on the carb body and measure to the top of the float. Float goes past 26mm bend tabthat is resting on needle to adj. Same for to short. Be careful not to break anything, the stuff is not bullet proof!..................hope it helps.

[cbRider]

tim
this sounds like a lean condition in all or more than likely your #3 carb. Do set up the float as brcree described, that's the right way to do it. You could also be sucking air at the intake boot on #3 carb.
Make sure you have nice, tight connections. Check that by spraying starting fluid onto the boots while the engine is idling. Listen for change in RPM while spraying. I would also recommend using an in-line gas filter.
Good luck.
Karl

[Re-run]

Have you checked to make sure your jets are completely clean and none of the holes has any crud? I know many clean with carb cleaner and compressed air but I find this does not work very well.
The issue you describe does not seem to be float related, it could but does not sound that way. Sounds like a fuel issue other than low level. I might also hazard a guess that your needle in that carb may need to be adjusted.

[KarlM]

I know this post is 4 years old but I have the exact same problem as MYUMI-2.
I am new to this site.
Can someone point me in the right direction?
Thanks
Karl

[Lucky 1]

All issues must be resolved BEFORE the carbs are worked on.

All electrical and mechanical issues must be corrected first.

[Lucky 1]

You did not say anything that could help me to help you find the problem.

For instance "72/74 CB750."  What does that mean??? 72 carbs 74 engine? 72 engine and carbs and 74 frame?
74 engine and carbs and 1972 frame????  Cheeeesshhh.

The #3 carb. "First, it was overflowing (I adjusted the float), then it seemed to develop a bloakage and no fuel was making it to the bowl......"

You need to put in 4 brand new float needs and seats. You cannot adjust or clean a worn out float needle. Either it works or it does not work. If it is more than 2 years old it probably needs replacing.

You need to buy a workshop manual.
The float level is determined by the type and year of the CB750 carb.
The 1972 or 1974 carb was 26mm or 1.023 thousandths. Measured with a ruler or a float gauge.
 Clymers manual Illustration #13, Chapter SIX, Page #125.

You may have surging if the float levels are off.
You may need rejetting too.

But I need all the fact to help you.

Do you have the 657A carbs or the #7A carbs.

[KarlM]

Thank you

I'm not sure what happened with my message. 

Sorry for my mistake.

I have a 1971 CB 750 K1

It has a studder/misfire between 3-4 k rpm.  Ok when standing still but it seems to be effected under load.   It does go away if I go to full throttle. 

I was wondering if this sounds like a carb issue or points/ timing / advance?

I just bought the bike and had it serviced  400 miles ago.  I guess I was waiting for it to go away.  Wishful thinking!

Sorry for the confusion.

Any help would be great.

Thanks

Again

Karl

Sent from my iPad

On Jul 23, 2012, at 6:51 PM, "Lucky 1 [via Honda CB750'S]" <[hidden email]> wrote:

You did not say anything that could help me to help you find the problem.

For instance "72/74 CB750."  What does that mean??? 72 carbs 74 engine? 72 engine and carbs and 74?
74 engine and carbs and 1972 frame????  Cheeeesshhh.

The #3 carb. "First, it was overflowing (I adjusted the float), then it seemed to develop a bloakage and no fuel was making it to the bowl......"

You need to put in 4 brand new float needs and seats. You cannot adjust or clean a worn out float needle. Either it works or it does not work. If it is more than 2 years old it probably needs replacing.

You need to buy a workshop manual.
The float level is determined by the type and year of the CB750 carb.
The 1972 or 1974 carb was 26mm or 1.023 thousandths. Measured with a ruler or a float gauge.
 Clymers manual Illustration #13, Chapter SIX, Page #125.

You may have surging if the float levels are off.
You may need rejetting too.

But I need all the fact to help you.

Do you have the 657A carbs or the #7A carbs.

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NAML

[Lucky 1]

Service? that usually means oil change and check fluids and tires .

It does NOT mean tune up and replace parts.

Actually it all means nothing.

What you have now has to get a complete work up and all here say and stories mean nothing.
Just actual facts that can be checked.

[Lucky 1]

Fact: 1971 CB 750 K1 right?

Check the side of the front of the frame steering head to make sure what you have.
Write it down.

Then tell me if it is true.  Then we will have a solid FACT.

Then see if you can find any numbers on the carbs. You need to get
 a Clymer's workshop manual so you can read up where the numbers
are located on the carbs. Lots of other info.

Tell us if the motorcycle is stock configuration.
A photo is always nice and creates interest in your story.

[shinyribs]

Lucky, KarlM does in fact know he has a '71K, it is the OP (Myumi-2) that mentioned about owning a 1974/1974 CB750.


KarlM,just to be sure,are you having the overflow/blockage issues Myumi-2 was having, or the odd power loss at certain RPM's? Or both?    I dont know alot,but I will do all I can to help

[KarlM]

Thanks
Yes same running problem.
I do have a float/needle problem but I think I have answere to that.

My bike has the EXACT same propblem as the Older post.
Problems are identicle.

I did order a Clymers yesterday.

Just wondering if anyone had a suggestion for the 3/4k rpm studder/misfire.

Thanks
Karl

[Re-run]

You might be getting a clymer manual, but download the large service manual here. Much better.

[KarlM]

Thanks
I did , unsuccessfully try to download it.
I wll try again

Karl

[Re-run]

http://creativeconceptcomputers.com/CB750/sohc6978large

This link should start the download for it.