Topic: After two years, finally fixed hesitation, stuttering and fouled plugs

[lucky]

Mine does the same......will not cruise at 30.......................

Hard as heck to get it to idle on start up, have to keep flipping the throttle, to keep it running.

Scribed......................

This part may not be the solution to problem.
You did not give any other facts.

BTW that needle jet he is talking about lets in fuel but the needle does not actually touch it.

[JimJamerino]

Wood is good   

That's what she said.

[lucky]

My bike does the stuttering when cruising, but my emulsion tubes are clean. Should i grab some new ones, or would it make a difference since all holes are clean?

You do not need new emulsion tubes bassguitar333.
 Just clean them if you think they are dirty.

[Dammerung]

After two years of fighting with my '73 Cb750 I've finally solved nearly all of my problems with one very simple, very cheap part. I'm super psyched to have finally fixed my bike, but upset at the amount of money I spent in the process........And the answer to all my problems was....Needle jets!

Not the needles, not the main jets, not the pilot jets, and nothing to do with the idle circuit. I'm talking about the little press-in tube that the needle slides into in the bottom of the throat of the carb. These little jets don't come with many of the rebuild kits sold online, but completely changed my bike. I found cheap rebuild kits on the rusty riders website and included this part. This is literally the only part in the fuel system I did not replace aside from the carb bodies themselves.

Don't believe that all 750s just hesitate a little bit off idle. My bike is 100% smooth now. All of my air screws are finally all at the same setting. I used to have them all over the place from 1 turn to 3 turns out just to get it to run. If you are having any of the problems I was having, then I strongly suggest replacing your needle jets. Maybe it will save you some time, money and frustration.

Cheers!

Can you please post a pic of the actual item please. Sorry noob here and i have a 73 as well and has the exact same plroblems you are talking about.

[longshanks]

Not to thread hijack but I feel like I should post some answers I found out about my carb problems too. I just recently figured out why number 1 and 4 carbs were fouling and running like crap. The play in the slides were out of spec. Ended up being .007" on #1 .003 on #2 .003 on #3 and .006 on #4. Put new bodies on and it runs much better.

[iron_worker]

How did you measure the play in the slides?

IW

[ekpent]

We got to get TT in on the slides thing.They ride in a groove and are more of just a mechanical way to get all the good parts working together.There are ways to modify and alter the bottom and shape of the slide but not sure on the rest as far as wear would go in a bike with less then a lot of miles unless they were damaged from a varnish seizure by a mechanic etc.

[longshanks]

How did you measure the play in the slides?

IW

Feeler gauge.

[dicklinn47]

Have a 400f with what appears to be the same problem.  The carb rebuild kit shown at Rusty Riders does not show a separate needle jet.  Is it possible that that function is combined on the 400f carb?  The carb kit I bought and installed, with standard jets, didn't have the emulsion tube included. 
Thanks for any help
DL

[DH]

BTW that needle jet he is talking about lets in fuel but the needle does not actually touch it.
[/quote]





What then, causes the jet to wear/malfunction and flow incorrectly?

[Spanner 1]

I would venture to say that a lot of reports of lack of 'response' in the idle circuit. i.e. adjustments to the air screws ( early carbs ) or mixture screws ( later carbs ) might be worn needle jets leaking fuel with the slides bottomed-out at idle and allowing a too rich idle/slow mix from the main jet circuit  ... So yes, the needles MUST close the needle jet at idle rpm, or the main circuit will be adding fuel to the mix... not the design A.F.A.I.K.   

[lucky]

How did you measure the play in the slides?

IW

Feeler gauge.

How would you get a flat thickness gauge between the slide and its cylinder wall?

Where would you find specifications for that?
It is not like a engine piston and cylinder where the piston is traveling thousands of feet and there is a lot of wear. Was there wear marks on the piston ?


Even on dirt bikes in the desert I never heard of anyone measuring the carb piston slide clearances.
Besides the cover on the top of the carb has a gasket to seal that area anyway.
How would it make a difference?


I have never see specifications for that even on car engines with slide carbs like SU's.

[hondaseven]

Hey Jimbo, Hondaseven from Oz remember me, If  I remember correctly I contacted you in 2010, yes 2 years ago, and told you what was wrong with your bike, how come it took you 2 years to try it, told you it would fix it.

[fmctm1sw]

Interesting thread.  I'm pretty sure I understand the part you are talking about but I don't think I've ever seen it come out of 350F carbs much less seen that piece for sale anywhere.

Edit:  You're talking about the little brass fitting that goes right where this arrow is pointing?  Hmm, that guy looks like he took his out.

Edit:  I think maybe that fitting IS the emulsion tube on my 350F.  Perhaps some bikes had a separate brass fitting there that the emulsion tube butted up against?

[Bodi]

Many of our carbs have the jet and emulsion tubes as one part, I think some 750 years and all the 400s and 350s I've worked on. I don't remember the 500/550s but a look at the carb diagram would tell the tale.

[longshanks]

How did you measure the play in the slides?

IW

Feeler gauge.

How would you get a flat thickness gauge between the slide and its cylinder wall?

Where would you find specifications for that?
It is not like a engine piston and cylinder where the piston is traveling thousands of feet and there is a lot of wear. Was there wear marks on the piston ?


Even on dirt bikes in the desert I never heard of anyone measuring the carb piston slide clearances.
Besides the cover on the top of the carb has a gasket to seal that area anyway.
How would it make a difference?


I have never see specifications for that even on car engines with slide carbs like SU's.

I wasn't going off any specs, just measuring the difference. The slide needle was also loose which started me on that trail.

[Cb400fnoel]

You may have cured my problem!
I've mine stripped and cleaned many times, havnt renewed anything but I suspect I need new emulsion tubes, my bike has same symtons as yours, great if you gun bike in every gear but splutters at low revs on full throttle, you can see from my pic that mine were really bad, there's 50k miles so I bet I need a pair, I was kinda heading towards ignition/spark issues until I read your post because I've these carbs spotless. So I'm going to get on and order up a refurb kit. I'll have to report back but in the mean time I Thankyou very much.
I've searched everywhere and found yours by chance.
Honda CB400Four 1977

[Cb400fnoel]

Spent the day replacing all jets, needles, carb spotless, floats set to 21.5,(some recommend 22, some 21 so I went in middle), guess what, still the same symtons, the needle clip is in middle grove