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

[jimbojangles]

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. Electronic ignition, coils, exhaust, air filters, all new rubber parts and dozens of spark plugs were not needed. All of the hours cleaning carbs was pointless without this part. I even bought an ultra sonic cleaner and was considering tearing down my engine in search of the cure.

I've made several posts on this forum trying to find out why one cylinder ran rich, then it became two rich cylinders. I also had problems with it fouling plugs at throttle openings just off idle. It also idled like crap and stuttered and blubbered just off idle. I really actually began to hate my bike and felt like it was more of a curse than a pleasure. I've spent hundred of dollars searching for the answer and probably just as many hours. I can remove a set of carbs, take them apart, reassemble, reinstall and sync them in less than 3 hours now. I even have certain set of tools set aside just for working on the carburetors.

A lot of people tried to help me and that is why after nearly a year of not posting I wanted to log in and try to help other people that might be in my position.

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!

[lucky]

What was wrong with them?
Were they the wrong size for the needles, or was it the O ring???


BTW.. I have never believed that all CB750's should hesitate from idle.
I would not stand for it.

[lostmykeys]

 I want to help you keep this thread allive!
 You have to read between the lines on any mechanical websight,trust me I know.
 I will be watching this thread.
 Great point!!!!

[jimbojangles]

From what I understand, they just wear over time. I've read that you shouldn't need to replace these parts unless you have a ton of miles, but I had this problem when my bike only had 19000 miles. The needle is constantly sliding in and out of the needle jet. I don't know about any o-rings involved here. From what I understand, the needle and and needle jet regulate when and how much fuel come from the main jet. At idle, the needle should fit firmly into the needle jet and cut off the fuel flow from the main jet. When the needle and needle jet start to wear, the fit gets loose and causes the carb to pull fuel from the main jet too early. I had to lean my mixture screws out so much to fix my off-idle problems that my idle got really bad. If I adjusted my mixture screws for the best possible idle, the bike would become impossible to ride at slow speeds and would kill spark plugs. This problem got worse for me when I was too lean at full throttle. If I fixed my jetting for full throttle, I was getting way to rich at idle and part throttle.

I'm sure a hondaman or twotired can probably explain this better. All I can say is that after way too much time and money I completely changed the nature of my bike in about 2 hours. The needle jets were the only change I made and my problems are solved.

What solved my problem, might not help everyone, but it sure is cheaper to try this before doing all of the things I did.

[andy750]

Great feedback! I have a set of needle jets and all other new carb parts waiting to be installed on rebuilt carbs so looking forward to the result it will make on my 76,000 mile CB750 K4.

thanks for posting
Andy

[kajtek]

Just to confirm, is it the emulsion above the main jet tube you replaced

[kajtek]

Sorry I phone posted before I finished, is it the emulsion tube above the main jet that the needle sits in or is there a seperate jet above this tube?

Thanks

[Trevor from Warragul]

It's the emulsion tube.  On my Morini 3 1/2 Sport the jet needles & needle jets in the Dellorto's wore because of wear in the square slides.  This allowed the jet needle to move from side to side & accelerate the wear.  Are your slides okay?  If not, the problems will return in short order.  The fix with my Morini was to fit brand new Mikuni VM28's

Trevor

[Rigid]

Great post, obviously people don't even know those parts are in there and they are important as you explained.  I had a rack of carbs that the po put them in upside down.  Obviously a poor runner when I got it. 

[orange550]

On my 550 , I had my carbs apart 3 times trying to figure out why the bike didn't run as well as it should. Cleaned everything, set everything, replaced everything, soaked everything... nothing helped. Then I finally figured out I needed to check/clean my emulsion tubes. I didn't even know they were in there. Actually, I had heard the term, but didn't really know what they were. Originally I though it referred to the passage way for the idle screw... ha ha.

Anyway, they were not easy to remove. After all the soaking I had done to the carbs, the emulsion tubes were still varnished in there like a rock. I took the carb apart, including the slides, and pressed them out from the top. The tubes were packed with hardened gunk. The cross drilled holes were completely full. I soaked them, used some emery cloth, reamed out the cross-drilled holes, and voila!! Bike ran like a top. Amazing.

[ekpent]

Think he is referring to part #3 that the needle jet goes through.Lower right picture on view.Sits above the emulsion tube.

[Johnie]

I agree Eric...he is referring to the small brass tube that you only can see if you look into the throat of the carb. They are pressed into place. We are not talking about the emulsion tube here. Part #3 is very small. Jimbojangles...tell us how you got them out...a punch? As I recall they are in there pretty tight. I take it you have to buy a whole kit to get them?

[70CB750]

Subscribed

[brewsky]

My needle jet removal tool....
Insert from the top of the carb and push or tap out the bottom.
The rounded end fits against the emulsion tube.
(This one is from a 305, not 750, but is similar)

[ekpent]

Wood is good   

[jimbojangles]

Yes I am referring to part #3, not the emulsion tubes.

It is referred to briefly on the salocal website. http://www.salocal.com/sohc/tech/carb/dspg_mgs/rmvndljt.htm
They also mention it on cycle exchange. There was a lot of reading between the lines to decide this might be my culprit. A simple search of my problems would have never brought this up.

http://cyclexchange.net/Filters%20Page.htm  Go down to the part "Our 2 cents worth."

I just used a 1/2 hardwood dowel slightly tapered with a pencil sharpener.

For the first time ever my ride to work in the morning was perfect. I can now go through traffic and cruise at 30 mph all day long. 30 mph used to be like kryptonite to my bike.

[WhyNot2]

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......................

[jimbojangles]

Well, speaking of start up. There is a downside to this repair. I actually have to use the choke in the morning now. It used to be so rich at idle that starting the bike was a piece of cake. At least now I can feel like part of the cool kids club when talking about my bike being cold blooded.

[bassguitar333]

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?

[jimbojangles]

I'm talking about the small cylinder piece in the bottom right of this photo. Not the Emulsion tubes. If you remove you emulsion tubes and look up into the carb body through the hole the emulsion tube screwed into, you can see the needle jet. Remove the cap to the carburetor and remove the slide and needle. you can then push the jet out from above. It will fall out of the hole that the emulsion tube was screwed into.

I really wish I would have taken pictures during this process, but I was running out of daylight. Here's a picture of the part. 

http://www.smebike.com/index.php?route=product/product&product_id=2898

[orange550]

I don't think the 550 has these, no? My emulsion tubes go all the way up to the bottom of the slide throat.

[iron_worker]

I haven't looked into a 550 set of carbs but it's possible the needle jet is built into the emulsion tube.

[Roach Carver]

Anyone have a supplier for just those pieces?

[ekpent]

Anyone have a supplier for just those pieces?

When I posted the fische the #3 part is called "Needle Set,Jet" and is part Honda # 16012-300-014 and costs,are you sitting down,Gulp.  $19.28 each or $77.12 for 4.
  One would want to be sure that you have a good stock needle to go with that though for the two to work right together. Some of those aftermarket needles,jets and other brass bits can leave a little bit to be desired,if you know what I mean.

[jimbojangles]

I wasn't willing to spend 20 bucks a piece on these. I bought entire carburetor rebuild kits from Rusty Rider that happened to include this part. It's a Keyster kit, but only cost about $13 each. I just used the needle jet from the kit and stored the rest away for another day. I know keyster has a bad reputation and admittedly there were a couple of parts in the kit I would never use, but I don't have any complaints with the needle jet.

I also placed my order on Friday afternoon and got my order in the mail on Monday. Can't beat that.