Topic: 750 Needle Clip Position-- Please Clarify

[MCRider]

I don't think that's Mark's perspective on it. I'm thinking he's starting from the top of the hole. Back the screw out till it clicks. You're at the top of the hole. Then turn in 3/4 or 1.25. You'd be going from most lean to richer. Rather than starting at the bottom and backing it out which would go from most rich to leaner. But I'm making assumptions as to what HM means.

He never specifically says, start at the top of the hole, or start at the bottom of the hole. But based on the comment about cars etc being the opposite, that's how i read it.

Both Mark and TT are smart guys. This is a simple issue. More likely there is a misunderstanding.

[TwoTired]

1) It is very bad form to quote from a PM to the open forum without the PM originator's permission.

2)  Physics.
The energy to operate the pilot circuit is derived from differential pressures.
There is atmospheric pressure on the pilot fuel jet entrance.  And, nearly atmospheric pressure at the pilot air jet entrance located at the mouth of the carbs, which see atmospheric pressure minus the pressure drop across the air filter membrane + some length of duct.

Nothing flows through the pilot circuit until a partial vacuum (pressure drop) occurs at the pilot circuit exit in the throat of the carbs.  Then, both the air jet and the fuel jet transfer their respective goods.
The only regulation in the fuel only path is the jet orifice itself.
The air bleed screw is in the air jet path between the air jet and the pilot jet emulsion tube.
The air screw does two things, siphons off pressure differential between air jet and pilot exit.  And chokes off or restricts the volume of air from air jet to the pilot jet emulsion tube.
Both these things control the air allowed to premix with the fuel before entering the carb mouth.  Meaning if you seat the air screw you have restricted the air to the fuel mix making it richer, and you have presented maximum pressure differential to the fuel jet orifice, making it flow the maximum amount and making the fuel mix richer via this method as well.

Note, the PD Carb's IMS is different.  It controls the fuel air mix at it's delivery point to the carb throat.  Closing these screws make the mix leaner and opening them allow the mix to be richer, to the limits of the air jet size and the pilot jets size flow capabilities at a given pressure differential.  I've no doubt other carb models can arrange the pilot mixture screws differently, as well.  I'm not going to talk about these without specific references or details of their specific arrangement.

3) Practical testing
I have an exhaust gas analyzer that reports hydrocarbon density at the exhaust.  I once minimized the hydrocarbon count out of the exhaust by turning the air bleed screws outwards from seated 4 or 5 turns more than Honda settings.   The more I turned the screws out, the leaner the exhaust mix, until a point where the screws provided little more effect.  By 5 turns it didn't seem to have more effect.
It idled beautifully.   But, opening the throttle slides makes the carb throat pressure drop, which causes the fuel jets to stop or reduce their delivery.  Without fuel for the air ingested the engine stumbles/wheezes until the throttle is returned to a point where the vacuum can again draw fuel into the carb throats.  So, I had to incrementally turn the air screws back in until the throttle response was returned to what is needed for street driving.  Turning the air screws in made the hydrocarbon count increase for idle speeds where the pilot circuit is dominant.  When throttle performance was again acceptable, I checked the turns count of the air screws.  They were exactly at Honda specified value.

I did further experimentation with turning the screws in even more.  The meter read a higher hydrocarbon count and I could increase the throttle travel for sudden opening and get good engine pick up.  But, the spark plugs started showing soot build up.  I turned them back out to book values and considered it a very interesting learning experience.

What I learned is that book values work well for the stock exhaust and induction, and that turning the air screws out made the pilot mixture leaner, while turning them in made the pilot mixture richer, matching the physics analysis perfectly.

With all due respect to Hondaman, I can't make his version of the physics work as described, and my exhaust measurement/testing seems to invalidate his analysis.

Best regards,
Lloyd

[Popwood]

Two Tired. I gave a lot of thought to posting Hondaman's PM. I'm a journalist with years of experience and know the rules of the road here. Hondaman's PM is derived from his published book and therefore public information. In fact, he references his book in his PM on this subject. I don't consider it a betrayal of confidence, something journalists with any ethics hold dear.

I don't dispute the science you site in your previous post. I don't dispute Hondaman's science. However, they seem to be at odds on a very fundamental question that deserves clearing up for all those who may search this question on this forum.

I'm truly hoping that MCRider is correct saying both you and Hondaman are saying the same thing in a different way. Making controversy for it's own sake is the absolute last thing I'd ever want. Clarity is what I seek. Again, if I've misinterpreted any posts or PMs due to my admitted lack of knowledge I sincerely apologize.

The last thing I'd want is to damage my reputation on this forum and forego the benefits of the resource it offers. It is an amazingly wonderful, valuable place.

Most Sincerely,
Steve Shanesy, aka Popwood

[72hondacb750]

Earlier in this thread it's mentioned that if you don't have honda needles to do the best you can to get some ... Anyone have a source for buying them online?

Thanks!

[Popwood]

So as you may know I started this tread because I was trying to solve the problem of carbon fouled plugs, especially on cylinders 1 and 4. Over the Labor Day weekend I have solved that problem, but I think it has come at the price of some performance. Before I explain what the "solution" was, I'll just say that my question now is needle height relative to throttle slide valve height.

It occurred to me, finally, that one of the changes from way back when I cleaned the carbs and carbon fouling got worse, was that the throttle valve was adjusted higher than before I removed the carbs for cleaning. When first removed, there was between 1 and 2 threads showing above the lock nut on the adjuster rod (reminder, this is a '75 750k). After cleaning and re-assembly, bench sync to allow just enough opening on the engine side of the slide to allow light to pass, left 5 to 6 threads on the rod above the lock nut; effectively raising the slide and the needle. Eventually, I raised the needle clip, lowering the needle in the needle jet. None of this provided any improvement with regard to fouled plugs.

So what I did over the weekend was adjusting the adjuster screw on the slide rid so that is was closer to the top of the rod, just 2 or 3 threads below. With no other adjustments, I suddenly found a lean, in fact very lean, condition. (also note I did a carb sync after these adjustments). Test ride showed performance "boggy" in the mid-range throttle position, indicative of I believe lean condition relative to needle and needle jet.

So I adjusted slide height again, this time allowing about 3 to 4 threads to show on the top of the rod. Re-did a carb sync. Performance improved to "reasonable" riding, plug condition much improved. However, I'd have to say the throttle response is notably slower and acceleration is slower as well.

Overall, the engine just doesn't seem as "smooth" as it should be, and clearly not like my 550k. (The comparisons are somewhat difficult because the 750 is much louder since the PO removed the very last part of the baffling system at the end of the silencers).

So, getting back to what I think is my question . . . the relationship between throttle valve slide height relative to needle position? One thing that baffles me is the illustration on the cb750 shop manual, supplement to 750K1-4, is the illustration on bottom of page 194. It shows a spec of 1.8 to 4.8 mm space between the top of the carb and lowest part of the adjuster mechanism on the rod. This clearance is supposed to be there with the throttle stop screw backed out to have no effect. This seems impossible since the throttle valve lifters with springs will pull the entire valve mechanism all the way down.
What am I missing about this spec and how do you know where the throttle valve height is supposed to be before fine tuning with vacuum gauges?

Sorry this seems complicated, BUT IT IS?

[mathias2803]

So as you may know I started this tread because I was trying to solve the problem of carbon fouled plugs, especially on cylinders 1 and 4. Over the Labor Day weekend I have solved that problem, but I think it has come at the price of some performance. Before I explain what the "solution" was, I'll just say that my question now is needle height relative to throttle slide valve height.

It occurred to me, finally, that one of the changes from way back when I cleaned the carbs and carbon fouling got worse, was that the throttle valve was adjusted higher than before I removed the carbs for cleaning. When first removed, there was between 1 and 2 threads showing above the lock nut on the adjuster rod (reminder, this is a '75 750k). After cleaning and re-assembly, bench sync to allow just enough opening on the engine side of the slide to allow light to pass, left 5 to 6 threads on the rod above the lock nut; effectively raising the slide and the needle. Eventually, I raised the needle clip, lowering the needle in the needle jet. None of this provided any improvement with regard to fouled plugs.

So what I did over the weekend was adjusting the adjuster screw on the slide rid so that is was closer to the top of the rod, just 2 or 3 threads below. With no other adjustments, I suddenly found a lean, in fact very lean, condition. (also note I did a carb sync after these adjustments). Test ride showed performance "boggy" in the mid-range throttle position, indicative of I believe lean condition relative to needle and needle jet.

So I adjusted slide height again, this time allowing about 3 to 4 threads to show on the top of the rod. Re-did a carb sync. Performance improved to "reasonable" riding, plug condition much improved. However, I'd have to say the throttle response is notably slower and acceleration is slower as well.

Overall, the engine just doesn't seem as "smooth" as it should be, and clearly not like my 550k. (The comparisons are somewhat difficult because the 750 is much louder since the PO removed the very last part of the baffling system at the end of the silencers).

So, getting back to what I think is my question . . . the relationship between throttle valve slide height relative to needle position? One thing that baffles me is the illustration on the cb750 shop manual, supplement to 750K1-4, is the illustration on bottom of page 194. It shows a spec of 1.8 to 4.8 mm space between the top of the carb and lowest part of the adjuster mechanism on the rod. This clearance is supposed to be there with the throttle stop screw backed out to have no effect. This seems impossible since the throttle valve lifters with springs will pull the entire valve mechanism all the way down.
What am I missing about this spec and how do you know where the throttle valve height is supposed to be before fine tuning with vacuum gauges?

Sorry this seems complicated, BUT IT IS?

Why did this thread die. Well I learned something, but I wish that more threads would have a final conclusion

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