Mixture screw direction

[scottly]

Page 13 from the carburetor manual:

[DomP]

And yet, no one has mentioned his sources for his evidence of his opinions? Not read them myself as I’m on a phone but the sources appeared to be genuine Honda bulletins specifically for carbs of that era. Seen more than a few myself so I’m familiar with there type. So do they prove or disprove Mark?
I’m personally of the opinion that in is richer on these carbs BUT show me evidence I can’t dispute, as in from the manufacturer themselves, I’m willing to be convinced otherwise.

So what’s the verdict? Are people saying Honda were wrong or that Mark interpreted incorrectly or actually he’s right and we were all wrong.

Wasn’t much of a rant btw, if he hadn’t tagged it as a rant I’d just put that down as some sort of frustration coming to the surface. He mentioned no names, blamed no one. How is that worthy of having a thread locked?

To be fair Ken, i used the word rant but it was more in my own frustration that I was being told by two different camps that their way was correct to a problem ive tried on and off to solve for months.  Id assumed Mark locked the thread given how immediately it was locked.

[Sesman]

“Theory is knowledge that doesn't work. Practice is when everything works and you don't know why.".

Herman Hesse.

[K2-K6]

It's easy to quantify in practice, regardless of any reference to instructions published.

Just turn all screws in to touch baseline, then start the engine, it'll probably start without choke. You can see from this that has now zero air ingestion through the idle system passage, full fuel in other words .... which should tell you something   

Also, with exhaust measurement, you can see clearly what happens when you move the screws while running.

[K2-K6]

Anyone can conduct a representative schoolroom science test though.

Glass of water, straw in water, suck on straw, water and only water comes up straw.

Now put a hole in the straw above the water line and see what you get ......

My understanding is that the difference in specific gravity of air/water is the controlling characteristic here. Any air available will be favoured in this scenario. the more air (size of hole) the less water will be lifted.

[DomP]

When you say with exhaust measurement what do you mean?  Temperatures?

[K2-K6]

When you say with exhaust measurement what do you mean?  Temperatures?

No, exhaust gas analysis, you can see mixture, unburnt fuel content (rich running) remaining O2 etc.

[DomP]

Ah ok, I might resort to finding somewhere to look at it

[Sesman]

Dom, just in case you weren’t aware, the 069A F2 carbs were equipped with a tapered air screw. Might be worth checking you have the correct ones in yours?

Other than that, if the carbs are correctly jetted, clean and have the correct Air screws it isn’t really necessary to go hi tech? Just twist and observe engine reaction. If you have to go beyond 11/2 to 21/4 turns out there is something else seriously amiss. Vacuum leaks on the inlet manifolds for example!

Apologies in advance if you’ve considered all this.

[K2-K6]

Ah ok, I might resort to finding somewhere to look at it

I was using to illustrate that it will prove which way the screw affects the mixture definitively by measurement of output.  Interesting for these bike to set up, but not essential to good adjustments.

As mentioned above, if there's no or not detectable shift when turning the screw by 1/2  each wsy, for example, then something is wrong somewhere and needs correction before they can be competently set.

All working OK though, and Honda has a neat routine in manuals to set them correctly.

Essentially, both high speed full load jetting & low speed jetting are ultimately metered by the jet sizes in place in the carburettors. That's main, needle, pilot specification that aren't adjustable "on the fly" but selectable for absolute air fuel ratio.

What the air screw enacts is the feed in rate of the idle circuit, along with air fuel ratio at closed throttle running.

The routine is to fully warm the engine with screws set at recommended (one and 1/4 out for example) then starting on one carb only, turn screw out slowly (the engine needs to feed the difference through combustion) perhaps 1/8 turn at a time.
That should speed up that cylinder, eventually it should start to just missfire (its the air fuel ratio going too lean to fire consistently) stop there as its showing virtual parity in 14.7 part air to 1 part fuel (stoichiometric value for petrol) and making clear the reference point for engine and carb equipment, at the altitude your working in.
THEN turn the screw back inward slowly to give a 100 rpm drop for the engine speed. What this does is, from that stoichiometric position (it wont accelerate properly like that) is to now bring the air fuel ratio back to correct range, and likely about 12.7 to 1 air fuel ratio. Honda suggest an accurate rev counter to judge the speed difference.

This is now the reference screw setting for this cylinder.

The overall tickover speed can now be corrected if too high or low. Then, individually, the next three cylinders done the same one by one.

This loops round to getting a response from each screw, if they don't respond as expected, then the error needs identifying and correction before this, very fine tuning can take place.

[DomP]

Thats a great breakdown of the procedure!  Thanks, I'll dust off my desk fan and annoy the neighbours later this week

[Oddjob]

Dom, just reread what Mark has written and one thing that stood out for me was the advice to check the inlet manifold orings. Are they new, genuine Honda? I've seen some supplied that wouldn't seal as they were incorrectly sized. You think they are new so they can't be the problem but they end up being the problem when you check.

I hate to say it but I also strongly suspect the galleries feeding the air screw on one or more of the carbs are blocked or partially blocked. If they are then that would cause the mixture to go rich, wouldn't matter much what you did to the airscrew if that's the case. I always squirt liquid oxygen down these galleries and observe where it comes out, then check all the carbs are showing the same signs when they are checked. The air bleeds in the 069a carbs are bigger than those on the 627b carbs off the 500, clearly Honda did that for a reason, they also changed the airscrew itself as Phil says, so they were clearly concerned about something happening at idle speeds, most likely the pollution level laws which were being implemented at that time in the USA.

[K2-K6]

As Ken's observations above, absolutely critical that every route for fuelling is accounted for and verified.

Overall rich running though would usually tend toward wear or failure in metering to deliver too much fuel. Essentially uncontrolled delivery that will obscure any fine adjustment the idle setup is capable of controlling.

This thread https://www.sohc.co.uk/index.php/topic,32423.60.html  recently looked at inspection of main jet emulsion tubes along with needle profiles and their wear/condition, which was significant in it's variety.

The main jet should be all but "closed" by that hardware arrangement with throttle fully closed, to leave the fine/accurate metering to the idle circuit alone. Much in the way of discrepancy there will leave you with overwhelmed idle control and showing no response effectively to adjustment.

I believe Skoti found similar wear of some parts in 750 F1 necessitating replacing to resolve ongoing rich issues.

[deltarider]

Thats a great breakdown of the procedure!  Thanks, I'll dust off my desk fan and annoy the neighbours later this week

I remain sceptical. It's mere theory. I actually would welcome the first video that succesfully demonstrates this procedure... In other words: prove me wrong.
I have a gut feeling best method is to listen by a rubber hose to the pops the 4 individually exhausts produce at idle and compare them. Any of you that has managed to arrive at a perfect idle - meaning driveability fine and acceleration linear! - is invited to make a sound recording of all 4 exhausts to the benefit of all.

[DomP]

Hi Ken, I replaced the inlet manifold orings that were new due to a severe hanging idle, I realised that the full Vesrah kit I bought for my rebuild had tappet cover orings that are the correct size for this which cured the hanging.

I could be wrong but I blasted carb cleaner through all orifices and got it ejecting out of the relevent places but happy to revisit that.  Just to clarify, all correct Keihin jets etc and appear in good condition, new Keihin pilots.

One thing I have done in an effort to solve it that I think I shouldnt have is dropping the needles one notch to their last available setting, might reset this.

Hoping to get time this weekend to make some progress