CB750 K1 Valve Timing Question

[K2-K6]

Just thought of another key aspect of the carb idle setting routine.

Although written in the sixties, it will take account of fuel difference including any formulation changes and ethanol content.

It's using the fuel you have in use to properly calibrate the whole low speed running ability on these engines.

Very clever 

[cbxman]

Nigel,

Thank you for your in-depth analysis of what is going here.

Yes, the procedure should make allowances for varying qualities of fuel, bearing in mind these bikes were sold all over the world.

Interestingly, Hondaman on .net has written about how the ethanol content, that Mr Honda was probably not planning for, has affected the way ethanol alters the burn rate and temperature.  He suggests that the main jet emulsifier holes which have two different diameters, the smaller ones should be drilled to the size of the upper ones....I think.  Opening the holes up allows more air to the mixture without reducing the fuel rate through the main jet....I think.....

As I am lucky enough to live in an area where the premium fuels are slated as having no ethanol in them, I will reserve judgement.

Anybody seeing this information should check very carefully before altering their emulsion tubes.  They seem to be quite scarce items.

Cheers,

Jerry

[SteveD CB500K0]

Where in Suffolk are you Jerry?
We are all on the way.

Actually, I’m there already as mother-in-law lives in Woodbridge.

Berkshire Superunleaded is 5%


Sent from my iPhone using Tapatalk

[cbxman]

Steve,

I'm very near Needham Market.

Jerry,

[K2-K6]

Jerry, do you have a link to the hondaman thread about ethanol?  I'm interested in reading it but couldn't readily find it on that forum.

Thanks, Nigel.

[cbxman]

Nigel,

http://forums.sohc4.net/index.php/topic,164119.0.html

This is a long thread.  Checkout reply #27, then go on to #55, then #60.

All very interesting, particularly the arguments over the what the airscrew does!

Also mods to Emulsifier.

Cheers,
Jerry

[K2-K6]

I see what you mean on airscrew direction  I'm with MRieck on this one, turning outward makes leaner mixture on these carb.

It's opposite on PD and many CV type as the screw sits between the venturi and the inlet valve in comparison to older type slide carb with screw next to air filter.

[K2-K6]

Two tired #62 post is absolutely correct, and checked with exhaust gas analysis too 

Amazing they are all missing the Honda setup that brings combustion parity (at tickover)  to the engine via idle circuits and not through absolute setting of carb synchronisation.

[Oddjob]

They both can't be correct though Nigel, either it's leaner going in or leaner coming out. My understanding was that going in makes it richer but that's directly contradicted by Marks statement and also by Hondas reps during that time if I've read right, as in #70.

[K2-K6]

They both can't be correct though Nigel, either it's leaner going in or leaner coming out. My understanding was that going in makes it richer but that's directly contradicted by Marks statement and also by Hondas reps during that time if I've read right, as in #70.

Yes Ken, he's wrong as most of us understand it. There's contradiction in the explanation of how a jet flows by reducing below atmospheric in venturi that allows fuel to flow through the jet to the lower pressure area (correct) but that's ignored in explanation of this interaction.

If you had no air at all, then the idle jet would be full flow as soon as you started the engine, and giving no gradient of fuelling at all.
Bringing air into that stream (between float bowl and carb throat) allows air volume to mitigate that full fuel flow as the vacuum would rather suck air (less resistance) than fuel.
Passage more open  (screw out) just allows more air and less fuel. More closed and vacuum portion to fuel line increases and sucks more fuel in proportion to that restriction. 

[cbxman]

Not being an expert at all.  See my bumbling earlier in this thread.

The theory may be fine if you are familiar with fluid dynamics and can understand it, but simple test surely is to check plug condition against an airscrew setting.  that should settle it....

Jerry

[K2-K6]

Yes and easy to check. Turning in until it stops the cylinder just goes far too rich and black smokes like it's on choke (unburnt HC) and the opposite as you back the screw out.

The others on thread do offer a more scientific basis with anecdotal supporting analysis though. All academic though as thread age is older in that case.

Jets are more akin to siphon though rather than anything more sophisticated. Try sucking liquid up with a straw, then put a hole in the side of the straw, you'll just get air bleeding in if air enters above liquid level.

I don't understand the reasoning behind changing emulsion tube holes in response to ethanol content. They are just there for atomisation of fuel droplets in the mixture (left as they are would seem reasonable)  and doesn't doesn't deal with combustion characteristics that seem associated with ethanol mix.

Essentially "emulsion" arrangement helps produce a stratified charge to burn effectively.  Meaning, each fuel droplet would be surrounded by 14.7 parts oxygen content air, the more evenly and homogeneous this is the more consistent the burn rate across the combustion chamber. It wouldn't appear to need any shift there to accommodate it, and certainly would need some very concise observations to understand the effects.

[cbxman]

Nigel,

It seems that the air entering the throat by way of the twist grip is the air that sets the 14.7, demand from the needle jet. I guess the emulsion tube makes the petrol flow out of the needle jet in smoother and, as you say, homogeneous way.  However enlarging the emulsion tube holes even ever so slightly must add air to the mix, which I guess means leaning the overall mixture.  Mark suggests that ethanol in the fuel makes the combustion burn cooler.  So maybe the extra leanness is compensating for the coolness and makes the combustion run hotter,   maybe ?

Perhaps a reduction in main jet size is too crude for such subtlety?

Jerry

[K2-K6]

Obviously it's a complex topic, interesting piece for discussion

"Ethanol molecules include oxygen atoms, whereas gasoline molecules don't. That's part of the reason why ethanol has less energy than gasoline. Another effect of the oxygen from ethanol is that ethanol blends tend to run "leaner" than pure gasoline because there is more oxygen available in the fuel-air mixture. If your engine is not able to compensate by reducing the incoming airflow, the resulting combustion conditions in the engine cylinder may be less than ideal. Newer vehicles are generally designed to take care of this automatically, but older engines may need a bit of manual adjustment to get the air-fuel mixture just right.

Some people have reported engines overheating when ethanol blends are used, suggesting that ethanol burns "hotter." This is a bit mysterious since ethanol contains less energy per unit volume than gasoline, and the flame temperature of ethanol is more than 40°C cooler than gasoline. The most likely explanation is related to the air/fuel ratio. Most engines are designed to run with an excess of fuel relative to the amount of air (a "rich" mixture); experience has shown that this leads to higher power output and cooler engine temperatures. When ethanol blends are used, newer engines are equipped with sensors to adjust the air/fuel ratio automatically. Older vehicles and small engines may not be equipped to do this, resulting in a "leaner" burn that may increase engine temperatures and/or reduce engine power. A simple adjustment to the fuel system to "richen" the mixture can often fix this problem."

This effectively contradicts adding more air.

[cbxman]

Nigel,

Interesting stuff, so why open the emulsifier holes.  That would make sense of Mark's explanation of adding extra air with the airscrew which enriches the idle mixture.  However, we know that adding extra air via the airscrew to the idle jet actually leans it out.

Maybe the difference between how an idle jet works and how a main/needle jet works is the answer.

Somewhere, the emulsion tube function is explained, maybe in that same thread.  The lower holes are under the fuel height in the bowl, the upper (smaller ones to be opened up) are above the fuel height.  This would imply weakening the mixture if we are observe the equivalent in the idle circuit.  Marks explanation of making the fuel bubble up faster would make sense in making it richer.....

Confused

Jerry