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Stock carb fuel circuits traced

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There's been some discussion recently about the air diverter valve, its diaphram and circuits, how they work and its effect on the fuel/air ratio in a closed throttle, decelerating scenario.

This is the diverter valve. The vacuum port in the intake side of the carb, next to the slide routes to the small oringed port under the diverter valve cover. The related hole in the cover routes to the outer side of the diaphram, so that side of the diaphram has a vacuum enacted on it, and under certain conditions, is strong enough to raise the diaphram against spring pressure and close off the spring loaded check valve of the center port. Normally, the center pin of the diaphram holds that check valve open. That center port runs to an air port at the edge of the airbox side bell housing.

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Center diverter valve port.

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This port runs from the bottom/inner side of the diaphram to the pilot jet circuit.

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This port runs from the bottom/inner side of the diaphram to a port in the airbox side bell housing of the carb.

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This is the fuel screw passage.

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So, lets get some thoughts going as to how this thing is working.

I have always been under the impression that it augments the fuel/air ratio a little with fuel from the pilot circuit to prevent a lean condition when decelerating under closed the throttle conditions. And if the diaphram has pin holes in it, that will prevent a vacuum from being created on the outer side of the diaphram, keeping if from being able to rise and close off the center check valve.

One of my questions is, how does the center check valve effect how fuel from the pilot circuit is pulled into the system? Initially I'm thinking, with the center check valve normally open and in concert with the open passage of the edge port, there's balance/no vacuum on the underside of the diaphram. When vacuum raises the diaphram and shuts off the center check valve, there's enough vacuum under the diaphram to draw fuel thru the passage that goes to the pilot jet and then from there it makes it way into and thru the edge port and out thru the connecting port in the bell housing. This sounds kinda odd, so I'm thinking there might some small passages somewhere that I can't account for.

There is speculation on both sides as to if the diverter valve entrains air to the system to prevent a rich condition, or fuel to prevent a lean condition. With it being directly connected to the pilot circuit, I'm leaning toward the latter. But exactly how it does it, I'll be interested to hear your thoughts on.

Edited by Trailryder42
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As for your first point, I do believe you are right.  The valve is there to compensate for a temporary high engine vacuum when decelerating, enrichening the mixture.  And yes, I do believe it brings in more fuel to temporarily compensate for a lean condition.  At least that's what I learned years ago working on Hondas.

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TR42

Thanks for running those wires.

 

Here is my take on it. When the engine is at high vacuum, (thinking at idle and deceleration), the vacuum port pulls the diaphragm, opening a second air port to the primary jet, giving a lean mixture there. 

 

As soon as you open the throttle, the vacuum drops closing the diaphragm, reducing the air flow to the primary jet in effect richening the off idle response. Honda can set the mixture at idle fairly lean but the vacuum diaphragm prevents the engine from going terminally lean when you open the throttle.

 

Going down a big hill you close the throttle and the mixture leans out a bit.

 

Makes me think that by fooling with the spring you might be able to effect when the mixture loses it's leanness at idle.

 

Your carburetor is spotless, do you actually run fuel through it? ;)

 

I have a crf150 that had an external vacuum pot with 3 rubber tubes going to various ports. One port to the intake vacuum, a second to airbox side and the third to the pilot circuit.  I removed it and plugged the ports. One time I tried jumpering two of the ports with a tube and was rewarded with instant flooding. I had evidently tied the vacuum port to the pilot jet port.

Edited by lump_tha_thump

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The Honda decel system is just that, a decel system.  When manifold vacuum is high, like in closed throttle situations above idle speeds, the diaphragm can overcome the spring tension and open.  When open it allows more air across the slow jet emulsion circuit which draw a little more fuel into the engine through the slow speed circuit to help eliminate decel popping cause by excessive leanness.

 

There is no off idle enrichening or anything else associated with the valve.  If you alter the spring tension you will change the response speed of the decel circuit, nothing more.  If you go too soft on the spring it might affect idle by coming on at idle speed.   If the diaphragm tears you  will get no decel richness and noticeable popping, it will not affect anything else since the system is normally shut and vacuum operated.  Without enough captive vacuum it will just simply stay shut.

 

Some would be tuners recommend disabling the system for unknown reasons, I guess they like decel popping.

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The Honda decel system is just that, a decel system.  When manifold vacuum is high, like in closed throttle situations above idle speeds, the diaphragm can overcome the spring tension and open.  When open it allows more air across the slow jet emulsion circuit which draw a little more fuel into the engine through the slow speed circuit to help eliminate decel popping cause by excessive leanness.

 

There is no off idle enrichening or anything else associated with the valve.  If you alter the spring tension you will change the response speed of the decel circuit, nothing more.  If you go too soft on the spring it might affect idle by coming on at idle speed.   If the diaphragm tears you  will get no decel richness and noticeable popping, it will not affect anything else since the system is normally shut and vacuum operated.  Without enough captive vacuum it will just simply stay shut.

 

Some would be tuners recommend disabling the system for unknown reasons, I guess they like decel popping.

Very true about the popping.  It should be noted that the system being disabled, whether on purpose or by damage to the diaphragm, will not cause any discernable harm to the engine, at least within my experience.  The 03 XR250R I currently ride sat for around 2 years before I revived it.  I rode it for a year or so without fixing it, the popping is annoying but, like I said, didn't seem to cause any harm,.

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I've been reading quite a bit about this recently due to my own issues and other posts on this board.  Thanks for opening this discussion.

 

 

My take is that the pressure differentials discussed above are correct, and that the diverter effectively enrichens the mixture.  The difference in my theory is that the valve actually closes in a decel situation rather than opens.  Here are the two likely scenarios IMO:

 

1.  Valve opens on decel

The above responses argue that opening the valve allows more air above the pilot circuit, and thus drawing more fuel (simplified explanation).

 

2.  Valve closes on decel

My theory is that the valve closes, not allowing air to come from the "to center diverter valve port" (pic 5) and through the "diverter valve port to pilot jet" (pic 6), effectively lowering the ratio of air:fuel.

 

I don't find the first scenario as likely, because this would add even more air to the mixture.  More air + more fuel = same mixture.  Plus, the physical attributes of the carb points me to scenario 2.

 

 

TR42: can you tell whether the valve is open or closed when just sitting on the bench?  IMO this is key info.....at least to my understanding.

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TR42: can you tell whether the valve is open or closed when just sitting on the bench?  IMO this is key info.....at least to my understanding.

Because of the big spring under the cap, the center port is open. The little rod of the diaphram sticks down in that hole. The rod pushes on a what looks like a plastic check valve. I had to push the valve down to reveal the hole in order to get the wire thru, so that means the valve is open when the diaphram is installed and seated under spring pressure.

If you look closely at that little rod of your diaphram, you see near the end of its tip it is machined with a groove around it. I suspect this is so when the diaphram in its normal position of pushing the check valve down to open the port, the little rod doesn't block off the small hole. Air is able to move around it because of the way the rod is machined with that groove.

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In this situation if it opens the air bleed to the slow circuit when the valve is closed. Then when the valve is opened it would drop fuel downstream from the throttle valve much like some chokes do.  I don't know if the 600 carbs are different in this respect, I have never traced the passages, rather just went with what I had heard.

 

If it does work in this manner and the diaphragm ruptures it would draw fuel at an idle.  I know for a fact from past experiences on the 600 anyway, if the diaphragm ruptures it idles just fine and runs just fine, but the decel pop is noticeable and irritating.

 

I would think controlling an air bleed rather than a fuel bleed would be the desired way to avoid issues with diaphragm failure.

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I would think controlling an air bleed rather than a fuel bleed would be the desired way to avoid issues with diaphragm failure.

Wouldn't it be nice if engineers designed things taking into account failure as an option.

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I would think controlling an air bleed rather than a fuel bleed would be the desired way to avoid issues with diaphragm failure.

 

I think this is what is actually happening.  At idle, the air bleed screw is allowed to flow air along with the fuel/air mixture of the pilot circuit.  When the throttle is snapped closed from open, the diverter valve closes, and the air bleed screw no longer feeds the mixture.

 

In fact, on the parts schematic, the cover to the diverter is even called "air cut-off valve cover".

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That makes sense to me, as I said I never studied the passage configuration on my 600 carb, but it makes no difference when the diaphragm tears other than decel pop.  I had to put a new one in once to cure that very problem.  I put it off as long as I could since it was mounted between the carbs and I hate pulling those carbs off, but it just became unbearable after a while with the popping.

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The Honda decel system is just that, a decel system.  When manifold vacuum is high, like in closed throttle situations above idle speeds, the diaphragm can overcome the spring tension and open. 

 

We need to be careful here what we're calling "open" and "closed".

 

I agree, When manifold vacuum is high, like in closed throttle situations above idle speeds, the vacuum on the diaphragm overcomes the spring tension and is pulled upward, pulling its center pin out of the hole, letting the spring loaded check valve down there rise too, "closing off" the port, not opening it.

 

We also need to be careful when talking of something adding "more air" when it might actually be vacuum, or visa versa. That's what's messing with my head in some of the theories expressed so far.

 

What would be neat is if I could come up with a diagram of the circuits with color coded flows and directions for fuel, air and vacuum. I like visual aides. lol

Edited by Trailryder42

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Let's just let it be said that when vacuum is high the diaphragm is moved and magic happens.

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Did some more tracing and found that the center diverter valve port (pics 4 & 5 above) remains the size you see in pic 5, all the way thru to the check valve, meaning, its huge, a low pressure signal, compared to the edge port that narrows down to bread tie-wire size.

 

I was also trying to figure out, when the pin is holding the check valve open, how pressure was moving past the pin to have an effect on the underside of the diaphram. I was thinking the pin was an interference fit in the hole, but its not. I now see that pressure can move past and around the pin, up the hole and that's how it has an effect on the chamber on the underside of the diaphram.

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In this situation if it opens the air bleed to the slow circuit when the valve is closed. Then when the valve is opened it would drop fuel downstream from the throttle valve much like some chokes do.  I don't know if the 600 carbs are different in this respect, I have never traced the passages, rather just went with what I had heard.

 

I assume by "throttle valve" you mean the slide? And "down stream" would be in the direction of airflow thru the carb, so on the intake side of the slide. There are no passages on that side of the slide besides the vacuum port to the top side of the diaphram and the fuel screw drilling, so I don't how or where it could drop fuel as you describe.

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In fact, on the parts schematic, the cover to the diverter is even called "air cut-off valve cover".

 

Yes, as when high vacuum during decel raises the diapham and shuts that center check valve, flow thru that passage is "cut off ".

 

This is what makes me think that, with size of the center port being so big all the way thru, normal operation with the check valve being open doesn't allow the small drilling of the edge port to pull enough vacuum on the underside of the diaphram to pull fuel thru the pilot jet port. This goes towards my pressure differential theory.

 

Then upon decel and the high vacuum raises the diaphram, closing the center check valve, cutting off that low pressure interference, the edge port can then draw a vacuum high enough to draw fuel into the chamber from the pilot jet port. This is assuming all 3 of the airbox side bell housing ports are under negative pressure.

 

 

For the theory that the system introduces air to lean out a rich condition on the intake side of the slide during decel, this is how I see that working:

The only place for the carb to draw air from a source besides the closed slide cutout on the intake side of the slide, is the fuel screw drilling. So, when the diaphram is raised from high vacuum pressure and the center valve cut off, that low pressure interference is cut off and so the fuel screw drilling on the intake side of the slide, thats under high negative pressure (vacuum) and pulling fuel thru the pilot jet, is also pulling a vacuum on the lower diaphram chamber thru its connecting port with the pilot jet, which in turn pulls a vacuum on the edge port and that extra air is entrained with fuel up thru the fuel screw drilling and into the airstream, leaning it slightly. 

 

Shoooooey! My brain hurts. Time for pizza and beer.

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Did some more tracing and found that the center diverter valve port (pics 4 & 5 above) remains the size you see in pic 5, all the way thru to the check valve, meaning, its huge, a low pressure signal, compared to the edge port that narrows down to bread tie-wire size.

 

I was also trying to figure out, when the pin is holding the check valve open, how pressure was moving past the pin to have an effect on the underside of the diaphram. I was thinking the pin was an interference fit in the hole, but its not. I now see that pressure can move past and around the pin, up the hole and that's how it has an effect on the chamber on the underside of the diaphram.

Edit: wrote this before reading your last post.

 

 

Here's a little more descriptive explanation of what I think is happening.

 

 

Note that at all times, ambient air pressure is at the airbox side.  The diverter valve is closed (spring compressed) when there is sufficient vacuum on the intake side of the carb.  The access for this vacuum is the ports in pics 2 and 3.  Air coming to the diverter valve (and through when open) from the airbox side via the diverter valve port in pic 5 is again at ambient air pressure.  It's this difference in pressure that sucks the valve closed.

 

Idle & open throttle

Not enough vacuum to close diverter valve.  Therefore, diverter valve is open.  Air is passing through the diverter valve onto where the pilot jet hole comes out (pics 6&7).

 

Snapping throttle closed

Enough vacuum to close the diverter valve.  Therefore, the extra air above is not allowed into the pilot circuit, effectively richening the mixture.

 

Note that there also should be a passage between the pilot port/jet and the fuel screw.  This is missing in your pics, not sure if it's assumed or not.

Edited by Redpoint

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