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gas soaked diaphragms
- Thread starter xschris
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- carburetors
My XS1B did that once long ago. A slow (very slow) weepage of fuel from one of the petcock mount screws would drip down onto the carb top. Fingering around the petcock base mount found the wet screw...
Haha, well, clears up THAT question.
That side of the diaphrams is exposed to venturi vacuum. Vacuum signal pulls on the diaphram and lifts the slide. Vacuum drops, the spring and weight of the slide lowers the slide. When the slide lowers, the airspace above it is filled with whatever is near the slide's bottom opening, usually an air/fuel mix coming from the needle jet.
In normal operation, CV type slides hover in their demand zone, with little oscillation.
In abnormal cases, the slide can flutter wildly up/down, and this pumps a lot of air/fuel mix in/out of the slide's bottom opening.
I've seen this fluttering caused by two conditions:
Strong inlet vacuum pulses, caused by leaky intake valve, or severe reversion.
Some CV carbs have a shared opening to the choke (enrichener) system, at the bottom cavity, the atmospheric side of the diaphram. Unwanted vacuum signal from this enrichener port (like from unclosed valve) can cause slide flutter...
That side of the diaphrams is exposed to venturi vacuum. Vacuum signal pulls on the diaphram and lifts the slide. Vacuum drops, the spring and weight of the slide lowers the slide. When the slide lowers, the airspace above it is filled with whatever is near the slide's bottom opening, usually an air/fuel mix coming from the needle jet.
In normal operation, CV type slides hover in their demand zone, with little oscillation.
In abnormal cases, the slide can flutter wildly up/down, and this pumps a lot of air/fuel mix in/out of the slide's bottom opening.
I've seen this fluttering caused by two conditions:
Strong inlet vacuum pulses, caused by leaky intake valve, or severe reversion.
Some CV carbs have a shared opening to the choke (enrichener) system, at the bottom cavity, the atmospheric side of the diaphram. Unwanted vacuum signal from this enrichener port (like from unclosed valve) can cause slide flutter...
I used another set of carbs and had the same issue. Although no fuel is shooting out of the carbs like I have seen from bad intake valves, it does seem to be amount of air leak from the intake valves to be causing this. The bike had sat for about 4 years and I have put about 400 miles on it after a go through. The motor has great compression and uses no oil. 
I may just swap in another motor I have till I can get the valves lapped.
I may just swap in another motor I have till I can get the valves lapped.
With airbox/filter stuff removed, you can watch the slides as you work the throttle, see if they're fluttering. I'm not familiar with that bike at all. Don't know about it's exhausts, inlet configuration (vacuum lines and such), valve train setup, ...etc.
Flying blind here. I'd wait until others can post up...
Flying blind here. I'd wait until others can post up...
Same exhaust as the 80-84 xs650 same carbs as the 80-84 xs650. But has a 180* crank. I ran the bike with the stock air box off so I can see the slides. At idle they set still but when I rev it, they do appear to vibrate a little.
Do you see any signs of "hovering fog" in the carb venturi area during this test?
If yes, maybe leaky valves or exhaust reversion/blockage.
If no, you might want to check float heights and needle seating/sealing in slide bottom...
If yes, maybe leaky valves or exhaust reversion/blockage.
If no, you might want to check float heights and needle seating/sealing in slide bottom...
If you mean a misting out of the venturi, no. There was no sign of that. It did feel like a bit of air from throats but no gas mist or very much. Both sets of carbs are rebuilt and floats where set to spec. I did a little more testing. I did a dry compression test on both sides and got 135 on each. Not bad, but new is 160 and the bike has only 14k on it. The next thing I did was to clean off the intake valves while they where in an open state. I just sprayed them with carb clean and let them dry for a while. Then I did a compression test and got 147 on each side. Next I set each side to TDC and sprayed a bit more in each intake port. The both held the fluid. I then dried them out and put the boots and carbs back on. The bike fired up with no issue ( like it always had) and re-synced the carbs. I did notice the mercury was now higher in the scale so I believe the compression is still up. Took it for a good 20 mile run and it felt very good. I didn't check the diaphragms but the plugs look good and the head temps are 15* cooler than before.
Haha, I guess I slip back into '70s lingo too much. Maybe there's a modern term for this phenomena.
When you view down the throat/venturi of a carburetor during full-throttle operation, you'll normally see fuel spraying out of the jet(s) like paint out of a paint gun.
With reversion, when the reflected exhaust pulses travel back thru the intake valve during overlap TDC, this 'spray' appearance becomes cloudy.
With severe reversion, at a specific rpm, this cloudy appearance takes on the appearance of a dense fog, that seems to just hang there, almost as if it isn't making it into the engine, just kinda 'hovers' there. In fact, the engine torque will be way down at this point, because it's not ingesting fuel.
The upside to this is that when at the higher appropriate rpm, the engine inlet/head/exhaust pulses combine to provide scavenging that can increase the engine's volumetric efficiency well beyond 100%, and torque skyrockets.
You can see the HP/Torque curves of these engines and see the telltale dip and peak of the torque curve. The depth of the torque dip is an indicator of this 'reversion' zone, the peak being the 'scavenging' over-ingestion zone.
An engine that ingests the same air/fuel mix at all rpms would, theoretically, produce a flat torque curve. But, since sound/acoustics figure into the breathing equation, you'll get these low/high flow zones, depending on the system's harmonics. Engine system designers do things to mitigate these effects, like that 'back pressure' thing. Racing engine builders go the other way, to maximise the effect.
(Sorry, brain ran outta steam)...
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