MKIIA 850 carb needle: bean can vs peashooter

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Deets55 said:
After adjusting the fuel level to the proper height, I noticed that when the float is in the full dropped position that it still does not lift up on the float needle. That indicates that fuel delivery depends solely on fluid pressure from the tank.

As an experiment I set the fuel level and then bent one tank up to pull the needle up off the seat as the fuel level in the bowl drops. I have close to 2,000 miles since I made the adjustments and not seem any negative affects. On the plus side I was able to get a higher fuel level and it will tickle a lot faster.
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Thanks for sharing Pete, very interesting! I was too absorbed in the fuel level to notice the effect you mention – nor did it occur to me to compare the two different types of float for the height between the tangs and the tops of the floats, that might be interesting too and I'll check both things next time, maybe tomorrow. Cheers.

L.A.B. said:
Try checking the fuel level with the float bowl removed from the carb.
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Good idea L.A.B., thanks again, I'll do that! Cheers.
 
L.A.B. said:
Try checking the fuel level with the float bowl removed from the carb.
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Good idea L.A.B., thanks again, I'll do that! Cheers.

Later: I've now had the 'problem' bowl from my right carb, plus Stay-Up float and aluminium float needle, off the bike and set up with the fuel hose attached plus the sight hose up the side for good measure – I got around the issue of only having a fuel hose for two carbs by setting up an old, spare float bowl alongside. Having done it this way, I can see that this kind of 'bench testing' is the only really reliable way, and I'd never have tried it but for L.A.B.'s suggestion, so thanks again for that. Problem is, as Pete reported, I found that the Stay-Up float doesn't lift the needle off its seat, so I felt that there was no way of knowing at precisely which point the float stopped rising as I injected fuel into the hose, short of shelling out for a dial gauge – sorry, I don't have one. I held the float spindle down with a cocktail stick btw, so my fingers were well out of the way. I guesstimated the setting for the float by poking the float down into the fuel with another cocktail stick and listening/feeling for the float clunking against the needle as it popped back up. Anyway, whether bending the float tabs or moving the needle valve seat, I couldn't get a fuel level up within the desirable range without the float reaching for the sky. I have to admit that I'm now very tired of the fiddle-faddle, mostly because I have to do it all in my kitchen (with doors etc wide open) while leaving tools etc in the street, vulnerable to passers-by. I'm sure this contributed to my problems. I'll try again another time, when my patience levels are restored! I expect I could improvise a measurement of when the float stops rising with a bit of bent wire ...
 
NortonMKIIA850 said:
Problem is, as Pete reported, I found that the Stay-Up float doesn't lift the needle off its seat, so I felt that there was no way of knowing at precisely which point the float stopped rising as I injected fuel into the hose,
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Even with a reasonably low head of fuel, the alloy needle should be forced off the seat by the head of pressure, it shouldn't rely on the float tang physically lifting the needle off its seat as is often the case with the heavier brass needle.


You reported the low fuel level problem before you fitted the StayUp float so I doubt the float is the actual cause. It's also possible to reproduce the same 'no needle lift' situation with the white floats by raising the seating too high so I don't exactly agree it only applies to the StayUp foats, after all, the other carb fuel level is where it should be.
When you bent the StayUp tangs, are you sure you bent only the tangs as it's important not to bend the arms of the float?



NortonMKIIA850 said:
short of shelling out for a dial gauge – sorry, I don't have one. I guesstimated the setting for the float by poking the float down into the fuel with another cocktail stick and listening/feeling for the float clunking against the needle as it popped back up. Anyway, whether bending the float tabs or moving the needle valve seat, I couldn't get a fuel level up within the desirable range without the float reaching for the sky.
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I would say there's a fair chance the seating in that particular float bowl, for whatever reason, is too high. Maybe tap it out and check there's no dirt trapped beneath it? You didn't happen to measure the depth of the needle seating by any chance?
 
Even with a reasonably low head of fuel, the alloy needle should be forced off the seat by the head of pressure, it shouldn't rely on the float tang physically lifting the needle off its seat as is often the case with the heavier brass needle.
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Les,
One thing I did notice was the issue I am having was less pronounced with a full tank of gas as opposed to a half of tank. So I will assume that head pressure will affect how much fuel flows pass the needle.
As far as the float not needing to lift the needle, I wonder if maximum fuel flow can be obtained by simply relying on fluid pressure as opposed to the needle being opened completely by the float tang. When cruising on the highway and opening the throttle I don't seem to get a starvation issue, but when doing a WOT 1/2 mile sprint I have a problem. I'm in triple digits by then so every test is a risk I really don't want ( or need) to take. It just seems that the bowl runs out of fuel. Nothing scientific here just seat of the pants observations.
I contacted Amal about just buying float seats ( I have extra bowls) to experiment with but have yet to get a response from them.
Pete
 
Deets55 said:
Even with a reasonably low head of fuel, the alloy needle should be forced off the seat by the head of pressure, it shouldn't rely on the float tang physically lifting the needle off its seat as is often the case with the heavier brass needle.
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One thing I did notice was the issue I am having was less pronounced with a full tank of gas as opposed to a half of tank. So I will assume that head pressure will affect how much fuel flows pass the needle.
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Yes, I think it does.


Deets55 said:
As far as the float not needing to lift the needle, I wonder if maximum fuel flow can be obtained by simply relying on fluid pressure as opposed to the needle being opened completely by the float tang. When cruising on the highway and opening the throttle I don't seem to get a starvation issue, but when doing a WOT 1/2 mile sprint I have a problem. I'm in triple digits by then so every test is a risk I really don't want ( or need) to take. It just seems that the bowl runs out of fuel. Nothing scientific here just seat of the pants observations.
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I suppose it's possible that the float valve can't keep pace with the Commando engine's 'thirst' under those conditions with the available head of fuel?

Try a (low-pressure) fuel pump? :wink:
 
L.A.B. said:
Even with a reasonably low head of fuel, the alloy needle should be forced off the seat by the head of pressure, it shouldn't rely on the float tang physically lifting the needle off its seat as is often the case with the heavier brass needle.
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That is my feeling too, especially given that there's a whole lotta shakin' goin' on! Not saying Pete's wrong though, either ...

L.A.B. said:
You reported the low fuel level problem before you fitted the StayUp float so I doubt the float is the actual cause. It's also possible to reproduce the same 'no needle lift' situation with the white floats by raising the seating too high so I don't exactly agree it only applies to the StayUp foats, after all, the other carb fuel level is where it should be.
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Yes indeed, I'm not blaming the Stay Up float, my problem is I haven't got my head around what's going on yet – btw I eyeballed the Stay Up vs the white plastic floats, to compare heights between tangs and tops of the floats, and they seem the same, so I don't see why it should be a problem with the Stay Ups.

L.A.B. said:
When you bent the StayUp tangs, are you sure you bent only the tangs as it's important not to bend the arms of the float?
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Good question, I was aware of that possibility so did my best!

L.A.B. said:
I would say there's a fair chance the seating in that particular float bowl, for whatever reason, is too high. Maybe tap it out and check there's no dirt trapped beneath it? You didn't happen to measure the depth of the needle seating by any chance?
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Another great idea, and no, sorry, I didn't measure that depth. Thanks for the support, I'll keep on at it, hopefully having another stab later this week. Cheers!
 
Try a (low-pressure) fuel pump? :wink:
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I have giving that a thought.
Not in my budget this season, but a nice pair of flat slides may be in the future. But then I would have to bin my plastic air box that I really want to keep (and might be part of the problem).
Pete
 
Deets55 said:
... a nice pair of flat slides may be in the future. But then I would have to bin my plastic air box that I really want to keep (and might be part of the problem).
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I'm pretty convinced the plastic air box isn't a problem, so long as the rubber venturi that fit over the bottom intakes are ditched. I discussed this years ago with one of the tech team for the NOC, and that was his view anyway, that it's a carb thing, although he didn't mention the bean can silencers. The plastic air box is a dirty great thing, and so long as the venturi are gone it has much more intake area than the carbs. I don't see the filter itself as a problem either as it's pretty big too. I think it's about 8"x6", less a corner segment to fit around the frame, and looks very breathable compared to a paper filter that has to be junked. It just needs to be washed and re-oiled occasionally, I don't know how often, maybe someone has a spec for that? On the other hand I want to keep mine too, so maybe I would say all that! :D
 
If the plastic air box without the snorkels on the inlets was a cause of air restriction then the main jets would not need to be increased match the MK2 at 260 when fitting peashooters to a MK2A but 240 or 250.
 
If the airbox is restrictive doesn't that mean the main jets and the rest should be set leaner ? Fitting more or less restrictive exhaust system also creates the need for a review of the jetting.
 
I was pretty sure that's what kommando's saying.
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Yes exactly my meaning, as the 260 mains with peashooters and black plastic air filter is also the same as the MK2 fitted ham can air filter with peashooters then that suggests the black plastic air filter is NOT more restrictive.
 
I think that's settled then? If so, in future when anyone's worried or dismissive about the plastic air box, can we just point them here? :wink: Btw I just did a 70-mile round trip to see my folks, man once it gets going the bike's singing, I swear! It even idles ok once it's warmed up properly, but it still pops a bit on the overrun, and is liable to hesitate picking up from a very low throttle opening – I've got some swanky anodised slides coming from AN (cue someone telling me that they're rubbish ...!), if that low-throttle weakness persists then it's back to the fuel levels ...!
 
In the days before we had air-boxes on bikes we sometimes use to put our fingers over the bell-mouth to richen-up the mixture while the bike warmed up. If the air-box has got a hole in it, you cannot expect the mixture to stay correct. In fact, when we were production racing small two-strokes, one of the dodges was to cut a hole in the air-box up under the seat, so the mixture would be leaner and the bike much faster.
 
Ok, so now I've fitted the hard-anodised throttle slides (3½). From cold there seemed no change, but once warmed up it was idling too fast! That was a dramatic change. And rpms were slow to drop on shutting off the throttle. After adjusting for idle, the rpms now drop as they should. The left pot's giving a nice, regular tickover, but the right's wiffly-waffly at its best, and that's the carb where I've yet to get a good fuel level. So I'll get back in there soon and have another try. The slides I replaced must've been quite worn thought I, and indeed a gauge shows them to be a bit oval. Another step in the right direction, at least. Also I've yet to try probing the pilot jets, but I think they must be contributing, as there's a response in both carbs to adjustment of the air screw. Maybe that ain't necessarily so, we'll see ...
 
As I understand it, the pilot jets provide fuel supply right across the whole range of throttle openings. But their main function is to provide fuel when the throttle is snapped shut - important in two-strokes to stop seizures.
 
That figures – I know that this is essentially all basic physics, on paper I understand it, but I get fuddled in the doing. But, now I think of it, I had to screw the air screws in quite a lot to approach a decent idle – I thought it was out, but now I'm pretty sure it was in. So my guess at this point is, that they'd been way open before to encourage air to go that way at idle, picking up fuel along the way, when it was easier for it to just go around the slides ... With the new slides, too much air was going through that circuit, and causing the fast idle/slow drop in rpm. Make sense? Or exactly wrong? I really need an assistant to film what I do so I can remember it accurately later ... Which way to the retirement community?! :roll:
 
Ok another minor update: I confirmed today that the new slides were not quite reaching the idle screws on shutting off the throttle! Strange ... I'm fine with the idea of new carb parts = new settings, but I don't think I've ever come across this before. The old slides were standard Amal that came with the carbs many years ago, the new ones are the 'hard anodised' type, that are much more solid structurally, but who knows yet how long they'll last ... Presumably there's a difference here. Anyway I just adjusted the throttle cables via the adjusters in their caps, screwing both adjusters in two full turns in the hope that I wouldn't disturb their balance, and the slides are now contacting the idle screws. But it's about to rain here so further experimentation and news must wait for another day! Or two. Or three ... Off idle though everything seems very happy! :)
MKIIA 850 carb needle: bean can vs peashooter
 
NortonMKIIA850 said:
I confirmed today that the new slides were not quite reaching the idle screws on shutting off the throttle! Strange ...
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All four anodised slides I've fitted in part-worn carbs needed (the carbs) 'opening up', (and not just by rubbing down the odd high spot in two of them) before the slides would drop as they should, although afterwards, they were like new carbs again.

NortonMKIIA850 said:
The old slides were standard Amal that came with the carbs many years ago, the new ones are the 'hard anodised' type, that are much more solid structurally, but who knows yet how long they'll last ...
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They don't seem to wear anything like as quickly (so probably reduce the carb body wear) so that's certainly one good thing about them.
 
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