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#715512 - Yesterday at 01:51 PM Re: Bonnie Carb Jetting help needed [Re: BrettF]  
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Nick H Online content
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I know there are other long threads on float level here and elsewhere. At the risk of making this another one....It seems to me that any float level higher than .08" would hit the bottom of the carb or the tickler and create a constant tickling situation. This is what happened to me as I was setting my stay up floats.


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#715533 - Yesterday at 05:54 PM Re: Bonnie Carb Jetting help needed [Re: BrettF]  
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koan58 Online content
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Brett,
I'm impressed that you put in the methodical time and attention to get this right, and that you compared the 2 ways of measuring to calibrate the easy manometer method against the fiddly but convincing direct method, rather than just assume the easy method is accurate to avoid doing the hard work!

I like your float spindle retention method, I will adopt that assuming you haven't patented it yet! With the tweak of using a bent staple to hook over the spindle, so it is easy to swap floats. This is so much simpler than my "bent bit of metal clamped by nut/bolt"!

It is obvious how you levelled the bowl laterally, though small errors in this won't affect your measurement much at all at the centre of the bowl, the bowl and float have symmetry laterally.

How did you level the bowl front-to-back? This is much more important as float and bowl are totally asymmetrical about the centre in this direction. Say the bowl leans toward the engine side, more fuel will move to that side, where the vast majority of the float is, raising the float (and hence closing the valve) earlier than it would if the bowl were level. Vice versa of course also applies. A small inaccuracy in levelling in this direction could invalidate fuel level measurements.

To level the bowl in all directions, I made 2 rubber cups that held the float bowl in the vice below the flange, with careful tightening I was able to hold the bowl from moving while being able to make fine adjustments to it in all directions.

I hear folk saying "the bowls are at an angle on the bike anyway". True, but to reliably set things you must have a reliable, repeatable baseline to work from, and level is an easy baseline to work from, and is what AMAL would have used in their technical advice, though their carbs are fitted to bikes at all sorts of angles.

Also, how did you actually measure the fuel level in the centre of the bowl accurately? It is easier said than done, as I'm sure you know.
I farted about with all sorts of ideas that required too many hands, none of them producing reliable REPEATABLE results, until settling on a bridge of straight machined 12mm square bar across the bowl. From this I could drop the depth leg of a vernier down until it just made contact with the fuel, several times to check for the smallest reading, and again several times after refilling the bowl. Only then was I sure of my results.

On your using 6" head of fuel. In my calcs I used 0.74g/cm3 so same as the stuff you're using. I found that the alloy needle lifted from the seat at ~12-13cm head. So 6" should be ok, just. However, if that is just in a tube, rather than a header tank, it maybe borderline for accurate testing. I used a funnel in a tube that could be kept at different heights between nails in a board to see the effect of different heads - no smoking! Then have your manometer plug as a drain, so you can drain and refill the bowl with ease, by lowering the tube.

There is much confusion over the phrases "fuel level" and "float height". The important thing is fuel level, that is what the engine sees, float height is only of secondary help, and only if certain conditions apply, namely that you are talking about the float and needle used in the recommendation.
Because of changes to float needle weight, and more recently the advent of the stayup float, the time-honoured relationship between FUEL level and FLOAT height no longer applies.
The .080 Float setting came (I believe) from the US Triumph agents in the early days of Concs, when they had plastic float needles and floats. I'd call it a "rule of thumb", because it is a crude, indirect method of setting Fuel level, but it was good enough for most.

Introduction of the brass needle changed the relationship between float and fuel height bigtime, and I think that is where a lot of this tapping the seat up and down has come from. Anyway, all praise to the alloy needle! It behaves close to the original plastic one, so as I found, putting it in with the old plastic float restored Fuel level to about where it was intended by AMAL.

Now you've put in a stayup float, made of different materials, not just air inside, and a different balance about the pivot. I think it is also deeper from top to bottom.
There is no reason to think that it will be bouyant to the same level as the plastic float. Forget float level, remember fuel level.

The bowl flange is just a reference point, the gasket and recess in the body allow the float to rise considerably further, so as long as it doesn't contact the roof before closing the valve (which will be obvious with flooding) it will be fine. Do use your setup to compare all your bowl components to make sure you haven't got a "freak".

Because an engine with taps closed can idle till it dies at tickover doesn't help much, would you do that at full throttle? An engine will run on a wide range of air/fuel ratios, but I'm sure we all know the difference in disaster between weak at tickover to weak at full throttle?

Brett, do you get any pinking when you open the throttle from say 2500 rpm, not wide open but say to half?

Enuf for now, Dave

#715571 - Yesterday at 10:38 PM Re: Bonnie Carb Jetting help needed [Re: BrettF]  
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Again thanks for all the input folks,

Hillbilly, I have a good idea of the relation between fuel level and manometer height now so could do it on the bike But I found the angle of the carb when on the head made it very hard to know where to measure it. When running I found the vibration made the level jump all over the place. I know some folks can balance an old pound coin on the tank when running but not me.

Nick H, thanks for the heads up - I plan to test a little tomorrow to see how high a float can go before topping out- I suspect it might be a bit higher than the top of the bowl. I will report back.

Dave, some synchronicity here. This morning I retested the two bowls. Previously I was measuring the depth between the float and the edge of the bowl. I then tried it - as you say - with a flat rule over the centre. My depth reading increased by 1 to 2 mm,,, So I'm around 9 mm now.... The meniscus between the float and the edge pulled the fuel up by that much (1 to 2 mm).

I'm focused on fuel level now - the float level is out the window,
One of the valve seats is already on the bottom /bottomed out so I will have to bend tangs to get the level where it must be on that one - the other still has some space to be lowered.

No pinking was heard.

BTW is it normal that the valve seat hole is not centred on the entry hole in the carb body? (ignore the swarf there- not sure where it came from...) Here is a pic of what I mean. (one can also see the seat as already flush against the base). If I ever were to knock the seat upwards, I'd probably ruin the seat beating on only one side of the hole like that...

[Linked Image]

Last edited by BrettF; Yesterday at 10:41 PM.

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#715582 - Yesterday at 11:40 PM Re: Bonnie Carb Jetting help needed [Re: BrettF]  
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koan58 Online content
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Where the hole ends at the bottom doesn't matter, as long as the seat has been properly made for the needle.
Would you like all my primitive scratchings on this subject? Happy to email them, don't wanna annoy anybody here.
Anyway, I think that you have begun to see the subtle aspects of this important matter for the engine, which most peeps skitter over hoping for the best.
If sure you have no intake air leaks, then the only way to proceed is to establish a fuel level, then jet from there.
I quite agree, you must measure in the centre, between float and bowl invites high error!
I reckon you're well on the way to understanding already! Dave

#715585 - 13 hours ago Re: Bonnie Carb Jetting help needed [Re: BrettF]  
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Hillbilly bike Online content
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Originally Posted by BrettF
Again thanks for all the input folks,

Hillbilly, I have a good idea of the relation between fuel level and manometer height now so could do it on the bike But I found the angle of the carb when on the head made it very hard to know where to measure it. When running I found the vibration made the level jump all over the place. I know some folks can balance an old pound coin on the tank when running but not me.



A manometer measures pressure difference. I'm talking about a simple fuel sight tube. The tube and bowl are both vented to atmosphere so there is no pressure...If you had used one on the tilted carbs you would actually see the fuel level....But .any way that works is the best way....


650 Triumph modified production LSR record holder 133.1 MPH... Twin 650 engine Triumph LSR that goes sorta fast...
#715590 - 13 hours ago Re: Bonnie Carb Jetting help needed [Re: Hillbilly bike]  
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koan58 Online content
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We're only using the word "manometer" as shorthand for the tube technique you describe. It is often recommended, but have you checked its accuracy as we have?
P;ease read the first paragraph of my earlier post. If you've never checked it, you don't know how accurate your tube is, do you? You just assume, which those who have taken the trouble to check, have found to be erroneous, and the reasons for the discrepancy easily understood and explained in preceding paragraphs.

#715595 - 12 hours ago Re: Bonnie Carb Jetting help needed [Re: BrettF]  
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koan58 Online content
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Brett, in 35 years of concs I've never knocked a seat up or down, and I doubt that AMAL expected the user to have to do it. I was set by the factory to be within the range of resulting fuel heights, using the original components.
Using alloy needles, you should not have to stray from the standard setup. Again compare needle seat depths (using the same needle) of your 4 carbs.
I can't imagine why yours would be different to mine, I had no problem setting the stayups, do be careful to only bend on the needle side of the spindle.
And as I said earlier, try one of your old floats for comparison. What fuel level do you get?
If, for whatever strange reason, you can't get the fuel level without the float banging through the roof, just set it as high as you can equally on both bowls. And jet from there. Though I'm less than happy with that - something odd going on!
Dave

#715597 - 12 hours ago Re: Bonnie Carb Jetting help needed [Re: BrettF]  
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koan58 Online content
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Actually, the more I look at that last pic, the less sure I am. I haven't any carbs out in the open to compare with, are your others as off centre as that? Just thinking of the banjo bolt that goes in there, it might cover some of that hole?

#715610 - 3 hours ago Re: Bonnie Carb Jetting help needed [Re: koan58]  
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Originally Posted by koan58
Actually, the more I look at that last pic, the less sure I am. I haven't any carbs out in the open to compare with, are your others as off centre as that? Just thinking of the banjo bolt that goes in there, it might cover some of that hole?



The banjo bolt does not seat in the bottom of the hole, so it won't restrict flow there.


Amateur Loctite enthusiast.
#715614 - 2 hours ago Re: Bonnie Carb Jetting help needed [Re: koan58]  
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Hillbilly bike Online content
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Hillbilly bike  Online Content
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Originally Posted by koan58
We're only using the word "manometer" as shorthand for the tube technique you describe. It is often recommended, but have you checked its accuracy as we have?
P;ease read the first paragraph of my earlier post. If you've never checked it, you don't know how accurate your tube is, do you? You just assume, which those who have taken the trouble to check, have found to be erroneous, and the reasons for the discrepancy easily understood and explained in preceding paragraphs.


So the fuel level shown on the sight tube with the carburetor mounted on the engine in the bike frame isn't an exact level of the fuel in the float bowl but setting the float in a mock up situation on a bench is?? Have I done it? Yes..


650 Triumph modified production LSR record holder 133.1 MPH... Twin 650 engine Triumph LSR that goes sorta fast...
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