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Series "A" Twin front carb float level


greg brillus

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Hi there all, I don't know what sort of response I will get to this question, but I have Rodney's twin here, plus another I have been building. It has had carb issues from the start and I'm not sure Neal's is 100 % right either. It appears to me that the float level for the horizontal mounted carb needs to be to the center of the bowl banjo bolt, the carb body is then tilted slightly down at the bowl end so the main jet is always covered in fuel. I know the inlet port at the bottom of the jet block is in a different position to compensate for the orientation of the carb body, but this is only for the idle and part throttle fuel supply. I think one issue here to is that the original twins basically had 276 carb's whereas Neal and Rodney have new 289 Shadow type carbs on........You might think, well big deal, that should not matter too much.........Only thing is that because these carbs are quite a bit larger than the 276, there is far less room to move (rotate) the carb without the banjo bolt hitting the rear cylinder exhaust valve spring. Also for those who know a bit about these early Amal carb's, the lower brass cap that holds the jet block into the body come in different heights, at least three that i have here, but I'm not sure for the 289's. Can someone who knows give me some idea as to float heights for these front (Horizontal mount) carbs............Even checking the fuel height is quite difficult because the usual clear hose and "water level" procedure as per the post war bikes does not work on these carb/float combinations. Many thanks and cheers for now.....................Greg.
 

nkt267

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Also for those who know a bit about these early Amal carb's, the lower brass cap that holds the jet block into the body come in different heights, at least three that i have here, but I'm not sure for the 289's.
Sould the type of bottom nut make any difference to a horizontal carb as in theory it would not alter the height of the float chamber.
 

greg brillus

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It would move the bowl and the banjo bolt further in toward the center of the carb, this would allow more room so the carb can be rotated on it's stub changing the float attitude. I doubt that these carbs are meant to be mounted with the body truly horizontal, but somehow the main jet still needs to be immersed in fuel.
 

Robert Watson

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I guess I am just lucky...........

When I built my A twin it came with 1.3 carbs and they were completely trashed. I ended up giving them to Glyn Baxter for his very original restoration (1936 show bike). My bike had been owned by 6 different teenage boys from the mid 50's to the early 60's so you can just imagine.

Anyway I had a pair of worn Shadow carbs, so stuck them on. They worked OK but of course the idle was crap. To make a long story short, I bought a pair of new shadow carbs, less floats, and just stuck them on. I ended up swapping the front slide out for a bigger cutaway as shown for an A Rapide in Richardson. It starts well and runs well, and idles just fine. I have never changed a thing on the jet blocks. It could be I am just insensitive and/or just lucky.
You are right about them barely squeezing in there. I have to take the bell mouth off the front one to get it to go on and then fiddle it past the valve spring and screw it on after. And you better hope you hit the idle correctly as it is near impossible to change once the tank is installed.
 

Viny4

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I made my 1 1/4" Amals for my A+. No float issues, put the hole in the Jet block on the bottom, both slides are .360". Runs well and idles. The jury is still out on the cutaways,might change to .375+.
 

greg brillus

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Thanks for that, but I'm still interested to know where the actual float level should be.....If it is about the center of the hole in the float chamber, then this would be level with the main jet..............On these bikes I think Neal has set them up so the float level is bellow this, and I don't see how the engine cannot starve for fuel. If the carb could be tilted slightly so the float end is down a bit, this would stop fuel running out of the jet and down the mouth of the carb. On the front carb we also have added a small brass thumb wheel to the mixture screw, and I put a piece of wire in the slot of the idle stop screw so it is easier to adjust both.........Can't be worse than an original 276 on the front of a post war Rapide to get to.
 

chankly bore

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Greg, I vaguely remember seeing a Meteor in about 1970 with a rear head and an 89 carburettor. The owner told me he set the float level so he could just see fuel at the level of the holes at the union nut. At least that may give you a starting point. Clarification; he set the float on a standard chamber and transferred it.
 
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greg brillus

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It is somewhat difficult to understand how exactly these horizontal carburetors function, as the mainjet is dead center of the mixing chamber, so if the float level is bellow it, to me it might starve of fuel, whereas if the level is up with the hole in the jet, then what stops fuel continuously flowing through it and ultimately straight down the inlet port and flood the cylinder. My thoughts are that the float level should be somewhere near the center of the jet , but with the carb body slightly tilted so the bowl end is down just a touch......Enough to not let fuel run, but simply rely on the suction from the main venturi to draw fuel from. I set the front carb up in this way this afternoon, and the bike started and ran well. Whilst I had the jet block out I also ran a number 76 drill through the idle circuit passageway just to be sure of good fuel delivery at low engine speed like I do on all the new Shadow carb's and this makes the low speed tuning much better. I also did this to the rear carb as well, and checked the float level in the normal way with a fitting in the banjo bolt and a piece of clear tube. The level for the rear carb is tricky too as the rear one angles slightly upward, so the level needs to be about 3 mm bellow the center top of the large lower brass cap nut, or else fuel will dribble from the small hole where the locator peg for the jet block locates. Sorry for all this useless dribble..................
 

Marcus Bowden

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Twice on Route 66 I had engine fill with fuel ! I had been fiddling with bowl heights as unbeknown to me the left hand fuel tap was being blocked with a sludge like substance (new tank rust free) ? the last time was at John Hugel's last day run into Santa Monica high speed all the way (Bill Easter was waiting) never faltered once. But I also never understood the front carb set up but Jeff Smith did a very comprehensive explanation of it when he was section organizer of Aldashot and was building up a twin at the time. If it floods the carb is in a down draft position with inlet valve open bingo. Checki the plug first time round Harvey was holding the plug to check spark and fuel gushed out of the plug hole !!!! good job the spark wasn't there.
Havey was very pleased too.
bananaman
 

greg brillus

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Thanks for your response Marcus, I was wondering when you'd chime in. Ok my thoughts are that if the float level is correct, the fuel level is actually bellow the center of the main jet and after discussions with Neal, we both conclude that the fuel is actually sucked into the mainjet from the engine. If you tickle the carb's fuel bowl it will easily start to flow additional fuel which will poor down the intake and flood the front cylinder. I feel that it is probably best to have the float level per standard, assuming the groove in the needle is right, and have the body of the carb tilted so the bowl end is down slightly. Because the slot in the carb body that locates the jet block is on the top side, then fuel should not run out of there, provided the lower cap gasket is in good condition and seals correctly. I have had Rodney's bike running several times and ridden around the block, but it has a low speed hunting of the engine and vibrates quite badly running more like a 500 single. I feel it is ignition related, still looking into it at this stage remember that they are running the Powerdynamo units on these bikes.
 

Marcus Bowden

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Neal & Rodney had starting problems on the Tassie Tour back in March. I rode Rodney's and it felt new (which it is) and it went well, their ignitions did give problems. Now going back to the carb set ups, the slot in the body for the jet block to locate, mine has the air screw & throttle stop on top and I've fitted them with wire extended wing nuts so they can be tweeked at the L.H. side of the fuel tank. My bother Harvey's is underneath. As I wasn't sure and use to instrumentation I turned up 1/4"BSF banjo bolts and banjo with clear plastic tube and cable tied them to the side of the float bowl, also the front tickler is bloody awkward to get at so made with some springy s/s sheet an "L" bracket inverting it upside down, 1/4" hole so the bowl top pinch screw at one end then a hole to fit over the 1/4" Whit hexagon kinking it up so it clears the tickler and the upside down "L" side is hanging down the bowl, easy to find the bowl with gloved hand and squeeze it. The only thing I worry about is the air passing over the tops of the plastic tubes and wicking out my fuel but as I'm getting 150 plus miles to a tank and Harvey only 120 tops I'm not going to worry about it just yet. But it is nice to see the levels.
 
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Marcus Bowden

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DIDN'T READ RIGHT THE BANJO BOLTS WERE SCREWED INTO THE BOTTOM OF THE BOWL WHERE THEY ARE DRILLED INTO THE BOWL THEN THE SCREW TO BLANK IT OFF. SORRY.
bananaman
 

greg brillus

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It would seem that the level for the front carb is at the base of the large banjo hole in the float chamber itself, or just above, but certainly not near the center. It is a very odd arrangement from the usual carb/float set up. The ignition has been revised so it now works more like coil ignition to help with the hard staring, with a switchable advance and needs an on/off switch.
 

kettlrj

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Hi Greg.
In a normal vertical carb, the fuel is drawn through the main jet by the vacuum created by the choke. If you set the fuel height of your horizontal carb at just below the main jet, then the vacuum will pull fuel up to the jet because the jet is in a sealed chamber and nature hates a vacuum. There have been many bikes fitted with horizontal carbs including Rollie Frees bathing suit Rapide so they do work. So set the level below I the jet which will prevent fuel running into the cylinder.
Good luck.
Richard
 

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