The BSA A7-A10 Forum
Technical (Descriptive Topic Titles - Stay on Topic) => Amal, Carburation, Fuel => Topic started by: Butch (cb) on 21.07. 2011 16:26
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I seem to suffer this when the bike is cold - and possibly only after a dampening. The bike is fitted with a new concentric carb. I've had the carb apart several times as I also had to remove some casting flash and nibble the float bowl gasket to stop the float sticking down. I have a (nasty) after market twist grip fitted, same style as original. I've replaced parts of this from another (nasty) one I got on ebay, and tidies up the castings on that too. I've fooled with the cable routing several times. I'm suspicious the problem might be with the slide itself - I used to see this with the late model concentrics that were fitted to my '79 Bonnie and an Armstrong Rotax I had - really hateful carburettors those, I always thought.
Thoughts? I'm not enjoying the revving when I fire up my 1000 mile motor from cold.
Thanks.
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Just a thought, you probably had it already, but concentrics are notorious for distortion and slide seizure if the flange nuts are tightened up 'properly'. (As well as the other defects you mention!) They want to be not far past finger-tight, using a low-strength loctite or some fancy nuts. Loctite's what I use anyway and they seem to stay on while being lightly tightened. Also, decent correct spacers between flange and manifold, which must keep some of the heat off the delicate little swine I guess. They really are a pain ITA compared to Monoblocs I reckon. Plus the floats are going to be eaten by ethanol, some people say . . .
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Have you got a tight bend in the cable were it exits the carb and across the frame top tube?
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Got my money on the overtight flange nuts.
Cheers
PS when running methanol at Eastern Creek the carbs would freeze full open by the end of the straight. Fun going into turn 1 at warp speed.
Cheers
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I also think the flange nuts are most likely the problem.
I have found that throwing away the "O" ring helps. Just use a gasket. I think the "O" ring swells up and makes the problem worse. Have to admit to not being a fan of Concentrics.
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Interesting what you say about the O rings Brian. Seems all wrong, but I have ditched them on the pair of 626s on my hybrid G9 (for the bad reason that I didn't have any new ones to hand last time the carbs were off). Just used paper gasket, thicker spacer, paper gasket, loctite on the flange nuts done up lightly, and it's been fine.
On that bike it's the oversize non-factory manifolding that tends to slacken one side or the other when thrashed hard enough - but better to have the carbs still attached when they do!
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The local Pommie Iron bike shop did some thick composite carb gaskets which he cajoaled me to try.
Now I use nothing else.
Replacing the O ring with a soft gasket allows the gasket to deform into the O ring grove and still make an air tight seal while the flange edges are actually supported.
If you always fit a nice new soft O ring then there is usually no problems but old hard ones become a fulcrum point to warp the flanges.
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Gents,
My concentric 930 slide sticks only at the top of travel if the throttle is wide open (thankfully, never while riding). The nuts are dead loose and I have manipulated the cable to the best bend I can work out. As I've chanted before, I'm not a carburetor specialist (or any kind of specialist, maybe, but definitely not carburetors). I was wondering what the harm would be in just sanding a couple of thou off the slide diameter near the top, or honing the cylinder a bit. It seems that a perfect fit near the top of the slide in the carburetor's cylinder might not be that important. Any comments or other solutions are appreciated.
Richard L.
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G'day Richard.
One of the only things I dislike about the concentric's. They distort very easily.
With the top off, push & pull the slide in & out a few times. You will see the shiny spot where it binds. Dress those spots with 800 wet & dry.
Cheers
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Thanks, Muskrat. I suppose having to whack it with the butt end of a hammer handle to shake it loose also doesn't help.
Richard L.
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I'd forgotten about this one. 3000 miles up now and the problem is long gone. I think I went for a lot of bathroom sealant and not too much turn on the mounting studs in the end.
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So, last night, I undertook taking the sticky out my concentric carb slide. Not having machinists' bluing, felt-tipped pen did well at showing the spots that rubbed. The 800-grit wet/dry paper (used wet) did a good job in three stages. Following the advice of one member, I used my vise to apply some backwards force on the flange, with the intent of reversing the bending that distorted the slide cylinder in the first place. This did seem to ease slide movement a bit, but I didn't have the cajones to apply much more force than I did, for fear of fracturing the flange.
Now, to the twist. When starting the reassembly of the carb, I went to look in the float bowl for any dirt. When I pulled out the float, the needle dropped out and, I thought, went to the floor. Not having a replacement I was at risk of having to wait for the post (and, maybe, spring) before I could ride again. This meant hands and knees, moving everything out from under the bench, whisk broom under the bench, all unsuccessful. Just before bed, hoping to eliminate one more possibility, I decided to sift (almost literally) through my trash can. Usually, no big deal, but this time there was about 2-1/2" of used oil-absorbent compound at the bottom. After removing the big stuff, mostly soiled paper towels, I went about inspecting the oily and rocky sand like a paleontologist searching for prehistoric mouse bones, about 1/4-cup at a time. Down to the last cup-and-a-half of the oily grit, OMG!, there it was. Damn, that thing is smaller than I remember from when I first rebuilt the carb. Unable to make a long story short at this point (or ever, maybe), I will just say, I slept well.
Richard L.
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...When I pulled out the float, the needle dropped out...
...I went about inspecting the oily and rocky sand like a paleontologist searching for prehistoric mouse bones...
...I will just say, I slept well.
*smile*
I love 'like a paleontologist searching for prehistoric mouse bones'.
Is there some kind of psychological weakness that compels us to drop things that we should not drop?
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I always seem to drop the important expensive or tasty things. *problem*
Cheers