Author Topic: Concentric carburetters  (Read 5233 times)

Online Brian

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Concentric carburetters
« on: 09.02. 2008 03:06 »
I have been reading through some of the older posts on carby's and see that there has been a lot of different things tried and different types of carby's fitted to A10's.
I thought I would explain what I have done with my plunger A10 over the last few weeks, its probably somewhere between a Tech Topic and a story but I went with Tech Topic as it may generate some discussion about carburetters.
I am not suggesting anyone should fit any particular carby to their bike but just saying what my experience has been with this particular bike.
When I bought my plunger A10 it was fitted with a totally knackered sidebowl 276. When I dismantled the engine I discovered that a previous owner had enlarged the inlet port to 30mm so seeing as the carby that was on it was no good I decided to buy a new 30mm Concentric. When I ordered the carby I asked if it was a genuine English made Amal and was told it was. When the carby turned up it was obviously poorly made but after a bit of cleaning up I fitted it to the bike. The bike went quite well with this carby fitted but lacked a bit of top end performance and spat back through the carby a bit, not a lot but enough to be a bit annoying. I rode the bike like this for about 2 years.
Eventually I decided to see if I could do anything about the spitting back through the carby and decided to try a slide with less cut-away, it had a 3 1/2 in it and I wanted to try a 3. I didnt have any spare slides but did have another  older Concentric with a 3 slide in it but it had been sleeved so I couldnt swap the slides. I decided to put the complete carby on just to try which I did. This didnt cure the spitting back, it was the same, but it gave me about another 10mph on the top end  and pulled much stronger through most of the rev range.
I decided this carby had to stay on the bike but still wanted to cure the spitting back, it was way to lean on the pilot mixture. To get it to idle I could only have the air screw out about 1/8" of a turn. This is where I decide to take some drastic action.
If you take the air screw out of a concentric and look inside with a torch there is a small brass sleeve that acts as a jet, I drilled this out. Next I drilled and tapped the body where the pilot mixture fuel comes up from the bowl and fitted a pilot jet. I tapped a 5mm thread as I have heaps of pilot jets out of japanese bikes and they are all 5mm. I also had to drill a recess in the bowl to take the head of the pilot jet. I fitted a #57 pilot jet and put it all back together.
The result ? Absolutely perfect. I have a 220 main jet, 3 slide and the air screw out 2 turns. It starts, idles and runs perfectly, I couldnt be happier.
Apparently concentrics that are meant for two strokes sometimes have pilot jets in them but I was unable to find any jets.
Why the original new concentric lacked top end performance I dont know, I used the same main jet, needle jet and needle in the older carby plus I have measured everything to try and find a difference but there are none that are obvious.
Well there you have it, as I said I am not suggesting anyone make any changes to their carbys but that is what happened to me. I guess one thing to come out of it is that a brand new carby may not always be the answer.
                                                                 Brian.

Online groily

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Re: Concentric carburetters
« Reply #1 on: 09.02. 2008 16:46 »
What a good topic Brian. I can't speak with any authority or experience re the A, because it came with a slightly tired but OK Monobloc and that's what it's keeping for the time being. But what you say about sensitivity of these instruments is so right. I have just this past week spent many bad-tempered hours playing with my AMC twin which, having been very expensively treated to a reconditioned Type 76 sidebowl promptly started being a pain. The Monobloc was worn, yes, but the darn thing always started, always ran. With this very expensive new toy - and it is a real one and the right one for the motor - it ran sweetly and ticked over like a clock (despite primitive tickover arrangements of the pre-Monobloc range) . . . but was no good at all under serious power (as in on the main jet or near enough).  Just died of fuel starvation after about 5 seconds, until the throttle was closed a bit and all was restored except road speed. All for the want of fuel, obviously, but whether from a lack of float height (unadjustable basically although I tried and cost myself a few litres of precious liquid all over the magneto from the overflow hole), or for want of a larger main jet I do not know. All is EXACTLY per book, just like your Concentric was, and all is brand spanking new or good as. It ought not to need rejetting unless I were running open pipes or megas. But, it just wouldn't deliver.
So, having got ***** off, I've lapped a new slide that was lying around into the old Monobloc (amazingly it was a tad bigger than the bore to start with - did they make oversize ones I wonder?), and pressed it back into service despite the fact that it's a B to fit for lack of wiggle room, and despite the fact it's wrong for the model. (The 2 very reasons why I replaced it.)  I may bother to try again with a larger jet or do mods to the float/needle arrangements, but frankly I bet I'll just leave it as it is - working tolerably well or better. I'm too lazy to keep messing when it ain't broke, just for the sake of 'Originality' and a bit of shine. Plus the fact that the leak-prone float chamber to carb body union and the fuel supply from underneath aren't attractive features of the old instruments. Shame to have spent the folding stuff, but one lives and learns. One day, a concentric maybe for something - cheaper and simpler - but with the flaw you mention on the pilot air side, plus the fragile bendy flanges which often can't be done up properly without making the slide seize in the bore, plus the literally whacky method of sorting out fuel level inside by tapping things. Groily
Bill

Offline fido

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Re: Concentric carburetters
« Reply #2 on: 09.02. 2008 17:21 »
I have been very happy with the Indian made Mikuni copy on My Enfield 535 and would like to try one on the A7. The only snag is they don't have a flange fitting and there's not much room for an adapter on the rigid.

Online Brian

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Re: Concentric carburetters
« Reply #3 on: 10.02. 2008 01:31 »
Groily, your experience is what I was concerned about. Since I restored my bike I have been able to get a head that has not been molested. My intention was to recondition this head and buy a new 276 and put it on the bike sometime in the future but now I think I will leave it all as it is. The bike is running perfectly as it is and even though I had intended to buy a new 276 maybe that would be the start of a heap of new problems. Has anyone else bought a new carby and had problems ? Are these new carby's that are being made as good as the originals ?    Brian.

Offline a10 gf

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Re: Concentric carburetters
« Reply #4 on: 10.02. 2008 11:55 »
Hi, some amal 276 content here, the new one looked perfect, and did have problems....
http://www.a7a10.net/forum/index.php?topic=90.0
We may start a new 276 topic.


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A10 GF '53 My A10 website
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Online groily

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Re: Concentric carburetters
« Reply #5 on: 10.02. 2008 17:54 »
Wouldn't let my hassles put you off trying Brian - the big prob I had was that, despite getting what the book says from Surrey Cycles in the UK - who are really very good, say most people - and a recon one not a replica - I don't know what my AJS might have originally had in terms of jets, needle, slide etc. I stupidly swapped the tired original carb for some bits I needed when I didn't have any money. So its secrets went with it 25 years ago.
I am quite sure a) that my new one will work/would work properly (it's done a few hundred miles at reduced pace already), it's just that I need a selection of jets, needles and maybe even floats if there are choices, to play with, and I don't have any. And b) that a bit more patience than I am prepared to invest would sort it.
The fault's simple, in theory - not enough juice when you want it! But I'm not about to buy in a load of stuff so I can play, particularly if the net result wouldn't be much better than the 376 with its new slide.
One slightly interesting thing is that on this other bike the needle in the Monobloc is not a C (which the 76 takes, per book and per Surrey Cycles) but is unmarked, and a LOT shorter. The engine will only splutter and bang with a C in there, although it will start right enough. It clearly needs to get off the needle a lot quicker than some other engines - which is reflected in the (normal, say other owners) fuel consumption, which is worse (only a 27bhp 500) than the A10. 55ish mpg is about it. Partly also because those engines rev - max 7000, which was a lot for a just-post-war design with 7:1 cr - and need to be allowed to if you want reasonable torque.
As far as the basic quality and fit of my recon 76 goes, it seems fine - all goes together well and no fuel leaks at weak points mentioned in earlier note. But of course, in light of some of what I've just read courtesy of this most excellent forum, I can't say whether the fuel flow through from the float chamber to the body is spot on, or whether there are any other internal faults on any remanufactured parts which may have contributed to my problem. Haven't even looked, to be honest.
Today I simply put the whole thing, covered in WD40 and wax paper, in a box marked 'Amal Type 76, AJS, Main Jet or Other Problem' and stuck it on the shelf next to boxes of other redundant or dead parts which might come in useful. . . my kids will probably find it there when I'm dead! Then I went for a thrash on the thing while the sun was still up. Groily
Bill

Offline beezalex

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Re: Concentric carburetters
« Reply #6 on: 13.02. 2008 20:49 »
Brian, was some of the new concentric carbs have had problems with distorted or leaking floats.  I wonder if this could have had anything to do with your problem.  I've personally never bought a new carb since you can get better than new by sleeving an old carb.  The pilot jet thing sounds odd, since most concentrics, except for the very first ones have the pilot jet pressed into the body.  I've been running a 928 concentric on my '53 Golden Flash and it manages to break the ton regularly (with 8.5:1 pistons).  I have now rebuilt a 276 and have gotten a stock airbox, so we'll see how that goes.
Alex

Too many BSA's


Offline Nitti

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Re: Concentric carburetters
« Reply #7 on: 15.02. 2008 10:53 »
Hi,

My newly bought -53 Golden Falsh also has a 928 Concentric fitted. When I opended it up I found it has a plastic float-needle, something I have never seen elsewhere. What's your recommendation, to keep it, it looks OK, or to replace it with a new metallic one with rubber-tip ?

On the topic, I would like to hear your views on the some of basic settings, the engine is a non modified, standard pistons etc. The only difference is that the previous owner has constructed quite a neat metallic and angled tube to fit a standard Amal air-cleaner, facing sideways, it is fitted with the pancake type with paper filter.

The needle clip is at the first grove , counted from the tapered end, so quite rich, what is the standard  position ?

What is the ""starting poistion" of the airscrew, 1/2 turn or ?

Is it enough to only have the O-ring between the carb and the inlet manifold, or should I stick a thick gasket there as well ?

I have not really run the bike, before starting to dismantle things like the carb, why I would appreciate some hints and tips on the basics, knowing that I will still need to fine tune it later.

Nitti
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Offline beezalex

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Re: Concentric carburetters
« Reply #8 on: 15.02. 2008 20:41 »
My newly bought -53 Golden Falsh also has a 928 Concentric fitted. When I opended it up I found it has a plastic float-needle, something I have never seen elsewhere. What's your recommendation, to keep it, it looks OK, or to replace it with a new metallic one with rubber-tip ?

If it doesn't leak, there's no reason to replace it, though the viton rubber-tipped ones are superior.

Quote
On the topic, I would like to hear your views on the some of basic settings, the engine is a non modified, standard pistons etc. The only difference is that the previous owner has constructed quite a neat metallic and angled tube to fit a standard Amal air-cleaner, facing sideways, it is fitted with the pancake type with paper filter.

The needle clip is at the first grove , counted from the tapered end, so quite rich, what is the standard  position ?

There is no "standard" position since these carbs never came on this model.

Quote
What is the ""starting poistion" of the airscrew, 1/2 turn or ?

1-1/2 turns

Quote
Is it enough to only have the O-ring between the carb and the inlet manifold, or should I stick a thick gasket there as well ?

The iron heads don't need the insulator due to Cast Iron's poor thermal conductivity, but what you want to be very careful of is not to overtighten the flange nuts.  These will warp the carb body and make the slide stick.  I install split lockwashers and tighten them just to the point where the flatten out, no more.

Here are the jets and settings that work for me (Standard motor except 8.5:1 compression):

Slide: #4
Needle Position: #3
Main Jet: 190
Needle Jet: 106

Cheers and have fun.
Alex

Too many BSA's


Offline Nitti

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Re: Concentric carburetters
« Reply #9 on: 17.02. 2008 15:45 »
Thanks Beezalex,

A lot of good information in your reply, however I would appreciate some minor clarifications ,

Needle position ; when you say #3, is that from the top of the needle or from the tapered end of it ?


Airscrew : Should I interpret your answer as 1/2 to 1 turn, or 1,5 turns ?

Nitti


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Offline beezalex

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Re: Concentric carburetters
« Reply #10 on: 27.02. 2008 16:30 »
Thanks Beezalex,

A lot of good information in your reply, however I would appreciate some minor clarifications ,

Needle position ; when you say #3, is that from the top of the needle or from the tapered end of it ?


Yes, counting from the top.  #3 is the richest position.


Quote
Airscrew : Should I interpret your answer as 1/2 to 1 turn, or 1,5 turns ?

Yup, 1,5 in eurospeak.

Alex

Too many BSA's


EGAVAS

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Re: Concentric carburetters
« Reply #11 on: 10.07. 2010 11:57 »
I have a 928/R300 fitted to my A7 Shooting Star .What is the corect jetting I have a 260 Main a 106 needle and middle needle setting

Offline muskrat

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Re: Concentric carburetters
« Reply #12 on: 10.07. 2010 14:46 »
G'day Egavas, Welcome to the forum.
                                                          As the 928 wasn't the std carb for that model it's a bit suck and see. I have a 932 on my '51 A7 with a '57 SS top end. It's a bit wild, so my settings won't suit you. In the book a '68 A50 RS used a 626 with 200 mains, #3 1/2 slide, .106 needle jet, clip position 2.. You don't say what slide you've got, you will best start with a #3. Plug chops are the best way to test, or use the choke at a certain throttle opening to see the effects, if it goes better go up in jet.
Lots of luck.
Cheers
'51 A7 plunger, '57 A7SS racer now a A10CR, '78 XT500, '83 CB1100F, 88 HD FXST, 2000 CBR929RR ex Honda Australia Superbike .
Australia
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Offline old53

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Re: Concentric carburetters
« Reply #13 on: 19.09. 2010 08:35 »
Hi Brian can you tell me if you are running an air cleaner wit the 220 main jet thanks

Online Brian

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Re: Concentric carburetters
« Reply #14 on: 19.09. 2010 08:55 »
G'day old53,

No mine doesnt have a aircleaner. Its almost impossible to fit a filter to the plunger models as there just isnt enough room. You could revert to the original air filter set up if you can find the correct right angle rubber boot but even then there would be difficulties as the threaded section the bellmouth screws on is larger in dia on the concentric than the original 276.

There is nothing wrong with the 276, I only used the Concentric because mine has had the inlet tract increased to 30mm in size. Oneday I will put a unmolested head back on with the original 276.

I am just about to fit a new monobloc to another plunger A10 I'm building but that ones a fair way off being a runner.