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Spray tube is angled.

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Originally Posted by Douglas Clark
Spray tube is angled.

This is the 2-stroke type. You want to squire a squared off one, carefully tap the existing one out and carefully tap the new one into place. Taking note that the spray tube and it’s recess have a tooth like appearance. So pressing it by feel into position with a drift of the right diameter is required.


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Thanks for the extra bit of advice Allan. I've not changed a spray tube before, so nice to get some tips👍

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Originally Posted by Douglas Clark
Thanks for the extra bit of advice Allan. I've not changed a spray tube before, so nice to get some tips👍

No problem Douglas, just be careful not to damage the visible part of the tube. It is after all a jet of sorts, as its shape determines the effective vacuum draw on the needle jet as you transition from slide to needle jet positions.


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Will do Allan, thanks👍

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Just a quick update as I don't like to leave these threads unfinished.

I got side tracked into wanting to sort out the timing before going any further with the carb. Long story short, I ended up fitting a Boyer EI unit, which so far works great. It has made quite a dramatic improvement to the general running, lovely mid range now.

However, I still have the cough and stall at low rpm. It improves by screwing the mixture in, but even with it fully in it doesn't cure it. The idle is still erratic.

A number of years ago I fiddled with my Bantam carb for a few weeks, but never got it quite right, so I bought a new one and it's been good ever since, so I have today purchased a new AMAL Monobloc in the hope that it will cure my issues. I will update next week when I have fitted and tested it.

On a related issue, I noticed today that the inlet on my barrel is extremely tough. I think it is unlikely it left the factory like that. I've posted a link to a picture below. Not done that before so hope it works!

https://www.dropbox.com/s/tckxk3x3b4b9yfu/2021-04-08%2012.31.53.jpg?dl=0

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So the new carb is fitted, but the cough and stall plus the yo-yo tickover remain.

The fitting of the EI and new carb have totally transformed the bike in every other way, and has made the bike a pleasure to ride, but I'm now more keen than ever to sort out the two issues that I originally set out to cure, the tickover and the cough and stall.

I probably should have mentioned this before. My plug, after just 30 seconds of tickover becomes black and sooty. After a couple more minutes of ticking over it becomes thick with deep black soot. I'm now suspecting that I'm getting oil into the combustion chamber somehow to cause sooty blackening this quickly.

While riding today I pulled the plug cap at about 4,000 rpm (a guess). The plug was showing on the lean side, but not badly. A light grey/brown. Once home I let the engine tickover for a minute, and the plug was sooty again.

If oil was occasionally dripping into the combustion chamber from a valve guide/seal, I guess this could cause the tickover to fluctuate, blacken a plug at tickover, but what about the cough and stall?

Very little in the way of smoke from the exhaust, even at tickover.

Anyone any thoughts on this?

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Hi Doug,
Is that pic of the top of the cylinder? So you’ve recently removed the head?

If that is what it is, then it does look dreadful (though pics on the net can be deceptive, it appears to be have been done with a half-round file!). It looks extremely coarse and nothing like a proper honing pattern.
If it truly is that rough, god knows what its doing to your rings and piston.

While such a surface could allow more oil than usual into the combustion chamber, I’m doubtful that this is the main cause of your dry sooting at idle. Oil usually results in “oiliness”, not dry soot.

This is more likely due to the idling mixture being set too rich, possibly when the engine isn’t fully warmed up (say at least 5 miles riding), especially as the plug seems to be pretty good at 4000 rpm (presumably somewhere in the mid-throttle range).

Note that the recommended idle air adjustment is 2.5 turns out on a Monobloc, not the 1.5 turns of the Concentric.

Have you strobed the Boyer ignition? Until that is done, carb tuning will be “pointless” (a silly jest). But that is the order in which to do things.

Good luck.

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Hi Koan.

Sorry if I confused you, but that picture is of the inlet. I've never taken the head off as yet. I've had the carb off a number of times, but only noticed how rough the inlet was the other day. If I poke my finger down the inlet, the roughness continues all the way to the bore, but I presume not into the bore itself! It doesn't even feel normal. It has the feeling of an alloy rather than steel. Anyway, the way the bike is going right now, I don't think it is causing a major problem.

I did wonder about the dry soot as opposed to oilyness, so thanks for clearing that up. I was already planning pulling the engine apart!

I've been trying to look up the air screw setting unsuccessfully, so I've set it at 1.25 turns out. I had no idea the Monobloc starting point was 2.5 turns out, so thanks for that👍 I'll reset it first thing tomorrow and see what difference it makes.

"Pointless", I like that🙂,
Yes, the Boyer ignition has been strobed. It only took a couple of minor adjustments from the Boyer "starting point" marks to get it right. It was quite a pleasure to see the mark move as the revs rose to being exactly in line with my 33.5° mark and then stop. Very pleasing.

Yes, the plug cap was pulled at around mid throttle, perhaps a tad more. A little hard to judge when you're hanging off the side of the bike.

I think changing the mixture screw tomorrow will be interesting. I'll update here with any news.

Thanks for your informative answer.

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I interpreted that photo as showing the inlet port, if so it is the ideal surface finish


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Hi Andy.

That's interesting. I can't claim to have made a study of inlets, but certainly my Bantam ones are a lot smoother. Maybe because they are 2 strokes?

Thanks for alleviating any concerns I had about it👍

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“I interpreted that photo as showing the inlet port, if so it is the ideal surface finish”

For many years the prevailing tuning technology was to “port and polish”.

There is a more recent school of thought which tends toward smaller ports for increased gas speed, and a rougher port surface to promote turbulence in the outer flow region.
This makes sense perhaps, more so for a racer, but as far as I know no production engine had ports dressed in that way.
That is something the PO has done, perhaps for the turbulence reason (on a C15?) or to fit a slightly larger carb to the manifold.

Is the hole still the same diameter as the carb throat?
The rough edges look as if the inlet has been filed somewhat.

This shouldn’t matter much as long as a good seal can still be made between carb and flange.

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The carb throat and the inlet width are almost identical in size. Can't remember the measurements now, but something like a few thou difference. I can't even remember which was the biggest now. So they are both essentially 7/8" (22.22mm).

When I first touched it, I instantly thought it was some sort of flexi alloy liner being used as a reducer back down to standard size. I believe some of the other C15s had a slightly larger inlet, so maybe the head from one of them, but I'm not so sure now. As you suggested, possibly just roughed up by a PO for the extra tiny bit of power?

I am definitely from the era when polished ports were all the rage. I even polished my Suzuki AP50 ports! I was unaware the tide had turned back to rougher ports.

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"Note that the recommended idle air adjustment is 2.5 turns out on a Monobloc, not the 1.5 turns of the Concentric."


An interesting morning.

I turned the air screw out to 2.5 turns as Koan suggested. I then warmed the engine thoroughly. At 2.5 turns out I have been able to turn the tickover down much lower than before. As I turned it down it became much more steady until I reached a point where it was happily thumping away with no yo-yo whatsoever. Very pleased with that.

The only problem with this, is that it has accentuated the cough and stall. It's now much more difficult to open the throttle without it cutting out, even when opening it gently, whereas before I could at least open it gently and get away with it. If I raise the tickover, the cough and stall improves, but then the tickover starts to yo-yo again.

Another point is that after 10 to 15 minutes of tickover, the plug is nowhere near as sooty as before. In fact it's not sooty at all, just black. I put the plug back in and then ran the engine at maybe 2,000 rpm for a short while and cut the engine. The plug was then heading back to a browner colour, although still quite dark.

Beginning to wonder if this cough and stall business is going to be with me forever!

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Try a slide with the next size smaller cutaway, if it has a 3 .5 fitted , try a 3.


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This is where a gunson colortune can come in useful, it shows the colour of the combustion inside the chamber using a glass window, the right idle colour is Bunsen burner blue but as you start to open the throttle you want a momentary rich mixture with yellow showing, if its blue then you need a slide with a richer cutaway. Float height may be an issue to check first as fixing that is a lot cheaper than a new slide.

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I've just checked the float height and it seems to be correct, although I had a bit of trouble with air locks in the tube I used. I ran a tube from the pilot jet around to the float bowl cover and taped the tube to the cover. When I switched the fuel on, the bottom of the tube fills up but then fuel flow stops with a 2" air lock. I had to use the tickler to get it going again, let the fuel flow past the dot on float chamber cover and then wait for it to go down. It would stop about 1/32" above the float height marker.

I understand the logic of trying a richer cutaway, but wouid find it slightly annoying after having bought a new carb with standard settings, although I have to admit that I am running a slightly richer cutaway on my Bantam, and I believe this is necessary due to the ethanol in our fuel nowadays. I will order one up tomorrow.

A friend has a colourtune that I can borrow, so I'll give that a go tomorrow first thing.

Thanks for all the help and advice I'm getting here. Much appreciated.

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Its possible it may be too rich on the cut away, if its too lean the cough has a spit back through the carbs more like a sneeze, if its too rich it may be OK under load, if it doesnt accelerate and just dies, its too lean, make sure the wee holes in the floor of the venturi are clear , they are whats happening delivering fuel just after closed throttle when it coughs, if one is blocked it might just be that. Is your fuel supply clean?
Its fair to say someone has been fiddling with the port, between that and modern fuel , different pipes and filters maybe STD doesnt always work.

Last edited by gavin eisler; 04/12/21 11:05 pm.

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Hi Gavin. Thanks for your input.

After a cough and stall, there is often (90% of the time) a sneeze through the carb, so that does sound lean. A sneeze is a great description. Just what it sounds like.

It's a brand new carb, so one would assume the tiny wee holes are clear, but I do appreciate that occassionally they may not be, even on a new carb. However, given that the tickover is now very nice, I would think they are clear. I don't doubt the carb will be off again some time very soon, so I will send a jet of carb cleaner up the little hole at the rear to make sure.

Fuel supply is new and clean, but to be frank, this cough and stall has been happening for years on many different tanks of fuel. Only just recently decided to try and fix it.

I'm keen to try your suggestion of a richer slide. I'm actually going to get the next two sizes to make sure!

A friend has also suggested I re-check my timing, although I'm pretty convinced it's spot on. This feels like a carb issue to me.

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90 percent of carb faults are electric
The slide is worth a try if you are happy with the sparks.is it the original ignition switch?

Last edited by gavin eisler; 04/12/21 11:50 pm.

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I'd like a £ for every time I've heard that........and another £ for every time it turns out to be true!!😂

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Originally Posted by Douglas Clark
A friend has also suggested I re-check my timing, although I'm pretty convinced it's spot on. This feels like a carb issue to me.

Many timing lights have an advance dial on them, depending on the model you have depends on how easy it is to knock.

When you stobe time the bike just check that the dial is turned all the way anti-clockwise (turned back if you prefer). The one on mine is quite free to turn (I have the chrome one with the rpm readout etc) I used to have hell of a job timing it up until I got some duck tape and fixed it in place.

Last edited by Allan G; 04/13/21 9:20 am. Reason: Didn’t want to give incorrect info

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Thanks for that Alan, but no dial of any sort on my Gunson timing light. I must have the basic model!!

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Originally Posted by Douglas Clark
Thanks for that Alan, but no dial of any sort on my Gunson timing light. I must have the basic model!!

No worries, ‘twas just a thought.


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Originally Posted by Douglas Clark
It's a brand new carb, so one would assume the tiny wee holes are clear, but I do appreciate that occasionally they may not be, even on a new carb. .

A few years ago it was a very frequent problem with swarf being left in small galleries on the new Concentric's, so worth a look. However if the same cough has been there for years then its likely the wrong cutaway.

The Tiger cub has a similar issue, I cured this by fitting an 850 Commando slanted spray tube combined with the special shorter needle, this was on a 600 series Concentric carb so would not carry over to a Monobloc without some testing and adaption.

The origins of the 850 slanted spray tube and shorter needle was a noise test which required the bike to be moving slowly in 4th and the throttle opened sharply and noise levels taken as the bike accelerated. With the std spray tube and needle the 850 did not accelerate at all and so did not pass the test and AMAL found the solution. The theory is the slanted spay tube retains a reservoir of fuel rich air that's taken up on fast opening of the throttle giving a momentary mixture enrichment for the acceleration to kick in.

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