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I normally fit a 6mm insulation spacer using longer studs if needed, easy to check the effect or even if the 2mm one is working by going for a run to get engine to temp and then putting a hand on the carb to see how hot it is.

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Next time she is hot I will feel the carb.

Should get time to do the coil swap this afternoon.

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Originally Posted by kommando
I normally fit a 6mm insulation spacer using longer studs if needed
Same here, 6mm minimum to have a reduced heat transference to the carb.


Now let’s all have a beer beerchug

68’ A65 Lightning “clubman”
71’ A65 823 Thunderbolt (undergoing restoration)
67’ D10 sportsman (undergoing restoration)
68’ D14 trials (undergoing transformation)

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Tried a known good coil but no difference.
Tried a new plug cap and lead, again no difference.

I ran the bike for 15 minutes until very well warmed up and the whole carb, even quite close to the head is cool. The bowl itself was just room (shed) temperature.

I'm borrowing a compression tester for the weekend, so I won't be doing anything else until I've tested that.

I may have a go at filing my number 3.5 slide to something richer next week when I'm more in the mood for it. Need a short break from it.

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didnt see this before .
Quote
 I noticed recently that the clamp has slightly mis-shapen the coil body. Maybe I should try a coil swap with one of the Bantams?

a dented coil can contribute to a misfire under load .
i would replace the coil . the coils on other 6 volts bike should be
Compatible ... but the mounting diameter may not be the same .
they come in 2 common diameters 48mm and 40 mm
sometimes this difference in diameter contributes to coil denting ,
sometimes they just get dropped
the dents ... opened up the possibility that the coil can arc to ground
... maybe not all the time , Maybe only more likely when loaded under acceleration

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Quote
I may have a go at filing my number 3.5 slide to something richer

I don't know if you meant filing or filling, but if it's filing as in removing metal with a file then this will result in a leaner mixture as more air can enter.

On my B44 I'm running a PWK 28 which is a Keihin PWK clone, I had loads of trouble at the start as it was running rich at low rpm. I filed the slide cutaway a bit and it helped lean out the mix.

Another thing to try is to check the throttle cable routing, maybe there is not enough slack and the slide cant bottom or the cable is lacking lube?


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“I may have a go at filing my number 3.5 slide to something richer next week when I'm more in the mood for it. Need a short break from it.”

If the old #4 slide is not excessively worn (say compared to the new #4), I’d be tempted to experiment with that, rather than the new #3.5.
I say that because you will emulate a #3 slide cutaway by removing 1/16” from the bottom surface of a #4 slide (or 1/32” from a #3.5), which will allow you to see if it improves the “cough and stall”, but it changes the slide for ever.

What I’m alluding to is the needle position in the needle jet. This would have to be raised by the corresponding amount removed from the slide. If you have needle slots available to lift the needle by this amount, such a modified slide could still be used.

I am only suggesting to try this experiment with the old #4 merely to see if it makes any improvement to the issue. If it did improve the issue, only then could I support buying yet another slide (#3).

As it seems to run well in all other circumstances, including under load, I consider it unlikely to be an ignition fault (but I appreciate all avenues must be explored).

It has been a long time since I’ve had a Monobloc carb in front of me. Do they run the same 2 hole system (just before and just after the slide) as the Concentric?
On the Concentric these handle the transition from idle to cutaway.

Best of.

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yes to what gunner said .
file-ing a 3.5 slide will quickly turn it into a 4 ... but probably a slide that makes no sense at all
... you would have to add material ( fill ) to turn a 4 into a 3.5

.. the bottom root angle is started from the same spot on the slide ( that will live next to the idle air Jets )
and the height only increases at the back of the slide .. and is surprisingly small steps
The difference between a 3 1/2 and a 4 ... is 1/32" on the back edge
( the incline angle of the wedge is increasing with slide size ... but measured in 32nds at the back )
from AMAL;
[Linked Image from gallery.myff.org]
a # 2 is richer <<< >>> a #4 is leaner

( more air accross ' the 2 idle Jets' changes how these 2 holes vent fuel or atomized fuel in the idle circuit )

the air screw 'IN' is richer <<< >>> idle air screw 'out' is leaner

there is crossover to what the pilot air screw can do ... with what a change in slide cutaway does .
Opening the idle Jet screw mimics what a larger slide does .

If the change you looking for can't be effected in the the normal operating range of the idle air screw
then the slide is changed ( and idle air returned to default start position )
if neither of these get to where you're going
The problem is probably somewhere else ?
... like between carburetor and inlet valve .(port size)

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Gunner. Yes, filing. My number 3.5 slide has more cutaway than my number 4 slide, indicating that the more you file off the smaller the number gets.

I've copied this from earlier in this thread:
"Slide cutaways get leaner with higher numbers."

So my number 3.5 has more cutaway than the number 4 slide and theoretically creates a richer mixture. So if I file off some more making it a number 3 slide, surely that is even richer?

Some clarification here would be great.

Koan. The slides are both brand new. The number 4 slide came with the brand new carb and is one of the fancy (expensive) hard anodised type, so I'd be reluctant to chop that one about.

From what I've read recently, nothing is taken off the length of the slide when filing. Just the required amount off at the inner end of the slope, graduating down to zero at the outer end of the slope. So the length of the slide is unaffected, as would be the needle position. As far as I can tell, my number 3.5 and number 4 slides are exactly the same length.

Yes, the two tiny holes in the throat of the carb are the same as the Concentrics.

Compression tested tonight. 120psi. That sounds ok to me, but I need to check what it is supposed to be when I get a chance.

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Quinten: I'm going to have to compare my two slides again. I was pretty convinced my number 3.5 has more cutaway than my number 4. Maybe I got that back to front? I will check in the morning.

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Theres some detail about slide cutaways for Concentric AMAL's on their website Here, near the end of the article which is about rebuilding the carb.

I know you are using a Monobloc but I believe the principle is the same, i.e. the larger the cutaway number the leaner the mixture.

120psi cylinder pressure sounds good, see this article from the BSA OC website where someone has measured their C15 - Link


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Not unsurprisingly, after all this, you have become confused.

If a slide had no cutaway at all, it would be a “0”, and it would exert maximum draw on the fuel in the venturi (ie as rich as it can get).

From there the cutaway goes in increments of 1/32” between increments (say between 3 and 3.5), getting progressively leaner as the numbers rise.

Whichever slide you choose to modify, in order to emulate a #3, you will be removing material from the BASE of the slide (1/16” from a #4, 1/32” from a #3.5).

I appreciate what Quinten has posted about the different angles of cutaway in original slides, but that shouldn’t matter much with your rough-and-ready filing experiment to see if a still richer slide helps the issue.
As I see it, it is only a test to see whether buying yet another expensive slide (a #3) is a worthwhile gamble.

Hopefully you are clear that I’m talking about removing material from the BOTTOM of the slide, not the cutaway.
A lathe would be ideal, but careful marking and filing would do it for test purposes.

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Apologies to all, I definitely got myself confused for a while.

I was clearly wrong about my 2 slides, I was just the only one that didn't realise it!

The 4 slide does indeed have a greater cutaway than the 3.5, as shown in the picture. Number 4 slide being on the right.

[img]www.dropbox.com/s/hxs1jnzd0jm1xev/Screenshot_20210416-225722_Gallery.jpg?dl=0[/img]

I'll give it some thought over the weekend as to what course of action to take next.

After reading the article in gunner's link I can see 120psi is very good. I will run a compression test on my Bantams tomorrow as a test for the test gauge, just in case it's reading high.

Last edited by Douglas Clark; 04/16/21 10:25 pm. Reason: Link not working
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Originally Posted by Douglas Clark
Gunner. Yes, filing. My number 3.5 slide has more cutaway than my number 4 slide, indicating that the more you file off the smaller the number gets.

I've copied this from earlier in this thread:
"Slide cutaways get leaner with higher numbers.", A, a number 4 slide is leaner than a number three, a number 4 has more cutaway to let more AIR in.

So my number 3.5 has more cutaway than the number 4 slide and theoretically creates a richer mixture. So if I file off some more making it a number 3 slide, surely that is even richer? .
A. No, Your number 3.5 slide has Less cutaway than your number 4.

Some clarification here would be great.
A. see above.

Koan. The slides are both brand new. The number 4 slide came with the brand new carb and is one of the fancy (expensive) hard anodised type, so I'd be reluctant to chop that one about.

From what I've read recently, nothing is taken off the length of the slide when filing. Just the required amount off at the inner end of the slope, graduating down to zero at the outer end of the slope. So the length of the slide is unaffected, as would be the needle position. As far as I can tell, my number 3.5 and number 4 slides are exactly the same length.

A, you can only file a slide leaner,BUT, you cant file it richer, taking metal from the bottom will make the cutaway richer but it will drop the needle at the same time cancelling some stuff out.


Compression tested tonight. 120psi. That sounds ok to me, but I need to check what it is supposed to be when I get a chance.


if it is a slide cutaway thing, you cant file up a number 3, what happens if you choke the carb intake, try a hand over half to start with, and then open the throttle, if choking with your hand helps then get a no.3.

Quite often if the port has been opened out in the past normal carb settings dont apply, a hogged out port has low speed air at idle compared to std , lower air speed over the jets gives a leaner mixture, AFAICT

Last edited by gavin eisler; 04/16/21 10:39 pm.

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I try the choking idea tomorrow Gavin. Nice idea.

Just another thought. My air filter is the original type in the battery box area. By the looks of it, it offers very little in the way of filtering. I don't think it would offer any resistance to air flow either. A filter working properly is going to restrict air flow to some extent, acting a bit like a choke. Could this perhaps be part of the problem?

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If I’ve understood correctly, the only problem you have is coming up from idle (maybe 1000 rpm and zero throttle) to perhaps 1/16 throttle?

It idles well, and runs fine on the road in normal conditions? So just that transition from idle to running?

I don’t see that pointing to an air filter problem (it should starve at higher demand), try it without for thoroughness.

I don’t see it pointing to an ignition problem (it would have to be very specific in rpm).

The compression seems reasonable. I don’t see beyond carburation.

That the PO did that strange roughing of the inlet is a mystery, but I don’t think that would cause this problem, but who knows?
Many thousands of C15’s must have been happily enjoyed over the years, has anyone heard this as a common complaint?
I’ve heard generally a little dull, but dead solid little bikes. So what is different about your bike?
Nothing that I can see, but for that scoured inlet.

Best of.

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its easy to get backwards or sideways .
i may gets it sideways myself tomorrow .

the slide # is
based on the height of the 'air-wedge' at the air cleaner side
Measured in 1/32 inch increments .
if the cutaway were modeled as a triangle
point A remains fixed ... near the idle jets ...
only line B-C needs to change ... to describe a different slide #
( this is my interpretation of AMAL speak )

( angle-A and line A-B do not need to be found or described ,
they change , but are functions of the height over the
fixed-radius of the slide )
[Linked Image from study.com]
to measure ,
the slide can be placed on a flat surface
and the height of the air-gap at the back measured
( this may seem counterintuitive , measuring the missing part )
but this allows
The change in air volume for various slides
to be gauged simply by the height of the high-side
[Linked Image from mathworld.wolfram.com]

Last edited by quinten; 04/17/21 1:36 am.
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I think you're right Quinten, except that point A is about the mid-point of the slide for the smaller cutaways, and further back for the 2 larger ones. (according to your own pic).
The cutaway is always restricted to the inlet side of the slide (of course), so it has to be a slice through the cylinder between that height (say 3/16" for a #3) and the centre, at maximum.
Its only the smaller inside dimensions of the tapered surface of the slide cutaway that matters, the taper may help with gas flow though.
Most applications use the 3 middle options.

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Koan. Your understanding of the situation is spot on. The bike could be ridden as it is, as long as I'm prepared to have a high tickover or keep the revs higher with the throttle at traffic lights etc. The higher rpm alleviated the cough and stall somewhat.

I have read in the past things like "the dreaded cough and stall" or "the curse of cough and stall", both relating directly to the C15, which lead me to believe this was a common problem. However, I can no longer find those comments on the interweb.

Regarding being "generally dull', I'll bet a lot of that comes from AAUs not working properly after not much use. Also by 1966, faster more exciting Japanese bikes (especially 250s) were starting to arrive making the C15 look like an old plodder. The EI has given me back a proper reliable (hopefully) advance curve, and a much more pleasing bike to ride.

Another thought regarding that rough inlet. Is it possible that the roughness is trapping fuel on tickover, and then releasing some of it (or all of it) as the throttle is opened causing greater suction? This would cause a momentary over rich situation which could cause stalling once the engine is warm. Clutching at straws here!

Quinten. Thanks for the further clarification. I think I have the basics of slide cutaways now. I'm a slow learner, but hopefully I'll retain it now that I know it! The main thing I will concentrate on is: higher number = more cutaway = more air.

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Have you tried the Colortune yet, opening the throttle at tick over and observing the colour of the combustion will tell if its a lean or rich issue or even if the problem is the lack of spark.

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That's a good point Kommando. The guy I borrowed the compression tester from also has the colourtune. I will try and get my hands on it later today.

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I am sure you will have already done this but ........
Before you start hacking bit off the slide I would just one more time go through this procedure

http://amalcarb.co.uk/downloadfiles/AMAL/amal_tuning_guide.pdf

don't rush it and don't try and skip steps! .

It strikes me that the issue is the transition between the pilot and the cut away, so either, or both, could be the cause (on indeed something else entirely!).
The cut away only comes into play over about 1/8 throttle. So if it "fluffs" at less than this it isn't the cut away, its the pilot and visa versa over 1/8th. Marking the twist grip (with tape) so you can actually see at what throttle position the "fluff" or "cough and stall" actually occurs might be useful. In my experience even a 1/8 turn on the pilot can make a big effect.

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Thanks for the link to the tuning guide Dave. I'll put that on my list of things to try. I'm not planning to attack the slides at this point. They're expensive and there's no going back, so I'll save them for another day.

I am about to go and get a colourtune kit from my friend. It will be interesting to see what that throws up.

Gavin mentioned choking it using my hand over the back of the carb. I'm hoping to take that a step further. The C15 never had a choke (air slide) fitted. If my Concentric air slide fits in the Monobloc slide, I am going to fit that. I have a lever and cable I can use. That'll make it much easier to experiment with choking.

Busy afternoon ahead!

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My first ever bike (apart from a Yamaha FS1E DX) was a BSA C15 SS80 I rebuilt from parts and a wreck when I was 18.

It was a great bike and I learnt a lot, it pains me now to think that I sold it all those years ago for £40, wish I could have it back.

I wish you luck with the colourtune and hope it solves the problem.

If the colourtune doesn't help then I would start checking things like valve timing. It wasn't unusual for BSA to miss stamp some of the timing wheels which means your timing may out one tooth advanced or retarded. This is mentioned in the rupert ratio unit singles Manual so could be a possibility.


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If you'd kept the FS1E DX (maybe you have) you'd be able to sell it and comfortably buy 2 X SS80s. Crazy world!

I will update here re colourtune, the tuning guide and choking the carb a little.

Then as you say, I might have to start looking deeper into the mechanicals. Valve timing, sticky valve etc.

I'm still interested in my theory that fuel may build up in the roughness of the inlet port, it really is very rough. To sand it all smooth would add a good 1/16" to the diameter. Another way would be to fill it with something that flows well and sand back to 7/8". A tricky job for DIY, but there maybe a professional out there that would do it?

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