Idle W/NG

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simon.holyfield
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Re: Idle W/NG

Postby simon.holyfield » Fri Aug 09, 2019 5:22 pm

IMG_20190809_161613.jpg


My bottom nut & screws...
cheers

Simes

'51 Square Four,
'58 Huntmaster,
'42 W/NG, '78 CX500
http://ariel-square-four.blogspot.com

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simon.holyfield
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Re: Idle W/NG

Postby simon.holyfield » Fri Aug 09, 2019 5:25 pm

alan.moore wrote:You've probably read all this but Hitchcocks have some great info re the premonobloc carb here

http://accessories.hitchcocksmotorcycle ... e-Monobloc


I haven't seen that, I will have a read. Currently the pilot air system & bypass are a bit of a puzzle.

You'll see that my 0.9" throttle stop screw length includes the head. A bit shorter than yours.
cheers

Simes

'51 Square Four,
'58 Huntmaster,
'42 W/NG, '78 CX500
http://ariel-square-four.blogspot.com

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alan.moore
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Re: Idle W/NG

Postby alan.moore » Fri Aug 09, 2019 5:42 pm

Simon,
Here's another link to info on the carb which includes explanation of the pilot system...bit better explained than previous link

http://www.wdbsa.nl/download/Amal%20Carburetter.pdf

Re the throttle stop screw...in that case its screwed in about the same as mine on the 276
1939 VH Redhunter;1942 RN WNG;1951 Triumph 6T Thunderbird;1970 BSA B175 Bantam;1986 Yamaha SRX600 single
http://cloggymoore.wix.com/triumph-pre-unit-6t

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simon.holyfield
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Re: Idle W/NG

Postby simon.holyfield » Fri Aug 09, 2019 7:28 pm

alan.moore wrote:You've probably read all this but Hitchcocks have some great info re the premonobloc carb here

http://accessories.hitchcocksmotorcycle ... e-Monobloc


Banging in the exhaust... I wonder if the pilot air screw is too far open.
cheers

Simes

'51 Square Four,
'58 Huntmaster,
'42 W/NG, '78 CX500
http://ariel-square-four.blogspot.com

Paul Slootheer
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Re: Idle W/NG

Postby Paul Slootheer » Fri Aug 09, 2019 8:53 pm

Banging in the exhaust... I wonder if the pilot air screw is too far open.

Simon, banging mostly means to much air is allowed to pass by due to a worn slide, or a air leak somewhere...
Sometimes also in combination with a retarded ignition.
1. Check your exhaust pipe, especially the conection on the head stub.
2. Are you realy sure there is no air leak between the slide and the body...
3, Two turns out should be a very good starting point for the pilot air screw.
4. Does the banging increasing or decreasing when you alter the pilot air screw?

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simon.holyfield
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Re: Idle W/NG

Postby simon.holyfield » Sat Aug 10, 2019 10:30 am

Screenshot_20190810_003656_com.android.chrome.jpg


I am harbouring a fanciful notion that this Amal air screw has a shorter taper than the one I have fitted.

IMG_20190809_161602.jpg


I'm wondering if my air screw bottoms out against the brass plug and does not meter properly. Adding to this thought is the sight of a witness mark on the brass closing plug at the bottom of the pilot air passage.

If this is the case, I wonder if the pilot air passage is always set lean, and as Alan suggested I am 'idling' on the needle jet.
cheers

Simes

'51 Square Four,
'58 Huntmaster,
'42 W/NG, '78 CX500
http://ariel-square-four.blogspot.com

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alan.moore
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Re: Idle W/NG

Postby alan.moore » Sat Aug 10, 2019 11:14 pm

Simon
My air screw looks identical to yours, here it is alongside a new one from Amal (new one at the bottom)

Looking at the parts lists the screw appears to be the same part for all the pre monoblocs
SDC13000.JPG


The new one is slightly different on the taper measurements. Whilst the taper is the same length : origional 0.132 down to 0.072, new one 0.123 down to 0.063 (the taper appears to be the same angle just slightly different in diameters)
SDC13001.JPG


I don't think this is your issue. Mine also seems to just contact the brass plug if you screw it all the way in, but at that point it is fuly seated inside the internal taper

Cheers
Alan
1939 VH Redhunter;1942 RN WNG;1951 Triumph 6T Thunderbird;1970 BSA B175 Bantam;1986 Yamaha SRX600 single
http://cloggymoore.wix.com/triumph-pre-unit-6t

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simon.holyfield
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Re: Idle W/NG

Postby simon.holyfield » Sat Aug 10, 2019 11:29 pm

Thanks for putting that together Alan. I had a look at that today as well, and now I'm not sure if the needle really does contact the bottom of the drilling, although I'm sure it comes close.

I did some measurements of port and and screw, which doesn't conclude anything; I also put some engineers blue on the taper, and that showed contact with the seat about halfway down the taper.

It's all back together now, and it seems to tick over reasonably well with the timing about 1/4 - 1/3 advanced. The mixture screw is about to 1 1/2 turns out. I've only tested it in the workshop, so it's not been really hot.

We have 55 mph winds here, so not brilliant for biking...
cheers

Simes

'51 Square Four,
'58 Huntmaster,
'42 W/NG, '78 CX500
http://ariel-square-four.blogspot.com

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alan.moore
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Re: Idle W/NG

Postby alan.moore » Sat Aug 10, 2019 11:41 pm

So...is the air screw making a difference to the tickover now when you adjust it?.
1939 VH Redhunter;1942 RN WNG;1951 Triumph 6T Thunderbird;1970 BSA B175 Bantam;1986 Yamaha SRX600 single
http://cloggymoore.wix.com/triumph-pre-unit-6t

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simon.holyfield
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Re: Idle W/NG

Postby simon.holyfield » Sat Aug 10, 2019 11:49 pm

No - I can close it and if the throttle stop is high enough and the timing advanced enough it will still run. Actually it will idle pretty quickly with the throttle shut if it is advanced enough.

I'm beginning to think I am chasing ghosts here. In my view, it ought to idle relatively slowly & reliably with the timing retard most of the way and the air screw a couple of turns out, with the throttle almost closed.

I guess I'm thinking - 'what should it do?'.

By the way, I still haven't had a proper look at the float level. Between shopping, walking the dog and making some shelves for my office I have been getting on with the FH mudguard.
cheers

Simes

'51 Square Four,
'58 Huntmaster,
'42 W/NG, '78 CX500
http://ariel-square-four.blogspot.com


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