Throttle cables for twin carbs on a G12 CSR

Information relating to the Matchless G12 or AJS Model 31 650cc twin
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Throttle cables for twin carbs on a G12 CSR

Postby CAB » Tue Jun 26, 2018 9:31 pm

Hello
I'm putting a 1961 G12 CSR back together and it is fitted with twin "Speed Kit" carbs and manifolds but I don't have any cables. I'm going to make some but before I do it would be handy to know the approximate lengths of the cables between the slides and 2:1 junction boxes and the twist grip/air lever and if any in-line adjusters are fitted. Does anyone have a set that they can measure or photograph?
Thanks in advance
CAB

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Re: Throttle cables for twin carbs on a G12 CSR

Postby robcurrie » Wed Jun 27, 2018 2:57 am

I have a 62 CSR with Speed Kit that I can measure and photograph for you. I removed the chokes from the carbs, but left the cables in place.

Rob C

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Re: Throttle cables for twin carbs on a G12 CSR

Postby robcurrie » Wed Jun 27, 2018 6:31 am

Here are photos of the cables, the length from the twistgrip to the adjuster is 150mm, from here to the splitter (seen above the rocker cover) is 400mm, from splitter to carbs is 275mm. The first length from the twistgrip has been shortened twice when doing roadside repairs due to the cable breaking at the nipple (make sure the drum in the twistgrip has a nice smooth transition to the exit point). The choke cable length to the splitter is 620mm and then they should be a similar length as the throttle cable to the carbs (mine are missing). As can be seen in the photos, the choke cable adjusters are soldered closed to prevent air leaks. The throttle cable is not routed along the handlebars so that it has a nice free loop to prevent unintended opening of throttle when turning; this loop should not be too big as to be a danger of snagging.

Rob C
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Re: Throttle cables for twin carbs on a G12 CSR

Postby robcurrie » Wed Jun 27, 2018 6:36 am

Photo showing splitter and choke cable adjuster that is soldered closed. Ignore ignition coil behind the splitter - yours will not have this ... yet ;-)

Rob C
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Re: Throttle cables for twin carbs on a G12 CSR

Postby CAB » Tue Jul 03, 2018 8:41 pm

Hello Rob

Many thanks for the photos and measurements. What was the reason for removing the air-slides? My bike came with electronic ignition (Boyer 6v I think it was) housed in a replacement body for the mag but I replaced it with a newly rebuilt mag after dad left the ignition switched on which fried the coil.... I recently stripped it to cure a load of oil leaks and blowing head gaskets; the sump plug had been welded up and re-tapped but the heat locally distorted the cases creating a gap. I've also had an O'ring groove machined around the filter tunnel and decided to fit the speed-kit kit manifolds and carbs instead of the single carb. I've had the heads rebuilt and ported by The Cylinder Head Shop and it's also got new speed-kit cams and followers and re-ground rockers. I've also got a set of Thunder Engineering rods to go in it when the new crank turns up.... I've decided to put it back together with the standard crank and rods (bottom end has about 3000 miles on it) rather than wait any longer - I expect it will turn up next week now that I'm ready to drop it in the frame.

Thanks again

CAB

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Re: Throttle cables for twin carbs on a G12 CSR

Postby robcurrie » Wed Jul 04, 2018 4:16 am

Sounds like it's going to be a beast! Mine has the 10.5:1 pistons and composite gaskets don't last long; I made some copper gaskets after measuring up the head and barrel interface to make sure oil channel and pushrod tunnels lined up. In the composite gaskets, the pushrod holes were in the wrong position and would have rubbed on the pushrods.

Regarding the air slides (I call them chokes), they are not needed in my area as the tickler provides enough richness for cold starting and I can't have a problem with them coming on unintentionally.

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Re: Throttle cables for twin carbs on a G12 CSR

Postby CAB » Fri Jul 20, 2018 12:53 pm

Hello rob
Sorry for the late reply - I have been on holiday at my mum's/in-laws and when we got back our brodband had gone U/S. The engine is assembled and back in the frame - just need to fit and time the mag, dynamo, carbs & make the cables and finish off the return oil filter lines.
I sent a spare head and barrel to Ferriday engineering and they scanned them and made me a set of head gaskets in copper. The fit is perfect - no misaligned holes unlike the club composite and copper gaskets (advertised as 1.5 mm thick but actually only 0.9 mm thick). Ferriday will make them in various thicknesses - I had to get them made slightly thicker than usual (2mm) because the barrels that I am using have liners fitted and the "top hat" made the spigots higher than usual. Maybe your composite gaskets aren't being clamped properly if your heads have been skimmed several times or you have liners or maybe both? I have used an aerosol copper gasket spray to help seal the gaskets as well as a small ring of "slug tape" (very thin copper self adhesive tape that goes around plant pots....) with the OD cut out with a hole punch around the rocker feed holes for additional localised clamping in this area.
Ferriday can make you gaskets from my patterns (1961 CSR) if you contact them. I don't know the price but it should be cheaper now that the design has been done. AMC Classic Spares also supply copper gaskets but they didn't have any when I needed them.
Regards
CAB

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Re: Throttle cables for twin carbs on a G12 CSR

Postby robcurrie » Fri Jul 20, 2018 4:20 pm

Thanks for your reply, you may not have noticed that I am in South Africa. I have access to a waterjet cutter and made my own copper gaskets which I also supplied to a friend for his G12 CSR. One thing I tried was to reduce the area of the gasket to increase the clamping force (more pounds/square inch). The composite gaskets also had a fault in that they overlapped slightly at the inner edges. I haven't used the composite gaskets again since changing to copper. I seal the oil and pushrod holes by putting a smidge of high temp silicon on the copper gasket around these holes and let it cure before assembly - it's very little and you can hardly see it.

I think.the main problem with the composite gaskets is that each surface sticks to the metal surface it is in contact with and because of the differential expansion between the ally head and cast iron barrel, the gaskets eventually delaminate from the metal mesh and this allows the gas pressure to stretch the gasket between the stud holes and finally fail. My engine is still standard bore with original high compression pistons and the heads have not been skimmed.

Hope you have many happy miles on your machine once it's back on the road, these CSR's have grin inducing acceleration, even though I changed to a 19 tooth final drive pinion for more "relaxed" touring!

Rob C

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Re: Throttle cables for twin carbs on a G12 CSR

Postby CAB » Wed Aug 08, 2018 9:28 pm

Hello Rob
How much ignition advance do you run with your setup? One of the later manuals that I have states "reduce to 29 deg BTDC if speed kit parts are fitted" but I'm not sure if that is everything - cams, twin carbs and 10.25:1 pistons. I have the cams and carbs but not the high comp pistons.
Thanks
CAB

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Re: Throttle cables for twin carbs on a G12 CSR

Postby robcurrie » Thu Aug 09, 2018 6:41 am

CAB,

I use 11/32" or 35 degrees for my timing and I can even go more because I left a bit more at the end of the advance lever. When riding, I can't tell the difference after about half travel on the advance lever - I can only start the engine on full retard though because of kick back. I ride quite hard and can be found mixing with the more modern classics.

I am at 1350 metres above sea level and I use unleaded 95 octane, we don't have any ethanol in our fuel.

HTH

Rob C

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