Idle Issue and Oil cap leak

Information relating to the Matchless G2 or AJS Model 14 250cc Lightweight
Iain_NCL
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Joined: Sun Jan 10, 2021 4:05 pm
Location: Newcastle upon Tyne

Idle Issue and Oil cap leak

Post by Iain_NCL »

Hi All, wondering if I could get some advice…

I was starting to get on well with my G2 following some jobs (forks, fuel tap, kickstarter oil leak), but that seems to have gone into reverse recently.

It rides fine on the few occasions I've managed to do so, most things seem to work, but I seem to have two bigger problems now:

Idle
When I got the bike originally, it started well and ran fine from cold but took a bit more of a knack to start when hot – when hot it ran very slowly and I noticed a bit of black smoke when left to run really slow but it never actually cut out. I figured it was running rich and decided to have a look. I found a sooty exhaust, sooty (and maybe oily also) spark plug so that seemed to indicate it was the case. As keen to stay away from fiddling with the carburettor as I was, I figured I would take a look.

I have so far succeeded in making it much worse! The horizontal screw (which I believe is the air screw) was previously screwed (near enough) all the way in so that would seem to explain the richness my attempts to change that and the idle/throttle screw seem to result in either a far too fast idle or it running so poorly it quickly cuts out. The idle speed also seems to wander all over the place. When I tried to slow it would often become very unstable and cut out – this seemed to be due to a bit of carbon stuck between the electrodes of the spark plug which I cleaned off each time.

Currently I have the air screw one turn out from fully home and the idle at a point that seems to keep it running but it's it seems to run quite fast but if left it might then slow down a lot, a quick touch of throttle and it'll go back to fast then slow and start to run rougher and so on. Slowing the idle further and it'll cut out or near cut our on applying any throttle. The only constant is it's not idling great!

I am tempted to try and put it back where it was before at least but that's probably not a good long term solution!

Is there an idiots guide to getting the carb idle set right or is it something more fundamental wrong?

Oil cap
After extended running, oil comes out of my oil tank cap when the oil gets hot and the engine speed higher. At cold, the oil is about ½ inch below the return hole but this rises as it heats up. When in motion it noticeably seems to escape down the front of the engine. I tried adding a felt washer as seems to be shown in the parts list, but this doesn't seem to help – the cap itself has a rubber O ring.
Iain_NCL
Posts: 19
Joined: Sun Jan 10, 2021 4:05 pm
Location: Newcastle upon Tyne

Re: Idle Issue and Oil cap leak

Post by Iain_NCL »

I read ColourNCoat's carb question thread with interest as it might be relevant to my issues.

I have a plain old 1962 Matchless G2 in theory, however the engine doesn't appear to be the original and is marked 14CSR/serial and appears to be 1962 from the serial number. I would assume this means the engine was from a 1962 AJS Model 14? The carb is marked 376/99. As such, I wonder if this is the source of the issue and it's been disguised to some extent by an odd idle setting. I assume I have to remove the carb and measure it to know for sure? If it is incorrect, is the best solution to buy a 389 carb?
dodger
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Re: Idle Issue and Oil cap leak

Post by dodger »

Could be the mechanical advance/return, they are usually worn out and unfortunately no longer available. Electronic ignition would be the way to go if this is the case.
Iain_NCL
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Joined: Sun Jan 10, 2021 4:05 pm
Location: Newcastle upon Tyne

Re: Idle Issue and Oil cap leak

Post by Iain_NCL »

dodger wrote:Could be the mechanical advance/return, they are usually worn out and unfortunately no longer available. Electronic ignition would be the way to go if this is the case.
Hi Dodger,

Funnily enough I had already had a look at this and I think it's ok. I can pull the weights out and the cam advances/retards smoothly in response - I put a drop of oil on each pivot after. No idea how to check it beyond that though!
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Taid
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Location: Shropshire UK

Re: Idle Issue and Oil cap leak

Post by Taid »

dodger wrote:Could be the mechanical advance/return, they are usually worn out and unfortunately no longer available. Electronic ignition would be the way to go if this is the case.
Does electronic ignition not depend on the mechanical adv/ret for its timing? ..

I thought it was to provide a healthier spark than the magneto would produce, especially when starting
___

Now sold ... 1956 AJS 16MS Bitsa .. HSU 414 .. rebuilt/re-registered 1987
Mick D
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Location: Aberdeenshire

Re: Idle Issue and Oil cap leak

Post by Mick D »

Taid wrote:Does electronic ignition not depend on the mechanical adv/ret for its timing? ..
Not normally - it uses a programmed electronic advance curve.

Regards Mick
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Taid
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Re: Idle Issue and Oil cap leak

Post by Taid »

Mick D wrote:
Taid wrote:Does electronic ignition not depend on the mechanical adv/ret for its timing? ..
Not normally - it uses a programmed electronic advance curve.

Regards Mick

Umm .. too clever for me ..

So the adv/ret is removed and the magneto (If the timing is taken from the magneto cam) is locked directly to the engine position and the electronics calculates the engine speed and, from that, calculates the time before the next expected pulse to fire the coil ...

or something like that?
___

Now sold ... 1956 AJS 16MS Bitsa .. HSU 414 .. rebuilt/re-registered 1987
Mick D
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Re: Idle Issue and Oil cap leak

Post by Mick D »

Hi

Being under the G2 forum I assume this is for coil ignition and the normal modification is the remove the points and auto advance mechanisms, these are replaced with an electronic sensor and interrupter. The signals generated correspond to engine revs and this data is used to select where along the advance curve to fire the spark.

Regards Mick
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Taid
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Re: Idle Issue and Oil cap leak

Post by Taid »

Mick D wrote:Hi

Being under the G2 forum ......

Regards Mick

Oops ..

Sorry about sort of hi-jacking ..

I picked up on advance/retard and electronic ignition rather than G2 specific ..

I was thinking specific to magneto (a la 1956 16ms)

Taid ..
___

Now sold ... 1956 AJS 16MS Bitsa .. HSU 414 .. rebuilt/re-registered 1987
Iain_NCL
Posts: 19
Joined: Sun Jan 10, 2021 4:05 pm
Location: Newcastle upon Tyne

Re: Idle Issue and Oil cap leak

Post by Iain_NCL »

Yes, it's a coil on my G2. The contact breaker and advance seem ok so I'll save the electric wizardry for later.

I have taken the carb off and measured the bore and the induction port on the head and as guessed the carb bore is 1 1/16 and the port 1 1/8 - is that going to throw it out and make it appear rich like in the IPO article? Anyone?
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