Idle Issue and Oil cap leak

Information relating to the Matchless G2 or AJS Model 14 250cc Lightweight
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Re: Idle Issue and Oil cap leak

Postby clive » Thu Feb 18, 2021 8:18 pm

Iain_NCL wrote: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?


That's how I read the article..................
clive
if it ain't broke don't fix

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Re: Idle Issue and Oil cap leak

Postby clanger9 » Thu Feb 18, 2021 8:28 pm

Yes, if you have a step like that it'll make fuel droplets - resulting in a duff idle and plug fouling.
Best try and get a 389 carb...
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Re: Idle Issue and Oil cap leak

Postby Iain_NCL » Thu Feb 18, 2021 9:40 pm

Thanks, a 389 it will be.

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Re: Idle Issue and Oil cap leak

Postby Iain_NCL » Tue Feb 23, 2021 8:42 pm

It seems what I was measuring was the carb spacer as 1 1/8 :oops:

The port into the head itself appears to be 1 1/16. Yet if it's a 14 CSR engine like it says on it, surely it should be 1 1/8? Is there anyway I can actually check engine it really is?

I guess I should get a new 1 1/16 spacer and probably a new 376 carb...

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Re: Idle Issue and Oil cap leak

Postby clanger9 » Tue Feb 23, 2021 10:48 pm

It’s possible that it’s a CSR engine with a 250 head. The only difference with a CSR head is a bigger inlet valve and 1 1/8” inlet tract.
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Re: Idle Issue and Oil cap leak

Postby Iain_NCL » Wed Feb 24, 2021 8:36 am

clanger9 wrote:It’s possible that it’s a CSR engine with a 250 head. The only difference with a CSR head is a bigger inlet valve and 1 1/8” inlet tract.


I think you're probably right. Although the registration date for my bike is '62, looking at the frame and original engine number suggests 61 (http://archives.jampot.dk/general/Matchless_Frame_Numbers_1950-1966_AJSMOC.pdf). The current engine number is a 62 but I guess entirely possible the head and maybe barrel are from the original 61 engine and then throw in the 1 1/8 carb spacer to make it more confusing - what a jumble!

Based on http://www.jampot.com/TechTips/carbs/matchless_carbs.pdf the cutover from 376 1 1/6 to 389 1 1/8 on standard 250 G2 seems to have been 1962

Hopefully the whole path at 1 1/16 and a new carb will do the job.

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Re: Idle Issue and Oil cap leak

Postby WiltshireMike » Wed Feb 24, 2021 9:32 pm

A 1/16" mismatch on the carb would unlikely cause rich running at idle. As for the ignition, so long as the springs are good, and rotate the weights smoothly to the 'closed' position the spark should be correct and obviously won't affect the mixture. Electronic ignition will often regulate idle speed via spark timing, but not applicable in your case.

As you probably have an intake valve guide oil control screw on the head, make sure this is set to no more than ¼ turn open to limit the amount of oil drawn down the intake valve guide. Oiled plug suggests this might be a culprit.

Assuming the carb is correctly built – 180 main jet, 25 pilot jet and needle clip in the central slot, the float isn’t leaking, and the float valve is correctly seating, you should be ok. Do check the intake system for air leaks at the carb to spacer, or spacer to head, as they'll require an over-rich carb setting to compensate.

Back in the day, conventional wisdom was to initially set the air screw around 1 – 1 ½ turns open, warm the engine, reduce the idle speed on the idle stop screw to a moderately fast idle, set the airscrew to obtain the highest idle speed, then re-set the idle screw. Then repeat.

The oil issue sounds as if the engine is slowly leaking oil from the tank to the sump when it’s standing and it’s that oil which is pumped back to the tank once the engine is started. You want to check and adjust the oil level only after the engine has been running for a short while and has scavenged all the oil from the sump back to the tank. Best to start from scratch, drain both the sump and the tank then refill with the specified 1.4 litres of 20W50.

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Re: Idle Issue and Oil cap leak

Postby Iain_NCL » Thu Feb 25, 2021 3:37 pm

Hi Mike

Thanks, good advice there that I'm going to work though. I have the cap leak sorted anyway - new cap, new oil and got the level right.

Got the bike in bits at the moment to look at a different oil leak, but getting there!

I am wondering if there is or was an air leak as that would figure some of the idle issues I saw.

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