My sons "new" AJS Model 20

Information relating to the Matchless G9 or AJS Model 20 500cc twin
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Re: My sons "new" AJS Model 20

Postby shifter » Thu Jul 20, 2017 5:24 am

Have a look at the carb take it off the bike and check all the internals jets needle position notch and so on strip and thoroughly clean (ultrasonic is good) even if its been done. Its quite an easy task to strip/reassemble and set up (tune) with all the help on the internet
?

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Re: My sons "new" AJS Model 20

Postby SPRIDDLER » Thu Jul 20, 2017 8:29 am

In your photos.
If the lever on the right is the choke then it is now set 'choke off' (as for normal running).
If the one on the left goes to the magneto it is now set at 'Full advance' (as for normal running).

Since you have been starting th'engine with them set as they are I suggest you do not alter them at all. (Once the sooty issue is resolved you can play with them if you wish).

The sooty plugs.

From closed to nearly a third throttle open th'engine will (only) have been supplied with fuel from the Pilot jet and needle. I expect most of your riding has been in this throttle range.

I guess that your mechanic and you have been tickling the carb and starting and stopping th'engine, leaving it ticking over etc., therefore the plugs will not be giving a true indication, so before stripping the carb.......

First remove the plugs and (if you don't have a new pair immediately available) clean the soot off with a soft (i.e. fibre-haired or brass) wire brush. It is particularly important to avoiding scratching the centre porcelain insulator. Definitely not a steel wire brush. You can use a blade to clean up the outer steel threaded rim.

Identify the Pilot air screw. This will be a horizontal 'screw' with a knurled slotted head on the r/h side of the carb and should have a spring. Refer to my previous link to the AMAL website just to identify the parts. Screw it in (clockwise) and count the number of turns before it stops. (Report here how many). Do not force the screw in as this may damage its pointed end or the internal seat.

Then unscrew the pilot screw one and a half turns. Start th'engine and proceed as in the AMAL Tuning Tips link already advised to make the final tweeks to the Pilot screw setting. This basically means screwing it in (richening) until th'engine slows and falters, then screwing it out (weakening) until th'engine speeds up and falters, then returning the screw to midway between those two positions. You may need to set the tickover speed higher so the th'engine doesn't keep stalling whilst you are adjusting the pilot screw. The screw to set the tickover (slow running) speed is the longer and nearly vertical screw (slotted knurled head) on the r/h side of the carb body. After adjusting the pilot screw you can reset the tickover speed if necessary.

Take the bike out for a say, 10 miles run (more if you wish) out of town (i.e. not pottering around in traffic). Stop th'engine immediately when you return then inspect the plugs again. This will only give you a rough guide but it'll be a start. Report how the bike ran. E.g. sluggish, cutting out, hesitating, surging etc. etc.
Last edited by SPRIDDLER on Thu Jul 20, 2017 10:23 am, edited 6 times in total.
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Re: My sons "new" AJS Model 20

Postby Daveinater » Thu Jul 20, 2017 8:41 am

These are great instructions. I will do all this when I get home and report back! Thank you!

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Re: My sons "new" AJS Model 20

Postby Daveinater » Sat Jul 22, 2017 11:29 pm

Good evening (morning?) all. Here's the latest update.

My mechanic asked me to not mess with the bike but, instead, to bring it back to him so that he could tune the carburetor. I took it back and he agreed that it was running rich. He needed the day to find out what "part" of the throttle movement was causing the plugs to foul so badly. (I've learned by your previous comments and looking at the carb manual that has been posted here that there are several stages of carburetor movement....so I nodded as if I knew exactly what he was talking about). After having the day to work on it, and test drive it several times, he said we are good to go. I haven't yet had a chance to test drive it, since here in the Southeastern US, we have a lot of thunderstorms this time of year and it's currently raining cats and dogs. I guess our test drive will come at a later time.

While there, I asked him if he could install the new chaincase seal, obtained from the No. America AJS Club spares. He did so and later reported that the outer chain case cover didn't seem like it was true. Being tin, he said there was not a lot he could do to fully straighten it. After struggling with getting it all sandwiched together, he did get it all bolted together and, when he began to add oil, it all began to pour out of the bottom of the clutch cover, from a screw hole. He found that the boss inside the cover was very stripped. He managed to get an oversized bolt in, but said it would probably have a slow leak....and it does. We talked about "options" and he didn't seem to think there were any. Sure, he could pull the outer chaincase cover and put a through bolt through the primary cover, with a nut on the back side, but then you'd have to pull the chaincase cover every time you'd inspect or work on the clutch...not a great idea.

He sent me home with the small but steady leak so that my son (Matt) can enjoy the bike for the next month before returning to college.

MY "great idea" was to just find a new chaincase cover. Well, it appears that's not an option. I cannot find one online anywhere. I see the part number for it is 018963 (and 018964 for the clutch cover). Nothing on Walridge, nothing in the Jampot spares listing...nothing on EBay.

What do you all do if you need to replace the chaincase cover?

I've heard about belt conversions. I'm wondering if I should spare ourselves the current and future agony of all this and just convert the thing to a belt and be done with it. What about the clutch rollers and engine shock that I hear also need the chaincase oil?

The bike is also burning just a tad bit of oil. I may have my guy redo the top end this winter. My thinking is "I have a guy that knows these bikes. He's not doing this just part-time because he's focusing on his family. I should probably take advantage of this opportunity while I have it and have him put new piston/rings, etc. in while I can". Any thoughts from the experts out there?

Thanks!

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Re: My sons "new" AJS Model 20

Postby robcurrie » Sun Jul 23, 2017 3:07 am

Have you tried Steve at AMC Classic Spares for the primary outer cover? You can download his parts list for secondhand spares on his web site.

Rob C

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Re: My sons "new" AJS Model 20

Postby Daveinater » Sun Jul 23, 2017 11:32 am

No, I hadn't Rob. Didn't know about them! THANK YOU. I sent them an email.

I am intrigued by the belt drive. I found one guy in Pennsylvania ("Quiet Power Drive") who manufactures complete kits, to include a dry clutch with a sealed bearing. I sent him an email but I've read that others have inquired but with no response. We will see.....

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Re: My sons "new" AJS Model 20

Postby SPRIDDLER » Sun Jul 23, 2017 5:20 pm

Daveinater wrote:My mechanic asked me to not mess with the bike but, instead, to bring it back to him so that he could tune the carburetor.

He sounds very sensible ;)

Sure, he could pull the outer chaincase cover and put a through bolt through the primary cover, with a nut on the back side, but then you'd have to pull the chaincase cover every time you'd inspect or work on the clutch...not a great idea.

Can you/he not fit a captive (Araldited) nut in place of the stripped thread?

MY "great idea" was to just find a new chaincase cover.

That's a bit extreme just to stop a drip! Chances are another one will be distorted or have a stripped thread (or worse). Those outer chaincases with the removable clutch dome are much sought after by owners of earlier bikes which didn't have a removable dome.

I've heard about belt conversions. I'm wondering if I should spare ourselves the current and future agony of all this and just convert the thing to a belt and be done with it. What about the clutch rollers and engine shock that I hear also need the chaincase oil?


c.£300+ (my guess) to stop a drip? It really isn't 'agony' but one of the endearing charms of these old bikes. Once fixed it'll see you out. Yes the clutch rollers do need at least some oil mist. Can you get a conversion compatible with your Burman B52 gearbox/clutch?

The bike is also burning just a tad bit of oil. I may have my guy redo the top end this winter....new piston/rings, etc. in while I can".

Depends how much is 'a tad'. They all burn oil to some extent. I'm not at all conversant with twins but I can say that the only certain way to know if an engine needs new valve guides or pistons and rings or just a hone and rings, or maybe a rebore etc. is to measure the bores and ring gaps. Check spares availability before taking it apart.
TBH you haven't really got to know the bike yet so I'd suggest you get it running right first then assess the oil consumption in the Autumn.
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Re: My sons "new" AJS Model 20

Postby Daveinater » Sun Jul 23, 2017 8:37 pm

Hi Spriddler!

Well the drip was more than just a drip. I'd call it a very slow pour. Lol. When he loaded the bike up in my truck, by the time we had the front tied down, there was a puddle of oil about 3" in diameter under the bike. It was flowing from that hole.

Having said that, I took the bike put on a run today and ran it at a good speed and through all the gears. It ran much better than before. It didn't bog down and remained at a consistent power. I did have one incidence of gears really grinding badly when I attempted to shift back up into first gear at a stop. It ground badly. The engine seemed to be idling at a bit higher speed at that point for some reason so perhaps that's the reason. I managed to force it into first and it shifted fine at subsequent stops.

After riding for about 30-40 minutes, around town and at speed (about 55 mph....didn't really want to go any faster than that), I pulled the plugs. Here is what they look like now:

Image

Image

I think these look much better and the mixture issue appaears to be resolved. I didn't have a lot of oil slung on the bike but I haven't checked the chain case oil...it was just too hot outside (95 degrees, 90% humidity). He had filled it up to about the mid point of the bottom chain. I'll report back with how the level now looks.

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Re: My sons "new" AJS Model 20

Postby SPRIDDLER » Sun Jul 23, 2017 9:47 pm

Glad you had a good run. I know you've only done a few miles around town but the plugs (which appear to be new ones) look far better than the previous photos where the earth (ground) electrode and the central insulator were thoroughly sooted up. It's not wise to generalise but 'ideally' the central electrode *should* be straw to dark brown colour, but the plugs' colour isn't such a good guide with modern fuels as it was with the petrol of 30 or so years ago, plus there's ethanol added now.

The oil leak does need sorting so something needs to be done (like fitting a captive nut inside as mentioned previously). It's recommended to add oil so that the oil is just touching the bottom of the lowest chain links and no higher. If the oil is coming out past the screw maybe a fibre washer or some sealant under the screw head would help (but I can't recall if there's enough room to fit a washer without going down the shed to look). If it's coming out from between the dome and the chaincase because the cork gasket isn't sealing you could try removing the dome and putting some non-setting sealant (bath silicone sealant - AMC black!) on both faces of the cork gasket but you don't seem to be the bodging sort to me ;)

I wouldn't be too concerned about the occasional crunch going into 1st. It might be that the clutch adjustment needs a tweak or that the clutch plates aren't lifting equally but most of the Burman clutches make a bit of a clunk or crunch when engaging 1st. Bear in mind that when stationary and even though the handlebar lever is pulled in the clutch basket is spinning (driven by the primary chain) but the gearbox final drive sprocket isn't, so the 'free' clutch plates are being dragged around by residual friction (even though you are in 'Neutral') until being stopped suddenly when you select 1st gear.
It is difficult occasionally to move the gear lever up into first. In this case it helps to roll the bike backwards or forwards say six inches whilst maintaining upwards pressure on the gear pedal.
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Re: My sons "new" AJS Model 20

Postby raffles » Mon Jul 24, 2017 6:48 pm

As has been suggested remove the chaincase degrease it and araldite a nut to the inside using the screw to hold it in position until it goes off
I did this on my own bike and have covered 7,000.00 miles since so it works
Tony

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