AJS & Matchless Owners Club

Well I've had this carb off yet again and checked all passage ways including pilot bypass, all seemed ok. Tomorrow I'll be kicking it up. It seems however, you were right Paul this carb is from a different bike. Initially the jets were wrong including a 470 main, long since corrected. But I decided to measure the port and it's 1.18 mm which is 1 3/16 which is for a 500 cc, should be 1 1/8".
Does this make a significant difference or can I live with it?

Ah. If the carburettor port size doesn't exactly match the manifold then it'll never run right. Sorry.
A smooth inlet manifold is essential for proper running.

Here's a really good article on why: http://archives.jampot.dk/technical/Gen ... mplin).pdf

(check out the 1956 BSA story on p.3, which might sound familiar...)

That is an excellent article, I learn again. The symptoms and carbonation of the carb throat are exact same. A PO must have run this bike quite a bit to cause such a carbon build up, I wonder was the 470 main jet his answer. I'm hoping I can dress the phenolic spacer to solve the problem. I'll try today. Thanks.

vigo wrote:I'm hoping I can dress the phenolic spacer to solve the problem.

Hi

Yes, you could smooth the intake tract by adjusting the insulating spacer but this is very unlikely to solve your issue.

The carb works by creating a depression in the venturi, this depression is dependent on the speed and mass of air flow through the venturi. A 500cc engine has a higher mass air flow than a 350, hence the larger bore of the carb.

By careful selection of carb components you could probably get the 350 to run on a 500 carb but it will always be a compromise, better to bite the bullet and get the correct carb.

Regards Mick

I tapered the step out of the phenolic packer a total diameter diff with the carb of 1/16" removing 1/32 all round concentrating on the bottom 1/3 of the circumference. Refitted the correct 25 pilot started and ran with good throttle response, a totally different bike. Enough success for one day before too much euphoria. Tomorrow out for a test run and some carb tuning.

Sent from my SM-T580 using Tapatalk

Great problem solving guys. I'm rebuilding a Model 8 and the engine came already disassembled 'for inspection purposes' so I may well be having similar carb issues in the near future and using your tips to troubleshoot.

I'm waiting for some new carb flange o rings, the ones I ordered have too thick a cord at 4mm, waiting for 3.5s. In the meantime have had the (unavailable) centrestand welded up by an excellent little one man workshop here in Germany, €30.

After receiving new flange o rings and working the phenolic packer some more I remounted the 389 and tried to start and start and start. Eventually started, clouds of smoke then out. So Mick I think you're right a replacement carb and packer necessary.
Amal 389/18 3632 is specified on a 1 1/8" bore. I may take a chance on an ebay used (fool) which is a 389/21. The bore and flange hole spacing is good and as I bougt new jets etc anyway maybe worth a risk? Why the difference in number 18 and 21, what do these mean? Could be suited to a different manifold and bores not line up?

An 018 when made new was for AJS 350 MCS/Matchless 350 G3LC 57-59, the 021 was made to suit a Panther 100 57-66, both 11/8 but each being set up to suit different machines.

389/042 AJS 350cc 1960-61; Lightweight 1 1/8" 25 3.5 220 0.11 3

Colin

Thanks Colin, so are we saying the dash number setup ie jets etc. Or are there fundamental differences like throttle valve size and drillings? As you detail dash 42 would be for the lightweight 350 (workshop manual). And dash 18 (club spares) I then assume is setup for a G3.
So in your experience could I buy the 21 and rejet it?