AJS & Matchless Owners Club

1954 G80s - I brought a brand new monobloc 389 carb (I got that one as the 89B, which is original eq for the '54 is not made anymore) from club spares and I was advised that it came with the correct jets for my bike , the main jet fitted was 160.

However, she was running very lean so I had to put needle up as high as possible but still too lean (pinking) when up to temperature. So, even though I thought this was going up way over standard I put in 190 MJ. That has improved things but still pinking when hot and opening the throttle at higher speeds (accelerating from 50 mph say to 60). I've done a plug chop check and looks lean still.

Then I checked my copy of Pitmans for my bike (but '55 mod) and it says the main jet for that carb (no air filter) should 240!!! so, no wonder she is lean (all the other jets and the needle (D 3rd groove) are as per Pitman.

So, am I correct I need 240MJ? (I presume Spares looked up the MJ spec for my bike year '54, which is 160, but of course that would have been for the 89B carb which is not available anymore).

Anyone running a G80s 54 with monobloc 389 carb?- what main jet are you running?

I'd go with the 1955 settings for the 389 monobloc: Main jet 260,(without air filter, 240 with) Pilot 30, Slide 3, needle centre notch.
Dave.

Thanks Dave.

dave16mct wrote:I'd go with the 1955 settings for the 389 monobloc: Main jet 260,(without air filter, 240 with) Pilot 30, Slide 3, needle centre notch.
Dave.

I recently bought a 1.5/32 389 Monobloc from the club spares. I specified a 260 Main, 30 pilot and No. 3 slide. The carb was fitted with a 160 main, 25 pilot and No. 3 slide but the 260 main jet and 30 pilot jet were supplied separately in a plastic bag. It only took a couple of minutes to change them over.
The bike, an AJS M18S runs beautifully with the new carb. (It had previously been fitted with a 1.1/16 376 Monobloc)

Ah- just realised my brain has been fuddled - I was wrong, the carb was supplied with 260MJ and that is what she was running - still running lean though.

I found a 300MJ after I had posted this thread, so pooped that in and I've just got back from 25mile run - lovely - no pinking now and A LOT MORE POWER . So I guess extra 40 thou on the standard MJ compensates for the ethanol in our fuel. Jobs a good'n

PS - my sincere apologies for casting aspersions in Club Spares direction!!!! - I should have known better, they always provide a top notch service - its been a long frustrating day until latterly.

Stephen G80 wrote:So I guess extra 40 thou on the standard MJ compensates for the ethanol in our fuel. Jobs a good'n

Point of pedantry:

Carb main jet sizes indicate their flow in cc / minute

Regards Mick

pedants are good - they lead to education - I did not know that

Mick D wrote:

Stephen G80 wrote:So I guess extra 40 thou on the standard MJ compensates for the ethanol in our fuel. Jobs a good'n

Point of pedantry:

Carb main jet sizes indicate their flow in cc / minute

Regards Mick

The flow rate also varies according to the material the jet is made from. Nylon jets have a bigger hole than brass for the same flow rate.

Cheers Harry

Hi

Not to mention the shape and length of the bore

Regards Mick