Carb jetting - confused

Information relating to the Matchless G80 or AJS Model 18 500cc Heavyweight.
StephenG80
Posts: 264
Joined: Sun Apr 30, 2017 10:42 pm
Location: ARGYLL UK

Carb jetting - confused

Post by StephenG80 »

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? :roll:
User avatar
dave16mct
Member
Posts: 3234
Joined: Mon Jan 01, 1990 12:00 am
Location: LANCASHIRE UK

Re: Carb jetting - confused

Post by dave16mct »

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.
StephenG80
Posts: 264
Joined: Sun Apr 30, 2017 10:42 pm
Location: ARGYLL UK

Re: Carb jetting - confused

Post by StephenG80 »

Thanks Dave. :beer:
User avatar
Pharisee
Member
Posts: 1126
Joined: Tue Mar 15, 2016 8:51 pm
Location: Lincolnshire UK
Contact:

Re: Carb jetting - confused

Post by Pharisee »

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)
I'm from the Fens.... Gimme six.
StephenG80
Posts: 264
Joined: Sun Apr 30, 2017 10:42 pm
Location: ARGYLL UK

Re: Carb jetting - confused

Post by StephenG80 »

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 :twisted: . So I guess extra 40 thou on the standard MJ compensates for the ethanol in our fuel. Jobs a good'n 8-)
StephenG80
Posts: 264
Joined: Sun Apr 30, 2017 10:42 pm
Location: ARGYLL UK

Re: Carb jetting - confused

Post by StephenG80 »

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.
Mick D
Posts: 2886
Joined: Mon Mar 24, 2014 5:44 pm
Location: Aberdeenshire

Re: Carb jetting - confused

Post by Mick D »

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 :geek: :geek:

Regards Mick
StephenG80
Posts: 264
Joined: Sun Apr 30, 2017 10:42 pm
Location: ARGYLL UK

Re: Carb jetting - confused

Post by StephenG80 »

pedants are good - they lead to education - I did not know that :beer:
User avatar
Harry44
Member
Posts: 279
Joined: Sat Dec 17, 2016 5:26 pm
Location: SOMERSET UK

Re: Carb jetting - confused

Post by Harry44 »

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 :geek: :geek:

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
si is does non opportunus vos postulo a maior pango
Mick D
Posts: 2886
Joined: Mon Mar 24, 2014 5:44 pm
Location: Aberdeenshire

Re: Carb jetting - confused

Post by Mick D »

Hi

Not to mention the shape and length of the bore :geek:

Regards Mick
Locked