Hitchcocks Forum

Hey
I am the owner of a 1995 Bullet, and recently bought a 1957 500cc triumph speed twin (5t) which is fitted with a 5/32” 689 Monobloc. According to a friend 689 carburetters are rare as they were only fitted to the BSA 650cc Lightning in 1966 and two Royal Enfield Models and are no longer made.

Currently the carb is fitted with a 220 Main Jet, 106 Needle Jet and a 30 Pilot Jet however the bike is not performing good at 1st and 2nd gear, the carb needle is set at 3rd position (try to put it at 2nd and 4th position) but still the bike is not accelerating well at low gears, it is like coughing…like it is getting to much petrol…

Any suggestions as to what jets size shall I use…shall I go for lower, i.e. 180 or higher i.e. 250

Thanks
Pat

Amal recon it should be this,

Carburettor Specification: 376/25
Make: Triumph
Model: 5T Speed Twin
Engine Size: 498cc O.H.V.
Carb Type: 376 Monobloc
Year: 1955 to 1958
Number of Carbs: 1
Needle Position: 4
Internal Bore: 15/16in

The 689 was fitted to,

AJS 750cc 1966; Model 33CSR 1 1/8" 20 3 360 0.11 3

Matchless 750cc 1966; G15CSR 1 1/8" 20 3 360 0.11 3

Norton 750cc 1966; Atlas 1 1/8" 20 3 420 0.11 3

5/32" that's a tiny carb

A 689 is too big for a 500. Jetting won't fix that. Sell the 689 on Ebay, as it is for lnterceptors it should get a good price and then get a new 376 as it should have. REgards, Mark

A 689 is only a mirror image of a 389 and they are not too big for a 500, I have one on my mildly tuned 350 and it runs fine. There is a 210 main jet in mine BTW. 389's were fitted to Continental GT 250's and Matchless 250 G2 CSR models, as well. Regards, Paul.

It's not my experience of over carbed singles but if Paul says it'll work on a Speed Twin fair enough. I still think you could sell that carb and buy a new one of whatever size you like! REgards, Mark

Sorry, Mark, I have just re-read the question - I saw 1995 Bullet and thought the carb was being tried on that for some reason. A Speed Twin, being a rather mild pair of 250 singles with one carb would probably prefer a smaller one, as you stated. Regards, Paul.

Many thanks to all of you ....i will sell it and buy a 376