I have picked up a fully refurbished monoblock (by Bratby) and am thinking of fitting it to my 99 . The issue is that it’s 1 1/8” bore instead of 1 1/16 , It’s going to fit ok and will be filtered with a decent paper car unit . Interestingly the main slide chamber is also a few mm bigger than a 376 casting, I was told they were the same body size , not true ! How do you think the bike will perform?, it has 650 sized valves and probably 8.25 comp ( 650 ss pistons).
A bigger carburettor is a…
- Log in to post comments
Hello again Bob - Your 1 1…
Hello again Bob - Your 1 1/16 inch monobloc will be a 376 type whereas the 1 1/8 inch monobloc will be a 389 type. These carbs have a bigger body and larger throttle and choke slides. The return springs are also bigger so you may need longer cables to suit. It depends on how much slack there is your inner cables at present. You may need to play about with your jetting. A good start will be a 320 main, 25 pilot and 106 needle jet. Try the needle in the centre position. Also go for a number 3 throttle slide. Are you using an air filter ? These are also a bigger diameter mounting than a 376. Good luck, Howard
- Log in to post comments
Sounds like it’s going to be…
Sounds like it’s going to be a journey of discovery .
- Log in to post comments
A bigger carburettor is a good idea, if 99s were sold new with carburettors that were too small.
You may be able to tune it to run satisfactorily, following the usual Amal procedure.
Wouldn’t be surprised if it’s tricky to get right at small throttle openings: idling and transition from pilot jet to slide cutaway. You’ll be running on the slide cutaway at speeds which would have the mixture controlled by needle height in the smaller carb.
You may also find full throttle is no faster than 3/4 throttle, especially at moderate rpm.