Amal carbs on CB72

Sooty521 · Post by **Sooty521** » Sun Dec 13, 2015 10:41 am

Hi

I have finally finished the restoration of my 1963 CB72 cafe racer

I have fitted Amal concentric carbs, and while I can achieve a good smooth tickover and easy starting, under load the the running is poor, with misfiring.

I am fairly certain that it is a fuelling problem, and know that I need to set up the jets for the different throttle openings, but was wondering if anyone had experience of the best set up, to save me too much experimenting!

Any advice much appreciated

Many thanks

Hahnda · Post by **Hahnda** » Sun Dec 13, 2015 12:36 pm

Have you read through this?

http://www.honda305.com/forums/amal-carbs-t28.html

Sooty521 · Post by **Sooty521** » Sun Dec 13, 2015 1:14 pm

Thanks, I did read through the thread, but there is not much info on jetting or slides. Think I might need to change the slides to a 2.5 or 3.0. It's not the best time of year to be test riding unfortunately!

Hahnda · Post by **Hahnda** » Sun Dec 13, 2015 1:17 pm

Also some info here.
http://www.honda305.com/forums/mikuni-c ... 35-10.html

G-Man · Post by **G-Man** » Sun Dec 13, 2015 3:17 pm

Sooty

Not terribly helpful with actual starting settings but a fun project for those winter nights...... :-)

http://amalcarb.co.uk/downloadfiles/ama ... _guide.pdf

Take a look at Vince Lupo's thread on "Japan meets Britain"

G

Sooty521 · Post by **Sooty521** » Sun Dec 13, 2015 3:24 pm

Thanks G Man. That looks like fun!

Will check out the post

Vince Lupo · Post by **Vince Lupo** » Sun Dec 13, 2015 4:31 pm

I have been using the 2.5 slides (the hardened anodised versions) and they've been running great. My mechanic has them set a bit 'fat', which he says is definitely preferable to having them a bit lean! I do not remember the exact jets in there, but I could ask my mechanic to see if he remembers, as well as needle position.

Misfiring? Is it 'bogging down' too? If so, sounds like it's running rich (but I am definitely no expert!). When my mechanic and I dialed-in our Amal 626's, we did a lot of plug chops. Plug chops -- basically riding at different set speeds, then shutting the bike off, pulling over to the side of the road, pulling the plugs and seeing what they looked like. Went through a lot of plugs! So imagine getting the bike up to top speed and then shutting it off and coasting to the side of the road -- that is what I did many many times to get it right.

Now if you really wanted to do it right and not go through the trouble of all the plug chops (as well as delving into the black art of plug reading), you might consider bringing it to a place that has a dyno. Might save you a lot of time, but if you did do that, be prepared with a number of different jet sizes and slides.