Jetting carbs

Joined
Dec 13, 2006
Messages
26
My recently acquired 1969 750S needs serious carb work and I require some sage advice.

She will be ridden mostly at sea level (California) with an ocassional higher elevation day trip here and there, not going to be racing any pocket rockets, not going to do a cross country jaunt. (Well not yet)

I am a little "fresh" when it comes to jetting carbs, twin Amal 930's in this case.

Any pointers would be most appreciated.
 
Joined
May 19, 2006
Messages
1,612
Country flag
The stock settings are a good starting point. Do you have a note of them?

What exhaust and silencer systems are you using and what sort of Air Filter ?

It goes without saying that timing needs to be accurate and the carbs in good condition if meaningful tuning results are to be obtained.
 
Joined
Dec 26, 2006
Messages
48
Clean carbs thoroughly. Make absolutely certain ALL the little orifices are clean. A set of gas welding tip cleaning wires works well for clearing the orifices, just don't get too aggressive with their usage. Spray can carb cleaner to further clear them out, followed by a blast of air. Stock factory jets and settings will work fine. Needle jets are often worn and cause poor running. Check and adjust float bowl height. Make sure the float bowl gasket does not impair the movement of the float.

Don't trust the timing marks on the primary cover. Check it against a degree wheel before setting ignition timing.
 
Joined
Dec 13, 2006
Messages
26
Thanks for the input.

Some further info........

Boyer ignition, 1 or 2 kick start.

From what I can gather the mufflers are called early megaphones or "pea shooters".

The airfilter is a round paper type squished to fit the oval assembly, it is in excellent clean condition. I need new rubber boots to go between the carbs and airfilter, the ones I have are definitely showing their age.

In tuning one cylinder at a time I can get the right dialled in beautifully but the left stays rich no matter what.

So I am thinking new Amal carbs for now and put the originals on a "to do" list. I want to get her street worthy ASAP.

From the Norton workshop manual it suggests a 220 main jet, .107 needle with middle position and a #3 throttle valve, for the 1970 model (my frame is stamped December 69, matching numbers).

A .106 needle is spec'd for 1971 and 72.

So I am going to go ahead and order carbs jetted to the manual specs for 1970.

Sound good and thanx??
 
Joined
Nov 12, 2006
Messages
44
There have been a lot of quality control issues with the new Amals. Before you plonk down for new ones,you should check out http://www.amr-of-tucson.com/They will take your carbs,and completely recondition them.New sleeves,and slides,pilot circuit cleaned,float set, mounting flange flattened,o-ring groove recut,ready to bolt on. Not only will they be your original carbs,but they will outlast,and out perform new Amals.

I have had three pairs of carbs done by them over the years,the first way back in '86,on my '69 T120. The next,a few years later,on my '73 Interstate,and the latest,in about '97,on my ,73 T140. I have been satisfied in each case. There are lots of shops doing the re-sleeve thing,but these guys are the only ones I have personal experience with.

By the way,even though my poor '69 Bonnie blew up years ago,those carbs are still running around on my friends '72 BSA Lightning!
:D
Cheers
Bruce
 
Joined
Dec 26, 2006
Messages
48
Please let me offer a little correction to the previous reply. AMR does NOT resleeve the carb, but bores them slightly oversize to accommodate a hard-chrome-plated brass mikuni slide. This has some distinct advantages over the pot metal amal slide, as it GREATLY reduces the wear between the slide and carb body. It also has considerably more mass and is thus subject to less bouncing due to vibration. I then use Mikuni springs in the carb to reduce throttle pull effort.

I've run these quite successfully on all my Nortons, and my BSA B50.

The only downside is the Mikuni slides do not accommodate the choke slide, so you do not have chokes. I've always found that a well tuned Brit bike can do without the chokes, by using the ticklers.
 
Joined
Nov 12, 2006
Messages
44
Quite right,Norflog. How silly of me! I remember now that the sleeveless repair was the main reason I went with AMR to begin with.The sleeves had a reputation of loosening,and rotating,in the bore.:roll:

As far as the choke slide goes,I don't know very many people who are still running the choke in concentrics,and the ones who are,in my opinion, are just courting trouble.However,there are some well respected Brit bike tuners who will argue that if the bike can be started,and ridden,from cold,without the use of a choke,it is running too rich. :?
:D
Cheers
Bruce
 
Joined
Dec 13, 2006
Messages
26
Carbs

An update.

I had already ordered the new Amals so will send those on to AMR to get reworked, my originals will grace a shelf as objet d'art for now.

My pet cocks look good but the fuel lines seem really brittle, their integrity is fine but flexibility is seriously nil. It is as though they are glass.

With the reworked Amals my choke lever becomes redundant...... any suggestions or do I just put it on the shelf?
 
Top