Britbike forum Forums British Motorcycle specific Brands Bsa bulletin board 1952 B33 Rich Mixture issue

After much time spent in research, I've come up short on how to correct this problem. Here are the symptoms:

- Wet, fouled black spark plug. Plug is an NGK B6HS
- Slight puff of black smoke when throttle is applied from idle; no smoke upon startup, and idle is smooth after warm-up with spark slightly retarded.
- Exhaust note is loud, with hesitation, when throttle is applied. Rather than a increasing rumble, it more resembles a snare drum; it's sharp, snapping and resistant.

A few other details:
Starting is easy; one or two kicks without any choke.
I have a foam air filter fitted over the carburetor.
Carburetor is an AMAL 376/40
Main jet: 260
Needle jet: 106
Pilot jet: 25

I'm really just at a loss as to whether this can be corrected with different jetting or if I'd be better off with a whole new carburetor. According to my research, this isn't even the stock carburetor, but the jetting seems correct otherwise. I haven't had any previous experience with Amals and my understanding is that they have a mind of their own sometimes. Any suggestions are appreciated. I haven't taken it out on the street for fear that this is a major problem. I've driven it around my property at less than 10mph and it seems fine with it, but any more than the slightest, slowest application of throttle yields the third symptom. Also, strangely, a slight application of the choke when throttling up seems to make this symptom less apparent, but only with the spark retarded also. With the spark fully advanced, the engine will run fine for a moment, then cut out and spit a cloud of grey smoke from the carburetor.

A 376 on a B33 should be 1 1/16" bore.

No air filter, main jet 260, needle jet .1065, needle position 3. Pilot jet 25

With air filter - 210 main jet, the rest the same.

Coming off idle, if the pilot jet is correct, check float level and slide cut away.check wear on the needle, verify needle is correct length for a 376 carbie.

Modern fuels do affect jetting from 1950's specs, but the change is incremental, not drastic.

Your bike should have a 289, which is a seperate float bowl carbie. But the 376 was used on later B33's

Forgive my ignorance, but is my current 106 needle jet the same as ".1065"? I searched the latter and couldn't find any results (unless I'm just looking in the wrong place)

AMAL's site lists new 376/40s shipping with 106s for needle jets because the 1065 is no longer made, so it looks like I'm good. Lets hope the 210 main does the trick.

What throttle slide cutaway do you have? The book say 3.5

Also is the carb in good condition with little wear?

The .106 needle jet will be slightly leaner than the .1065 and from what you describe isn't therefore the source of your problem.

It might also be useful to take the bike for a run so you get up to normal road speeds and then see how things go.

Doh, I forgot to give him the slide cut away. Thanks.

Try replacing the pilot jet, it may have been cleaned out over enthusiastically and enlarged .
What setting is the pilot air adjuster screw at?
What slide number?, it sounds like it may be too lean.

All of my symptoms seem to point to a richness issue though, unless I'm sorely mistaken. The pilot jet appears in good order, but I suppose replacing it outright couldn't hurt. The adjuster screw I originally set at 2 1/2 turns out from its seat per some source I read that I can't currently remember.

As soon as I've got my replacement primary chain cover gasket. Someone decided to make their own out of Permatex and that had to go.

2 and a half turns out seems excessive, normally , the correct setting is somewhere around 1 to 1 .5 turns out, this setting must be fine tuned when the engine is warm. At 2.5 turns out with a good pilot jet you would be very lean at idle, given the richness you are experiencing , either your pilot jet has been enlarged or you have other issues like a sinking float or poorly seating float needle valve causing high fuel levels in the float bowl.
,

hey it's been awhile, but when i lived in arizona i had to take my old bonneville in for emission testing every year to get it registered. it failed every year, too, but i developed a successful work-around.

anyway, the little paper they gave me when i failed told me the CO ratings, i believe. can you wangle a test at a licensing facility? lots of carbon monoxide means rich, and it might substitute for an A/F gauge. if you bring in a box of doughnuts in the morning, they might do a series at different rpms for you on the rollers.

just an idea.

That's what's so good about the UK, bikes are not tested for emissions

Well only a visual inspection. Is it chucking out smoke? Yes. A lot? No! Pass and advise.

I don't think this would work for a few reasons. To eliminate possible bribes it appears the process leaves no room for an employee to activate the test without first entering the vehicle's VIN at the time they confirm they've placed the payment for the test in the drawer. The test itself is automatic at a fixed road speed so there's no room for getting readings at a variety of rpms.

Since for years everything on a car has been controlled by an ECE only old fashioned garages probably still have functional A/F mixture instruments. But, those that do can determine -- no doubt for a fee -- the mixture at idle. You could operate the engine at higher rpm as well, but I'm not sure a no-load reading would be all that beneficial.

p.s. Arizona eliminated testing of motorcycles a decade or so ago so the anxiety of pulling into the testing bay with an AMAL-carbureted bike is gone. At least with a Bonneville you had a fighting chance of passing emissions, but with a Trident...

How does it run without the air filter?

I tried oiled foam filters and concluded that they have to be pretty big, to avoid restriction leading to over-rich mixture.

It ran better without the filter, but riding in Phoenix without an air filter wouldn't be a great idea. I've been trying to find another option, but the clearance between the carb inlet and my oil tank is less than two inches and the carburetor points at about five o'clock so anything round would have to be really small. I tried an offset Vokes style filter and the back plate by itself wouldn't fit. Tried a 90 degree elbow, but then I hit it with my leg and the weight causes it to sag. A full sized sock filter bends upwards and rests on the oil tank and gets oil on the back of my thigh. But I digress.

Right now, the filter is thick enough that it fills the gap between the inlet and the oil tank. I'm sure that it's causing the rich mixture condition. Tomorrow my 210 main jet will get here so we'll see where I'm at once I get it fitted.

I have found that a restrictive filter requires a bigger slide cutaway and an adjustment to the pilot screw and throttle stop.

If that improves running at slow speeds, but it's still rich at over 1/3 throttle, drop the needle by a notch.

my bonneviile always failed arizona emissions, but at that time if youcame back for a retest and showed a 40? percent improvement you qualified for a waiver.

I would show up on my bike and while I was inline id pull out a screwdriver and richen the idle, then retard the spark a few degrees. I would fail horribly. so I would then go home and put in new plugs and correct the idle and timing.

on my retest I would still fail, but as it was improved the minimum amount they would issue the waiver.

In Jooooisey they stopped the testing of bikes about 5 years ago.
Before that there were exemptions for old bikes and even for newish bikes if you went to the right private test station and slipped the guy a tenner you passed anyway.
This is Joooisey after all!

Thanks for the tip. I'll keep it in mind tomorrow after I change the main jet and readjust everything.

Main jet has 0 influence on Idle and just off idle mixture. It is all bout float level, pilot jet, slide cut away and mixture screw setting......

Just sayin'

Id take the air filter off and try it then.

As I've said, Arizona is too dusty to drive without one, and taking it off only reduces the symptoms; it doesn't eliminate them completely.

That's ask very well. If you'd care to elaborate on a solution, I'm open to suggestions. Everything except my main was as you listed before. So, can a float be adjusted on a Monobloc? Would getting a different pilot jet help? If so, do you have a recommendation? As I said, I've got no previous experience with Amals.

There is another active thread right now about the Monoblock float jet and float height. Lots of good will info, no sense me repeating.

Is there by chance a fiber washer under the needle seat that screws and into the float bowl? Original Monoblocks should not have a fiber washer.

An AMAL is like any other motorcycle carb. It has 4 seperate circuits - idle, off idle, mid range, and WOT. That is the simple explanation. In reality, at least in my experience, there is some overlap and other influences.

Idle and off idle is your current problem. Idle is controlled by the pilot jet and mixture screw. Off idle is slide cut away with some influence of the idle components. Both are affected by float height.

BTW, does your carb have a choke? Is there it on by chance?

Is your tickler sticking holding the float down?

Just some things I would verify first

Getting a new pilot jet may help ( its cheap and easy to check, theres a very good chance the old one has been so poked that instead of a tight metering hole it is now oversize and instead of the correct fit it will now be like throwing a sausage up an alleyway)
, doing this from thousands of miles away is difficult though,

there are several things already mentioned here which may also cause richness at low throttle openings.
Wrong float needle banjo height ( no fibre washer is correct),
Sticking float. heavy float ( filling with fuel)
Running with choke on, the choke is off with a tight wire, counter-intuitive.
Wrong slide cutaway. I dont think you have given us the slide number ( may have missed it though ).

Your issue is not about Main jet size. Not yet any way. You need to be flat out with a wide open to 3/4 open throttle before the MJ has any real effect.