make shift choke on monobloc 389 carb

[WayneBergman]

Sorry I am sure this is a dumb question and idea but my 52 B33 is firing up easy enough but then dies shortly aftter, maybe 5 seconds if I am lucky and then it dies and I am thinking its because of the now cold weather. The specs say my motor should have a Type 289 carb but I have in fact a monobloc 389 and when I got the bike it did not come with a choke assembly. Now in the cold I am thinking this may be my start up issue. Photos show my work around test to see if it is a carb related issue. The little black plastic dish fits snug into the air intake so I figured if I punch a small opening like a 1/2" circle to restrick the air intake it may act like a choke to see if I can attribute the hard start in cold to the carb. So a couple of questions please....

1..will a simple tempory blockage over the air intake as shown work like a choke, I am guessing so but not sure? 

2. If this work around blockage for air flow will work similar to a real choke assembly, what would be an appropriate sized opening in the plastic dish to be  to be close to the amount of air that is allowed through this area with a reg funtioning choke?

[LightningAndStarfire]

I think it would act like a choke but half an inch probably too large. Difficult to say ideal size, a lot of trial and error and probably less work to have a proper choke installed.

[AdrianS]

Get the proper choke fitted which is adjustable and can be tailored to the engines need.
Very easy to do especially if you have got the spare opening in the top of the monobloc carb.

[martinh10]

See attached photo of my B31 carb. I would say at full choke the slide covers about half the area of the intake bore. A half inch hole is only about 25% of the area of a one inch bore so I would say that's probably too small. I would start at 7/8" dia.

[MGI]

On the other hand, the B series engines generally do not need choke at all, I have two and they both start easily without choke. The method is well known, petrol on and depress tickler until petrol drips out of the bell mouth, retard mag slightly and kick! Usually that does it occasionally a second kick is required. This practice applies with any conditions likely to be encountered in the UK and it probably helps if the machine is garaged when not in use!

[DAVE BRADY]

Hi,

I would agree that if you want a choke fit the proper bits then you have the flexibility to gradually open it as the engine warms as you ride along.
When both the slide and choke are fully down which roughly the starting position, there is almost no gap so the hole needs to be small.  I have bit of Heath Robinson idea that would work, so get a bath or basin rubber plug that fits the bell mouth on your carb. Drill small holes in it until you get the desired choke affect.  Use the chain so that you could ride off with the choke plug in place and then once warmed up the engine is choking just pull the plug.
Maybe not, better to fit the proper choke.

Dave.

[martinh10]

Sorry, in my earlier post I meant 3/4" dia.

[idie]

I never use the choke on my B33 or M33. I just flood the carb retard the ignition, gently turn engine over once to draw in some mixture then a long kick and away they go.

[WayneBergman]

Thanks for all the tips guys.

Yes I realize a proper real choke is simple and more presice but I dont really plan on much if any cold weather riding, at this point I am just tring to trouble shoot the motors shut down right after start up.  So my cobbeled together choke testing is more of a test to narrow down as to what is going on and why it stops running because a few weeks ago in warmer conditions it was fine and showed no signs of wanting to shut down at start up.

The previous owner told me a choke was not needed and he knows the bike better than I do (perhaps he was a fair weather rider also) so I may just go the route of no real choke installed. In the mean time i am hoping the manual pluging of the air intake may just tell me a few things about how the motor will run in the cold weather and if my current issues are related to not having a choke so this is more of an experiment I guess at this point rather than a working solution if this makes any sense.

I will try the 3/4" hole punch and see how it goes. If it works atleast I know what is going on and then I can decide if I want a real choke or perhaps  go with Dave's elegant "rubber drain plug and chain" route. I kind of like that actually

[LightningAndStarfire]

As an alternative you could pour some petrol into the carb inlet through a thin plastic hose, to regulate the amount just squeeze the hose to get a stable idle and let the engine warm up. Cheers

[WayneBergman]

On start up this morning I made a real effort to keep the revs up to perhaps help with it not dying right after start up. To my surprise the motor was back firing and really sounding bad and then I realized what I think is my problem and its most likely not the choke.  I purchased a new timing adv/retard lever & when I installed it the other day I am thinking it probably needed a longer cable meaning my timming adjustment cable is always under tension even wtih the lever backed off full. So timing is too far advanced when trying to start. When I was installing the new mag lever I had to really pull hard on the cable houseing to fit the ferrule into the barrel of the new Mag lever as it was a differnt model that my old lever and surely needed a longer cable for proper slack in the cable. I was so happy I got lever to accept the cable I did not make sure I had enough slack in the system. I will take the cable stop out of the handlebar lever and try and work my way down to the other end of the cable at the mag itself. Hopeing I can just remove the fitting circled in red in the photo to pull out the cable and its stop from the plunger (I think its called a plunger?) in the mag to make up a new cable. Can someone tell me please if the area circled in red will let me get at the cable stop or do I need to take apart the mag for removing and installing new cable. ......thanks wayne

[Dennis49]

Hi, you can remove the cable from the mag  whilst its on the bike. I may be wrong about this but I think your magneto cable needs to be pulled tight when running. If the cable goes in the other side i.e. front then it runs slack.

[WayneBergman]

I have made up a  new longer cable to give me the proper slack in the system. The spring assembly and the plunger that engages with the cam ring look to be moving smothly and the spring is returning the plunger to its slacked position as far as I can tell - this is an assumtion just by looking at the top of the plunger and moving things aroung when I replaced the new cable, I have not removed mag cover however to look inside at the cam ring itself as it was not necessary to do so just to swap out the cable. Ok so cable installed and things look good but....

on trying to start now up with the normal full retarded lever set up, I was surprised to still have the issues with back fireing and the motor quiting on me right after intial fire up. As nothing has really changed that I can think of since it was running just fine a month or so ago other than the cold weather and my replacing the mag lever with its cable.

One thing comes to mind & this may not be worth worring about but I have a question please..... when I was first installing the new mag lever with the original cable that was too short I had to tugg on things pretty hard to get enough slack in the system to get my cable ferrrul inserted into the mag lever barrel. I have also seen on magneto drawings there is a slot/stoping mechanism on the cam ring itself to just alow around 3/8" able cable movement between full retard and full advance but I am wondering if when I pulled hard on the cable is there any chance of me changing anything regarding my timing settings. Perhaps I forced something to move from leverage against the cam ring stop by my tugging hard on the cable?  I have never timed an engine before so I have yet to look inside the mag at its workings hoping that someone can shed some light on things for me.........thanks again wayne

[Dennis49]

Hi Wayne, BSA changed the cable entry on the magneto. If the cable enters on the right looking at the end of the magneto, the cable needs to be pulled tight to run. If the cable  enters on the left the cable needs to be slack when running. Very confusing I know. Try to start the bike with all the slack taken out. My B31 starts easily and hardly needs any advance at all. Dennis

[WayneBergman]

Thanks Dennis, photo attached shows my set up. Slack cable to my understanding means fully retarded on my set up. This is also what the previous owner told me. So slack cable for start up and then tension up tight for running after the motor has warmed up. I have tried starting with cable under full tension and it was worse , kick back as well so this confirms to me that slack cable on my set up is fully retarded but even though I am full slack at start up it is not running like it was just a short while ago. Mystery for sure. Hard to start and rough and then dies.