Ok, is this normal?

I have finally fixed the oil leaks with my bike and the weather is starting to get nice. Took the old girl out yesterday, and after a few months sitting around, she started up no problem and off around the neighborhood I went.

She runs good under 30 mph, but if I press it, she just would not go. lack of power. Was thinking carbjet issue

This is a new bike to me, had it since the fall. Please keep that in mind if this is a stupid question...

For starting the bike and at idle, the choke slide is fully down at the carb. On the bars, the lever is parallel to the bars. Starts and idles fine. If you mess with this during idle, she dies.

Today, I took her out and once I started going, I moved the choke lever toward me, thus opening the choke at the carb. The freaking bike ran like a bat out of hell, I got it to 60 mph (ok, was going downhill at the time). Other than some backfires once I let off the throttle, it worked nice, but coming to a stop, I have to adjust the choke to the closed position or she dies.

Is this normal for a WM20? Not like any other classic bike I have owned, or other type of machine. My other bikes you don't have to mess with the choke once the bike is warm.

Thanks

email (option): sctclark1@msn.com

No. Idle jet holes blocked in the jet block. Or badly out of adjustment.

On the subject of chokes, it's surprising how many people don't understand them, I had a customer come in who came in on a Triumph twin who thought the choke lever operated the other way so was riding with the choke on all the time! Another thing I see a lot are people who use half choke when starting which has absolutely no effect on the carburation what so ever as the throttle slide is sitting lower than the choke slide and even opening the throttle a little as you kick it over wouldn't expose the choke slide, so in reality the choke really should be all or nothing unless you adjust it in tandem with the throttle every time you rev the bike up
Even if you have a mixture enrichener on more modern carbs these are usually a separate circuit in the carb which are overridden the second that you open the throttle as it then draws air in through the main Venturi instead of the choke circuit. It surprising how many people start the bike on the choke, then promptly rev the engine thus bypassing the choke without realising that the choke is bypassed as soon as you open the throttle.

email (option): davmax@ntlworld.com

Hi Scott..You shouldn't have to use the choke at all with a warm engine and if you use the 'tickler' to flood the carb when cold you don't really need it then either unless the bike is operated in Winter conditions....

Having to run with the choke 'closed' indicates you have a weak mixture. If the bike 'spits back' in the carb just as you open the throttle off the stop (without choke) that is a good indicator of a weak mixture...Also a 'white' colour at the spark plug electrode (as opposed to a darkish 'mushroom' brown) indicates the same.
However, the latter will not neccesarily be the case depending on where in the carburettor range the weakness is or whether it is occurring throughout the range of operation...
Overheating is also symptomatic of a weak mixture but this may be difficult for you to identify if you are not familiar with the 'normal' operation of the bike.

The weakness could be caused by either a restriction in the petrol supply and/or incorrect carb settings...

You should have a 170 main jet, 106 or 1065 needle jet, a no 6/4 slide (no. 4 being the slide cutaway size and 6 the carb type) and the needle clip should be set in the the No2 groove in the needle (this is the standard setting, No.1 being the top groove). Finally, the pilot air adjustment screw should be screwed in all the way and then backed off 1 - 1 1/2 turns as a 'base' setting...

Alternatively, if the petrol supply is sufficient you may have an air leak which is weakening the mixture...

Possible causes for this..

A leaking gasket between the cylinder and carb (note you should have a tufnol heat insulation spacer fitted here with a gasket on either side of it)
A leaking head gasket.
A leak around the spark plug seat.
A leak past the cylinder head timing bolt (this should have an annealed copper washer fitted under it.
A badly worn carburettor slide and body.
A badly worn/burnt exhaust valve/valve seat.

It is even possible a combination of any of the above may be causing the problem....

If you are not familiar with the bike, settings and the details of the carburettors design I would suggest it is imperative you obtain a copy of the BSA factory worksheets which cover the operation of the carb in detail and the other aspects which may be having an effect.(The worksheets are available in reprint.)

The carburettor functions in four distinct stages which each cover a different part of the rev range and each stage is affected by a different carb adjustment...Also there are the possible 'external' influences detailed above.

It is therefore unwise to start making any adjustments until you have the information to hand, from which you can acquire a degree of familiarity with the structure of the carb and an appreciation of how it all works....Ian

email (option): ian@wright52.plus.com

Thank you everyone, wealth of information.

I have the manuals, and just about everything else I could find off this sight about adjusting the carburetor because it just didn't feel right, but since she starts easily and ran, I didn't think I would have to.

This changes that.

Luckily, I am somewhat familiar with the Amal carbs and since I have the day off, I just might have to take the thing apart and see where the needle is and make sure there are no clogs. I will check the gaskets as well.

For testing leaks, get the bike running and spray starting fluid around the areas I am testing? Increase in revs = leak?

Thanks and I will keep you posted.

email (option): sctclark1@msn.com

Scott, any chance of a photograph of your handlebar controls ? Normally, the lever would be parallel with the bars when the cable is slack and the choke in operation. With a tight wire and the choke raised, i.e. 'off' the lever should point back towards the rider.

It's possible to convert the advance / retard from tight wire to slack wire operation but an Amal choke has to be off with a tight wire.

If you have got blocked pilot drillings then you'll need to gently warm the carb and tap out the jet block in order to clean it properly.

email (option): 79x100@gmail.com

Yes, I have pictures. How do you upload them, copy and paste?

I also need help identifying my bike but will start a new tread for that.

Sorry if I misspoke, when the control is parallel to the bars, the little slide in the carb is down, cable slack (blocking the airflow). This is how I start and idle the bike. Once going, I have to raise the slide for proper operation.

If I stop with the slide up, the bike will die.

email (option): sctclark1@msn.com

Blocked holes in the jet block will normally cause difficult starting and a misfire at low revs...If you haven't got those symptoms I wouldn't rush into removing the jet block....Just check all the carb settings and the condition of the internal components, ensure everything is clean and reassemble...

I didn't mention in my last post...Check that the carb mounting flange is flat with a straightedge...

If the carb mounting nuts have been over tightened previously a distorted flange is likely and can cause an air leak...Ian

email (option): ian@wright52.plus.com

This is what I found so far.

Everything looks to be in good condition, no excessive wear.
Have: 170 main jet, a 6/4 slide, the needle has a small 6 stamped on it.
The clip was on the first (top) groove. Moved this to second.
Disassembled and cleaned. It was dirty in there, like a fine rust powder, but nothing seemed clogged.
Set the pilot air.
In between the carb and head, there was one thick/hard gasket, with only one paper gasket on the carb side. Crispy. Cut two new gaskets.
There was no washer under the small timing bolt, corrected this with a copper one.
Tightened the spark-plug.


Bike now starts with the slide lifted all the way up. Took it around the neighborhood and she seems to run great. Idle seems faster when the clutch is pulled in, or in neutral.

I will probably let it cool for a day or two and try again, but seems worlds better. Thanks for the help here again.

email (option): sctclark1@msn.com

Hi Scott...Good news...glad to hear you have made some progress...

You will need to readjust the slow running after replacing gaskets and adjusting the pilot screw...

Remember that the Amal settings in the book are not 'cast in stone' but are intended as a base setting which may require further fine tuning....
Variations between individual engines (and carbs) at the production stage and varying degrees of subsequent wear mean that a 'fixed' set of specifications for all bikes are not actually possible...

When setting for slow running (tickover) do so with the advance/retard lever set about 1/4 retarded...The engine won't tend to run 'evenly' with full advance at such low revs..
Also, make any adjustments when the engine is at full working temperature after a ride.....Ian

email (option): ian@wright52.plus.com