I have a recently redone BSA m20 1944. Runs well and tops out at 53 mph. I noticed that under heavy acceleration you can feel gas spit back out of the carb. Am I running to rich? The spark plug looks fine and I am worried about overheating due to the amount of ethanol mixed in with gas in California. USA. Once again thanks for any help

email (option): Jeff@infinitymedspa.com

Hi,
if the carb spits isnt that a sign of poor mixture ?
If it is, the engine would indeed run hot.

I was a a meeting of the Norton club recently where I was told that most engines run to poor nowadays, caused mij the new petrol. The advise was to use bigger main-jets and change the position of the needle.

Hi Jeff..If the carb spits right at the bottom of the throttle range just as you come off the 'stop' then it is weak on the pilot mixture..If you are getting petrol 'blowback' at higher revs as opposed to spitting i.e. petrol vapour blows out of the carb onto the oiltank, then the mixture is probably too rich at whichever stage of the throttle movement it occurs..An alteration would then be required to either the throttle cutaway, needle position or main jet size depending on where that point is.
If you are running a standard main jet (170) that should be OK...Likewise with the throttle cutaway...A needle position alteration might be the answer...
This can all be affected if the carb is badly worn though. The needle, needle jet and slide can all wear badly in Amal carbs and this should be addressed if the carb has had no work done on it...Ian

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

Cam timing?

Hi Ian,

Unsure on what the difference is between blowback and spitting, but I can feel petrol on my hand while accelerating hard trying to get from 30 to 50 mph. (Well hard for that bike) I bought a new carb from Amal with a 170 main jet and normal cutaway, since my old carb was very worn. I was thinking of lowering the needle 1 line??? I think California consistently has 10+% ethanol.

I tried my best to time it correctly and it starts easy and runs well. But I never see any difference when retarding while climbing a hill. Is thre a noticeable difference?

email (option): Jeff@infinitymedspa.com

There should be a noticable difference with the spark fully rearded.
If not then your timing is out or your cam slip ring on the maggy is sticking ( like mine does ).

When you are on a flat secction of road and near full throttle slowly retard the throttle.
You should start to miss a bit ( called 8 stroking ) with some occasiional pops in the exhaust.
If it does not do this then the bike is too far advanced.

At the other end if you can easily start your bike on full advance then the timing is too far retarded.

Remember that an M20 should be tight to advance & slack to retard

email (option): wariron@tpg.com.au

Trevors comments are correct...However, there is a possibility your bike has the wrong mag/points housing fitted in the past, so ref. the cable setting for adv/rtd..If your adv/rtd cable enters the magneto to the right of the contact breakers it will be 'tight' cable advance. If it enters to the left 'slack' cable advance....It sounds like you have fuel blowback as opposed to spitting so I would try dropping the needle as you suggest initially..Note, however, that sudden large throttle openings will cause this effect anyway, as the motor cannot 'speed up' sufficiently quickly in relation to the degree of throttle opening...Ian

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

Hi Trevor,

Nope mine is definitely tight advance and it will start to 8 stroke if I retard. However I have heard some people say they will retard the bike on a hill climb to get more power, but I have never seen more power by doing that. Maybe I Am doing something wrong, but I am climbing a hill at 30 mph?!

email (option): Jeff@infinitymedspa.com

Some of the old timers ( owned the bike since new ) do this but you really need to know when.
As the engine just starts to labour you can retard the ignition a small amount but then again you can also go back a gear.

The ony time I retard the ignition is when I am on a heavy over run and the only reason why I do this is to stop the spark plug loading up

Bike Beesa
Trevor

email (option): wariron@tpg.com.au