Model 20 black plugs

[sunnybob]

Hi Folks, I need help for my 1958 model 20. I bought the bike, not running, a year ago. The magneto was not working properly and so my mechanic replaced it with an electronic ignition kit (coil) the seller alredy bought. The heads were also checked (oil leaks) and ONE piston has been replaced, both cilinder were in good shape and were just lapped. After that the bike starts fine at first or second kick but "kills" spark plugs in 20/30 km... They went both black, starting backfire and then they do not work anymore. As they went all black I supposed mixture was too rich. I went for hotter plugs (NGK B6ES and Champion N5) the I change the carburettor. Somebody put an AMAL 626 on the bike so I bought a brand new 376 hoping it was better but even setting it for a lean mixture black spark plugs problem is still here.

In conclusion, now the the bike starts fine either cold or hot, runs well until the plugs went black starting backfire and the loosing a cylinder ... Using in city driving they do not last long. Something as 20/30 km...Better at high rpm driving (motorway). When it starts to backfire I found I can run better if I use the air (richer fuel mixture).

Any suggestion, please? Thank you.

Roberto

P.S (sorry about my English... I hope is good enough to explain the problem)

[sunnybob]

I even tried hotter plugs: N7YC

Here their pictures after 30 km.

[JimFitz]

Hi Roberto

When you replaced the carb did you check that the jets were right for the engine? I am not sure of the right size jets but I am sure that someone will advise - it should be shown in the owners manual on http://archives.jampot.dk/
When you say that you adjusted the mixture to make it leaner did you mean that you lowered the needle and adjusted the pilot screw?
Also check that you have the correct slide cutaway in the carb.

Jim

[sunnybob]

Hello Jim,
thank you very much for the answer. I ordered the carb from AMAL for the 58 model 20. They send this part:

http://amalcarb.co.uk/carbspec/carburet ... s/id/4528/

So has main jet 220, pilot jest 30 and needle jet 106

As the bike has more problem in urban drivin condition I would try a smaller pilot jet, so I already order a 25 and a 20 but I don't know if it will help.
At your second question the answer is yes, my mechanic lowered the needle jet and adjusted the pilot screw. If I remeber correctly it has five position and now it is #4 from the top. In my opinion the bike seems s running a bit "lean" now but still has black plugs
I don't know what you mean with:"Also check that you have the correct slide cutaway in the carb." could you explain me, please?
Thank you

Roberto

[Rob Harknett]

Your 3 jets are correct the needle is set wrong it should be in the 3rd groove not 4th, that's in the middle of the grooves.

[JimFitz]

Hi Roberto

If you look into the air intake you will see the carb slide which rises and falls when you turn the twistgrip. Different engines need a differently shaped cutaway to help with the transition from pilot jet to main jet. The slide cutaway is marked on the slide body. If you ordered this carb from Amal for a Model 20 then this is probably correct. It may be worth checking the jet sizes are correct as shown in the owners manual.
There have been problems reported in the past when new carbs are fitted. Sometimes when they are machined small particles of aluminium swarf can block the small passageways in the carb. If the air passageway to the pilot screw is bocked then this will cause a rich mixture which will be worse at small throttle openings such as urban driving.
Do you get black smoke from the exhaust at low engine speeds?

Jim

[Winkie]

Hi Roberto. Just a passing thought - are you sure that you are using the choke (air lever) correctly? You mention in your first post that the problem improves if you adjust the air when the backfiring starts. This is odd as if the engine is already running rich making it richer by using the air lever would make things worse. The air lever should be pulled all the way back towards you for normal running.

[sunnybob]

Thanks everybody for answers and support.

About the slide: as the carb was ordered new at AMAL to fit a 1958 Model 20, should I check (and how) it has the right slide cutaway?

About the needle jet: first time we set the needle jet at 3rd groove. Then, to solve the black plugs problem we move to 4th but still has black plugs... Should I move it again at 3rd?

The choke is working right and well and I suppose I operate it in right manner. The bike needs choke at cold to start and keep the idle. With no choke it stops running at idle if not warm. As the engine is warm enough I close the choke (that means it has more fresh air going to carb) and the engine runs smootly untill the plugs last.

The strange thing is when the engine starts to backfire (warm) I can have better running for few miles if I operate choke (so it has richer mixture) . It runs rough but no more backfire for some miles, then the engine loose power and usually stops. New plugs and it runs fine again.

I do not have an air filter (just a bellmouth with flat gauze)

[clive]

I have had the same problem with my 600cc G11CS. Works fine out of town with light brown on a plug chop, but once I am in town for a number of miles it carboned up the plugs in a similar manner. Dropped the pilot jet to 25 and it is much better but still tending to too rich. I am running on Champion N5 and have yet to try going for a hotter plug. I do have an air filter. Good luck in sorting it out.

[sunnybob]

Clive, thank you. I really appreciate your post I'm not alone! I tried N7yc plugs but they did not stay more the N5. An old British bike mechanic told me to fit some n9yc if using in town drivin but I fear they are too hot..

Anyway I will try a smaller pilot jet. Now I have another problem, the dynamo stopped to do its job. I checked with a tester and it seems to be the armature (only 2 volts on meter) but I bought it new at the end of september

By the way...Do you have magneto or coil ignition? I was wondering if the Thorsparks ignition kit I have could be part of the problem... Maybe the spark is not good enough?