spitting back/baulking

Which ignition system have you got?

Hi Mike,

Spitting back through the carb is a sign of a weak spark and being unwilling to rev is also a warning sign of the same problem. My father once had a Sunbeam S8 (in the fifties)which would not accelerate unless you opened the throttle slowly otherwise it spat back through the carband everyone blamed the carb as being the cause. He faffed around with the carb which made no difference so he replaced the coil, what a transformation! Ran perfect after that, pity he didn't keep it though....

James

Hi Mike

I had exactly the same problem on my 920 Mk2A - started OK but then misfired, spat back etc. - I spent ages faffing about with the carb (single Concentric Mk2 via RGM manifold), looked for air leaks, checked float heightetc. In the end it turned out to be the ignition system (in my case a Tri-Spark Classic Twin system - Steve Kelly sent me a replacement unit FOC) all was OK after the system was swapped-out. It may be a similar problem in your case ( I dare say other ignition systems can exhibit similar problems).

Mark Woodward

Wish I'd known that a month ago

See Pazon and PVL Coils 850 MkIIa

R's

Graham

Spitting back through the carb is a classic symptom of weak mixture and this is confirmed by your spark plug colour. If you have richened the idle mixture by screwing IN the idle mixture screw and it still spits back you will probably need to increase the size of the pilot jet. If that does not cure the problem you may need to then fit a slide with a smaller cutaway. The settings for a single Mk2 concentric are: 25 pilot jet, 106 Needle jet, 260 main jet, three and a half slide and needle 2A1.

There are two possible positions for the pilot jet on the MK 2. The better one for a fourstroke engine is in the bottom face of the main body and not in the bottom of the float bowl. Try increasing the size of the pilot jet to a 30.

Was your bike running properly before fitting the new carb? if it was, then the ignition system is unlikely to be the problem.

In my limited (40 year) experience of messing with engines, and Commandos in particular, any new problems are normally a result of the last thing you were fiddling with. So fiddle with one thing at a time, prove it, then move on and fiddle with the next. ( you can change the word "fiddle" with another starting with the same letter.)

I "fiddled" around with a single Mk2 for some considerable time and with a velocity stack it went like stink but had a nasty habit of sticking open at the most inconvenient moment. An SU followed and was excellent apart from starting and I now have a single Mikuni. This spits back when cold but then improves and is good for most riding with excellent pickup and mid range. All the single carbs apart from the SU run out of puff at about 80MPH.

Happy fiddling

Previously wrote:

MKIII 850 1977 with RGM mainfold,Amal MK2 and KN filter.

Starts easy enough, went for a run Sept and starting to spit back a little. I took the plugs out they were a frightening yellow,almost completely. I thought that looks like very week. Screwed in the pilot screw and went for a short run....seemed like a different bike, pulling like a train.

Four weeks later end of Oct (air temp colder) started up no problem and ran 20 miles. Spitting back even after warm up andreluctant to let me open throttle without stalling. Putting on the choke briefly made things much better.

When I got back I took out the plugs which were black around the edge, the electrode a reasonable colour but that bent metal bit (tech term) that jumps the spark from the electrode to earth still a horrible yellow colour.

The pilot screw datum is recommend at 1 1/2 turns out is now at 3/4.

I will check for air leaks as suggested in the forum but can anybody assist, I have never seen plugs this yellow colour. This seems to be something that`s developed suddenly because it wasn`t always like this.

Mike Wallace

I had spitting back through the carb when the yellow/black ignition lead from the electronic ignition stator plate fractured. I repaired the wire and the bike ran fine again. At first I went for the carbs as the cause. At least they`re clean and balanced now.

Previously wrote:

Previously wrote:I believe its a Boyer but I didnt fit it and its been on the bike since my ownership 7 years. OK spitting back faulty ignition maybe but what about this horrible yellow plug weak colour this surely cannot be due to ignition. This fault has suddenly developed.

Which ignition system have you got?

If the stator coils are failing or have an unreliable connection then you can get a weak spark, a spark at the wrong time and all sorts of other variations. If it sparks when there is very little fuel in the combustion chamber then you will have all the symptoms of a weak mixture and, yes, it can develop after several years of use.