Amal settings

Previously wrote:

I've got a single Mk1 Carb on my 73 750 with a 250 main, 106 Pilot, No 3 slide and needle as low down as it will go(Top Groove) I'm running BP7ES plugs and it's running rich and fouling after 50-60 miles.

1st move was just to fit a 3.5 slide and see how that looked, Plugs looked like it was weak (Porcelain pretty white).

2nd was to bring the needle up just one notch, that made the plugs look rich again just like it was with the No 3 slide in.

3rd and current setting that I've not been able to test yet due to a crack in the oil tank bottom mount is to change the main jet for a 260 and put the needle back down again.

Can anyone see any fault in my logic if so I'd be very happy to listen to some advice with anyone who's been there and done that.

I chased a similar problem around on my dommie, albeit it has monoblocs fitted. After trying every variation I could think of I bought new carbs which solved the problem. In my case the carb bodies were worn, leaving the choice of having them sleeved or renew. The upside (try to think positive!)is I've now got a box of new bits from when I was trying to get the old ones sorted. I don't have any experience with concentrics, but hope this helps.

The larger main jet will gradually come into effect starting at 1/4 throttle and peaking at full throttle, The throttle valve cutaway is only effective over the first half of the throttle peaking at 1/4, the jet needle and needle jet start taking effect at 1/8, peaking at 3/8 to 3/4 and then rapidly having less effect after that. I will try to scan a graph showing how the various parts overlap in their operating range, Don't hold your breath. The 32mm Mk1 should give good low down punch but will run out of puff at higher settings

Hi Mac

After helping you with the valve stem seals perhaps I can try again !?

The needle 'as low as it will go' suggests something is wrong if you have rich, sooty (and not oily ?!) spark plugs. Needle brass jets are reckoned to be worn out after 6-10 K miles, typically they can be seen to be oval because of the vibrating needle. But I doubt this is the reason. I spent nearly two years chasing and over-rich mixture that linked to chronic misfiring at certain throttle openings, and only when warm. I finally found that it was nothing to do with the mixture but was an ignition misfire - I my case the well-known dodgy Boyer pick-up wires. When the spark is weak then there is unburnt fuel left, hence it appears to have been running rich. Change the carburation a little bit and it will run weak, but that is just allowing the spark to jump. Close the plugs down to 12 thou and see if it runs better on your 'rich' mixture. I'm not saying this IS your problem, but it is one item to check before buying new carburettors.

Regards

Norm

Hi Mac

Just as you have found, the problem came into play after some distance, 20-60 miles depending upon ambient conditions: often perfect running up until then. This, of course, is the time when the Boyer pick-up wires have picked up all the nice casing warmth and become fully soft. Expansion of the plastic wiring outer allows the broken copper inner to have intermittent contact, made even more intermittent by vibration. Nothing to do with cumulative sooting or plug fouling.

The breaks occur at the at the place where the pick-up wires are soldered to the paxolin board. Boyer should never have soldered wires but supplied screw points as you see on the Pazon. A defence engineer said to me that in the days of British missile manufacture copper wires were never soldered, they all had to be crimped or screwed. When you solder a semi-hard copper wire the heat tempers (softens) the wire just at a point a few thou beyond the solder. This soft/harder boundary become a focus for vibration movement until work hardening causes the wire to fracture. It is still held in contact by the plastic sleeving so you remain unaware. Until you do a long run and it is all very warm! This is why later 'improved' Boyer pick-ups had araldited wires and nylon ties to stop the vibration. This problem was a well-known plague back in the 1990's, but it also affected my early model 'micro-digital' type.

Hence, a partial misfire and weak spark (although to be honest I do not know if a weak/intermittent trigger signal will result in a lower spark voltage, I am only surmising). What I did find though was that now the engine was playing up it was highly sensitive to throttle opening when on the move. One hilarious outing in very hot weather had the engine sort of pulling below 2,000 RPM, and above 4,000, but not in-between! I have seen it reported in some old text that a weak spark will jump a weak mixture gap and not a rich one. My theoretical physics cannot explain this but I do recall something about sparks jumping bigger gaps as a vacuum is applied.

Mac

You missed the message in my 13 November post. When you took off the Boyer plate you should have REPLACED the two wires that were soldered in (use fine cored cable, tie down and araldite), assuming you want to remove any possibility that the fault is in those wires. If there is a break it is hidden in the copper a few millimetres up from the solder connection. You will not easily detect a break with a test meter. I strongly suggest that you eradicate this possible fault before chasing others. Slit the wires open after removal to see whether there was a fracture. Check also that all bullet connectors in the trigger cable up to the amplifier have been crimped and not soldered. Alternatively, if you send the trigger plate to Boyer they can test and replace the wires.

Or, borrow a spare Boyer system from a friend and replace the whole thing, and cables, to test the effect. Or, as I did, buy a new one and you at least have a spare setup when you have found out what is and isn't working.