Forum

[Marko]

This may be a magneto or carb problem, don't know; I'm usually pretty good with carbs (especially concentrics) but long story short a 28mm amal concentric (premiere) was fitted (because I had one & it matched the intake manifold choke size) to replace the very clapped out pair of mismatched mponoblocs originally fitted to the bike by previous owner. I set it up with a 230 main jet, 4 slide, needle pos 2 (mid), no air filter fitted (no room) but a wire strainer/gauze on the end of the short bellmouth. Magneto is K2f with manual advance; valve clearances were set at .002" inlet .004" exh (I know the book says nil).

Symptoms: sometimes when accelerating hard, and I do mean hard, the carb will spit audibly once, if I ease off the throttle immediately, the bike seems to recover - sometimes. However sometimes when accelerating hard the carb will spit & die, the engine refuses to restart. It has happened a few times, most recently this morning (when testing brakes - thankfully I only had to push the bike home on my home road, unlike yesterday).

I've always considered a "spit" a sign of weakness in carbs so I adjusted the needle richer - same thing, accelerated hard, then a spit, engine dead, wont restart.

On all occasions: after a while (say after pushing the bike home or smoking myself to death)the bike will start.
FWIW the position of the advance/retard lever, well, its so finicky, some days its an absolute pain to start.
I'm on the verge of throwing a hammer at the thing;( its driving me nuts

Mag? Ign timing? Carb?

[Mark M]

Slide too weak, try a 3.5. REgards, Mark

[vince]

Hi, check if you have a blockage in the fuel lines, or the petrol cap vent is blocked. It ties in as while your pushing home the carb has time to refill hence the restart. Check the banjo connection. Vince

[Marko]

Mark, i'll try a 3.5 slide, but I was always under the impression that the size of the cutaway in the slide was of revelance just when coming of idle? I could of course be wrong in my understanding... this bike has me flummoxed, no sooner do I sort one problem out then another rears its head... oh if I had hammer.... i'lll get there if it kills me

[Dennis C]

Marco, it sounds like carb but?, get any old spark plug and take the outer electrode off so the spark has to jump from the centre to the outer ring, lay it against the engine and kick the bike over you should get a good fat spark, if not you mag is weak.

[Mark M]

Marko, in my experience (and this is just my finding, others may/will have different opinions,) the cutaway comes into play on Amal carbs much sooner than the tuning guide would suggest. I find this particularly on Concentrics and although Monoblocs are better they are not immune either. If you care to look at the original specs for Concentrics fitted to Interceptors when Concentrics were new, none got a slide weaker than a 3.5 and some even had a 3.

REgards, Mark

[Adrian]

While I'm tempted to agree that a No. 4 throttle slide does sound a bit too lean, I'd like to pick up on Dennis' point. What is the situation with the magneto? Has it been there untouched for a long time or has it been rebuilt/serviced within the last few years? If the bike won't start when hot I'd get the mag tested. Too lean a carb slide AND a failing magneto wouldn't make for a pleasurable experience, though I'd leave hammer throwing to field athletics, temper tantrums around classic bikes never end well.



Are you using suppressor plug caps and/or resistor spark plugs?



A.

[Marko]

Hi Adrian, the bike when I got it in November had supposedly been fully restored (ho hum), amongst the wad of paperwork receipts there is a rceipt that the mag had been rebuilt/rewound (whatever) earlier in the year. Resistor plugs & caps not fitted. I will check the mag for a fat spark as Dennis suggests and take it from there - I do have a thorspark electronic conversion which I purchased but never got around to fitting - if I get a weak spark from the magneto when checking then that's it the thorspark is going on. Mark, yes been through the books regarding slides, Paul Henshaw on the Realclassic fb page also thinks the no4 slide too weak.. anyway I've ordered a new slide... lets see how it goes when i'm in a better frame of mind.. best regards to all Mark

[Dennis C]

Never rule anything out on an old bike?, As Adrian mentioned, don't use any sort of resistor with a magneto it overloads the mag coil and weakens the spark, a good mag will make a spark jump 6mm, don't try any more than this or it will jump internally instead and that's not good.

[Dennis C]

Oops our posts crossed.