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[Thack]

Oh, Wilf, could you tell us what carb you've got?

[Thack]

Sorry - I keep remembering things. If the engine won't behave when following the procedure I described (for example, it you can't find the air screw position that gives the maximum engine speed, with a reduction when turning the screw in either direction from that point), then it suggests there may well still be a problem with your carb.

[jaffa90]

Spit back? Wilf are you talking backfire? Mine did that when I used a bit of throttle on start up, it blew the conicle freeflow air filter off. Mine has a mikarb crap fitted. I just turn the tickover screw in a full turn and cover the cone with a hood and remove when it starts after priming with kick start and decompressor.

[Thack]

Jaffa90, I guess it must vary, but here in the UK the common parlance is that a backfire is in the exhaust system (a loud bang), and a spit-back is in the inlet system (the still-burning previous charge shoots back up the inlet tract when the inlet valve opens).

[wilf]

Hi Thack, it's a micarb. I managed to get the auto ADV springs returning fully, still no improvement, so back to the carb. I'm wondering if it could be the small O ring on the pilot air screw not sealing properly, which may explain the slackness. Starting is now a real challenge, but once warmed up and running in mid-range all is well. Tick-over is barely affected by the pilot air screw, so I think the problem is somewhere in that region.

[wilf]

Jaffa 90, that sounds like a good idea to see if the air screw is at fault.I'll give it a try.

[Thack]

Hi, Wilf. Yes, I'm sure there is a problem with the idle or slow running circuit on your carb. Something is definitely not right. I'm sorry I can't be of more specific help, but I don't have a Mikarb to hand to look at.

[jaffa90]

I`m from east Lancashire and I`m with the U.S. version on the BACKFIRE / FRONTFIRE, my present Enfield did it and my 1973 Norton commando crap did it on kick start up,(a bit too much throttle). They both fired the engine backwards to reopen the inlet valve and fire back out through the carb. A front fire is un burnt fuel in the exhaust system firing up more so with a wasted spark.
Spitting back/coughing to me is a weak mixture causing high temperature and dieseling which I had on a brand new KLV 1000. upon which a power commander sorted the mixture. Each to there own.

[simon]

I shagged around for months with the idle mixture settings trying to get my bike to idle properly. I had the settings right a couple of times but didn't know it because I had a head gasket leak. Painted detergent and water around the barrel and head join whist the bike was running and spotted a stream of bubbles. I definitely suggest that you eliminate the possibility of a burnt valve or a head gasket before you start changing bits of the carb.

[Thack]

Jaffa90, that's a good distinction. I've never heard the term "front fire" before, and I've been interested in engine technology for 40 years, but it is useful to distinguish between the two types of what I've always called "spit back". As you say, there's the type where the engine "kicks back" on starting, most likely due to over-advanced ignition. This reopens the inlet valve exactly as you describe. Then there's the spitting back that occurs when the engine is running forwards, most often caused by a weak mixture that is still burning when the inlet valve opens for the next charge. Finally, in my experience the traditional "backfire" bang in the exhaust is caused by a combustible mixture accumulating in the exhaust due to a failure to ignite - then being ignited when the engine next fires. Sometimes it's enough to blow open the exhaust pipe. Anyway, whatever we call it, it sounds like Wilf is suffering the weak mixture variant, as I guess he would have told us if the electric starter, or kickstarter, was kicking backwards.