G80 - spitting and backfiring

[Samuel]

My 1949 g80 is still giving some problems. Spits back through the carb and also detonates in pipe on overrun. Tried with a Mk1 concentric to see if the pre monoblock was the problem. Starting slightly better but still some of the above symptoms.
To cap it all the kickstarter detached itself and went missing on the last test run and now I have to replace that also.
Any ideas ?

[TommoT]

Hi Sam,
on my 1956 G80 I had the same symptoms after the rebuild. I tracked it down to a lean idle mixture. When you open the throttle, to increase rpms, the transition from idle to actual throttle openings must take place without fuel starvation. A classic symptom of this is backfiring and spitting back through the carb. You must check the entire idle circuit, which includes the drilled channels at the bottom of the carb bore, the idle mixture screw and the throttle stop screw. Remember that on the monoblock the idle screw controls the air volume, so turning it in reduces the amount of air = richer mixture, and vice versa. Adjust your idle as per the manual, and check out the effect. If this turns out only to be a partial cure, next step is the throttle slide cut-away. After getting the idle right, I had to change my throttle slide from a #3½ cut-away to a #4, to get the transition from idle to throttle opening right. With a little patience and care you should be able to get it right. And then a final advice, if your carb is old and worn out, don't throw any more new parts at it, get a new one. You can buy both new pre-monoblocks and monoblocks. I dit it in the end and it was the biggest single improvement I made did to my bike. Calm and steady idling, crisp pick-up from idle and no more sooted plugs!

Banging and clanging through the exhaust is usually due to an air leak in the exhaust system itself. An air leak will allow air to be sucked in causing ignition of the exhaust fumes, with the familiar bang as a result. Have fun and let us know how you get on.

TommoT

[Samuel]

Thanks for the info Tommo.
I currently have fitted a mk1 Concentric in reasonable condition. The original is an 89 Amal (separate float) which was fully refurbished by Surrey Cycles so should be good for active service. In both cases I get spitting and backfiring so its down to hunting for the air in the system.

[Chazzyb]

Sam, spitting back through the carb and backfiring don't generally have common causes. Spitting can be weak mixture from pilot setting /wrong slide cutaway/air leak at carb. Backfiring commonly caused by an air leak in the exhaust and unburnt fuel combusting, ie rich.

Spitting back: only fiddle with the carb after you've got the bike up to running temperature.

I'm not an expert, but let's have a chat on Thursday at The Foley?