My newly built C15 Trials bike backfires like a b****** on overrun. The exhaust/silencer clamps seem good, the carb and inlet are new, so could this be down to lack of back pressure? The short silencer is a token gesture and the outlet is approx the same as the headers..? (its been modified since this pic)
Howdy Dave: Beautiful C-15 Trials. I just finished a 65' C-15 in café mode with complete rebuild. After fitting a new AMALConcentric 626 (190 main jet), start-up with the air mixture screw at two turns out and open megaphone exhaust (no back pressure) I had only one experience of backfire on overrun because it was too rich. I've since reduced the main jet to 170, re-installed the meager megaphone baffle, air mixture screw 1-1/2 out, velocity stack (no air cleaner) and haven't had a backfire since. Idles and pulls evenly through all throttle advances. Important balancing act between the mixture screw and the throttle cable adjustment. Make sure you have no change in throttle when you move the steering head. Can get interesting when you least want it, and check the spark plug color after a hot run. Jeff
Thanks for the answers, I`ll have a play with the carb and see what happens. Jeff; the front end is off a Can-Am Bombadier, the forks are Betor with alloy triple clamps and I used the brake too. They were all in military green but once cleaned up look great.The brake looks quite retro and polishes up well, and weighs about 6lb. The stem is the same dia as Bsa, just needs shortening.It can`t be shortened to totally the correct length but a spacer about 1/4" on top of the lower bearing sorts it out. Or just re-thread the top? Easy to understand once you have the parts in your hand. From the other side..
Also have a good look at your ignition. If you still have points, they can stick and not retard on overun so the sparks occur at the wrong time. Air leaks in the exhaust will also make it backfire on over run if the timing is correct. and then there is the problem of a sticking slide
Joined: Sep 2002 Posts: 7,812Alex
BritBike Forum member
BritBike Forum member
Joined: Sep 2002
Very nice looking bike. Backfire on overrun is almost invariably due to an air leak. Incorrect idle mixture (particularly if the idle circuit is plugged) can exacerbate this. Don't bother with any of the other carb settings as they are unaffected at closed throttle.
A smattering: '53 Gold Flash '67 Royal Star '71 Rickman Metisse '40 Silver Star '37 Rudge Special sixtyseventy Lightboltrocket road racer...and many more.
Hi, thanks for suggestions. Carb is new 622 with 120 main and .106 needle. Ignition is PVL type , and I`m sure that its not far off.I did read that engines with low back pressure exhausts can often pull a weak mixture through when the throttle closes? I`d accept the odd backfire, but these are pretty constant. ( Jeff, its a `66 )
Hi Larry and others, cheers for taking time to reply.The reason I suggested it pulling a lean mix thru on overrun is I was under the impression that backfires can be due to the pilot cicuit (closed throttle, highish revs) fuel passing thru unburnt as too lean, then firing in the exhaust? I`ll check the kill switch too, although it`s new, but I`m pretty certain its fuel related. Not been able to get out today and have a mess around ,but hopefully will one evening this week
...snip....I`ll check the kill switch too, although it`s new.............
Dave....don't just check it out...take it out of the circuit. Mine was new too, it had a weak spring and when I reved it up...the vibration would cause an arch....and a BIG backfire. When it settled down...it ran fine. When it first started...I pulled the rubber cover off and looked it over...and saw no problem. So....it took over a month of chasing EVERYTHING ELSE before I finally, as a last ditch effort...just unpluged it.
Just a thought...and easy to do.....proud to be one of the BSA unit single guys...Gordon Gray in NC, USA
(...proud to be one of the BSA unit single guys...Gordon Gray in NC, USA)
Hip, hip, hurray, Gordon Gray! We 'single-unit' guys are often lost in the world of numerous twins. A 'thumper" owner since 1965 (Ducati Diana MkIII, Bultaco Montesa, and Triumph Cub. Long live the Thumpers. Jeff