Backfire on slow down

[pottersrebel]

hi all
ive took my stock car out for a practice today and its backfiring thru the exhaust when i ease off the throttle going into the corners , ive replaced the dizzy, changed the coil, replaced the spark leads and plugs, checked the timing (twice) and its still back firing
Its a 1.3 vauxhall engine and we only run in second gear around the track, anyone got any ideas?

[MK999]

Give it a run up to the kinda speed you enter a corner, come off the throttle and then turn the ign off, check the colour of your spark plugs straight after. Sounds like it's developed a problem with leaning out.

tbh as long as it's fuelling ok under load I wouldn't worry though, which you can check in the same way.

[pottersrebel]

ill give it a shot mate when i pulled the plugs they were in good condition not black or sooty. i have recently changed the carb to one that should be completly standard (dont know if that makes a difference)
i recently had a problem with the intake lifters where it was backfiring thru the carb so they got swapped just wondering now if the exhaust lifters may be sticking or something??

[Andy]

put a pic of the plugs up.
If its running lean they will be really pale

[Stuart]

imho it sounds like its running rich and retarded.

[MK999]

ill give it a shot mate when i pulled the plugs they were in good condition not black or sooty. i have recently changed the carb to one that should be completly standard (dont know if that makes a difference)
i recently had a problem with the intake lifters where it was backfiring thru the carb so they got swapped just wondering now if the exhaust lifters may be sticking or something??

Possible I guess, never seen it personally, but if it is doing that I'd swap it asap before you start burning seats/valves out.

If you've changed the carb I'd say it's fuelling slightly different though.

[pottersrebel]

imho it sounds like its running rich and retarded.
if it was running rich would the plugs not be black? the day before i took it upto practice i`d checked the plugs and they were black and i noticed the dizzy was loose ( ive had a few big hits recently) so i re adjusted the timing an the plugs were alot cleaner on practice today (but they were paleish in colour). we do advance our timing but i set it at what i normally set it at and ive never had this problem before.
sorry if my info confuses things engines arent my strong point.

[pottersrebel]

put a pic of the plugs up.
If its running lean they will be really pale
ill post a pic up tomorrow, The cars down my mums so ill whip down and pull the plugs, i cant run it down there at speed as the neighbours get upset

[Stuart]

With retarded ignition the exhaust temperatures are much higher which could burn off the soot from the plugs

[pottersrebel]

That`d explain why my engine gets so hot then.when you say retarded ignition stuart is that the same as advancing the timing? I set mine at 26 degrees at 5000 revs which is what alot of the stock car racers set theirs at?
Ive just spoke to the guy who built my engine and he thinks its the valve springs so he`s sorting me a new head out so he can swap it in the pits at our next meeting,
so hopefully should sort out the back fire but now you`ve all got me thinking that maybe the carb also needs setting up aswell.
I think i need a session on a good rolling road once ive had the new head on. does anyone know any decent ones around stoke on trent?

[garyc]

Did it do it with the old carb....?

[Stuart]

That`d explain why my engine gets so hot then.when you say retarded ignition stuart is that the same as advancing the timing? I set mine at 26 degrees at 5000 revs which is what alot of the stock car racers set theirs at?
Ive just spoke to the guy who built my engine and he thinks its the valve springs so he`s sorting me a new head out so he can swap it in the pits at our next meeting,
so hopefully should sort out the back fire but now you`ve all got me thinking that maybe the carb also needs setting up aswell.
I think i need a session on a good rolling road once ive had the new head on. does anyone know any decent ones around stoke on trent?


Retarded ign is the opposite of advanced (or not advanced enough).

Sounds like it could be running lean though if its got a shed load of advance and runs hot.

[pottersrebel]

no gary it didnt but since ive swapped the carb ive had two big trips to the wall and also into a parked car, the carb i had on before was one borrowed off a fellow racer which had been set up on a roller to his engine.
just remebered another thing as well, when ive finished a race the tick over is erratic.