6 replies to this topic

[chargersjoe]

Hi,

My car (1990 mini 998) for no apparent reason began backfiring through the carb a week or so ago. The backfire is non-existant when the car's rpm is very slowly increased. The problem seems to arise at about 1500-2000rpm when quickly revving the engine from idle and consists of a huge loss of rpm followed by a backfire and finally the revs increase.

Since the problem arose I have tuned the carb and mixture, adjusted the valve clearances and timed the car to the recommended advance. None of this seems to have worked.

I did however fiddle with the timing last night and adjusted by ear to when the idle became fastest. This has now cured the backfire, yet a loss in rpm remains when revving quickly, this loss is now however around the 2500rpm mark.

So I'm basically asking whether anyone can guide me towards curing this problem?

Thanks,
Joe

Also, I am led to believe that the car has been fitted with a minisport cam prior to my ownership (not sure whether this would affect anything)

[Big_Adam]

Check the dizzy / overhaul the points and condenser contained within.

[mpihornet]

[my 998 did that when the head gasket blew between 4 and 3 but it is usually distributor. i went electronic, highly recomend that. if that does not cure it, check timing with light, mixture, and finally compression.

[dklawson]

Apart from timing issues, backfires out of the carb can also be due to lean conditions. You've adjusted your carb, but did you check the oil in its dashpot? Did the situation start suddenly or creep up getting progressively worse?

[Lillee]

Backfiring from carb means you're running lean. Have you changes ait filters lately or any other carb settings? fuel starvation from a bad fuel pump can cause this.

[Yoda]

As stated above, this sounds to me like lack of oil in the dashpot. This would make the carb piston rise too rapidly on acceleration, leaning out the fuel mixture, but would not be so noticeable on gentle throttle opening.

[chargersjoe]

Thanks for the advice guys.

The problem came about fairly suddenly.

I did refill the dashpot oil to the top when I tuned the carb using motorcycle fork oil.

I just replaced the airfilter a few days before the problem arose but have tuned the carb using colortune since.

I think that the problem must be in the ignition system as the only variable that affected the backfire was messing around with the dizzy.