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I have an old Mitsubishi Triton Utility 1993. It has a 4g54 4 cyl petrol engine with a carby. No computers or anything like that to check error codes.

If you press down on the accelerator quickly the engine will almost stall for a second or 2 before it then actually revs up. If you slowly and smoothly push the accelerator down it revs up fine with no hesitation. It only happens when it's warm. Also after sudden acceleration it backfires more often than not.

I've replaced the plug recently and it doesn't happen until the vehicle is warm so I don't think it's the electrics.

Given that it only happens when it's warm I am thinking there must be a vacuum leak and for a moment it sucks in too much air. I can only assume that once warm one of the hoses or connections expands a little and introduces a leak.

What else could be causing this sort of issue that I could investigate before I have to try and replace all the vacuum tubing?

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2 Answers 2

Backfiring is typically a too-rich condition. I would check any accelerator pump for proper operation, due to it being on acceleration only.

Engine's are inefficient when cold and will require a lot more fuel, so that may be what's masking the symptoms until warm.

It sounds like the engine is temporarily flooded on hard accel., and then when the gas is pumped out the right spark ignites it and all the fuel in the exhaust, causing the backfire.

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Does the carb have vacuum secondaries? If so, check the spring that holds the secondaries closed. If it's weak, then the secondaries are opening too early and flooding the engine. You might also want to check your air filter and its housing, it's possible the filter is dirty or you got some kind of obstruction in the cold air intake somewhere.

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