Brief Misfire after heavy braking - what causes it?
Turbo Mini Forums

Guys, I've had this problem for AGES, now over two different engines.

If I get back on the throttle, immediately after heavy braking, I get a REALLY obvious misfire, for maybe 2 seconds or so, then the engine starts running perfectly again.

I originally thought it was something to do with the servo'd brake system, somehow causing an inbalance by taking a vacuum feed from just one runner of my webber 45 manifold.

I've since converted to a non servo brake system, but the problem remains.

It's a 1380 swift tune engine, with an MED road comp head, 296 scatter cam, and 45 DCOE.

It also happened on my previous engine, which was a 1380, with MSPD trackday cam, and the same webber 45 DCOE.

It used to to it with a dizzy fitted, and it still does it now on MJ V4.

Any suggestions welcomed, as it really is bloody annoying at times!

Mr. Prawn, the friendly Crustacean- slowly making steps towards forced induction.

Fuel slopping away from the pump?

1. Has it been properly set up on a dyno?

2. If you get to mid range revs, let off the throttle then plant it, does it misfire? (I'm thinking acceleration enrichment is wrong)

Alloy tank in the boot is sumped, Currently got a Facet solid state (180bhp claimed capacity) pump, although previously had a redtop and same issue. 8mm fuel lines all the way.

Old engine was setup on the webber at SRB in launceston, it used to do it back then.

New engine was tuned by Slark on the dyno, made 114bhp.

No misfires if I let off and plant it at all :(

It's REALLY obvious though, if I have to stop in a hurry, then get straight back on the power, it stutters like hell and doesn't pull for 2-3 seconds, then it's perfect again!

Edited by Prawn on 30th Sep, 2011.

Mr. Prawn, the friendly Crustacean- slowly making steps towards forced induction.

float to high?

Have you got or can you borrow a sideband, check if it goes lean or rich when you get this misfire?

Bah stupid phone, Wideband obviously.

Gotta be something like float level incorrect..? G-force of braking causing fuel to slosh forward in carb.? Fits symptoms perfectly and only the carb remains the same across all builds.? Setup on dyno okay as vehicle static.?

For every action there is an equal and opposite criticism.

My black Metro used to do that, always getting a flame or two out of the tailpipe. Always put it down to timing.

I,m guessing that the fuel is loading up in the manifold and then causing an over-rich misfire until it clears. As said, a wideband will give you the answer.