I have a MJ v4 setup on an Austin mini with a supercharger setup. I'm getting a backfire through the carb when you switch the engine off and then re-start it carb backfires I wonder if this is a wasted spark issue as both cylinders are firing at once?
Anyone else had anything similar when swapping?
Thanks for any help
carb backfire
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Spockie-Tech
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I have little experience with supercharged engines, so this might be only half right..
Normally, a cam will have some overlap of the exhaust and intake valve openings at TDC on the exhaust stroke.. I think the idea is to use some of the exhaust gas leaving velocity to help suck the next intake charge in.. always seemed strange to me, but I suppose once the exhaust gas stream has got underway, its not likely to change directions and head up the intake in the last few degrees..
With a supercharged engine, its conceivable that with the inlet manifold under pressure, a fair bit of charge might start zooming into the cylinder as soon as the inlet valve opens, which could be significantly before TDC on the exhaust stroke, then the waste-spark fires.
If enough charge gets in there quick enough, the waste spark might light this up, which of course would then fire back up the open inlet valve.
This *could* be the reason for your backfire. I would imagine that a cam designed for supercharged use would have much less overlap than a normally aspirated cam would.. but if you are running a supercharger on an engine with a normal cam, this could happen.
I recall someone telling me a while back about this being a potential problem with supercharged waste spark systems, but there seems to be some megajolts out there working on blown engines, so I assume it can be done if the valve timing is right.
are you running a supercharged specific cam with minimal overlap, or a normal aspirated cam ?
Any blower gurus want to pop in with some more clues ?
Normally, a cam will have some overlap of the exhaust and intake valve openings at TDC on the exhaust stroke.. I think the idea is to use some of the exhaust gas leaving velocity to help suck the next intake charge in.. always seemed strange to me, but I suppose once the exhaust gas stream has got underway, its not likely to change directions and head up the intake in the last few degrees..
With a supercharged engine, its conceivable that with the inlet manifold under pressure, a fair bit of charge might start zooming into the cylinder as soon as the inlet valve opens, which could be significantly before TDC on the exhaust stroke, then the waste-spark fires.
If enough charge gets in there quick enough, the waste spark might light this up, which of course would then fire back up the open inlet valve.
This *could* be the reason for your backfire. I would imagine that a cam designed for supercharged use would have much less overlap than a normally aspirated cam would.. but if you are running a supercharger on an engine with a normal cam, this could happen.
I recall someone telling me a while back about this being a potential problem with supercharged waste spark systems, but there seems to be some megajolts out there working on blown engines, so I assume it can be done if the valve timing is right.
are you running a supercharged specific cam with minimal overlap, or a normal aspirated cam ?
Any blower gurus want to pop in with some more clues ?
I did get backfiring on the initial first startup with EDIS&MJLJ with base map,but not anymore. I think I adjusted the fuel mixture,but can't remember
Have a look at the spark plugs,should tell you?
Interesting setup you have,a blow thru. I'm aiming for a turbo blow thru soon.
I assume you are using the SU carb?
Have a look at the spark plugs,should tell you?
Interesting setup you have,a blow thru. I'm aiming for a turbo blow thru soon.
I assume you are using the SU carb?
I still have never managed to sort this. I think its defo a wasted spark thing as it never did it on a dizzy.
I think the only options I have is a different cam, or a anti run on valve tied to the inlet that cuts open to air when the car shuts off to stop fuel being pulled into the engine when the car has switched off? but these are still guesses
Backfire when running on an SC suck setup is normally too lean mixture.
I think the only options I have is a different cam, or a anti run on valve tied to the inlet that cuts open to air when the car shuts off to stop fuel being pulled into the engine when the car has switched off? but these are still guesses
Backfire when running on an SC suck setup is normally too lean mixture.