Backfiring GT4

Do you have the air pump connected? If you disconnect the pump (just pull off the belt), it will reduce/eliminate the backfiring.

 

The JimShadow GT4 is a 1974 Euro. My guess is that it will have no air pump...

Backfires on the overrun are VERY normal with these cars. Don't worry and enjoy the firework display.

(And if anything, it looks like an indication of a LEAN mixture low down, not rich. So lean it will no longer burn in your cylinders, but rich enough to gather in the exhaust and explode there...)

Sometimes this can get a bit extreme
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backfiring out the exhaust at idle, burping/coughing through the carbs, or popping out the exhaust on throttle lift off?

 

Sweeeeet

 

Good day,

A similar thing would happen to my old GT4 if I inadvertently flooded the car when starting. Upon doing so the spark plugs would become wet (with fuel) and then misfire. When this would happen I had to remove the plugs, clean them, etc and then the car would work as before.

Perhaps check each spark plug one by one to see if one or more are fouled/wet and if so, then clean and/or replace them and see if this remedies the situation.

Cheers,

Sam

 

on my carby 308, if i screw in the idle screws until the cyl starts to stumble, then back it out until the cyl recovers (ie, setting the leanest condition it will run smoothly at), then i get a whole bunch of popping on overrun - really quite spectacular and will clear the sidewalks. if i back it out another turn, richen it up a bit, then the popping on overrun goes away, but back it out too much and its too rich.

if you have popping on overrun, try backing out the idle mix screws.

I was goofing around the other day, and think i have a slight air leak at the carb manifolds.

it really is set up best with the 55 idles just by backing them out 3 turns like the manual says. That being said, I installed the euro screws without the shoulders, which i think is what the 3 turn recipe is about. if you have the US "shoulder" idle screws i think its more like 4-5 turns.

 

that is why i asked the OP the q about whether it was a backfire (rich) or popping (lean).

 

Early this year my car started backfiring loudly through the exhaust...turned out to be a failing crank sensor that was causing the front bank to cut out momentarily. As a consequence, unburnt fuel got forced into the hot headers and detonated.

Maye be irrelevant to your prob since my car is injected but replacing the sensor solved it.

 

Dr Kananga
Welcome to the GT4 mafia - these are fabulous cars, but anything with 4 downdraft webers will occasionally give some attitude.
I had a chance to experience your car when Jim owned it, and it was a rip snorting unruly beast. It ran great, but was raucus and raw. My US Spec '79 is much more refined. Interestingly, they both seemed to perform about the same despite very different 'personalities.' Vastly different feel, which was really entertaining. Neither one is better - it's just what you want out of the car.
If at idle you hear little "pooft" noises, especially when the car is cold, that's what most folks refer to as the carbs coughing or spitting. What's happening is that some fuel is burning at the wrong time, when the intake valve is open.
If you are getting louder, cracking "bang" noises on the overrun, that's raw fuel catching fire in the exhaust system (ala Hans' fire breathing dragon). I recall Jim's car doing this occasionally while being driven quite quickly, but it was NOT making those noises at idle.
If you are getting these noises between shifts, that's probably a little raw fuel going into the exhaust as well. If they are very loud and scary you must investigate.

If this is only happening when the car is cold, it is quite normal.

I highly doubt that the 400foot difference between Indiana and California could be at all responsible. I live at 5280 feet and there are plenty of hills in town that are more than 400 feet tall. I don't notice mixture changes until I change well over 2,000 foot altitude change, and even then nothing is really annoying until about a 4000 foot difference.

Perhaps some bad fuel, perhaps a lot of ethanol in this tank, or perhaps something in the ignition system - a loose plug wire or two seems more likely. The carbs on these cars are robust and once set they usually stay that way. Check the simple things: plug wires all tight, wires to coil nice and tight, etc. On this car you really don't need the "choke" in warm weather, so make sure the car is well warmed up without choke and see if it happens. If the problem is only at idle, you can take the airbox top off and see/hear which cylinder is misbehaving, and then double check that one for problems (loose wires, clogged idle jet, etc). I doubt you have any serious problem, as the car was in great shape.
For those reading, this is a '74 european spec car, so has no airpump, no cats, and no wacky vacuum stuff.

Jonathan
Denver, CO
GT4 sn 15302

 

Check timing and advance before making any carb adjustments.

 

A well-respected mechanic in my area recently told me: "90% of all carburetor problems are electrical".

That is starting to ring true.

 

Good day,

Thanks for the update! Indeed, this very same issue caused me problems with my old GT4, as I mentioned previously. I remember having a set of spare plugs just in case... and if not, then I had to "clean" the plug with a torch in order to burn off the fuel... After that, the car ran fantastic and I vowed to not be overly exuberant with my accelerator pedal when starting...until the next time I

Cheers,

Sam