Backfire

[GT350AUS]

I started the topic because it morphed from an earlier one re correct spark plugs and has had me chasing a backfire in my KR.

So in summary:

Engine starts, idles and cruises fine but it backfires when accelerating harder than just cruising.

Car was running fine until i had it stored in a workshop for 3 years slowly getting some work done when it developed a backfire once it was started.

Had the carby rebuilt

Changed points, condenser and coil, set timing, checked number of spark plugs and they're fine no sign of wear or fouling and the problem is still there.

Could be a vacuum leak causing engine to run to lean

Tilt away was fixed by changing the relay but now the relay is working but the tilt away wont tilt back so I'm thinking there's a vacuum problem in the system which could be related back to the backfire.

A couple of guys on the forum have offered some assistance which is much appreciated, thanks Shelbydoug and 1967 eight barrel

So this is the current position, I'm going to chase down a vacuum leak as my next step,

If anyone else has had a similar issue your feedback would be appreciated by me and also to assist for anyone else who may experience the same issue in the future.

Ross

[Rickmustang]

Easy stuff first. Change your cap and rotor. Solved my problem.

[GT350AUS]

Cap and rotor changed also

Sorry forgot to mention that.

[shelbydoug]

Bring it in. I'll fix it for free.

[GT350AUS]

Come to Australia I'll put you up for free 

[shelbydoug]

Come to Australia I'll put you up for free 

Let me get my Google driving instructions.

[GT350AUS]

Cool

I'll have cold beer waiting for you.

Ross

[Rickmustang]

Timing or carb issue is about all that's left. Would have to be a big vacuum leak....

[shelbydoug]

At this point, after installing a vacuum gauge, I'd disconnect the vacuum cannister for the tilt column and plug the hoses. Run the car. See if the problem goes away.

[Greg]

I would go back to the carb.  Just because it was rebuilt doesn't mean it was rebuilt correctly.  Get a new or proven carb, put it on and try it before you do anything else.  I suspect that is where your problem is.

[texas swede]

Does the car have Thermactor System? If so, it could be the backfire valve.
Another problem causing this can be the float level in the carb.
Texas Swede

[GT350AUS]

No thermactor system fitted. It was removed before my time.

Float level is set correctly.

Thanks for your suggestions.

[PR]

Go back to the basics, your missing something, it is a very simple system

[GT350AUS]

Go back to the basics, your missing something, it is a very simple system

Yep I agree...its probably staring at me but I can't see the forest  because of the trees....LOL

Once I get it sorted I'll post back on the forum.

[Side-Oilers]

I realize a stumble is different from a backfire, but I would still take another look at things that can cause both:

Carb, fuel filter, bad fuel, fouled spark plug(s), bad ignition wire(s).

You wrote that you checked a number of spark plugs. I'd check and replace them all.  Yes, it's a pain, but the one plug you don't check or change will always be the bad one.