TheSamba.com :: View topic

Fri Jul 11, 2014 12:38 am

Hi folks
Just wondered if anybody has experienced a slight backfire on idle, it's bugging the hell out of me, I've changed the bushes in the carb as they needed doing, checked for leaks around the inlet manifolds and joining rubbers using wd40,stripped the carb blown through with airline and fitted new gaskets,strobed the timing, reset the tappets,checked the exhaust manifolds are tight, checked the vacuum lines to the brakes,adjusted the fuel and air to all possible settings.
I have a 1972 bay with a 1600 tp engine, 34 pict 3 carb, svda dizzy with electronic ignition and a bugpack aftermarket exhaust.thermostat has also been removed.
It seems to be more prevalent when the engine is hot or been driven hard.
But occasionally disappears when left to idle for a long time.
Any suggestions will be much appreciated
Kind regards
Mike

Fri Jul 11, 2014 5:12 am

mikemango wrote:
It seems to be more prevalent when the engine is hot or been driven hard.
But occasionally disappears when left to idle for a long time.

I think you want to focus on the quoted text above.

By this symptoms, maybe the rise in temperature is bringing the carburetor to a lean or rich condition. When the temp colds down, it works as expected.

Fri Jul 11, 2014 5:31 am

Turn the idle mixture screw out and see if it stops.

Check for vacuum leaks using an unlit propane torch.

You will here the engine run different. WD-40 does not fire good enough.

Fri Jul 11, 2014 4:30 pm

How long has this been happening? What jets in carb? Exhaust leaks? The fact that you thing it is more when hot, leads me to think either seating areas of carb/manifold or exhaust system.

Fri Jul 11, 2014 4:56 pm

If it is running on 3 cylinders for some reason the pops may be the hot muffler igniting un-burned mixture. That would happen when it's hot. Also a exhaust leak could do it. Probably just a bad heat riser gasket. You can feel the exhaust gasses when those leak. (check when the engine is just started from cold before they get too hot). It could be a burned valve though. Just make sure it's idling on all 4 cylinders evenly. Perhaps it just needs the valves adjusted.

Wed Oct 30, 2019 9:40 pm

I had backfire issues under slight acceleration between 1,000 rpm and 1,500 rpm.I had the Zero valve lash gap, then I adjusted to .008 and problem was solved

Thu Oct 31, 2019 4:08 am

Assuming the engine is in good condition(compression and valve adjustment), there are two things that raise warning flags here for me:
mikemango wrote: a bugpack aftermarket exhaust.
Does this system have enough exhaust gas flow through the heat riser? Or any flow at all? Most aftermarket systems don't.
mikemango wrote: thermostat has also been removed.
WHY!?!?

How can you tune an engine that is at a different temperature every time you take your tools out? That and a lack of intake heat to vaporize the fuel will leave you tinkering and tweaking for the rest of your life and never getting it optimized.

Thu Oct 31, 2019 2:12 pm

blow out idle jets with carb cleaner