246 Backfire

Could be a lot of things. Is it difficult to start, does it pop and misfire when cold / warm / hot / under acceleration or de-acceleration. Does it pop and bang through the carbs or exhaust.

Could be incorrect timing, bad carb set up etc etc.

Tony

 

Sounds like a carb problem to me. Check fuel and mixture as it could be running lean, snapping out of the exhaust is normally a sign of lean mixture or blocked idle jet/s.

check timing too as this can cause all manner of symptoms as well.

 

what service was done? Did they take off the carbs or perhaps the intake manifold. If either was not reinstalled properly there may be an air leak. Stick one end of a 4 foot length of 1/4 inch fuel line in your ear and run the other end around the carb gasket and the base of the intake manifold (where the o-rings seal); If you get a loud hissing sound you have your leak. Also, are all the idle screws in their holes?

 

points ignition?
popping on overrun with throttle closed is likely due to spark retardation
if as you say feathering means that the throttle is partially open then probably carburation
crap in float bowls or idle screw setting....methinks....

 

Sounds like they didn't do proper testing/test runs. Many shops are lazy, aren't passionate but are more than happy to take your money. Sounds like you need a new mechanic who cares.

Freeman

 

Was evident the moment you dtove it away from the garage or did it develop later?

 

From original post. Sounds like a good fully warmed up test drive would have exposed this.

Freeman

 

backfiring on the overrun can be caused by a lean idle setting. If you feel brave you could open the idle screws ALL THE SAME KNOWN AMOUNT (so you don't change the balance and can put it back how it was if you want to) and see if it helps

A leak would also result in a lean condition because air would be entering the system
You can check for that by spraying starting fluid around the bases of the carbs. If there is a leak it will suck it in and the idle will increase

 

sounds like any kind of test drive would have uncovered it.
Mechanics must have been in a hurry to leave for the day...

 

Bet you felt really good when you realized you had it sorted!

 

Possibly a tiny amount of crud got stuck on the idle screw, it has a very fine tapered edge and can sometimes clear when its moved. Never known it cause backfiring though. sounds like the idle jet was blocked, that can cause all manner of problems. Most dont realise the idle circuit is still in play up to around 40mph

Tony

 

actually I notice a little popping when my car has been sitting (it sat for a bit before I bought it in august ) but after driving for 1/2 hour or so it clears up then is fine ,I wonder if I should run an additive in the fuel . like a injector cleaner .... (i know it does not have injectors.)
LarryH

 

Isn't it in play regardless of car speed on a fully closed throttle?

 

Yes, the mixture screw is purely there to keep the car idling on closed butterflies, when the throttle is opened then the progession ports on the idle circuit assist with a smooth transition in engine speed and also operating the acc pump circuit / main jets. Without it you would get a huge dump of fuel into the manifold and a lot of spluttering.

Most people think the mixture screws set the air/fuel ratio. It doesnt except at idle. Air/Fuel ratio is set by the jets and venturi and can only be altered by changing the them.

 

Hello everyone.

i am facing the same problem (i.e. backfire) with my GTS, but only when the weather is hot. During winter time, everything is ok. Any recommendation?

This being said, can someone give me the translation of "idle screws" in french? (My technical vocabulary is quite limited).

Thanks in advance.

Del.

 

The standard user manual is in french also.
Page 59
"Vis de réglage de mélange
de ralenti"

 

A literal translation to be sure.
The term idle in French may not be appropriate in this application.
I could be wrong, it is not my native tongue

 

Could be your idle mixture is on the border of being too lean, so when the air is cold it's OK but when hot it's too lean. I'm not sure of the physics behind that, but I have been told that can happen.

 

It would have to be very extreme for that to happen, i would think its more likely to be fuel starvation or vapourisation.

tony