backfire

[narrau]

If a 4stroke engine backfires, when trying to start the engine, does that indicate that it fires too early or too late?

[TerryLL]

If it's backfiring through the intake, then the timing is early. The plug is firing before the intake valve closes, somewhere near the beginning of the compression stroke instead of at the end of the compression stroke. The sequence is this in a 4-stroke:

Intake stroke, intake valve open, exhaust valve closed, piston moving down.
Compression stroke, both valves closed, piston moving up.
Plug fires near the end of compression stroke.
Power stroke, both valves closed, piston moving down.
Exhaust stroke, intake valve closed, exhaust valve open, piston moving up.

This is the simplified version, as there is some overlap on valve openings and closings, variable valve timing schemes, etc, etc.

If you've recently had the distributor out, and the backfiring started when you put it back in, then you're off by a tooth or two. Or if you've had the plug wires off and put them on wrong, then that could be your problem.

A little more info on the engine type would be helpful.

[narrau]

Engine type is a Clae F6(australian)

[Paul Pless]

Backfiring through the intake is as likely to be caused by fuel mixture issues (usually too lean) as it is to be caused by the ignition timing being 'off'.

[John B]

lean ?

[Paul Pless]

Yeah I think too lean is the more common reason. For example, I think in a certain engine you might have a limited range of rpm and load on the engine where in a certain 'instant' it runs too lean to promote good combustion. The charge that's in the combustion chamber continues to struggle to burn through the exhaust cycle and into the period of valve overlap. I think under this condition the engine is not experiencing a very strong intake charge effect due to stumbling combustion process. So this allows the still not burning very well mixture to ignite the next intake charge. That's my theory at least, I may be wrong on the why of it, but my experience with it happening has been the motor has been running too lean, and didn't really have much to do with the ignition timing.

edit to add: I'm not referring to a mixture that is just a little lean, which promotes a very hot combustion event, but one that is at least momentarily very lean.

[TerryLL]

Engine type is a Clae F6(australian)


I've got zero experience with this type of engine, so what I said above might have absolutely no relevance. Lots of issues can cause backfiring, timing being only one.

We still don't know if it's backfiring through the intake or the exhaust. More info may get you a better shot at the right answer.

[John B]

Yeah I think too lean is the more common reason. For example, I think in a certain engine you might have a limited range of rpm and load on the engine where in a certain 'instant' it runs too lean to promote good combustion. The charge that's in the combustion chamber continues to struggle to burn through the exhaust cycle and into the period of valve overlap. I think under this condition the engine is not experiencing a very strong intake charge effect due to stumbling combustion process. So this allows the still not burning very well mixture to ignite the next intake charge. That's my theory at least, I may be wrong on the why of it, but my experience with it happening has been the motor has been running too lean, and didn't really have much to do with the ignition timing.

edit to add: I'm not referring to a mixture that is just a little lean, which promotes a very hot combustion event, but one that is at least momentarily very lean.


I've only ever lost my eyebrows when its been too rich or flooded even. I think you call that empirical data;).

Burnt hair smells awful.doesn't it:rolleyes:

Ever blown a muffler off a car ... mate of mine did the 'watch this' thing when we were youngish.... rolling down the hill, switched the ignition off, left it off too long and turned it back on. idiot.:D

[Paul Pless]

I can understand that (rich condition) too, John.

Yes I once owned a car that would backfire through the tailpipe on demand. It was awesome, I never blew the muffler off though.:D

[Tylerdurden]

If you haven't messed with the timing do a leak down test on the cylinders. May have a hung or carbon fouled valve. If you have no access to leakdown disco the primary wire and hold a straight edge across the tops of the valves while hand cranking the engine. You should be able to see one or more valves hanging up unless you dropped a seat then a leakdown is the way to go.
Some times pouring some Marvel mystery oil onto the valve train and running it will unstick a valve.