Backfiring through Carburettor

pope101 · Post by **pope101** » Tue Nov 04, 2008 11:22 pm

Hey Guys

My Edis setup is backfiring through the Carby every 3-4 seconds on idle. It will also do this as it revs but not as badly.

I'm assuming this has something to do with the signal being sent from the VR sensor.

Is there any way of testing the signal without using an oscilloscope?

Are there any other things i could also check?

alexander · Post by **alexander** » Wed Nov 05, 2008 12:43 am

the most likely thing up with a vr sensor is not close enough to the wheel. i think brent actually commented on that from your photo, did he not? so a (possibly) easy thing to do is move the head of the vr sensor closer to the wheel, which would help if you are suffering an intermittent signal for that reason.

alexander

pope101 · Post by **pope101** » Wed Nov 05, 2008 2:34 am

Thanks Alexander but i had modified the mount as Brent suggested.

pope101 · Post by **pope101** » Thu Nov 06, 2008 6:16 am

Should I take the raging silence to mean "go get an oscilloscope and check the damn signal

"?

Post by **brentp** » Thu Nov 06, 2008 7:37 am

Unfortunately, yes: if you're experiencing mis-firing, it's likely the EDIS module is losing sync with the trigger wheel. Does it happen when running on the EDIS alone (MJLJ disconnected) ?

pope101 · Post by **pope101** » Thu Nov 06, 2008 7:55 am

Yeah stand alone EDIS does it as well.

I would have thought if it looses sync it would just not spark, but it sounds like its firing when the intake valves are open.

Dimbit · Post by **Dimbit** » Thu Nov 06, 2008 8:49 am

Lots of carbs spit back, it's not uncommon when they're less than perfectly set up. I'd look at your carbs first because as you say it should spark at the right time or not at all.

pope101 · Post by **pope101** » Thu Nov 06, 2008 10:04 am

Yeah but the same carb works fine with the stock ignition.

Dimbit · Post by **Dimbit** » Thu Nov 06, 2008 8:25 pm

Ah, ok, in that case it sounds like the things mentioned above (or the timing is very, very wrong).

EDIT: Just a thought, you say it's not as bad at revs. Could it be that your bracket resonates at idle and wobbles the VR sensor all over the place?

pope101 · Post by **pope101** » Fri Nov 07, 2008 12:22 am

Timing is good for the most part but every so often it will just backfire. I remember when i first put it together i couldn't get a clean timing signal from lead 1 however lead 4 was fine.

Bracket is rock solid.

Spockie-Tech · Post by **Spockie-Tech** » Fri Nov 07, 2008 12:43 am

Its difficult to tell from the angle of your photo there...

Are you sure the center metal pole of the VR sensor is aligned accurately with the center of the holes drilled in your pulley ?

It looks like the VR sensor may be aimed slightly *inside* the hole circles. that gap looks pretty wide too..

If they are off to one side, you probably wont get an equal positive and negative swing on the sensor signal, which will possibly confuse the EDIS.

Also, are the holes all accurately center drilled on a circle-line ?

For a hole-drill trigger setup like that, I would imagine that alignment and tracking would be very important, since the edges of the circular holes would give a more gradual sine-wave-like transistion signal between hole and metal as the circular hole sweeps past, whereas the usual square-cornered trigger-wheel setup would give a sharper edged transition.

A scope is probably the only way you are going to be able to check for certain you have equal positive and negative swing and minimal run-out on your trigger signal

pope101 · Post by **pope101** » Fri Nov 07, 2008 2:35 am

Yeah i think a scope might be the way to go.

MY trigger wheel is by no measure "accurate"

alexander · Post by **alexander** » Fri Nov 07, 2008 8:06 am

well... if you do buy an oscilloscope, go for a USB oscilloscope which plugs into your laptop. they are cheaper, smaller, portable, and a tiny fraction of the weight.

Oliver Sedlacek · Post by **Oliver Sedlacek** » Mon Nov 10, 2008 9:42 am

I had a lot of trouble with carbs backfiring, which I traced down to two problems. Firstly, the Megajolt was picking up noise, which also shows up as an unstable RPM reading. Secondly, I wasn't giving the engine enough advance on my very conservative initial map.

Dean924s · Post by **Dean924s** » Sun Nov 23, 2008 8:56 pm

A couple of things come to mind.

Have you checked your grounds on the shielded cables for both the PIP/SAW wires as well as the VR sensor?

Do you have enough cable to re rout the shielded wires?

Have you put a tining light on it? What is the timing doing when it back fires?

Have you checked the plug wires? Ya I know probably dumb but I have to ask.

How far off are you drilled holes? From my limited understanding this could be critical.

Have you tried wiggling wires with it running?

You could be getting what I call spark leek. It is when the charge from one wire will jump or leek to another wire and fire in a different cylinder. Have you looked at your wires how old are they? How are they routed in relationship to each other? Try separating them / moving them with the motor running and see if it helps. Just do it with something that is insulated. You could also try the ammonia / water trick (spray ammonia and water mix on wires while it is running and it is dark and look for the sparks.

Now the far fetched things:

If you had another coil pack to try I would give that a shot as well.

Since it is not doing it with the stock setup I doubt that it is a problem but could you have a cam lobe going bad and with the new timing it is becoming more apparent?.