Backfires through carb

[dcarey]

Car: 1978 Spider, ADFA 2 bbl
Issue: car idles great but backfires through carb when opening throttle (steppng on gas), particularly under load, this is happening well before the linkage opens the secondary

What I have done so far:
confirmed timing 0 degrees with timing light at idle
confirmed dwell
confirmed plugs condition
new gas
no vacuum leaks, 20-24 inches vacuum at idle
150 psi on all cylinders
jets are correct and carb is very clean
accelerator pump is working, diaphram ok, good flow into carb
fuel pump seems to be fine, I drained the float bowl and the engine restarted with almost no lagtime

I removed the EGR, charcoal cannister and air compressor, could that be the cause?

[So Cal Mark]

removing the emission equipment is not the problem. Popping out the carb is a lean condition and most commonly caused by the accelerator pump. The discharge nozzle may also be leaking around the outside due to a worn o-ring

[sptcoupe]

I agree with Mark that it sounds like lean condition in the acceleration circuit, but I noticed you have the timing at 0 degrees. Should be about 10 degrees.

[So Cal Mark]

I agree about the 10 btdc, I meant to post that and forgot

[ScoopMan]

You better make sure you have not slipped your timing belt. You should be getting only vacuum in your intake manifold, backfiring through the carb seems pretty unlikely to me if that is the case.

The other posters certainly have a lot more experience with Fiats than I do, but when this happened to me with an Olds V8 I had, the timing sprocket had gone bye bye....

[dcarey]

Thanks guys, very helpful. A few follow-ups...

This car submerged in about 18" of water after storm water and did not run for eight years, so there is plenty of rust in the exhaust system. I hear a lot of backpressure from the muffle/cat, could backpressure be the cause?

Regarding the timing, I have tried the timing at increments at -5, 0, 5, 10 and 15 and the backfire continues. I have it at 0 now because the label under the hood indicates zero for tune up. Also, isn't it too much advance that causes backfire at carb?

I know the valve timing is ok at the cam (it is easy to see), the mark on the crank is not easy to see and could be off one tooth, could that cause it?

[ScoopMan]

Well if it idles well the cam timing can't be far off. Mark is probably right about the carb, but I would put a vacuum gauge on there and check it while you rev it anyway.

[dcarey]

Thanks.

What do they say, there is a fine line between a hobby and mental illness.

Here are a few more points and remedies I have tried:

1. The vacuum, with the EGR and cannister removed, is very high, almost 25". Could it be too high?

2. I tried swapping the primary and secondary emulsion tubes and their air jets to provide more gas to the primary (those from the secondaries have fewer air holes) but that caused no apparent change.

3. I checked to see if excess exhaust system back pressure could be causing the backfiring, by loosening the muffler connection, but it caused no change in the backfiring.

4. But here is something I just found... the timing marks on the camshaft drive wheels appear to be off 180 degrees. But at that 180 degree position, the crank and #1 piston are at TDC and the distributor is firing on #1. In other words, when the timing mark is 180 degrees away from the pointer, the to cam lobes are pointing up and in towards the center line of the engine. Can the cam drive wheels be installed 180 out to the timing marks, I would have thought they should have a keyway. The auxiliary wheel only drives the oil pump, not the distributor, so its position does not matter, correct? When I line up the cam wheel timing marks, the exhaust tappet is just releasing its valve and the intake tappet is just beginning to open its valve.

[Mathew26]

You can very easily put the cam gears on the wrong cams and that will cause backfire through the intake. If you like I have a picture of cam gears off a car showing which one goes on with cam. But basically the cam allignment pin in the center of the gear should be under the cam timing dot towards the outside of the cam. If the allignment pin is above the timing dot then they are most likely switched.

[So Cal Mark]

with all of the timing marks aligned, the engine is firing on #4 so it makes sense if you line up your dist on #1 the cam sprockets will be off 180. One possibility not mentioned about backfiring through the carb is a worn cam lobe, a common malady in these days of reduced-zinc in the oil

[dcarey]

Matt, if it is not too much trouble to find the pic of the proper mounting of the cam gears, please send it along, that would be very helpful, thanks!

[ventura ace]

For the pulleys, if you rotate the engine so that that the dowel locating pin for either pulley is straight up in the 12:00 O'clock position, then the timing indicator marks for that pulley should be pointed inboard toward the center of the engine.

Alvon

[ScoopMan]

Sounds like you already had the valve cover off for a look. If the intake valve on 1 is just opening right when the rotor hits firing position for 1, I would say there is your problem. You are 180 degrees out of whack.

If so I am very surprised it runs at all....

[So Cal Mark]

it can't run 180 out

[ScoopMan]

Thus my surprise....

Yes he DOES say it idles fine, if it was 180 out it would not, but it might shoot blue flames out of the top of his carb when he turned the key.

I just don't see how he could be backfiring out his carb unless his valve timing was screwed up. Even if the carb is leaking, how would the gas ignite without either spark or excessive heat? Maybe I am missing something.