Cracking and banging of of exhaust

[advinnie]

What cc engine and turbo you running?
Mines only a 1600 with a baby T3 ( .42 cold & .36 hot side)

[buguy]

Mine is a 1600 with a little TDO4 from a Subaru.

[advinnie]

I remember you saying that before now.

[advinnie]

Hi all just an update she is not fixed
She starts the second the key is turned, idle fine and rev’s up in neutral fine, she accelerated fine if you take it very easy but the second up put your foot down she misfires and loses power. The misfire today is only when you put the engine under load. Now I’ve changed the plugs, leads, rotor and distributor cap the only other parts to look at are the magnetic pick up in the distributor and the coil. Can a coil on its way out cause a misfire when the engine is under load?
Like I say the past few days she has been great but last night it rained and this morning the misfire is back so something could also be getting damp.

[advinnie]

Some more information. With two of us in the car it’s made it easier so see what’s happening. I’m good in first gear it seems but in all other gears I’m good from 1000 rpm up to exactly 2573rpm while under acceleration then the misfire starts. If I very gently accelerate in all gears I can hit 5000rpm it’s just when I accelerate hard and hit this rpm that she misfires. Could it be a main jet? Or auxiliary pumps although you can see both pumps working as they should.

[buguy]

If you can go easy and its ok it's probably not the main jet. I don't know those carbs work so I'm not a whole lot of help. On my Holley that would be the accelerator pump shot isn't enough. I would need a larger one. Whatever that translates to on your carb.

[advinnie]

I’m now stuck on what to do. Is it ignition or fuel? My A/F hand held device is saying all is good and I’m back firing at 2500 rpm with a reading of 12.6:1 and as low as 10.4:1. Ive tried 8 difficult ignition maps now and it’s made no difference. The pump jets at .70’s but the weird think is it was working fine the other day no problems at all then I left her for a few days had a bit of rain and now it’s playing around.

[buguy]

Maybe just grab a cheap coil at your local FLAPS and see if it helps. Probably would hurt to have a spare in the trunk anyway. I bought one at my local place and it has a lifetime warranty. Can't beat that. I think it was $20 or $30.
Or throw your points back in and see if the pointless ignition is the culprit?

[Clonebug]

What is your plug gap at???

Re gap to .024" and try again.

[seabeebuggy]

I don't know that carb but banging and popping is always a lean situation. could be a leak somewhere too. I have never seen a pop in fat conditions.

[buguy]

I agree. Mine does that at around 16:1 AFR. When it's fat it's just sluggish.

[advinnie]

I’m getting a new coil today so will try that and have the carb in bits ready to clean out.
I just want to make sure when I say backfiring popping I mean it coming out the exhaust NOT the carb

[advinnie]

Some news And here’s some results below are some A/F figures running 50 idles and a pair of 55’s
Rpm. .50 idles. .55 idles
1000. 21.5. 21.2
1500. 16.3. 14.1
2000. 11.9. 11.2
2500. 11.2. 10.9
3000. 11. 10.9
4000. 10.7. 10.7

[advinnie]

Now some more information
With the ignition unit set to 15 degrees BTDC this is what the cylinders where firing at
Cylinder 1- 13 BTDC
Cylinder 2- 17 BTDC
Cylinder 3- 13 BTDC
Cylinder 4- 17 BTDC
So should I bring the higher reading cylinders down from 17 BTDC to match the ignition unit timing?

Next issue is that I’m getting good spark at the plugs on cylinders 1 & 2 all the way to the rev limiter but on cylinder 3 the spark starts to break down / miss at 2510 rpm at the distributor cap and has totally gone by 2573rpm and cylinder 4 at the distributor cap the spark start to break down at 2570 rpm and has gone by 2573 rpm the same as cylinder 3 ?
So considering plugs 1&2 are good I’m guessing it can’t be the coil. So if it’s not the coil, plugs, leads, rotor ( I’ve tried two different ones with the same results) or distributor cap ( once again I’ve tried two different ones) that as far as I can see only leaves the magnetic pick up in side the distributor that replaced the points, can you lot see or think of anything else it could be?

[Chip Birks]

What is controlling the timing?