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Old 08-12-2019, 09:55 PM   #1
vwbusman66
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Default Popping through carburetor

So, as discussed in a previous thread, I have been having a very strange popping through the carburetors, specifically, under partial throttle, or immediately after putting my foot down after an upshift.

It is certainly not a lean pop anymore. Under WOT in any gear, it will run fine and not pop. If I stay in a gear and let off and then hammer back down, it almost never spits.

I don't know how much power the car is supposed to have, but it doesn't feel very quick, even high up in the powerband.

Before I pulled the engine, the timing was set correctly. Tomorrow morning, I will re-adjust the valves and re-verify timing.

Also, the car will occasionally sound like it "drops" 1 or 2 cylinders at idle.

Could the rotor/cap be causing this issue?

For reference, 1971 142S B20B dual SU HIF6's D cam pierburg mechanical pump bosch distributor with vac advance. Points and dwell are set correctly, plugs are nice and tan as they should be.
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Originally Posted by stick70
It's all just lifter noise til it comes out the side.
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Old 08-16-2019, 09:59 AM   #2
matt b
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You don't mention the condenser ? When was it last replaced ? Are you confident that your vacuum advance is functioning properly ?
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Old 08-16-2019, 10:01 AM   #3
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Are the SU dampers filled with oil? They're what accomplishes the accel enrichment function on an SU.
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Old 08-16-2019, 10:37 AM   #4
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$12 with Moss Motors. It's worth it.
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Old 08-21-2019, 10:39 PM   #5
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Quote:
Originally Posted by matt b View Post
You don't mention the condenser ?
Generally, when a condenser fails...no go

RE: strange popping

Pops are related mostly to a lean mixture, or timing SNAFU.

With a vac distributor, at base idle with vacuum line removed and plugged from distributor, then timing is set. Never seen your distributor, but I suspect there is a mechanical advance mechanism inside it...is it freely moving...springs still attached. I've seen this advance mechanism stuck in one position, and with one spring missing.

On older distributors, the coil wire that attaches to point should be replaced. With mechanical advance, this can break strands of this wire internally as mechanical advance plate moves back/forth.

RE: plugs are nice and tan

On an EFI based engine, this would be expected....but the "tan" color may suggest a lean mixture. Give a pinch of choke, and ramp up under load.

RE: Points and dwell

When point setting is changed, so does timing change. I rarely used a dwell meter, just a feeler gauge, and timing light.

Your coil and plug wires need to be the right ones
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Old 08-22-2019, 08:05 AM   #6
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Quote:
Originally Posted by 84B23F View Post
Generally, when a condenser fails...no go
On complete failure, yes.
If it is not fully functional, it can cause issues with ignition causing misfires and such. You can test for leakage with a multi-meter.
If the condenser has not been changed, it wouldn't be a bad idea. For $10 - $20, they are cheap insurance.
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Old 08-22-2019, 05:43 PM   #7
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Or intake valve not closing all the way.
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Old 08-23-2019, 02:00 AM   #8
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Worn out exhaust cam lobe? Check valve lift.
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Old 08-23-2019, 07:36 AM   #9
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James replaced his cam recently.
If I recall, it was popping prior to the cam replacement.
Unless he flattened a lobe during break-in, the cam shouldn't be the problem.
Push rods all nice & straight? Rocker arms all smooth with no grooves?
I'm still going with the probably 20-25 year old condenser.
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Old 08-23-2019, 09:23 PM   #10
vwbusman66
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Cam is fine. I verified lift on all 8 as consistent and even.

The popping was a combination of poor rear delivery on the rear carb, worn-out points bearing block, way out timing caused by the worn bearing block, and a wasted cap/rotor.

I'm replacing the condenser this weekend to round out the ignition overhaul.
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Old 08-26-2019, 06:31 PM   #11
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Quote:
Originally Posted by vwbusman66 View Post
The popping was a combination of

1. Lack of Integrated Maintenance Processes
.
.
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