Looking for Insights in diagnosis of a Large POP On accel

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Looking for Insights in diagnosis of a Large POP On accel

Postby hexibot43 » Sun Oct 04, 2015 7:07 pm

Hoping to tap into all the minds here for some insight. Our shop seems to be changing direction and going back to the 50's and 60's. Usually working on cars where I can use a scan tool to great advantage. I think I've gotten a bit rusty with the older cars. I've been spoiled by being able to hook a scan tool up and usually having an answer within minutes.

We are working on a 1959 Chevrolet El Camino with a 350 V8 hydraulic tappet engine. 650 CFM Holley Carburetor. Automatic Transmission. Open Element Intake. HEI ignition. The problem is when you get on the freeway and open it up. It will have this very loud "PoP" out the intake. You won't get another "Pop" like the first one. If you continue to hold the throttle down you'll get what feels like very random misfiring. Unfortunately the exhaust is dumping just in front of the rear wheels. Consequently I cannot run this vehicle on our dyno. I scope it in the stall, and can't see a thing wrong. Power balanced showed very even power out of all cylinders. It runs great at idle, and slow cruising speeds. We started trying to rule things out. Base Ignition timing @ 12 BTDC. Total timing 42 degrees. Attempted to remove Vacuum advance no change ( 9 degrees out ). Fuel pressure is good at 6 psi. No drop off whatsoever while the problem is occurring. Tried another carburetor no change. The Tech I had on it thought it was a loss of fuel. I believe we have now ruled that out. Ran a voltmeter between power feed to the HEI and ground at the base of the distributor set to record MIN. Never went lower than 13.86 volts while problem occurring.
Set up Pico to record Ignition while at speed. Revving up engine in stall didn't seem to show a problem before and still not now. Got everything hooked up for a drive. Ok, we see something. Looks like we are loosing a cylinder. Realizing I'm not doing a very good job with the Sync signal. I used two of the Inductive pickups from the kit to get my signals. Perhaps there is a better way to get a Sync signal where the noise doesn't effect sync? Is there a better lead to use as pickup for sync? Or some way to filter it so that my sync signal is correct?
Here is an Animated Gif of the Ignition while the problem is occurring.
Chevrolet_El Camino_1959_Petrol_20151004-0001.gif


Here is view without the movement
Chevrolet_El Camino_1959_Petrol_20151004-0003.jpg


It is obvious were loosing a cylinder. It seems like the ignition is just dropping out. So we swapped in a different coil, and module. Same problem still existed.

I have a whole distributor ready to throw in and see what happens. I'm just feeling rather bad here. I should be able to diagnosis this without throwing parts at it. I'm wishing I had put a scope lead on the "Tach" output, and perhaps another on the power feed using a low amp probe. The reluctor looks good visually, but you never know.

I keep thinking something mechanical in the valve train. It just runs so well at lower speeds, and even if I lost pressure in the cylinder I should still have a firing line of some sort. I am obviously over thinking this....

Any other ideas? I'm thinking of pulling out that FirstLook just to put a view on the intake pressure/vacuum. I
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Re: Looking for Insights in diagnosis of a Large POP On acce

Postby Robski » Sun Oct 04, 2015 7:43 pm

Set up Pico to record Ignition while at speed. Revving up engine in stall didn't seem to show a problem before and still not now

Brake torque ?
Adjustable spark tester ?

Have a look in the Autonerdz shop for ignition secondary leads (look at a sync probe whilst you're there) I'm sure they did a set that improved upon the ones that come in the Pico kit, & if Pico designed some more I would expect them swapped FOC because there is a known issue with them not being accurate in the waveform they display - looking like an actual fault in secondary ignition when there is not !

http://www.autonerdz.com/ignition.html
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Re: Looking for Insights in diagnosis of a Large POP On acce

Postby hexibot43 » Mon Oct 05, 2015 5:29 am

Thanks Robski,
I'll be giving autonerds a call. Just got a dedicated pc for the Pico Scope. I didn't know there were better cables I could get for Ignition. I've been using a much older OTC automotive scope for Ignition. Only switched to Pico when I had to take it for a drive. I'd like to be using it all the time.
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Re: Looking for Insights in diagnosis of a Large POP On acce

Postby Steve Smith » Mon Oct 05, 2015 4:23 pm

Hello and thank you for the posts.

I think you already have the next direction in which to head when you mentioned

I'm wishing I had put a scope lead on the "Tach" output, and perhaps another on the power feed using a low amp probe. The reluctor looks good visually, but you never know.


Monitoring power supply but more importantly primary current may help here when under WOT at speed.

I was amazed during the case study below how “current” proved to be the revelation and not primary/secondary voltage

https://www.picoauto.com/library/case-studies/1961-jaguar-e-type-3-8-unstable-idle-speed

Here we could monitor the true dwell times under cruise and idle, then compare them to events that take place under WOT/Full load

A reliable sync will always be challenging in these scenarios regardless of the style of HT pick up used if the cause of our fault effects the secondary voltage we are attempting to sync from.

I think the First Look sensor or the WPS will prove invaluable here looking at activity in both the intake and exhaust when under the fault condition.

I know this will be challenging but if we can hold the fault condition at WOT we can hopefully reveal a pattern in the captured pressure waveforms

I hope this helps and any feedback would be most appreciated

Take care.......Steve
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Re: Looking for Insights in diagnosis of a Large POP On acce

Postby hexibot43 » Fri Oct 09, 2015 4:56 pm

Ended up under the gun for time. Replaced the Distributor as a whole. Problem gone. I will put the old distributor on the distributor machine this coming weekend and hopefully get some patterns. I had already changed the module, and coil wihout any improvement. The wiring didn't seem to have any frays or cuts in the shielding. It will be interesting to see if anything comes up from the pickup coil/wiring. Because everything else was new. Thanks for the help everyone. Enjoyed reading about the Jag. Why didn't I take a current reading from the beginning? I will in the future. I knew there was a reason I got the 4 channel scope.
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