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Mystery hesitation under load, worsens over time


rundwark

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Like Chickerman said, it's likely a fuel issue but still could be spark. Does the Mallory distributor use points or an ignition module? An ignition module can break down intermittently especially with heat.

It's an infrared LED-based trigger module that sits inside the distributor. From what I've read so far though is that the typical failure scenario is that the LED burns out. I can't find any stories of intermittent missed sparks that actually are fixed by replacing the module..

 

Still, it's another piece of electronics that gets hot over time when the engine is running, so it could be the source of my problems. Makes me wish I had another distributor lying around for this car to test it with.

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Chickenman might be referring to me. I asked what fuel filter you were running because I had a similar mysterious issue that plagued me and it turned out to be a faulty filter . Specifically glass type Mr Gasket

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Have you checked to see whether your electric fuel pump is working? The symptoms sound like perhaps your mechanical pump is drawing through a failing or failed electric fuel pump, and with only limited success.

Yeah, it is working. I have it on a switch in the cabin and the level in the under-hood fuel filter clearly changes based on whether it's on or off. I often drive the car with it off (just use it to prime the system) and that's always been fine too, so maybe it's obstructed..

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Yeah, it is working. I have it on a switch in the cabin and the level in the under-hood fuel filter clearly changes based on whether it's on or off. I often drive the car with it off (just use it to prime the system) and that's always been fine too, so maybe it's obstructed..

That method of operation is incorrect. An inline electric fuel pump must be operating when plumbed in series with another fuel pump ( Mechanical or electrical ) . Shutting off an inline pump creates a lot of fuel flow resistance. 

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That method of operation is incorrect. An inline electric fuel pump must be operating when plumbed in series with another fuel pump ( Mechanical or electrical ) . Shutting off an inline pump creates a lot of fuel flow resistance.

That's good info. Also makes me believe that if there's a flow problem it's not near the tank, but it has to be under the hood somewhere. 'cause when the car is hesitating, turning the pump on or off makes zero difference to how it runs but I can still see a difference in how full the fuel filter is.

Edited by rundwark
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Okay, the adventure continues.

 

Replaced distributor cap, rotor and ignition module to eliminate ignition causes: no change.

 

Opened up floats, checked and cleaned banjo bolt filters. Tiny bit of black crud inside the banjo bolts, nothing on one filter, one dark spot on the other that I was able to scrape off, bowls themselves looked clean and floats move freely, no obvious mechanical resistance on the float hinges or switches. Have yet to test drive after this last attempt but pretty sure there'll be no difference..

 

My latest hunch is that the return line has a clog. At temperature, the lack of circulation leads to some vaporization in the lines causing the lean condition and hesitation at load at high RPMs. After driving for a while at high speeds and then stopping at a light, the floats become overwhelmed by the high fuel pressure and sudden lack of fuel being sucked out of the bowls, causing shitty low idle and near stalling due to a rich condition. Does this make sense to anyone but me?

Edited by rundwark
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Here's a video I shot a week or so ago, showing what happens to the front fuel filter when I turn the electric fuel pump on after it's been off for a while.

 

https://youtu.be/q7rO57iCIn0

 

It doesn't look like the filter is bad, but I have another one on the shelf, would it make sense to replace that as well?

 

I have access to air tools and a lift, so I'm wondering also if it would make sense to disconnect the return and feed lines on both sides (one line at a time, using hose pinchers on the things they connect to) and blow them out with compressed air..

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