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Old 04-15-2018, 11:14 PM   #1
toolboxchev
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Engine Misfire & Stumble

So I have been pretty lucky with my older rebuilt engine so far, until today.

Going down the road under mild acceleration I get a ever so slight misfire. I have been having a problem with a lifter under cold starts for a while now but it will quite up after running for 10 seconds and after a couple of minutes it is virtually non existant.

Fast forward, under half throttle it is stumbling and a few more backfires I am thinking the HEI ignition module is going out again. I go through one about every 2 years due to the older style of charging system which can give out voltage surges.

Replaced it and I have the same problem, now I thinking I am losing fuel pressure or in need of a fuel pump. Have not checked pressure yet.

What is a good next direction to look at?

Carb is about 9 years old, Eddy, stock mechanical fuel pump (not leaking yet) Large can style fuel filter, I checked between fuel filter and carb by cracking the line open, fuel came spraying out.

HEI was just gone through last fall, new adjustable vac advance, cleaned weights, and made sure springs were functioning ok. Everything was ok until today until that little backfire. Ughh I start a new job tomorrow and this is my driver.

Stumped for now, looking for some hints, Any pointers are appreciated.
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Old 04-15-2018, 11:55 PM   #2
geezer#99
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Re: Engine Misfire & Stumble

Maybe your cam is going flat.
Pop the covers off and check the rockers for equal movement.

Or maybe it’s just flooding a bit. Eddy carbs don’t like more than 5 psi.

Or maybe you need more timing.
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Old 04-16-2018, 07:50 AM   #3
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Re: Engine Misfire & Stumble

Your description does sound more like carb popping than backfire. Backfires are usually nice and loud and out the tailpipe. I don't usually see multiple backfires unless the plug wires are crossed. Carb popping would have a nice rythem to it. Maybe a little more description can narrow it down more.
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Old 04-16-2018, 09:04 PM   #4
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Re: Engine Misfire & Stumble

Check all the suction lines from the tank to the pump inlet. Bad line can let air in and create fuel starvation.

If that Eddy carb is one of the 140x family, pull out and clean off the power pistons and give where they move a good spray of carb cleaner. They can get fouled and stop moving freely.

Don't overlook the simple possibility that you got a bad batch of gas. Rare but it does happen. If you have dual tanks, switch to the other one.
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Old 04-16-2018, 10:25 PM   #5
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Re: Engine Misfire & Stumble

Quote:
Originally Posted by cadillac_al View Post
Your description does sound more like carb popping than backfire. Backfires are usually nice and loud and out the tailpipe. I don't usually see multiple backfires unless the plug wires are crossed. Carb popping would have a nice rythem to it. Maybe a little more description can narrow it down more.
It did have somewhat of a rhythm to it, yet next time I laid on the fuel it would go away, then after accelerating ( normal ) it would come back.

Today being the first day of little to no rain here it did act up a little this morning. I laid on the throttle pretty heavy more than a few times. I has definitely kinda cleared up. On the way home from work it was even better with only one or two stumbles right after warm up and then all clear on the way home.

Weird??? I am going to work on the fuel line issues, they are the old rubber hoses. This weekend I will check fuel pressure and blow out those areas noted on the carb.

Thanks to all whom have gotten back, I was worried it would have died on me this morning going to a new position with BENEFITS the date of hire!
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Old 04-17-2018, 08:59 AM   #6
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Re: Engine Misfire & Stumble

If you have a vehicle that runs bad on rainy days, that is a classic sign of bad spark plug wires. Are yours original wires or parts store replacements?
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Old 04-17-2018, 02:41 PM   #7
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Re: Engine Misfire & Stumble

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If you have a vehicle that runs bad on rainy days, that is a classic sign of bad spark plug wires. Are yours original wires or parts store replacements?
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Old 04-17-2018, 09:54 PM   #8
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Re: Engine Misfire & Stumble

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If you have a vehicle that runs bad on rainy days, that is a classic sign of bad spark plug wires. Are yours original wires or parts store replacements?
They were just replaced last year, Accel Super Stock brand. Not leaning that way yet, but will check also.

Started sputting again under medium load and a misfire/backfire every now and then again today, its mostly dry out.

Still working the problem, thanks again. Fuel pump, carb, over 8 years old. Carb has never been dipped/cleaned and as a daily driver it has never sat more than 3 days. I run a fuel system cleaner between once a year to over 4 times a year.
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Old Yesterday, 08:12 PM   #9
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Re: Engine Misfire & Stumble

Update!

Just checked the fuel pressure, filter, and replaced line at carb. Filter is in place between pump and carb. Should I have more pressure under a hard rev in park?

Fuel Pressure, 5-6 at idle, drops to 4 under hard throttle in Park.

New thoughts? Checking Coil and Pick up Coil assembly, will update later.

Checked the Coil, and Pick up Coil assembly, both check within spec.

Noticed timing chain has about 2 degress of slop. Performed this check by simply turning the fan. Any thought on that?

Only have 70k on that gear and chain as it was a complete top end rebuild 8 years ago.

Last edited by toolboxchev; Yesterday at 08:50 PM.
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Old Yesterday, 08:34 PM   #10
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Re: Engine Misfire & Stumble

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Originally Posted by toolboxchev View Post
Update!

Just checked the fuel pressure, filter, and replaced line at carb. Filter is in place between pump and carb. Should I have more pressure under a hard rev in park?

Fuel Pressure, 5-6 at idle, drops to 4 under hard throttle in Park.

New thoughts? Checking Coil and Pick up Coil assembly, will update later.
I found this distributor on Amazon and it has worked like a champ for me. For $45 you have a brand new distributor that will rule out the coil, module, cap, rotor, etc...can't beat that deal.
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Old Today, 04:57 AM   #11
cadillac_al
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Re: Engine Misfire & Stumble

FWIW I've had good luck with Accel Super Stocks. I did read a thread somewhere recently where a guy had a sticking weight in the distributor. I run Skip White distributors in all my vehicles. I would never spend much time on an old one.
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