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 Post Posted: Mon Jul 08, 2013 1:01 pm 
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Last weekend, I drove my 1980 malibu about 1450 miles. It was entirely well behaved until I got about 300 miles from home. I was cruising at 80-85 when it suddenly behaved like it was out of gas, bucking and hiccuping. I started to pull over and it went back to behaving like normal again. I started to speed back up but as soon as I reached 3200rpm, or about 70-75mph the car would resume bucking and backfiring. I replaced the fuel filter, but that didn't help. The car will also sputter occasionally at lower speeds while I'm accelerating as well, but not nearly as consistently as it starts acting up when I approach 75. If I am going downhill or something for a minute and then hit an uphill, it will accelerate normally for a few seconds and get up over 75, but then begin to buck again (almost like it's sucking the bowls dry in the carb). I can't get it to stutter at all when it's parked, even if I hold the rpm above 3500 for a while.

I limped it the 300 miles home this way, sort of gently provoking and testing the behavior and I can't really make much sense of it. One strange thing I noticed is that the stereo crackles every time the car stumbles, which led me to believe it could be ignition related, but that could just be some strange coincidental symptom.

I'm running a holley fuel pump, with a holley regulator set at 6psi (I need to check whether it's still set properly though) and a demon carb. I have an MSD distributor cap, MSD coil and an older MSD box.

I thought maybe the altitude was the issue but the behaviour is identical at 4000ft and 6000ft (it started when I was at about 5000ft). It's identical with 85 octane and 89 octane fuel. It also started very suddenly when I'd been at 5000 feet for a while, so I'm thinking it's really not altitude related.

it SEEMS to me like something got picked up in the tank and partially blocked my fuel supply, so at a certain flow rate the carb stops getting enough fuel. I'm wondering if anyone has any other ideas? I really don't have a lot of spare time to spend troubleshooting this for the next few weeks, so I was hoping there'd be something easy to check for starters.

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1980 2dr
383 stroker with a phat roller cam and aluminum top end.
Stiff TH350
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 Post Posted: Mon Jul 08, 2013 1:43 pm 
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I'd swap out the fuel filter first. By far the cheapest to replace

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 Post Posted: Mon Jul 08, 2013 1:49 pm 
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Oops, I thought I mentioned that, I did replace that as soon as it started doing this, but that didn't help.

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1980 2dr
383 stroker with a phat roller cam and aluminum top end.
Stiff TH350
3.42 8.5" Posi

Loud, Quick & Ugly.


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 Post Posted: Mon Jul 08, 2013 5:20 pm 
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how much fuel is left in the tank.

Did it start doing this after a fill up?

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 Post Posted: Mon Jul 08, 2013 5:30 pm 
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It started doing it with about a half tank of 89 octane fuel. I thought maybe that the high octane coupled with the high altitude was to blame, but switching to 85 at the next fill up made no noticeable difference. I should also note that when it stumbles, it does backfire through the exhaust sometimes, so at least some fuel is being delivered, it's either the wrong mixture or there's an ignition problem. It didn't at any point backfire through the intake though, so I don't think I'm getting cross-ignition from bad wires or something. The crackle through the stereo every time it stumbled does make me scratch my head. I was also wondering if it was a mechanical/vacuum advance issue but I can't think of a simple way something could suddenly happen to cause these symptoms.

I should also note that upon parking, the fuel in the fuel filter was immediately boiling as soon as the pump switched off, so the temperature may be part of the issue.

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1980 2dr
383 stroker with a phat roller cam and aluminum top end.
Stiff TH350
3.42 8.5" Posi

Loud, Quick & Ugly.


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 Post Posted: Mon Jul 08, 2013 5:53 pm 
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Weak ignition. At high speed / moderate load the fuel/air mixture is leanish and cylinder pressure high -- tough to ignite. I'd look at plug wires first. Sounds silly -- but this happened to me too. The multimeter indicated "infinite" resistance! New wires fixed it.

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 Post Posted: Mon Jul 08, 2013 6:04 pm 
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I will check that tonight. I did have a similar issue with another car and it turned out to be bad wires, but that was my old POS beater. I think the ignition components in this beast are in pretty good shape but it's easy enough to check. The way I discovered it on the other car was actually just opening the hood outside in the dark with the car running. Light show!

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1980 2dr
383 stroker with a phat roller cam and aluminum top end.
Stiff TH350
3.42 8.5" Posi

Loud, Quick & Ugly.


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 Post Posted: Mon Jul 08, 2013 10:29 pm 
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Cam wrote:
I should also note that upon parking, the fuel in the fuel filter was immediately boiling as soon as the pump switched off, so the temperature may be part of the issue.


Classic vapor lock. You have a fuel filter that you can see into, and you can see that your fuel is actually "boiling"?!?! That, sir, is vapor lock. Nothing to do with octane, tuning, jetting, etc... Move your fuel lines away from the exhaust/engine, and isolate it the best you can. If that's not enough, many companies sell heat reflecting wrap for fuel lines and spark plug wires. Just re-route your fuel lines.


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 Post Posted: Mon Jul 08, 2013 10:39 pm 
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I don't know if it's that simple. I will have to report back whether I have the same symptoms when the engine is still cool or the weather is cool. Like I said I can see the fuel boiling in the clear plastic fuel filter after I shut the engine off and the fuel filter is sitting in the hot engine bay sucking up the heat. I think it might be part of the problem though, like maybe some debris in the fuel tank shifted and partially plugged the uptake, and now when the fuel pump pulls more than a certain flow rate it vapor locks.

Don't get me wrong - it certainly seems like a smoking gun... it just doesn't seem like it would be so consistent. It's defo something I'll check though. Might get a chance to take it for some test drives and tinker tomorrow.

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1980 2dr
383 stroker with a phat roller cam and aluminum top end.
Stiff TH350
3.42 8.5" Posi

Loud, Quick & Ugly.


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 Post Posted: Tue Jul 09, 2013 9:38 am 
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Check for a bad pick-up in the MSD Dist . .But I would lean towards vapor lock also. Take it out at night whan its not as hot and see if it acts up.. Get rid of that clear fuel filter before you have a big fire.


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 Post Posted: Tue Jul 09, 2013 9:58 am 
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pldmalibumalibu wrote:
Check for a bad pick-up in the MSD Dist . .But I would lean towards vapor lock also. Take it out at night whan its not as hot and see if it acts up.. Get rid of that clear fuel filter before you have a big fire.


Is there something I should know about using plastic fuel filters? I've never had an issue with one.

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1980 2dr
383 stroker with a phat roller cam and aluminum top end.
Stiff TH350
3.42 8.5" Posi

Loud, Quick & Ugly.


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 Post Posted: Tue Jul 09, 2013 7:29 pm 
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Cracked distributor cap. Been there, done that.

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 Post Posted: Tue Jul 09, 2013 9:04 pm 
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How is your fuel tank vented?

Tommy

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 Post Posted: Wed Jul 10, 2013 9:41 am 
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LS6 Tommy wrote:
How is your fuel tank vented?

Tommy


It's just got the factory tube that was hooked to the evap recovery system. If I recall it's got a filter on the end of it. Having said that, I did notice vapor hissing out of the tank last time I opened the gas cap to fill it up. Could I be building pressure in the tank and somehow causing issues that way? Wouldn't the regulator at the carb still do it's job?

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1980 2dr
383 stroker with a phat roller cam and aluminum top end.
Stiff TH350
3.42 8.5" Posi

Loud, Quick & Ugly.


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 Post Posted: Wed Jul 10, 2013 10:05 am 
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You shouldn't build pressure in the tank, you would create a vacuum as the fuel is sucked out but air cannot go in to replace the volume. This ends up starving the engine of gas.

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