Author Topic: 351W will not idle and backfires.  (Read 5073 times)

Last of the Machicans

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351W will not idle and backfires.
« on: September 20, 2009, 03:52:41 PM »
     I bought the car from someone who had it stored for 4 years. It will start and run if I hold it at a high rpm but it will backfire intermittently. If I drop the rpm it will die. It is a 351W auto with a 4v Holley carb, which I removed and cleaned. The gas has been flushed, new fuel and air filter. I checked for vacuum leaks.

     As I understand it, the car was running when parked 4 years ago, so timing shouldn't be the culprit, right? I am going to do a compression test today. Any thoughts and ideas would be greatly appreciated.

Thanks!

Soaring

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Re: 351W will not idle and backfires.
« Reply #1 on: September 20, 2009, 05:37:56 PM »
If it is backfiring, it is either timing or some other electrical problem like the coil is shot and needs to be replaced.  Do you have an electronic system on it, or do you still have points? 

Last of the Machicans

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Re: 351W will not idle and backfires.
« Reply #2 on: September 20, 2009, 08:51:45 PM »
Thanks for the reply Soaring.

It does have electronic ignition: HyFire VI-A, Mallory system to eliminate the points and a coil upgrade. 

I just now got the car to idle and drive via tinkering with the carb. It still isn't anywhere near perfect and thinks about stalling if I try to change the RPM's quickly.

 Though I felt confident in my visual vacuum tube inspection I went and bought a vacuum gauge anyway (never had one for some reason), hooked it up, and the pointer doesn't even leave the zero mark...  Which, I suppose, can only mean a massive leak somewhere. 

Back to the drawing board. >.<

Soaring

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Re: 351W will not idle and backfires.
« Reply #3 on: September 21, 2009, 05:24:50 PM »
Yeah, if you have a massive vacuum leak it will also act like you described.  Start with under the carb and your intake manifold.  Spray some carb cleaner around the gasket, and if the RPM's pick up, then there is your problem.  I would also go ahead and replace all the vacuum hoses on that engine because pin holes and rips in them can be disguised.  They are obviously old after being stored for so long, and God knows how many mice and other rodents have chewed on them.   

Last of the Machicans

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Re: 351W will not idle and backfires.
« Reply #4 on: September 21, 2009, 08:04:48 PM »
Once again, thanks for the reply.

I am going to the garage right now to find that darned leak.

I'll post back if I come up with an answer so the next guy with a similar problem can have some hints.

Last of the Machicans

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Re: 351W will not idle and backfires.
« Reply #5 on: September 21, 2009, 10:10:24 PM »
I'm getting closer to finding the problem.  The vac lines on this car are hooked up unlike any vac diagram I can find, so it is slowing my newbie butt down a bit.  :D 

There is a vac line from the "Power brake" port on my Holley carb that is hooked directly to a 3-prong loop attached to the firewall (the actual power brake can is hooked to vac supply on the intake manifold). The two other prongs on the loop have ancient-looking vac hoses that are routed down below the firewall. 

I removed the hose and plugged the "Power brake" port on the carb (which should remove the loop and those two old vac tubes from the vac system) and now with the car at idle I still have no vacuum, but if I bring it up around 2k-3k rpm (tach doesnt work yet or I'd be more certain) I get 10 -15 inches of vac. Before I plugged it I had no vacuum at any rpm.

Sooo, it seems I have a vac leak in one or both of those two tubes attached to the firewall loop.  The vac diagrams I have show only a 2-prong loop (mine is three) with only one extra tube that goes to the transmission.  I am assuming one of my two tubes goes to my FMX, which I will replace in the morning.  The other tube, I am unsure of, but I am thinking that it goes to the climate controls inside the car (will trace it in the a.m.)? The heater controls seem to have been rather hastily removed by the previous owner, which I am guessing explains some of the vac leak from one of those tubes.

The power brake can won't have another vac tube leaving it, aside from the line directly to the intake manifold, correct?  My diagrams do not show brake vac.

If my oil filler cap has breather holes, can I just cap my PCV valve at the valve cover and the carb just to eliminate one more vac hose?

Wow... that was long winded, sorry!  :o

 

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