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Stumbling and backfiring


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#1 mikeringland

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Posted 10 October 2017 - 02:48 PM

I have a bone stock 350 4bbl in my '72. I took it for a ride over the weekend and under acceleration it was stumbling awfully and if I gave it too much gas, it would backfire. When it's idling, it runs perfectly fine, only in gear under load it's doing this. Any ideas??

 

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#2 Rob Peters

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Posted 10 October 2017 - 05:23 PM

Possible vacuum issue at the distributor for one thing.  

Possible timing/dwell issue for another.

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#3 mikeringland

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Posted 10 October 2017 - 06:49 PM

By vacuum leak at the distributor, do you mean the vacuum advance? I attempted to replace that at one point and it almost looks like it was riveted to the body of the distributor. The replacement one I had looked different, so I never replaced it. Any thoughts on that? I just replaced the points and condenser and set the dwell and timing over the summer. I'm thinking maybe the distributor bolt isn't 100 percent tight and the distributor turned.

#4 Mo's70MCs

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Posted 10 October 2017 - 08:17 PM

If you have a separate as well as the uniset style points, condenser gone bad...also check the distributor cap for carbon tracks or cracks.

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#5 stangeba

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Posted 11 October 2017 - 07:00 AM

Last time this happened to me, the plug wires had started to go bad. I checked them with an ohm meter and some were way off. Replaced them and all was good.

Good luck!


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#6 cny first gen 71

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Posted 11 October 2017 - 03:14 PM

A lot of good suggestions here. When was the last time it had a good tune up? Could be a tune up issue. Good place to start.
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#7 stangeba

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Posted 11 October 2017 - 04:48 PM

If / when you have all the spark plugs out of any vehicle, with a performance problem, I strongly suggest running a compression test. I recently learned proper procedure is to open the throttle some (not at idle position) for proper air flow.

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#8 mikeringland

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Posted 12 October 2017 - 10:06 AM

I inspected all of the spark plug wires. they all had great resistance values, but the #1 had a tiny hole in the insulation, so I replaced it. In terms of a tune up, the plugs, wires, cap, rotor, points and air filter are under a year old.

 

The next thing I checked was the points dwell and timing. I put new points in back in June. I set them to 28 with a dwell meter. I checked them today and it was at 11. I hardly drive the car, how did the dwell change so drastically already? This is why I've never been a fan of points.

 

Timing seems to run rough when at 0 or advanced at all. I have the timing retarded now and that's producing the smoothest idle. Not sure what to think about that.

 

The next thing I'm going to look at it the vacuum advance. I took the car for a quick ride, it seemed a little better, but lurches and pulses when enough acceleration is applied, I'm thinking if the vacuum advance doesn't solve it, maybe running too lean? Thoughts?



#9 stangeba

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Posted 12 October 2017 - 10:32 AM

Question, was the vac hose removed from the dist and plugged when checking the timing?

I have heard of the pulley / harmonic damper slipping (outer pulley is attached to the inner hub via rubber) and making the timing marks way off.

Perhaps time it by ear, advance till it runs good, yet doesn't spark knock on heavy acceleration. After (if) it runs good then check the timing again.

I wonder if that set of breaker points was made in the USA or someplace else..........

Good luck!

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#10 mikeringland

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Posted 12 October 2017 - 11:44 AM

The vacuum hose was not taken off of the distributor and capped off. I forgot to do that. I have the vacuum line for the distributor coming off my carburetor. Is that correct or should it come off the intake manifold? The port I'm coming off the carburetor with does not have suction at idle.

Also in regards to the harmonic balancer, it is original and has definitely slipped. It now lines up with the #3 cylinder with the timing light rather than #1. I'm basically timing it by ear now till it runs the smoothest.

#11 mikeringland

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Posted 12 October 2017 - 01:17 PM

I ended up moving the vacuum line for the distributor to a different port on the quadrajet carburetor, one that has suction at all times. I also used a vacuum gauge and a portable tachometer to set my idle mixture screws. I had to back them out a few times, so I believe I was running too rich??

In any case, I took the car for a quick ride on the highway and the stumbling is gone, feels like it has good power again.

#12 stangeba

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Posted 12 October 2017 - 03:05 PM

Attached File  PC210002.jpg   161.71KB   0 downloads

 

Here is my 71 350 4BBL. I removed the TCS system years ago, in 1972.

I run my advance line from "ported spark" so it only has vacuum when you accelerate.

This hose is "Teed" in the fuel tank evaporative canister on the driver side ft. The diaphragm for that canister was leaking vacuum, I replaced it.

As for Ported vs non-ported vacuum, try it both ways for a while and see what works best for your driving style.

I believe for non-ported, as soon as you hook the vacuum hose up, the timing advances at idle and the speed increases so the idle needs to be readjusted.

Good luck.

Bruce


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