Help! 351C backfires & dies

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barkstang

Member
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Jun 11, 2016
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Location
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My Car
73 Mustang Convertible, mostly original

HELP! I had some work done recently because I needed a car lift and don't have one.



Work done: freeze plugs replaced, manifold gaskets replaced, new Holley 600 cfm, new distributor (Cardone), removed Pertronix and replaced with points condenser.


 


Ran nice for a while when i got it back except trans shift points went up too high. The mechanic advised i just adjust the FMX modulator, but i suspect some kind of vacuum leak. Now it starts and idles ok for a very short time, maybe a minute then starts to backfire (exhaust) really loud and then dies.


 


PVC is ok. Set points gap and timing, ( a couple times) wires should be ok, they are nice Taylors. was going to put my Pertronix back in but won't fit the Cardone distributor.


 


Any ideas?





 
Is it common for the modulator diaphragm to start leaking then rupture? Or does it sound more like a manifold leak? Where are likely suspects i should start looking?

 
I have seen the modulator leak, but the fact that you are getting backfire as well tells me vac leak especially since you had the manifold off recently and gaskets replaced. They are difficult to get on straight and sealed correctly. If you have a vac leak the modulator will basically be non-functional and rely on the trans for shift points, hence the high shift points. My guess is a leak at the intake manifold around the ends. What engine do you have?

 
Check all the vacuum lines for cracks and leaks. It's also possible that the work dislodged some gunk in your fuel line and it got into the carburetor and plugged some passages.

 
unplug all vacuum accessories and plug the vacuum tree to rule out a leak from them including the transmission modulator.

retest

possible choke issue on new carb.

you may want to disable the vacuum booster if you have power brakes as well for testing purposes.

 
Another possibility is that either your coil or the condenser starts breaking down as it warms up, which would cause it to misfire, allowing gas into the exhaust and igniting when it fires again.

 
Have you put a vacuum gauge on it?

You might double check that you have a resistor wire at the coil. With the ignition in the ON position check the positive side of the coil.

Also, make sure the negative wire from the coil is in good shape and not grounding on anything. This should go right to the condenser.

 
Just a suggestion... depending on the quality of the points that were installed, the rubbing arm that rides on the lobes may be loose, worn down or, the points themselves may be bouncing.. Whenever I install new points, I put a small dab of Standard Ignition Cam Lube on the lobes. A thousand years ago, the arm that ran on the distributor cam lobes was made of phenolic.. The problems began to arise when it was replaced with nylon.. Heat generated from a lack of lubrication would melt the nylon and wreak havoc with the points settings..

Just sayin

 
Thanks for all the suggestions. I will start with plugging all the vac lines and tree to see if that makes a difference.

Re: the points i did lube them and have checked them. I would really like to get the Pertronix back in. The new Cardone distributor has a different shaped seal where the wires go through the side. I would have to do some modifications but may see if i can make a new seal.

Since the car seams to start ok then quickly go down hill, you may be right. Might be an issue with the new choke. I'm about ready to give up on Holley. I think this is the 3rd or 4th one in the 15 years I've had the car. I can rebuild one and set it up in my sleep. I'd love to go with an efi set up but.....big bucks.

Lastly I will check the manifold gaskets. Can you tell I'm avoiding that?

How do you handle putting gaskets on with the valley pan? A little RTV under the valley pan then gaskets on the pan? I always put a dab of RTV on the ends of the end seals too. I've even just used a bead of RTV in lieu of cork end seals. Put on a bead then let it set for 10 minutes to skin over then drop manifold on. Works well and doesn't crush like cork.

Thanks everyone for the great ideas. I will keep you posted on the outcome.

Oh, It is the 351C. I think that question was asked. Didn't add it since the title of thread is 351C backfires then dies. Not trying to make you feel bad. I do stupid stuff like that all the time.

 
I can't offer much about the backfire other than your timing maybe off. What I can offer is my very recent experience with Pertronix and distributor issues. (See my other post)

First, do you have a full 12V to your Pertronix? In other words have you bypassed the resistor wire. You did not say which Ignitor you have, 1,2 or 3. The P3 MUST have a full 12V, but the earlier modules can run on resisted current (according to the instruction sheet), but are better with 12V. Your Coil may also be suspect as well if it's not a high output Pertronix. As for your Cardone distributor, I think you have the new one from Cardone Select that has the same slot as the Motorcraft Duraspark , in other words it is open and not just a hole as in the original dizzy. I would just "fix" this after placing the wires where needed by placing a bit of tape on the inside then using a dab of RTV to fill the top space, then remove the tape when cured. Also, the threaded holes are metric M4 (I think). If it's just a reman., threads should be 8-32.

There are many potential problems with the reman or "new" distributors that I was NOT aware of until I was forced to figure it out for myself. This could get very lengthy, but in brief is your timing correct, 34 deg no vac @ 3K? Do you get any pinging on acceleration? Is the vac advance set to add no more than 10 degrees?

In my case, I have a 71 351C 4V manual trans, so it will be a bit different to an auto set-up, but I needed to reduce the advance stop slot (welded it up and recut) to give me 20 deg on the crank, 14 deg initial and hardly any vac advance added. The new Cardone Select dizzy I have has an L15 slot, 30 deg on the crank. If you have 14 - 16 deg initial, then you would have 44-46 which for me was way too much timing.

Also, if you do have a Pertronix III (P3) I can tell you from experience it is a piece of junk. I sent mine back with a strong letter of complaint. Nothing wrong with the module itself, just the bad construction for the Ford distributor. It has a plastic sleeve/bearing between the two plates that is a poor fit, very loose and causes the rotating top plate to move laterally when vacuum is applied and the pick-up hits the cam lobes. I replaced it with a P2, no more issues.

There's a whole can-of-worms here, but while you may have vacuum leak issues and not timing causing the problem, I thought it worth going over as it has taken most of the summer for me to finally get a distributor that works. It took spending money to get it on a distributor machine to actually see what the problem was. Working in the car seamed okay, but was actually far from it. A timing light will only get you in the ball-park. I have found that today's distributors, remans or new are far from original spec and are "generalizations" that need work to get right, slots, springs, weights.

Anyway, I may have been going over what you already know, but others my benefit from my issues.

Good luck sorting it out and let us know what the cause was.

Geoff.

Edit: don't forget you still need a ground wire in the distributor between the movable top plate and the body when using a Pertronix Ignitor, 1,2, or 3

 
Last edited by a moderator:
The end seals are junk. Toss them and just use the RTV.
I used RTV and still got a leak. Then I used "The Right Stuff" and no more issues....... other than the cost of it and doing the job over!! I agree with you though, the rubber end seal are junk.

 
Weird. I've been running a Pertronix III for about a year now and have had no issues and fitment was good.
Mine had .020" play in it between the top plate and plastic bushing as well as up and down slop. Perhaps yours is not worn out yet if that proves to be the case with mine. It was 3 summers old. It has been returned to Pertronix for evaluation. I actually did not realize there was a major problem until it was put on a distributor machine where it could be seen. I had been chasing bad timing for a loooong time thinking it was something else, but like I said, a generalized slot plate, too much crank rotation for my engine, was the prime issue compounded by the Pertronix III.

 
I will have to keep an eye on it. Thanks.
Not to get off the original post too far, but yes, keep an eye on it. Perhaps it has been improved since mine was installed in my Motorcraft dizzy housing. I'm just relating my experience with this product. Also read my post in "Temperature gauge and Pertronix" in Electronics section.

 
Hi everyone thanks for all the ideas.

- Pertronix issues: it is a Pertronix I but doesn't matter. the distributor cams on the New (not rebuilt) Cardone distributor are smaller (shorter?) than the stock distributor. and the trigger ring doesn't fit correctly on the new lobes, (way loose, flops around). So I still have in my new pts/condensor. I've tried 3 coils, my new one (external resistor), Pertronix Flamethrower, and original coil. None seem to make a difference. The flame thrower is on it now and I noticed it was very hot. Could barely touch it. Is that normal? My electrical manual shows a resistor off the ignition. So, assume that it goes to the coil that way. Any comments?

_ Plugged all the vacuum lines and it seems to run ok. I put a new PCV valve and vac hose on and it started missing and dying again. Took it back off and plugged the carb. runs ok again. Not sure what that means since it is a new valve and hose. put on a new brake booster line and was able to drive the car reasonably well up and down the street.

- Still just shifting on internal shift points but then no vac hooked up. I think i will run a line directly to the trans and see if that helps with shift points.

- The whole engine seems to be hot. Timing is ok. I will check the thermostat. maybe that isn't opening. Might be the cause of all the problems as it runs fine for about a minute before starting to miss and backfire. Is it possible that has been the problem all along? Not sure if overheating could cause those symptoms. any feedback?

Thank again for everyone's help!

 
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