Yesterday I took my freshly frame-on restored 68 L-68 427 tri-power couple out for a ride. Everything's been gone through and the car has been performing pretty much flawlessly since completion, and I've put a couple hundred miles on it only needing to sort some very minor things you would expect considering the work that was done to the car.

I warmed it up and pulled out the driveway and went about a mile down the road and it suddenly made two VERY loud backfires, and died, and I coasted to the side of the road. I couldn't tell if they were on the intake side or exhaust side but they really scared the crap out of me. I popped the hood, pulled the shielding cover off the distributor and checked for loose plug wires or anything else obvious, since I did have a brief problem with the shielded plug wire shorting out intermittently on the + terminal of the coil, which was fixed by just shoving the shielding down a little with my finger. I thought maybe it had shifted back but it hadn't. I jumped back in and the car fired right up and I just drove it strait home and put it away since I had a family dinner to attend and didn't have time to mess with it.

I came home tonight after work and took a better look. It fired it right up. It ran fine and idled in my driveway for about 30 minutes with me looking things over and trying to make it happen again so I could see what had occurred. Nothing... car runs fine and fired right up each time I shut it off. Went up and down the driveway, no problem (long driveway). Didn't take it down the road though since it was getting late and I didn't feel like getting stranded in the dark. This is a 400hp L-68 tri-power car, with the original carbs that seem to be functioning fine except maybe for leaky floats causing me to crank the hell out of it to start it after sitting for a while since the bowls drain.

I can't figure out what the heck happened and being completely OCD, I'm now afraid to drive the car until I can positively identify what occurred. I know a lot of guys would just brush it off as a fluke thing but it's really bugging me. Anybody got any thoughts on what might have happened? I can't make it happen again for the life of me...


thx

 

take a look at the oil for metal flake. you may have a cam gear thats starting to go. typically you wouldnt get the luxury of that start up and 30m idle time, however sans that it sounds like a typical cam gear failure symptom; possibly one tooth failing. Lets hope not, theyre a pita to clean up.

 

WHERE the backfires occurred makes a big difference in providing a good diagnosis.

I ASSUME the engine was rebuilt???

During this 'warm-up' before you took it for drive when it backfired..was it idling very good?

How fast were you going approximately when the backfire occurred???

Were you coming off a higher RPM run when it occurred???

I 100% agree with 'AW IR C3'...because depending on the type of gear on the distributor...which can be influenced by the camshaft used and what the cam manufacturer want for a distributor gear...they can wear. NOT wanting to make you PARANOID....but you only turn over about four pieces of the Jeopardy puzzle. So a precise diagnosis is REALLY HARD to do.

With it cranking back up and not doing it since....it is hard to 'say' due to not knowing if the backfire came out of the exhaust or carbs.

DUB

 

It does sound like your ignition coil got shorted out because when that happens the cylinders pump a LOT of fuel and air into the HOT exhaust system.. When the coil becomes un-shorted the mixture in the exhaust system will go KA-BOOM and that's probably what you heard. I would check your shielding again to make darned sure its not touching your coil in any way.

 

Because you said you had to crank the engine a lot after sitting.I think you have a blown power valve in your center carb.Most of the time if you get a back fire through the carb it will take out the power valve.If the power valve is blown the fuel will leak through the vac port in the base plate. Just a thought

 

Little more information, I do not believe the motor has been apart and was not rebuilt under my watch, nor the previous owner who owned the car since 2007 and who started the restoration. (I bought it as an unfinished restoration project). Before 2007? who knows... It has 77k miles on it according to the speedometer and the original VIN number is still there.

I think I'm going to stop running the car and pull the distributor over my pending two week xmas vacation to get a better look at the gear, as well as the cam gear. With the distributor out I can shine down there with a flashlight and turn it over to see every tooth and know where to go from there.

Thx

 

Quick update, I pulled the dipstick a few times, the oil is clean with no sign of metallic anything under any light.

 

If by chance you have a small bore inspection scope you could try giving it a colonoscopy through the oil drain plug hole and make your way back to the cam gear without having to remove the distributor.

I wouldn't suspect power valve necessarily; typically you get permanent drive-ability issues when they blow. They also tend not to be the cause of a backfire, rather are the result of a backfire.

I think you'll find the cause in the ignition system somewhere. If its from the aforementioned coil situation, bad gear, inductive pickup having moisture or the rotor having rust, etc.

Good luck looking. Keep us updated on what you find.

 

One other thing that occurred to me is the ignition switch. If the contacts are corroded it might be dropping out here and there? Now that I think about it, it does seem like it lost ignition for a split second, then regained it and had to pop off a big charge of fuel somewhere, hence the backfires... FYI the entire harness is new from the previous owner who started the restoration. I've had the keyswitch assembly out but unfortunately didn't think to check it out or try to clean it up.

Also good idea on the boroscope, especially since they seem to have become very affordable these days. You can get them for $20 that plug into the USB port on your laptop. I've got one coming.

I agree the power valve might have been caused to be bad by the backfire, but it runs fine so I don't think it was.

 

I think you have a short. I've blown mufflers right off when this happened. It momentarily shuts off the ignition and fills everything up with raw fuel then when the igmition comes back on and fires....kablambo!!
I think it may be jumping the gun assuming it's the cam gear. Start with the easiest stuff first. Save yourself some money. If you look hard enough you'll find a crappy connection somewhere. Could just be a ground.

 

From left field......Advice I was given about Tri Powers about 35 years ago was that a very common problem is the end carbs not closing consistently/smoothly/quickly. I was told to think of them as a potential intermittent vacuum leak. A lean condition (from a vacuum leak) could give you a good backfire especially before the car was warmed up.

 

One thing to add, the only other thing that I just did to the car just before this was the installation of a remote Ford starter solenoid to alleviate the hot start problem that would not go away. So now the yellow wire goes to that solenoid instead of the one on the starter. The solenoid was a kit from Summit, so is there anyway that that solenoid (probably made in China) is somehow intermittently sending a full 12 volts to the coil instead of the reduced voltage of the resistor wire and would that cause a problem? I checked with the key on and car not running, it was around 6.7 volts at the coil.

Also could it be that my coil is damaged from when it was being shorted from the positive terminal to that ignition shielding? Is there any way to test the coil?


Thx

 

Yep, I split a muffler wide open years ago. Back in the 1950's it was fun to turn the key off and let the car coast for about 50 yards then turn the key back on and........................KA-BOOM. If your coil still produces a spark its okay.

 

Any chance the distributor hold down clamp is loose?

 

I doubt it...if the end carbs open they will draw fuel. and if they were slow or stick you would have idle problems, not a backfire.

it sounds like ignition cut out to me. if you've ever turned the key off and back on really quickly, that is exactly what happens. could be switch, could likely be the wire that had the shield grounding issue.

 

I had the exact same thing happen about 4 years ago. Large backfires and then nothing.
It was my transistor ignition, the pickup coil in the dist. failed.
After a flat deck trailer home, it started right up and ran fine. Credit to Dave Fiedler for diagnosis, he said most of the pickups fail this way.

 

Maybe my recollection is off but I thought the end carbs lacked idle circuits. This drives my previous comment vs the idea that they would just mess up the idle.

 

Knowing that you added more information about the ignition cutting out and then coming back...makes a BIG DIFFERENCE.

I doubt that you distributor gear needs inspection...but do as you wish.

I agree that it is a connection somewhere that is loose or needing repair.

AS stated above.....start simple...because chances are that it is going to be something simple.

DUB

 

My gut is taking me towards the ignition cutting out and suddenly coming back as the cause of the backfires, and I'm leaning towards the ignition switch being the culprit. Since the wiring harness is new I have my doubts on that but I plan to trace the wires out looking for issues anyway. I'm also leaning towards the ignition switch because of my hot-start problem, which lead me to the installation of the Ford solenoid. I'm wondering if the contacts inside are corroded enough to start causing issues with intermittent or diminished contact. I believe it to be the original ignition switch.

Based on that assumption, would it be possible to pop the tumbler assembly out and spray contact cleaner into the switch, then cycle it a couple dozen times to clean it up? I'm not sure how the switch is constructed or if the contact cleaner would get to the right part of the switch, does anyone have one out they could look at?

Alternately, NOS ignition switches are plentiful on ebay- although not exactly cheap... so if it comes down to it I'd be happy to pull the trigger on one for peace of mind.

 

If there is no way that the coil or wire is shorting out to the shielding, I`m going to say that the ignition coil does not like to get hot.