Need help from Gearheads with diagnostic skills

[roostwear]

This may be a little long, and not directly boat related, but this problem is driving me up the wall.
I bought a 79 Bronco to pull the boat on and off the ramp at the Shack. It has a a 400/C6, 153,000 miles, and has a miss. I replaced the plugs, cap, rotor, wires, vaccuum advance, Duraspark module, and EGR valve. Compression is 175-180 in all eight. It idled smooth, but on acceleration, I heard (but didn't feel) a miss. It failed smog with high CO and HC. Brought it home to check it out, and found it needed ported vacuum switches. One of them controls the smog pump which would explain part of the high HCs. I regasketted the carb (motorcraft 2150 2 bbl) just to make sure everything was good and to replace the power valve. I decided to power brake it to check for the miss under load, and sure enough, it's there. Checked the coil by pulling the plugs, cranking the motor and checking the spark. Looked a little weak, so replaced the coil. Same spark. A few have said the old ignition systems don't have the spark the newer HEIs have, and mine is normal. One plug showed little if any firing, so I put the plug on the #6 wire and cranked it... spark. Found one plug carbon fouled, cleaned it and started it up again. Idled ok, but when I powerbraked it, it stated missing badly. Pulled the valve covers to take a look. Spun the engine, and no broken springs, frozen valves... all look to have full lift and move smoothly. I'm pulling 19 inches of vacuum at idle, so there's nothing real obvious there.
There's probably more I've done and haven't listed, but I need some unbiased advice to help figure this one out.

[Fiat48]

Sounds like ignition miss under load and you have certainly covered the bases. First thing I would do is get a look at the timing and how it advances with a timing light. What I would be looking for there would be too much advance for some reason.
Next I would go buy another set of plugs. I have had brand new bad plugs before. A check of all plug wires to be sure they were made right.
Remember "new" is untested.

[victorfb]

i agree with Fiat48. sounds like the ignition advance curve is not working properly. maybe advancing too far too fast. run it with a timing light and see if the ignition timing is bouncing. slowly advance the rpms and watch the timing curve with the light on the timing marks. if its not a smooth advance you may have found your problem. check for weak or broken springs and the advance wieghts. check for improper gear alignment with the cam and dist. possibly too much lash (play between gear teeth). all in all it does sound like an ignition problem by the symptoms and what you have allready described. put some new plugs in and go after that dist. by the way, remember danhercules problem with a decent idle but stumble on accelleration? i told him his distributor was junk. so after countless attacks to the carbs he finally had the carb guy look at the distributor. sure enough it was junk. they replaced that distributer and he is now running strong.

[BrendellaJet]

A bronco with a 400 and an hei?

[roostwear]

A bronco with a 400 and an hei?
I wish. What I said was "A few have said the old ignition systems don't have the spark the newer HEIs have".

[roostwear]

I've suspected the dist might be a problem, but it keeps coming back to 2 things. The miss is consistently on one cylinder, and other than the mech adv, the only thing left is the reluctor/pickup and that shouldn't cause this.
I do get to do one cool thing I haven't done since I worked at dealers..... decarbonize the cylinders :D :cool: Nothing like laying out a giant cloud of carbon and having the neighbors think it's on fire!

[roostwear]

This may be stretching it a bit, but I noticed something odd. The 2 cylinders I have bad plug readings on are 180* out from each other. #1 is carbon fouled, and 4 plugs later is #6 that doesn't look to be firing much at all. I haven't been able to find any other correlation between them. They're not on the same plane of the intake, and they're on different banks. Hmmm

[victorfb]

try a differant distributor i tell ya...lol

[roostwear]

Sure, like I'm just gonna THROW the Mallory from the roadster in there.

[victorfb]

will it fit? id do it just to see if thats the problem. then you can go to kreagan and get a new for the bronco. will a distributor from a 78 460 work? i can bring you one of those to try out.

[Norseman]

Are you sure the new wires are good??
IS the new cap good??
I've had both bad out of the box..
Try another set of plugs!
I'd check them and then try a different dist

[roostwear]

Are you sure the new wires are good??
IS the new cap good??
I've had both bad out of the box..
Try another set of plugs!
I'd check them and then try a different dist
That's why I cranked the engine with a plug in the suspect wire to check for spark. I have another set of new plugs I'll put in after I decarb the engine.

[roostwear]

will it fit? I'd do it just to see if thats the problem. then you can go to kreagan and get a new for the bronco. will a distributor from a 78 460 work? i can bring you one of those to try out.
Yes it fits, but it's all detailed, and I don't feel like pulling the blower, wires, holders, etc to test, then reinstall it and detail again. Rebuilts are only $50. I might go that way.

[Havasu Hangin']

Did you try switching the missing plug and/or wire and see if the miss follows to the new cylinder?

[wsuwrhr]

I tell you what, I would pay to see Roost out in the street powerbraking in his neighborhood.
Brian

[Fiat48]

That's why I cranked the engine with a plug in the suspect wire to check for spark. I have another set of new plugs I'll put in after I decarb the engine.
But that does not load the plug...which is when it misfires.

[roostwear]

But that does not load the plug...which is when it misfires.
True, but this is part of the process of elimination. I spoke with the engineers and techs at work today (quite a few ex-Ford engineers), and will be bringing it in nest week for a group brain storm and put it on the dyno with a scope. One of the advantages of working for an OEM. :D This old stuff really strains the memory when you're used to diagnosing new cars. FWIW, the former Ford techs tend to lean toward the dist, being as there was a problem years ago with the reluctors building a magnetic field causing a misfire. Screw it..... it's off to the river for a few days,

[Just Tool'n]

When you get on the scope look at the firing line, do a raster partern, see if the are all lined up, this will help you detect if you have a bad dist, or if its worn.
Also you can check out the reluctor, & pick-up with a true lab scope.
Just giving you a little tool tip help.

[GofastRacer]

Well I'll probably make an ass of myself here but I didn't see anything mentioned about doing a resistance test on the plug wires, maybe I missed that???, I've had the same problem before and it drove me nuts, come to find out it was a faulty "NEW" plug wire!..

[victorfb]

distributor i tell ya. cmon cmon cmon, dont make me come by and smack that distributor with a hammer so you HAVE to replace it. :messedup:
thats cool mike, great idea talkin with the techs at work. can i bring my boat over there to use that scope too? lol

[Kindsvater Flat]

Well I'll probably make an ass of myself here but I didn't see anything mentioned about doing a resistance test on the plug wires, maybe I missed that???, I've had the same problem before and it drove me nuts, come to find out it was a faulty "NEW" plug wire!..
go ahead and be an ass Art :D
But actually spark is going to take the path of least resistance so you could have a bad wire or plug that could have high resistence causing it to cross fire in the cap. Once its starts in the cap you will have already created a carbon trail causing it to miss with less load each time.

[Fiat48]

But I said "A check of all plug wires to be sure they were made right."
So does this make Art an ass or does it clear his ass? :D
I know Art remembers the Champion Spark Plug Tester that used air pressure to simulate engine cylinder pressure to check if a plug was bad or not.

[roostwear]

Unfortunately, ohming out suppresion wire doesn't work. It may show if there's a bad terminal connection, but carbon core can ohm out ok, but still break down with increased load. The scope will tell all.

[Blown 472]

The ford dizzy is fine, what color ignition mod do you have, should be on the drivers fender with two big plugs and the color is what is on the gromet where the wires come out.

[Infomaniac]

I know Art remembers the Champion Spark Plug Tester that used air pressure to simulate engine cylinder pressure to check if a plug was bad or not.
I have one of those LOL And it has the blaster/cleaner on it. Fits aviation and automotive plugs.
A quick check with a timing light on every cylinder while running will show if the cyl is sparking or not.

[Norseman]

I tell you what, I would pay to see Roost out in the street powerbraking in his neighborhood.
Brian
The neighbors must have loved it when he cleaned the carbon out!!!! :yuk: :yuk:

[roostwear]

I have one of those LOL And it has the blaster/cleaner on it. Fits aviation and automotive plugs.
A quick check with a timing light on every cylinder while running will show if the cyl is sparking or not.
I did that initially, but not under load. I will do it again as soon as I find someone I trust enough to power brake it while I have my ass inside the engine compartment!
The Duraspark module has been replaced (more maintenance than anything considering how often they fail), and the condition is the same.

[roostwear]

The neighbors must have loved it when he cleaned the carbon out!!!! :yuk: :yuk:
De-carb is today. I'll take pics :D

[Norseman]

:D :D
De-carb is today. I'll take pics :D

[roostwear]

Just got done with the decarb and a couple other things, and WOW! 26 years old and 153,000 miles worth of carbon and I got lucky.... no neighbors out! I did smog out one guy waiting at a bus stop on a cleanout run around the block... sorry dude. GM doesn't sell Top Engine Clean in California anymore, so I went with Seafoam. I'll be doing it again before I smog it. I didn't take any pics, cuz this IS California, and I didn't want to take incriminating photos the environazis could use against me. The difference is like night and day...if there's a miss under load, I can't hear or feel it, but I'm still taking it in next week to scope it on the dyno. I got the EGR working properly, so now there are the normal driveability problems, but as long as it passes smog, I don't care. It's sole mission in life is to drive 1 block to launch the boat, and another block to retrieve it. I might wind up putting 50 miles a year on it!
I'll post the results from scoping it next week after I get back from the river.

[Oldsquirt]

Roost, is the EGR valve is mounted on an aluminum adapter sandwiched between carb and manifold? If so, those things were notorious for warping. This could lead to a vacuum leak or even a little unintended EGR action by way of a leak from the exhaust passage into the manifold.
A couple other things to consider....Valve springs that, while not broken, are weak. Intake gasket leak from inside the valley(bottom of gasket). Not common but I have run across that a few times in my career.

[Infomaniac]

If those are the original springs. THEY ARE DEAD !!!
No doubt about it.

[72 Hondo]

Roost, is the EGR valve is mounted on an aluminum adapter sandwiched between carb and manifold? If so, those things were notorious for warping. This could lead to a vacuum leak or even a little unintended EGR action by way of a leak from the exhaust passage into the manifold.
A couple other things to consider....Valve springs that, while not broken, are weak. Intake gasket leak from inside the valley(bottom of gasket). Not common but I have run across that a few times in my career.
Something about Fords and EGR valves dont mix. My 86 Mustang GT had a small leak in the gasket and missed on all of the RPM range. One thing diffrent, it whould surge at idle. Normal for about 5 seconds and then the surging began worse and worse until it died at most lights. All because of that small leak and a carbon build up in the valve (stuck open).
HEY ROOST, I want to know how to de-carbon an engine. My wifes Volvo has 267,000 miles on it and, its starting to miss a little. Its a 2.3L with a 16 valve alum. head also fuel injection.
I should buy her a new car but that means my boat money will be gone! :cry:

[canuck1]

Smog???....WTF is smog?
:wink:

[roostwear]

You can decarb an engine with Seafoam (what I used this time), GM top engine clean, or BG 44K (good stuff). In a nutshell, on a carbureted engine, pour the decarb down the throttle bores evenly and moderately. Give it enough throttle to keep the engine running until you are at 1/2 can. At this point, open it full throttle and dump the entire contents in. This will kill the engine. Remember, if you have a dual plane manifold, pouring down one side only cleans 4 cylinders.
The instructions say to let it soak for 15 minutes... I let it soak for 1 hour, more on higher mileage engines. Fire it up and let the fun begin! I will be doing it another time to get more carbon out.
Before I took off Saturday, I put the timing light to the engine to check advance. I didn't have a tach on it, but I brought the rpm up, and there was no advance until about 2500 rpm. This is about the point where there is a stumble, so even though it is advancing, it is not advancing smoothly.A rebuilt dist is in it's immediate future. :D

[roostwear]

Pulled the distributor last night and tore it down. All I can say is DUH. The pivot for one of the advance weights is loose, and the plastic bushing/retainer for the pivot is laying in pieces in the bottom. This explains the advance not coming in until 2500 (the rpm range during the emission test), and the misfire. A rebuilt will go in tonight. Hopefully the cat isn't junk..... :frown: