this is my last resort ... 5.0 giving me a hard time

[turbostang79]

over the past 2 weeks several mechanics tried to fix my 5.0, none of them really had an idea of what's going wrong. I'm not a professional mechanic either, I can do most repairs, solve some problems but this is freakin me out

The engine starts fine (carbed 5.0), idles fine without vacuum advance, timing is at 10°.
I connect the vacuum advance and the engine immediately starts knocking, so I give it a little less advance, idles fine, you can hear a little bit knocking at the high rmp's, but it's undriveable. It doesn't go faster than 40 mph. She has got so much missfire, and at WOT there's constantly backfire in the carb. I pull over and rev it high, everything ok, drive for 20 meters, problem is back again.
I took a mechanic for a ride, every 20 meters he stopped and adjusted the timing, but he cannot get it to run as it's supposed to be. He'd have to set the timing at almost TDC with the vacuum on the distributor.
There's a new Holley500 on the intake, the vacuum advance is connected to the manifold. I know it should be on the carb, but there's no connection for the vacuum advance on it.
Anybody ever tried it like this, connected to the manifold ?
I don't think buying a carb with spark advance will effect my problem ?
We had a look at the timing chain, there was a lot of play, so I replaced it, but didn't affect my problem. Cam and crankshaft are dead-on right.
I turned the crankshaft with my finger in spark plug hole #1 until I feel compression, balancer marks TDC.

What could cause this engine to missfire so much, and backfire at WOT (when driving)?

 

[stangbear427]

I assume it hasn't always run like this? Did it just come out of nowhere or were you doing something/changed a part, etc. Can you trace it back to a specific event? Also, what is the rest of your combo? I was having a similar problem with my '84 5.0, turned out ot be the distributor shaft was broken at the gear. Not saying that's what it is, but it does sound ignition related the way you describe it and not knowing your parts list.

 

[ashford]

check the dist cap for carbon tracking too

 

[turbostang79]

I assume it hasn't always run like this? Did it just come out of nowhere or were you doing something/changed a part, etc. Can you trace it back to a specific event? .

The engine comes from an other car. When I bought it, she ran and drove like a beauty. I swapped the engine into my car, using all electronics and engine related parts from the donor car. I drove the donor car for a week and a half, I was convinced that the engine was fine. I then proceeded to do the swap.

After putting the parts back in place, she started great, she misfired when revving it, but I just thought it would be lean or something.
I drove the car to my friend, who has a shop, and he wasn't able to make it drive right. We made some progress, when parked, the idle is good, and revving is no problem.

He's convinced that it's a ignition problem, but we tried everything for the timing, and can't find a broken part ?
I just can't make the link between being able to rev when parked, and misfire when driving.

Also, what is the rest of your combo? I was having a similar problem with my '84 5.0, turned out ot be the distributor shaft was broken at the gear. Not saying that's what it is, but it does sound ignition related the way you describe it and not knowing your parts list.

I checked the distributor axle, I can only move the rotor a small bit, I think there's a spring beneath it giving it some free play ? the shaft should be fine.
When we removed the timing chain, I had a good look at the gears for the distributor, they are still in good condition.

combo : holley 500, flowmaster american thunder, ford headers that were allready on there, not stock, they are like shorty headers, auto tranny, motorcraft sparkplugs, the rest is all stock.

 

[turbostang79]

check the dist cap for carbon tracking too

yeah, that's one of the first things I checked, there was some white dirt on the connections inside the cap, I just sanded it off.
For the rest the cap is really clean, and looks new. No visible cracks or other damage at all.

 

[BlackGT89]

I would try this:

- Check the float level in the carb (too low = too lean = misfire).

- With a carb, if you ever have a backfire there is a good chance the power valve in the carb has been damaged, so check that. But that would also be accompanied by a rich condition with black smoke out the pipes.

- Do a vacuum test (checks for leaks, fluctuations etc.) The lean condition could cause the misfire. Here is a good website for vacuum tests: http://www.fordf150.net/howto/diagnoseengine.php

- Open the hood at night at a place that is pitch black (watch out for the fan, etc. !). Give your eyes time to adjust to the darkness and look for errant sparks when the misfire begins.

Good luck !

 

[hllon4whls]

My suggestion to you is different approach. I had the exact same thing on my race car except it was valve float. I tried every damn ignition thing that I could think of and it ended up being valve float. It would rev to 6500 no load and be fine. It would break up and not run over 4k with a load on it.

Since you have tried everthing else, a new set oif springs or a valve spring tester would not hurt a bit.

 

[hllon4whls]

I forgot to mention the vavle springs problem I had was just as sudden. On week the car ran great, 2-3 weeks later after sitting it wouldnt run. Took me many tries at the track to get it straight.

 

[1990Coupe]

I had the same problem with my car, only mine is Fuel injected.. My problem was the same, over 40 mph it would misfire, studder, couldnt rev it over 5000 then eventually 4000 then eventually 3000 rpm's. Turned out to be my fuel pump that was on its way out. She would rev fine under no load, but once i had a load on her, she would misfire. I dont know if this is the problem, but its worth a shot to stick a fuel pressure gauge on there and see if it dies off.

 

[cjman15]

I would do this... scrap your engine and get a EFI motor. Or just scrap tha whole thing and get a new one

 

[turbostang79]

I would do this... scrap your engine and get a EFI motor. Or just scrap tha whole thing and get a new one

No go, I just spent all my free time last month on swapping the carb engine

thank's for the input, there is some great stuff mentioned here

- fuel pump is actually new, got it a few months before the swap, it's a Holley red pump. Makes a lot of noise, but the fuel gets in the carb :d

- valve float is a possibility and seems logical, I'll see with the mechanic if he knows a way to test these valves.

- carb float level is set by the book, when opening the screw in the float body, fuel just flows a little bit out

- the link for the vacuum diagnose is great, I'm going to hook up a meter on the engine asap !


I actually just got back from the shop. I worked a little on the car this evening.
To be sure the misfire was not coming from the carb, we took another one, and installed it, with vacuum advance for the distributor.
When the engine was started, it still didn't run great, but there was this enormous air sucking sound coming from behind the carb.
The gasket provided with the holley 500 didn't do his job, and air was leaking in between the carb and intake.
As a quick fix, I removed the gasket and put some silicone in place.

The misfire is almost gone ! The engine still doens't have much power, and at WOT she'ed still backfire.
When connecting the vacuum advance, coming from the carb this time, the knocking sound was back.
But at least most misfire is gone .... pfew, one step further than last night.


Thank's for the input, the support keeps me motivated, I'll get there, in time...

 

[dastang2]

have your heads checked or rebuilt and cleaned of all the carbon

 

[BarnStang]

There was also another post I read through a few weeks ago with simular problems that was failed intake gaskets. Put on new but leaked. Sometimes you can find that with starting fluid (ether), but if its leaking under the intake, nothing sprayed on the top will pinpoint it. You should be able to see that it was leaking when you pul them out. Lets hope that is the last resort. My first call was going to be the timing chain. Sounds like thats straight though.

 

[stangbear427]

The engine comes from an other car. When I bought it, she ran and drove like a beauty. I swapped the engine into my car, using all electronics and engine related parts from the donor car. I drove the donor car for a week and a half, I was convinced that the engine was fine. I then proceeded to do the swap..

When you say "all the electronics" do you also mean the computer? If so, what year was the donor car? (was it a Mustang engine?) Do you drive it everyday now? What kind of mileage are you getting? I'm sure you thought of this, but what are your plugs gapped at? Also, when using silicone as an intake gasket, make sure you let it sit overnight at the least before you start it.

 

[SmockDoiley]

Your timing and or A/F ratio is off. Your either way off on timing or your running too lean. How have you tested your A/F? Id try a new distributor too or rebuild it. Just some suggestions.

 

[ashford]

oh i just had a brain fart. a friend of mine had the same problem he worked endlessly on it to try to fix it. he had the firing order wrong

 

[turbostang79]

When you say "all the electronics" do you also mean the computer? If so, what year was the donor car? (was it a Mustang engine?) Do you drive it everyday now? What kind of mileage are you getting? I'm sure you thought of this, but what are your plugs gapped at? Also, when using silicone as an intake gasket, make sure you let it sit overnight at the least before you start it.

Computer has been swapped, I tried another one, still backfiring.
Can't drive the car like this, the backfire can't do any good, and doing 40 mph on the highway should also be interesting
Plugs are gapped at .032
Maybe my spark plug cables are too resistant, and at load they don't deliver enough spark ? I'm just going to get new cables to be sure.
They're 8mm cables now.

Id try a new distributor too or rebuild it

I wish I had a spare one laying around, I'm looking for a replacement, but those things aint cheap

edit : oh yeah, the firing order.
If I had a wrong firing order, could it run that great at idle ?

 

[r6bullet]

Replace the entire ignition system, Get MSD or something like it

 

[ECU5.0]

open up youre spark plug gap to like .045

 

[BarnStang]

Yes, it could run ok at idle. Depends on whats out of order. You can pull one plug wire off and it will not be immediatly noticable. Check and recheck this. You might find the problem.